Guitar Headphone Amplifier

[tech_builder]

Hi,

I am designing a simple guitar headphone amplifier for my son, and I would like to get some feedback on the design I have come up with. I've spent some time looking around at various incarnations of headphone amplifier and decided that I wanted to use jellybean parts and not specialty parts like an LM386 just to make it more interesting. I’m not interested in pristine audio quality as it is a for guitar, but I don’t want noticeable distortion when playing clean tones either.

It seems to me that one of the departures guitar headphone amplifiers have from a typical headphone amp design is the requirement to manage the input attenuation of the device as guitar pickups vary a lot in their output. I decided to gang opamps in series with the first one attenuating and the second one acting more like a volume once the guitar level was set. This method does not let me turn the follow all the way to zero though. Any suggestions on making this better would be welcome.

Once past the input stage the output is a conventional class AB amplifier with a simple Tl082 driving the transistors. I’ve seen some designs with an output cap and some without, I would like to use one to deal with possible DC offset, however I’m not too sure what size to use or what type. The ones I’ve seen seem large enough that they would need to be an electrolytic cap, but I’ve heard that electrolytic are not very good for audio applications (at least in the audio path). 220uF was one value I saw, but 47uF was used in the MXR headphone amp. I saw another one with 1000uF too.

The power stage is a buffered virtual ground design I scrounged on the internet. I’m not sure if it is overkill, or if a simple resistor and capacitor divider would work just as well. I thought it would be fun to try

I’ve built the circuit up on a breadboard and for the most part it sounds ok. Getting it to not distort at a decent listening volume can be challenging, which is something I’m hoping to get some advice on before committing to a circuit layout. I’m also happy for any pointers to existing designs that I can look at as well.

Thanks!

[DrGeoff]

You might want to set the input impedance of the input stage to around 47K, as this is what most guitar pickups expect to see.

[Keicar]

My understanding was that an input impedance around 1M? was more typical.

[Zero999]

The input has no biasing resistor, meaning the output will quite likely saturate at either supply rail, when the input is disconnected. Add a resistor from the input to 0V: 1M does seem reasonable.

Running the TL082 off a total supply voltage of 9V is marginal. It's designed for higher voltages (>10V) than that. Most of the datasheet doesn't specify it below 10V (+/-5V) and the lowest supply voltage in any of the graphs (figures 6 & 12) is 7.5V (+/-3.75V) which won't give you much battery life. It also depends on how much voltage swing you require. I'd consider using an op-amp which is fully specified to work down to 6V.

http://www.ti.com/lit/ds/symlink/tl082.pdf

[Buriedcode]

Most guitar amps have an input impedance of ~1M.  Lower and it can roll off the highs (not that electric guitars have much in the way of high frequency content).

I'm assuming by 'headphone amplifier' you mean a box you plug your guitar into, and headphones into, so he can hear the guitar - rather than just a headphone amp one would plug into the 'headphone out' of a guitar practice amp which isn't loud enough ?

In this instance, whilst starting off just 'making an amp' would be good, I think you should think about the possibility of adding some sort of overdrive and/or EQ.

If he's learning, then although it completely takes the fun out of DIY, there are some very nice cheap portable multieffects units about, which are pretty much designed for headphone practice, and have all manner of 'tones'.  I found when learning a touch of reverb completely changed how I played, and also.. the 'nicer' tone made me play more.

As for the opamp choice, I've seen guitar outputs hit +/- 4V for the higher output humbuckers, so if running off 9V you might consider something with rail-to-rail inputs.  But as you said 'jellybean' parts that makes it difficult.  A TL082 is just fine for guitar (many guitar pedals use the TL072, but a TL082/4 is in quite a few, electric guitars dont' really need much bandwidth). But as you noticed, clipping might be an issue (input or output).  You can always just use two 9V batteries, but I hate those things, expensive, and won't last long.

[tech_builder]

Thanks for all the replies!

DrGeoff: I’ve never heard that guitars expect to see 47k impedance. Where does that spec come from? Does it change from pickup to pickup?

Hero999: Thanks for the heads up on the input biasing resistor, it was definitely saturating! I decided to change the op-amp to something rail to rail. I found an interesting website that looks at a number of modern op-amps for application to audio:

http://www.cycfi.com/projects/six-pack/op-amp-shootout/

I went through his list and found a few that I thought could be suitable candidates: OPA1664 looks like it could be good, and it’s relatively inexpensive. I also found the OPA4171 as well.

Buriedcode: He has an amp already with headphone output, but he wants something that he can move around with and won't be a big distraction for his baby sister. He's not too picky, and it is a win-win situation for us both . He gets a guitar headphone amplifier, and I get to make something cool. I will probably iterate on it in the future sometime, but I have some other projects I would like to get to first. As I mentioned above, I decided to go with different op-amps that are designed to operate at lower voltages and are rail to rail.

I’ve attached an updated revision on the guitar headphone amplifier. I included the input biasing resister mentioned by Hero999, as well as the new op-amps (in this case the OPA1662, which was modeled in TINA TI). I also changed the volume portion of the input to be an inverting amplifier so that I could get a gain lower than one. I am having trouble getting TINA TI to run with these op-amps in the circuit though. It keeps telling me I have convergence problems, and I’m not too sure what that means. The circuit simulates fine with TL082s.

