Op amp based preamp supply. Charge pump, single vs symmetrical & ripple

[b_force]

Then I misunderstood. If you need 30V peak to peak, then you need a higher total supply voltage than 30V. A +/-16V supply won't do for the TL082, which needs a couple of volts of headroom on either supply rail, to avoid clipping, unless that's desired for the distortion effect

Yes, it's built to distort, for rock'n'roll purposes! 

More precisely, it's supposed to do both clean and distorted tones. That's what the gain pot is for

The OPA2196 will be able to give 30V peak to peak from a +/-16V supply. If does have a lower slew rate, than the TL082, but it will be the lower audio frequency components which generate the largest voltage swings, so this is a non-issue.
http://www.ti.com/lit/ds/sbos869/sbos869.pdf

I'm afraid it really needs to be an LM1458 as in the original circuit, or one of those TL082 with jfet inputs (which are supposed to clip more softly). Well, not really those 2, just one op amp with lots of mojo regardless of whether other types make more sense from an engeneering perspective. That's how guitar players roll 

You don't want any distortion/clipping from your opamps. Also really don;t see where "sound descriptions" come from.
I have building these circuits for years.

Only thing you need to take care of is be able to drive enough capacitive load if it's a pedal.

[dazz]

Do you have a specific type of distortion i.e. transfer function in mind?

Well, it should be as close to the original as possible. Would posting LTSpice screenies help? Or perhaps I should post the .asc? I'm using a couple of "custom" models but I can zip them

I think most op-amps will give fairly harsh clipping, as once the output goes beyond the limit, there will be no negative feedback. I suspect the clipping level may vary between op-amps with the same part number, because it's not a tightly controlled characteristic. By operating an op-amp at its clipping level, it's being driven past its deign specification.

Well, there are many guitar effects and preamps that use opamps for clipping, I also heard that they clip harshly but many Marshall amps use them. The Tube screamer too, although I think the clipping diodes do most of the distortion there. I dunno, the Lead 12 works like that, so who am I to question Jim Marshall 

Another thing with using charge pumps, is that your negative output won't be completely symmetrical.

As the application circuit shows, there will be the diode's forward losses, plus any losses in the converter itself.

And indeed if not filtered correctly, the plain 7660 will generate an audible whine when used with sensitive audio circuits.

Yeah, the asymmetry of the charge pump is what made me lean towards using it as a voltage multiplier to get from 9V to as close to 30V as possible, then use a heavily filtered virtual ground to ensure symmetry. I believe the load from the op amps will remain fairly stable so I don't expect the voltage supply to sag very much

[BravoV]

Alternative consideration ...

https://www.analog.com/en/products/ltc3265.html#product-overview

[Audioguru]

Yes, for the 50 years old extremely distorted sounds, you use 50 years old 1458 opamps (dual 741 old opamps). A fairly old but much newer TL082 or the lower noise TL072 are NEVER used for severe clipping because they produce the problem called Opamp Phase Inversion where the output suddenly goes positive when the input voltage goes to within a few volts from the negative supply. Its distortion is completely different to having ordinary clipping.

I see the circuit as an ordinary 2-opamps preamp with a passive tone controls circuit at its output. The 2 opamps can easily use a single polarity supply and the tone controls circuit can have an added input coupling capacitor.

[b_force]

Yes, you do proper clipping with diodes, not relying on how the opamp clips.
That is asking for trouble.

[dazz]

Alternative consideration ...

https://www.analog.com/en/products/ltc3265.html#product-overview

Thanks! I'll look into it

Yes, for the 50 years old extremely distorted sounds, you use 50 years old 1458 opamps (dual 741 old opamps). A fairly old but much newer TL082 or the lower noise TL072 are NEVER used for severe clipping because they produce the problem called Opamp Phase Inversion where the output suddenly goes positive when the input voltage goes to within a few volts from the negative supply. Its distortion is completely different to having ordinary clipping.

I see the circuit as an ordinary 2-opamps preamp with a passive tone controls circuit at its output. The 2 opamps can easily use a single polarity supply and the tone controls circuit can have an added input coupling capacitor.