[BrianHG]

Don't you have the + and - inputs of opamps U3&U4 wired backwards?
And wont the way you wired the volume in U2 only go from full volume loud to super loud? (Unless you intend to distort the sound)
Also, having a volume control knob wired into the feed-back circuit of an op-amp.  This isn't usually done in practice for interference and noise & performance/signal gain issues, but it can function.
C5 is also shorted out and of no use in this new schematic, it does not match the first schematic.
There were fewer mistakes in your first posted schematic.
Also, the output drive has enough drive for both ears of your headphones.  I recommend getting rid of 1/2 of your output stage if you want to save parts and a bit on battery life.  Just use a second 220uf cap from the transistor emitters to the second earphone channel to maintain low frequency response.

[MagicSmoker]

...
http://www.cycfi.com/projects/six-pack/op-amp-shootout/

I went through his list and found a few that I thought could be suitable candidates: OPA1664 looks like it could be good, and it’s relatively inexpensive. I also found the OPA4171 as well.
....

Op-amps for driving headphones need a bit more grunt that the usual "audio" op-amp like the venerable NE5532 or LM833. Personally, I really like the sound of STMicro's TS922AIN, and it has 80mA max output, which is extraordinarily high for an op-amp in a standard SO-8 package. It is a bit of a power hog, though, requiring 2mA supply current minimum.

The OPA1664 looks good, too. In fact, there are dozens of op-amps that will do an excellent job, and trying them out could be a worthwhile pursuit in itself. Use a socket for the op-amp to make it easy to swap it out.

[Audioguru]

The 10nF capacitors that are parallel with all negative feedback resistors cut all the mid and high audio frequencies.
For example when P1 is set to halfway, its 25k ohms and the 10nF capacitor cuts frequencies above only 640Hz.

[BrianHG]

The 10nF capacitors that are parallel with all negative feedback resistors cut all the mid and high audio frequencies.
For example when P1 is set to halfway, its 25k ohms and the 10nF capacitor cuts frequencies above only 640Hz.

I said C5 in his second updated schematic.  That one isn't wired correctly.  It's wired correctly in the first schematic.
All the other feedback 10nf caps are functional and fine in both schematics.

[tech_builder]

That's embarrassing...its what you get when you only have 20mins to work on something...it looks like when I did a swap for the U3 and U4 opamps they weren't oriented correctly, and the shorted cap

I was wondering whether one of the output transistors could power both headphones, so I guess it's worth trying out. I'll look through all of this again when I get another chance and hopefully produce something better

I have ordered the op-amps I mentioned. Can't wait to see how those work out!

[BrianHG]

That's embarrassing...its what you get when you only have 20mins to work on something...it looks like when I did a swap for the U3 and U4 opamps they weren't oriented correctly, and the shorted cap

It's alright.  You should see how many times I end up editing my posts after I do the post.  Dooohhh... 
As long as you catch 99% before you may a PCB....

[Audioguru]

Here is a simulation of the frequency response with the 10nF capacitor negative feedback capacitor cutting mid and high audio frequencies:

[danmcb]

what you have is basically OK, but as others have said, you want a resistor of somewhere between 470k and 1M to deck at the + input of the input stage to set input impedance. It is also common to place a -3dB point for the LF response with a blocking cap  before that. You can set the LF point at about 40Hz, as the E string of a guitar is round about 80Hz. You also want to set an HF 3dB point - around 10 or 15kHz is fine for a guitar pre. This is better done on the power amp stage. You might want some HF rolloff on the preamp for stability but it will vary with the feedback resistance, so push it up out of band, about 50-100kHz  so that the tone does not alter with gain.

You are making a bit of a meal of this though. On a thing like this there is not much point having separate gain and volume - unless you have a SIG LED or something to help set the gain. They are just going to do pretty much the same thing from the point of view of the user.

And why two output amps? for left and right of a headset? just drive them in parallel.

So you can do the whole thing with a dual op amp like a TL072 (FET type is better than 5532 for this to get the input Z high). One preamp stage with a GAIN pot, and a mini power amp (opamp with push pull o/p to give current drive,  as you have shown).

[MagicSmoker]

I'm embarrassed to admit I just now looked at the schematic... I agree with the other comments, but also want to add that you can greatly simplify this circuit by using one of the op-amps I recommended to directly drive the headphones. That is, no output buffer will be needed.

I'm not sure how much voltage gain you need in a guitar amp, but I suspect it is not so much you need to break the amplification into 2 stages (plus a unity gain buffer), which is what you have now. It is also best to put the volume pot in series with the input, and not in the feedback path. The capacitor across the feedback resistor is to prevent oscillation and roll off the gain at high frequencies; the voltage gain will be cut in half (ie - down 3dB) when the reactance of the capacitor equals that of the feedback resistor. If the feedback resistor is 10k then an appropriate choice of shunt capacitor is 470pF. Note, however, that this capacitance is seen by the output of the amplifier and can contribute to instability itself if it gets too large in value (and 10nF is almost certainly too large in value, both because of this reason and for rolling off the output way too early, as mentioned above).

Finally, when you use an active rail splitter to create a bipolar supply you don't need the big DC blocking capacitor in series with the speakers. You do, however, need to remember that the signal ground is actually at 1/2 the supply voltage and so might not get along well with other devices sharing a common chassis ground. In this case there shouldn't be any problems as a guitar is a floating signal source and headphones are a floating load.

[Zero999]

Here is a simulation of the frequency response with the 10nF capacitor negative feedback capacitor cutting mid and high audio frequencies:

But that's not what the schematic says. C5 is bypass with a piece of wire, so does nothing.

[Audioguru]

But that's not what the schematic says. C5 is bypass with a piece of wire, so does nothing.

The circuit has two parts that cut high audio frequencies. Before I showed the worst one that was accidently shorted by a wire and now I am showing the other one that is not shorted. Both filters added together destroy the audio.

[tech_builder]

Once again, thank you for all the feedback. I finally found some time this weekend to breadboard a new design I came up with after reading through what has been discussed and doing some more research.
The first thing I decided to change was the opamp responsible for volume change. I just swapped it out for a simple pot. I had a 100k on my desk and it worked well.

I still wanted to have a gain and volume, so I put together a simple clip circuit with a comparator to tell me when the output from the first opamp was over about 2.8V, which was where I figured the distortion would begin. I figured it might be difficult to see the clipping LED so I put a simple sample and hold type circuit on it to give it a little bit of persistence. Seems to work good.

The OPA1664 gave a much much better performance than the TL082 opamp did so I will move forward with this opamp. The OPA1664 is a 4 opamp package so I only need to power all of them once. I’m not too sure what to do with the unused opamp, any ideas?

The other thing I did was switch to a simpler power supply strategy by removing the transistors, which seemed to work fine in testing.

I changed the resistor values in the output (biasing?) stage from 4.7k to 1k in a bid to increase the voltage swing I could get out of the headphones before distortion occurred. Unfortunately this increases the current consumption...So far with my limited testing the whole circuit pulls about 8mA idle and about 12-16mA while strumming the guitar at a reasonable volume. A 9V battery with about 500mAh it should last 30-40 hours. Maybe I'll look into getting some small lithium cells and stringing them in series to get above 9V, but then I have to figure out how to charge them...

Hopefully this effort is better than my last one  I appreciate any feedback you may have on this iteration. Thanks!

[Zero999]

The unused op-amp can be connected as a unity gain buffer with its input connected to 0V.

Bear in mind that the clipping voltage depends on the supply voltage and it will reduce, as the battery discharges. The comparator's reference could be changed so it indicates clipping when the output voltage exceeds the supply voltage minus a three diode drops.

[Audioguru]

On your new headphones amplifier circuit I calculated that its maximum output level will be low if the transistors are not replaced for higher current gain.
If the current gain is low and the base-emitter on-voltages are high for the 2N3904 and 2N3906 output transistors then the peak output voltage will be far less than 1.6V.

100mW is loud in a headphone. For 100mW the RMS voltage must be 1.25V then the peak voltage must be 1.77V.

Your high value for the emitter resistors is throwing away much output voltage swing and is causing crossover distortion.
C1 is preventing the opamp from reducing output distortion.

I use my hearing as a clipping detector because I do not listen to RAP, instead I listen to music.

[Zero999]

On your new headphones amplifier circuit I calculated that its maximum output level will be low if the transistors are not replaced for higher current gain.
If the current gain is low and the base-emitter on-voltages are high for the 2N3904 and 2N3906 output transistors then the peak output voltage will be far less than 1.6V.

100mW is loud in a headphone. For 100mW the RMS voltage must be 1.25V then the peak voltage must be 1.77V.

Your high value for the emitter resistors is throwing away much output voltage swing and is causing crossover distortion.
C1 is preventing the opamp from reducing output distortion.

I use my hearing as a clipping detector because I do not listen to RAP, instead I listen to music.

Those are good points. I don't think it will be that bad, since the transistors will be much better than the minimum values, especially after they've warmed up a bit, as the current gain has a positive temperature coefficient.

I think the biasing resistors could all be increased to save battery life and the whole biasing network AC bypassed with two large electrolytic capacitors to couple the signal from the op-amp to the transistors.

[MagicSmoker]

Or, you know, the entire "booster" amplifier stage could be dispensed with by choosing an op-amp with a higher output current rating like the TS922AIN I suggested before. That op-amp's max output of 80mA can deliver over 200mW into the typical pair of 32 Ohm headphones all by itself.

[Zero999]

Here's what I was talking about earlier on. 1000µF may sound large but the capacitors only have around 700mV on them, so can be a low voltage rating such as 6.3V.

The 220R biasing resistors may need to be adjusted to give low power draw and low crossover distortion.

[Audioguru]

Hero has a good idea of using the capacitors so that the opamp can turn ON the output transistors with plenty of current instead of turning OFF the transistors.

[calexanian]

add say a 470 ohm resistor directly from the output of the op amp to the load for stabilization purposes. That circuit works fine for non audio but without a resistor there you may have noise or junk in the signal. I used to build that circuit all the time when a LM386 would not cut it and I had a  dual power supply. Oh, and you don't really even need those capacitors. If you want boot strapping of that to above the swing of the op amp there are other circuits to use.

[tech_builder]

Alright, time for the next episode. I’ve changed the clipping diode’s reference to be two diode drops from the positive supply, rather than being referenced from ground, as suggested by Hero999. This means that as the battery dies and the voltage begins to drop, the clip point will drop with it.

I made the changes Hero999 suggested with the bypassing capacitors, and what a change! I am really impressed with how well it worked. I tried a few different size capacitors and could not hear/see a difference between a pair of 1000uF caps and a pair of 220uF caps, so I stuck with the latter.

I made a few other small changes. The output resistors have gone from 5ohms to 1ohm as per Audioguru’s suggestion, and the 10n capacitor across the output feedback loop is now gone. I also used an OPA4171 quad opamp package, mostly because I had a smd to dip convertor for it, while I didn’t have one for the OPA1664 that I was originally going to try. This opamp worked great as far as I could tell, it was better than the TL082 I was using before at least.

Calexanian, I am curious about the other circuits you have used/come across for this purpose as you mentioned there are other circuits, what are they?

Thanks again, I think it's finally coming together. Next is the circuit board layout and enclosure making.

[Zero999]

I'm glad it worked. On reflection 1000µF was a bit overkill, 220µF is more than enough.

R10 is not needed, since the base current into the transistors is limited by the voltage change on the emitters. For example, if the voltage on T2's base increases, the voltage on the emitter will too, reducing the voltage between the base and emitter, thus limiting the base current. All R10 will do is increase the voltage drop.

[calexanian]

Going To have to dig for those. Have not used them in many years. I may have to take a picture of a schematic out of an old data book or re draw it or something.

[technix]

Maybe you should take a look on the LM386 chip as it packs the op amp and the power amplifier in one chip. Maybe you still need one op amp input stage for impedance matching but the second stage op amp and power amp can be merged.

Also when designing the PCB socket the chips. Different input stage op amps can have different performances so you can swap them in and out. Dual op amp chips almost always have the same pinout, and SO-8 adapter boards result in the same footprint as DIP-8, so you can safely use the TL082 DIP-8 footprint. LM386 when driven too hard for too long (like a big session of death metal) can fry if insufficiently cooled so socket them for easier replacement.

[akis]

I have looked quickly and maybe I am making a mistake, but can I just say that an electric guitar requires a very specific equalisation with serious chopping down the mids and amplifying the highs (5KHz) just to get the usual, clean sound. As was noted right at the start, a 47K bias resistor would steal all the high frequencies - it would sound *exactly* like when you plug your guitar into your stereo system, it really does not sound like a guitar at all. I believe the 1M resistor is too high and just asking for noise and offsets. I would use a 500K instead, it is very common.

But then guitar pickup amps have a ridiculous ceiling, and any amp will clip, unless you have equally ridiculous voltage rails. You cannot do it with 5V, or with 30V (2x15V). You have to account for clipping and make the clipping as symmetrical as possible and as smooth as possible.

Typical pre-amps found in quality valve amps have rails of 400V and two cascade input stages of Av=50 followed by another Av=50, letting the valves do the smooth clipping. Look out for the very common Fender tone stack to see how it aggressively cuts down on the mids allowing the highs to go through. That is more or less the sound you are looking for.

In terms of smooth clipping produced by a transistorised circuit, my best experiments have been with series of BAT48s.  I remember dozens of them, anti-parallel, and even then clipping can happen at any place (even inside the op-amp) so the design has to be tested on breadboard and check the scope to see the smooth curves (not square clips).

You are looking for around 15% distortion for the "crunch" sound and maybe 20-25% for the "metal" sound. Technically there cannot be "clean" sound (called "crystal" on some amps) because you do need valves for that or to turn down the pots on the guitar maybe.

[tech_builder]

Hero999: So I tried removing R10 and I got a lot of noise and waveform deformation, so I have decided to keep it in 

Calexanian: That would be great if you could post them here. Any format you can document it with is fine by me!

Technix: I know the LM386 is an easy way to make this thing, but I’m more interested in the design and build of a transistor amplifier. I don’t get to do much analog design at my job and I have forgotten most of it due to neglect, so I am reviving it with a fun project. Hopefully it will be one of many that I do, and having a great community like this one to help me along is fantastic! So bear with me if I take the hard way, it’s for a good cause.

Akis: Thanks for looking at the schematic. This first iteration of the guitar headphone amp is not going to have any designed in distortion. I am cutting it off pretty much where it is so that I can at least get something for my son to play through before he grows up and moves out! I’m interested in your comment about the input resistor R17. You mention that a 500k resistor to ground is common, where is this configuration typically used?

Thanks!

[Zero999]

Hero999: So I tried removing R10 and I got a lot of noise and waveform deformation, so I have decided to keep it in 

That's interesting. Don't you want to know why?

When you mean noise, does it produce a waveform, when there's nothing attached to the input? If you put a square wave in, does it ring?

Could it be that it's better with R10 because it reduces the open loop gain slightly, therefore stabilising the amplifier?

[akis]

R10 is a last ditch effort to save the current limiting of the op-amp. Get rid of those caps. Maybe reduce the biasing resistors to 3K each and remove the 220Rs.  I think headphones are 64R (are they?) so make the emitter resistors, say 5R each, even 10R each. If you do all that change R10 to something just for lip service, 10R, or remove it.

[akis]

I am not sure your ground is a good ground, trying to split the 9V supply into two. The op-amps draw a lot of current, as do the buffer transistors, and the 10K split supply resistors will allow what, .45mA, whereas each op-amp draws a few mA itself. In order for the split supply to work you'd need an impedance 10x, or 20x of the largest current draw, so for example if all the op-amps and all the buffer transistors drew, say, 12mA in total, you'd need a potentiometer of 1.2mA (10x) or 0.6mA (20x) to make some half-way house ground and even then. You could try to make a better ground by using a spare op-amp if there is any. Or better, do away with trying to split the supply. Or use 2 x 9V batteries, more headroom too!

[Zero999]

R10 is a last ditch effort to save the current limiting of the op-amp.

No, R10 shouldn't be relied upon for current limiting. If it's shorted the transistors would still have enough base current to allow a damaging collector current, made higher by the fact the current gain has a positive temperature coefficient.

Get rid of those caps. Maybe reduce the biasing resistors to 3K each and remove the 220Rs. I think headphones are 64R (are they?) so make the emitter resistors, say 5R each, even 10R each. If you do all that change R10 to something just for lip service, 10R, or remove it.

Perhaps you should read the thread more thoroughly? This is not a criticism, since I don't blame you for not having the time to read through a such a long thread. A similar configuration to what you've described has already been tried and found unsatisfactory. Here's a link to the schematic.
https://www.eevblog.com/forum/projects/guitar-headphone-amplifier/msg1214040/#msg1214040

Here's why:

If you remove the bypass capacitors then you'll need to also reduce the value of the biasing resistors to the point, where the quiescent current becomes very high to get the desired power output. Removing the 220R resistors may help reduce the quiescent current but it would increase distortion dramatically.

The headphones have an impedance of 32R per channel and are connected in parallel making 16R, so increasing the emitter resistors is a bad idea.

I am not sure your ground is a good ground, trying to split the 9V supply into two. The op-amps draw a lot of current, as do the buffer transistors, and the 10K split supply resistors will allow what, .45mA, whereas each op-amp draws a few mA itself. In order for the split supply to work you'd need an impedance 10x, or 20x of the largest current draw, so for example if all the op-amps and all the buffer transistors drew, say, 12mA in total, you'd need a potentiometer of 1.2mA (10x) or 0.6mA (20x) to make some half-way house ground and even then. You could try to make a better ground by using a spare op-amp if there is any. Or better, do away with trying to split the supply. Or use 2 x 9V batteries, more headroom too!

There is some truth in this but it's grossly exaggerated. The current taken by the op-amp and the buffer transistors makes absolutely no difference to the split power supply because it just passes from +V to -V and doesn't flow into or out of the 0V node. The biasing currents going in/out of the 0V node will also be insignificant. As long as the current from the +V and -V rails is equal, then no current flows in or out of the 0V rail.

The 0V rail is bypassed by two 220µF capacitors, which effectively make 440µF, which will present a low impedance path to 0V for the headphones and the input voltages, so that isn't a problem.

The places where the positive and negative currents are not balanced is what will cause the 0V rail to rise/fall. The most glaringly obvious one is R11, which is connected to +V, via a couple of diodes. Fortunately this is easy to fix: R11 could easily be connected to -V, without much change to the voltage across the diodes. R13 is more of an issue, although perhaps as it's a clipping detector, one would say it doesn't matter, since when the LED lights and starts to pull the 0V rail up, then the user should adjust the volume any way. That then leaves R27, which will leak a small current into the 0V point, more so at higher output levels.

[Zero999]

Looking at this again, DC can be avoiding in the 0V rail by referencing the peak detector circuit to 0V.

I would also consider upping the value of the bypass capacitors, to make a lower impedance path at AC.

[tech_builder]

Hero999 I had actually just made similar changes to the schematic. I had not increased the power supply caps though. I also increased R13 to account for the larger voltage drop across the LED. I haven’t specified an LED yet though, but I do want a low current one.

I am curious about the reason the removal of R10 caused the opamp to be so wonky. The output of the circuit when I remove R10 is shown in the first image below. There is quite a bit of noise in the output signal. I tried various resistors from 100 to 270ohm and found that 220ohm was the lowest value resistor that didn’t show any distortion; at least that I could see on the scope trace.

I remember classes talking about open loop gain, but I’ll have to dig a bit to pull that out of my memory. I do recall phase being important because I believe that as you get closer to 180 degrees out of phase your feedback becomes more positive instead of negative and no longer functions correctly.

Akis, as Hero999 already mentioned that was my original design. However, since I am using a fairly limited voltage I was running up against the rails and getting a lot of clipping during my testing. When I put the bypass caps on the output it really opened it up. I’m still not too sure how it is working though, perhaps someone can elaborate? I’m guessing it might be similar to how putting a bypass cap across a common emitter’s emitter resistor would increase its gain at signal frequencies? However, the capacitor is now across the base emitter...sorta. I have to think about it more.

Thanks again!

Edit: Yellow is input, Green is output.

[akis]

Hello

In my opinion

Supposing the load is 16R (someone said so above I think). My "pro" headphones are 64R but anyway. The op-amp cannot drive such a low impedance load, so we add a buffer stage. The low E is 80Hz. We need approx 15mW to drive good quality headphones. With that in mind and on simulation (rather than on breadboard)

1) set the frequency to 83Hz
2) try to get 15mW on the 16R load (around 500mV RMS, 1.42V p2p)
3) on my simulation it does not work

I believe it is because you are returning the power output from the speakers into your fake ground.

If we accept that the fake ground is unnecessary and does not work for returning loads to it, then I can suggest:

1) We can use a single supply design which basically only attempts to set the Vin+ of the op-amp to half the supply voltage , you can have huge resistors for that, eg 200K each.
2) We have to return the speaker to the V- and power the speaker through a large capacitor
3) Need a complementary pair to give us a lot of power without distortion, but we can get rid of the extra two transistors for slightly higher distortion
4) I attach a schematic for ideas

[BrianHG]

Hero999 I had actually just made similar changes to the schematic. I had not increased the power supply caps though. I also increased R13 to account for the larger voltage drop across the LED. I haven’t specified an LED yet though, but I do want a low current one.

I am curious about the reason the removal of R10 caused the opamp to be so wonky. The output of the circuit when I remove R10 is shown in the first image below. There is quite a bit of noise in the output signal. I tried various resistors from 100 to 270ohm and found that 220ohm was the lowest value resistor that didn’t show any distortion; at least that I could see on the scope trace.

I remember classes talking about open loop gain, but I’ll have to dig a bit to pull that out of my memory. I do recall phase being important because I believe that as you get closer to 180 degrees out of phase your feedback becomes more positive instead of negative and no longer functions correctly.

Akis, as Hero999 already mentioned that was my original design. However, since I am using a fairly limited voltage I was running up against the rails and getting a lot of clipping during my testing. When I put the bypass caps on the output it really opened it up. I’m still not too sure how it is working though, perhaps someone can elaborate? I’m guessing it might be similar to how putting a bypass cap across a common emitter’s emitter resistor would increase its gain at signal frequencies? However, the capacitor is now across the base emitter...sorta. I have to think about it more.

Thanks again!

Thant distortion is due to cross-over distortion + influenced oscillation.  Choose a different op-amp.

[Zero999]

Hero999 I had actually just made similar changes to the schematic. I had not increased the power supply caps though. I also increased R13 to account for the larger voltage drop across the LED. I haven’t specified an LED yet though, but I do want a low current one.

I am curious about the reason the removal of R10 caused the opamp to be so wonky. The output of the circuit when I remove R10 is shown in the first image below. There is quite a bit of noise in the output signal. I tried various resistors from 100 to 270ohm and found that 220ohm was the lowest value resistor that didn’t show any distortion; at least that I could see on the scope trace.

I remember classes talking about open loop gain, but I’ll have to dig a bit to pull that out of my memory. I do recall phase being important because I believe that as you get closer to 180 degrees out of phase your feedback becomes more positive instead of negative and no longer functions correctly.

Akis, as Hero999 already mentioned that was my original design. However, since I am using a fairly limited voltage I was running up against the rails and getting a lot of clipping during my testing. When I put the bypass caps on the output it really opened it up. I’m still not too sure how it is working though, perhaps someone can elaborate? I’m guessing it might be similar to how putting a bypass cap across a common emitter’s emitter resistor would increase its gain at signal frequencies? However, the capacitor is now across the base emitter...sorta. I have to think about it more.

Thanks again!

Yes, I think you're right about the resistor reducing the loop gain, thus increasing the stability. It's interesting how the oscillation only occurs on the negative going side of the waveform.

The AC coupling capacitors work by allowing the op-amp to the transistors both on as well as turn off, by allowing audio frequencies to bypass the diodes. Without them, the op-amp can only turn the transistors off, as the diodes prevent current flow in the other direction, so the drive current for the transistors has to come from the 10k resistors.

Hello

In my opinion

Supposing the load is 16R (someone said so above I think). My "pro" headphones are 64R but anyway. The op-amp cannot drive such a low impedance load, so we add a buffer stage. The low E is 80Hz. We need approx 15mW to drive good quality headphones. With that in mind and on simulation (rather than on breadboard)

1) set the frequency to 83Hz
2) try to get 15mW on the 16R load (around 500mV RMS, 1.42V p2p)
3) on my simulation it does not work

I believe it is because you are returning the power output from the speakers into your fake ground.

If we accept that the fake ground is unnecessary and does not work for returning loads to it, then I can suggest:

1) We can use a single supply design which basically only attempts to set the Vin+ of the op-amp to half the supply voltage , you can have huge resistors for that, eg 200K each.
2) We have to return the speaker to the V- and power the speaker through a large capacitor
3) Need a complementary pair to give us a lot of power without distortion, but we can get rid of the extra two transistors for slightly higher distortion
4) I attach a schematic for ideas

I agree about not using the false ground for returning power from the headphones but compound transistors on the output are a bad idea. They increase the voltage loss, clipping and could make the amplifier less stable, as they have more voltage gain than ordinary emitter followers. It's better to only have one set of emitter followers, which provide more than enough current gain to drive the headphones and AC bypass the biasing diodes with capacitors.

Thant distortion is due to cross-over distortion + influenced oscillation.  Choose a different op-amp.

I think you have a point regarding crossover distortion but why blame the op-amp when it doesn't have a class B output stage? That also doesn't explain why it disappears when the loop gain is reduced.

If there's crossover distortion, it will be down to the buffer and can be reduced by increasing the value of the 220R resistors in between the diodes slightly, which would increase the quiescent current drawn too. The voltage across the 1R resistors could be monitored, whilst adjusting the bias, to reach a compromise between quiescent current and crossover distortion, but since reducing the loop gain already fixes this, there's little point.

The OPA1664 is a 4 opamp package so I only need to power all of them once. I’m not too sure what to do with the unused opamp, any ideas?

You could use it to reduce the impedance of the 0V rail.

[Audioguru]

The application note for the LM380 little amplifier (similar to the LM386 little amplifier) says that the output produces high frequency oscillation on the negative going swing, exactly like we have here when R10 is removed. They fixed it by adding an external zobel series RC network from the output to ground.

[BrianHG]

Thant distortion is due to cross-over distortion + influenced oscillation.  Choose a different op-amp.

I think you have a point regarding crossover distortion but why blame the op-amp when it doesn't have a class B output stage? That also doesn't explain why it disappears when the loop gain is reduced.

If there's crossover distortion, it will be down to the buffer and can be reduced by increasing the value of the 220R resistors in between the diodes slightly, which would increase the quiescent current drawn too. The voltage across the 1R resistors could be monitored, whilst adjusting the bias, to reach a compromise between quiescent current and crossover distortion, but since reducing the loop gain already fixes this, there's little point.

My recommendation to try a different op-amp, is not a description of an exclusive problem being in that op-amp, it's just that some op-amps have a better time when feeding such a class AB buffer without going as haywire pronouncing that NPN/PNP switchover problem.

Another thing which might help is to tie a 100 ohm resistor between the output of the op-amp and the center of the 2 resistors inbetween the emitters of the 2 transistors.  This helps conduct a bit of signal inbetween the switch-on and off of the PNP and NPN transistors removing that distortion without sacrificing base drive current.

[tech_builder]

Thanks Akis. That is a neat idea. I’ve built it up in a very simple simulator (Everycircuit) and it seems to work pretty well. I guess it would still need another stage at the front to moderate higher level input signals since the pot wasn’t able to attenuate it to zero as a volume control (it just stopped it from clipping).

I like the idea of using the extra opamp to lower the ground rail impedance, it’s certainly better than doing nothing!

I’m not too sure what a zobel network is, but it doesn’t sound simple. Maybe I’ll take a look if I find some time.

I do have another opamp to try for this circuit BrianHG. It’s an OPA1664, which they state on the datasheet as being designed for audio. Not too sure if that’s going to make a difference here. I didn’t have a way to mount it into a breadboard since it only comes in surface mount packages. I did recently acquire the means to mount it into the breadboard, but now I just have to get motivated enough to rewire the spaghetti mess for a new opamp.

[Audioguru]

I like the idea of using the extra opamp to lower the ground rail impedance, it’s certainly better than doing nothing!

Yes, it is much better.

I’m not too sure what a zobel network is, but it doesn’t sound simple. Maybe I’ll take a look if I find some time.

A zobel network is a low value resistorat the output of an amplifier in series with a capacitor to ground. It provides a load at high frequencies where the inductance of a speaker or headphones is not a load.
Maybe your resistor should be 16 ohms and the capacitor can be 33nF. Here is an LM386 little amplifier with its zobel network:

"into a breadboard', "rewire the spaghetti mess"

A solderless breadboard has capacitance between its rows of contacts and the spaghetti mess of wires all over the place that guarantees a circuit like this to oscillate at a high frequency as you show. Use a compact pcb or a compact stripboard layout with the parts soldered on it instead.

[tech_builder]

Thanks Audioguru, I put the Zobel network on to the output of the amplifier and it cleaned up the signal as seen in the image below. I am currently going through the Learning the Art of Electronics book, which is a lab based learning tool that compliments the well known Art of Electronics text. I found an example of a simple speaker driver that they show as having a similar zobel network on the output, but they call it a “snubber”. It also has a 1k resistor to ground in parallel, which I haven't tried yet. Green is output, yellow is input.

You can see the spaghetti mess below. I have been wondering what the implications of such a setup would have on performance and how much I can trust the results I am getting. I am planning on making a PCB out of the design I have this far. Hopefully the performance will only get better!

Attached is a schematic of the state of the guitar headphone amplifier as it stands.

[Audioguru]

Like all solderless breadboard circuits, yours is a mess. There is stray capacitance and antenna wires that pickup interference all over the place.

The value of R1 and R2 that provide base current to the output transistors is much too high so the base current is almost nothing.
The 2N3904 and 2N3906 transistors cannot provide enough current even if R1 and R2 have low values.

Work it out:
1) The output voltage might need to be +1.6V or -1.6V. Then the current in an output transistor will be 1.6V/16 ohms= 100mA.
2) Look at the datasheet for a 2N3904 and 2N3906. The current gain drops sharply above 50mA. At 100mA the text says the minimum current gain at 100mA is 30.
3) The base current must be 100mA/30= 3.3mA.
4) The base voltage will be about 1.6V + 0.9V= 2.5V. Then the base resistor needs to be (4.5V - 2.5V)/3.3mA= 606 ohms, much less than your 10k.
You might need an output higher than 1.6V then the 2N3904 and 2N3906 will burn out.

2N4401 and 2N4403 little transistors have a minimum current gain of about 120 at 100mA so the base resistors can be about 606 x 4= 2424 ohms. Their maximum allowed current is 600mA.

You show a little 9V battery. It is too small to provide these high currents.
   

[Zero999]

The value of R1 and R2 that provide base current to the output transistors is much too high so the base current is almost nothing.

No. You've forgotten that R1 and R2 only need to provide enough current to bias the transistors into conduction when there's no signal to avoid crossover distortion. C7 & C8 bypass the high value biasing resistors at AC, thus providing a low impedance path for the base current from the op-amp's output stage.

[Audioguru]

No. You've forgotten that R1 and R2 only need to provide enough current to bias the transistors into conduction when there's no signal to avoid crossover distortion. C7 & C8 bypass the high value biasing resistors at AC, thus providing a low impedance path for the base current from the op-amp's output stage.

Yes, I forgot that the opamp drives plenty of base current into the transistors through the capacitors you added.

[alexanderbrevig]

I've been a long time reader of this thread but decided to reply mostly to get it in my "Show new replies to your post" section.
Also, to thank everyone! I've learned things reading this thread. 

[Zero999]

I thought I'd revisit this topic.

I've simulated both the compound transistor and emitter follower with bypass capacitor output stages. With both circuits, I adjusted the biasing to get rid of most of the crossover distortion, without too much bias current.

Both circuits have a gain of about 0.9, which is to be expected, as emitter followers always have a gain of less than one and some voltage is dropped across the emitter resistors.

The compound transistor circuit causes clipping at 1.2V less than the power supply rails, around +/-3.8V.

The emitter follower circuit doesn't clip. The output will be able to reach close to the power supply rails, minus the voltage drop on the emitter resistors and transistor saturation voltage.

[Circlotron]

Most guitar amps have an input impedance of ~1M.  Lower and it can roll off the highs (not that electric guitars have much in the way of high frequency content).

I read somewhere that a low input impedance can damp the string vibration somewhat as well, causing the sound to die away quicker. I suppose it's possible in theory at least.

[tech_builder]

It’s been a while since I last posted as I haven’t had much time to work on this project, but I managed to build up the op-amp based ground rail and do a few simple tests. One thing that I noticed is that my output swing is much more limited. Previously I could get over 3.5Vpk-pk output at 1kHz.



 I can get about 1.75Vpk-pk output (1kHz sine) at most before distortion, but the shape of the distortion is strange. It happens on the lower half of the waveform by sneaking in and cutting it off.



The upper half starts distorting when the output reaches about 2.5V.



I’m not too sure what would be causing this other than the op-amp may not be able to put out enough current to maintain ground? The point at which distortion starts is frequency dependant. At 200Hz I can’t get more than 1Vpk-pk output before distortion begins,



 but at 10kHz I can get more than 3.5Vpk-pk.

[Audioguru]

I think your fake ground is causing the output distortion. If you make it with a single supply then you should not have the distortion. Have you measured the battery voltage when the amplifier is distorting?
But you also might have the pins of the transistors mixed up or have some polarized capacitors connected backwards.

Since the problem occurs only at low frequencies then maybe a capacitor (or a few) values might be much too low.

[tech_builder]

I did some more testing, specifically with the bypass capacitors on the power rails. I gradually increased the size of the bypass caps till they were 100uF. This size allowed an output voltage swing of about 4Vpk-pk at 200Hz. It sounds pretty good with headphones as well. Not burning with tone that'll knock your socks off, but pretty good!



I noticed that the crossover distortion has increased, possibly because I can now drive it to larger amplitudes on the output. Although at 10kHz and using 2.2uF caps I didn’t notice any crossover distortion at a similar output (see my previous post).

[Audioguru]

Guess what? The voltage from your battery is falling flat on its face unless it has a pretty big capacitor parallel with it to help it.
I repeat, "Have you measured the battery voltage when the amplifier is distorting?"

[Zero999]

I noticed that the crossover distortion has increased, possibly because I can now drive it to larger amplitudes on the output. Although at 10kHz and using 2.2uF caps I didn’t notice any crossover distortion at a similar output (see my previous post).

Try increasing the values of R8 and R9 slightly. It will increase the quiescent current slightly but should reduce the cross-over distortion.

[Circlotron]

If OP1 doesn't have enough grunt to drive the speaker directly but needs a buffer, how do you expect OP4 to be strong enough to hold the "grounded" end of the speaker steady? Same current flows in both.

[Zero999]

If OP1 doesn't have enough grunt to drive the speaker directly but needs a buffer, how do you expect OP4 to be strong enough to hold the "grounded" end of the speaker steady? Same current flows in both.

Indeed. There's an error in the schematic I missed. The speaker's return should be connected to the negative rail, not the output of the op-amp.

[tech_builder]

I connected the ground of the headphones to the negative rail and replaced the bypass caps back to 2.2uF and I didn’t see any distortion. Everything seems to work great! Thank you so much for the help. This design wouldn’t be anything without this forum! So I think I am going to call a design freeze here and work on designing the PCB and enclosure. I can still make small modifications if I need to, but if I don’t call it soon it’s never going to be made.
Audioguru, I was testing it with a power supply (HP 6235A). I did measure the ground voltage on the scope and it was stable. I haven't made the same tests with a battery.
Attached is the schematic I will be working from. I will be using KiCAD for the PCB layout.

Thanks!

[tech_builder]

I found some time to get everything together and start learning KiCAD. Here is the schematic I have put together for the headphone amplifier. I added a guitar input connector and a headphone output, which I had to make. I managed to find a mono-jack but I couldn’t find a stereo jack. I searched for it to no avail online. It seems that the library is now auto managed through GIThub or something? It was a little confusing.

Another thing I added was a break in the negative power supply rail that cuts the power when the guitar is not plugged in, so that if it is left on accidentally it won’t drain the battery. This required me to reference the guitar input to the negative rail instead of ground. I might add one in series to the headphone as well, any thoughts on if this implementation is horrible? Any better ideas?

I will now begin the next step which is part footprint association, which hopefully won’t take as long as the schematic to do.

EDIT: Schematic updated to fix errors in op-amp orientation. Updated file dated Aug 7, 2017

[Audioguru]

R12 connects to ground then the output signal overloads the output of the voltage splitting opamp. Maybe R12 should connect to the negative terminal of the battery instead?

[Zero999]

I found some time to get everything together and start learning KiCAD. Here is the schematic I have put together for the headphone amplifier. I added a guitar input connector and a headphone output, which I had to make. I managed to find a mono-jack but I couldn’t find a stereo jack. I searched for it to no avail online. It seems that the library is now auto managed through GIThub or something? It was a little confusing.

Another thing I added was a break in the negative power supply rail that cuts the power when the guitar is not plugged in, so that if it is left on accidentally it won’t drain the battery. This required me to reference the guitar input to the negative rail instead of ground. I might add one in series to the headphone as well, any thoughts on if this implementation is horrible? Any better ideas?

I will now begin the next step which is part footprint association, which hopefully won’t take as long as the schematic to do.

Disconnecting the power supply when the headphones or guitar is disconnected is a good idea.

[tech_builder]

That took longer than I thought. Anyways, I have finished laying out the circuit board. Not the prettiest thing, but I want to have something to try out. I decided to fill the top layer with the +4.5V net and the bottom layer with the -4.5V net. Not too sure if there are any concerns with doing that. It made it a lot easier to connect everything.

In general I found KiCAD to be a pretty good program to use. The only thing I wish is that I could get the traces to stick to follow along other traces when drawing them, which is how some of the other programs I’ve used have worked. I really like the 3D rendering of the board. Super easy to use!