Oh, thanks for the info. Maybe Tl072 were being used in other amps, not this Lead 12. I'll socket them to try them both

[dazz]

Damn, I believe I made an incredibly stupid mistake. When I argued for the single supply charge pump instead of the symmetrical one because I would avoid ripple in the V- supply pin of the opamp, I completely overlooked the fact that the ripple from the positive rail would go straight to my virtual ground and to the opamps inputs. So actually the charge pump ripple will get massively amplified if I'm not mistaken. FML, am I dumb or what 

[spec]

Crossed Posts

Just so we can see whats involved here is the schematic for the Marshall Lead12-12W: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Lead12-12W-3005-5005-Schematic.pdf

My first thoughts are to use better RRIO opamps, so that the supply rails could be lowered and get the same or better dynamic range and open loop gain. That would possibly enable a simpler circuit with a better performance, including a lower noise, not that noise is an issue here.

A 12V, 15V, 18V, or 24V wall wart should do nicely for the PSU.

Thanks for posting the schematic, should have done it myself in the 1st post.
I'm still not sure how I could use lower power rails without significantly impacting the way the amp will perform. The thing has 80dB of voltage gain (with the gain pot at max) and the gain pot is part of a low pass filter so it changes the frequency response of the amp. So if the original one clips at 50% and mine clips at 10%, they won't sound the same. At least I think so.

Also, a 9V supply is pretty much a requirement, since this will be a guitar pedal and most pedal power supplies are 9V 

No probs posting schematic

If you have a RRIO opamp, the output will swing to the positive and negative supply rails. With non RRIO opamps, like in the Marshall amp, the output only swings to within approx 2.5V of the supply rails. There are other aspects too, but not necessary to go into that. Just to give and example, using +-15V rails a TIL072 will produce a maximum output of 25V peak to peak, whereas an OPA191, for example, will produce 30V peak to peak. So for the same dynamic range as the TIL072 you could run the OPA191
at +- 12.5V rails.

About the circuit sound, I get your point 100%. I was forgetting that this is a guitar pedal where the sound of the circuit is most important. The TIL072 is sweet sounding opamp, even in clipping whereas the OPA191 would give less distortion, but it would be much harsher in clipping.

I find all the above posts very interesting and, after all the discussions, can I summarize that the specification is to clone the Marshal circuit exactly, to get the same sound. This includes +-15V rails.  The power input to the circuit must be 9V DC to be comparable with standard pedals. I think you actually said most of this in your original post.

As well as the opamps, other factors affect the sound. These include: type of capacitor (especially), type of resistor, and the way that the supply rails perform dynamically.

One suggestion is to use turn-pin sockets to mount the opamps. That way you can do opamp rolling if you ever want to. To use surface mount opamps, just mount them on DIL headers that plug into the socket, like standard DIL opamps.

To summarize then, all you need from us is an approach to derive +-15 rails from a 9V input.

I would strongly recommend the use of symmetrical split supplies, for the reasons so well put by Benta in reply #1

One question, and apologies if this has already stated, but will you be running this pedal from a 9V PP3 battery?

[dazz]

No probs posting schematic

If you have a RRIO opamp, the output will swing to the positive and negative supply rails. With non RRIO opamps, like in the Marshall amp, the output only swings to within approx 2.5V of the supply rails. There are other aspects too, but not necessary to go into that. Just to give and example, using +-15V rails a TIL072 will produce an output of 25V peak to peak, whereas an OPA191, for example, will produce 30V peak to peak. So for the same dynamic range as the TIL072 you could run the OPA191
at +- 12.5V rails.

About the circuit sound, I get your point 100%. I was forgetting that this is a guitar pedal where the sound of the circuit is most important. The TIL072 is sweet sounding opamp, even in clipping whereas the OPA191 would give less distortion, but it would be much harsher in clipping.

I find all the above points very interesting and, after all the discussions, can I summarize that the specification is to clone the Marshal circuit exactly, to get the same sound. This includes +-15V rails.  The power input to the circuit must be 9V DC to be comparable with standard pedals. I think you actually said most of this in your original post.

As well as the opamps, other factors affect the sound. These include: type of capacitor (especially), type of resistor, and the way that the supply rails perform dynamically.

One suggestion is to use a turn-pin socket to mount the opamps. That way you can do opamp rolling if you ever want to. To use surface mount opamps just mount them on DIL headers that plug into the socket, like standard DILs.

To summarize then, all you need from us is an approach to derive +-15 rails from a 9V input.

I would strongly recommend the use of symmetrical split supplies, for the reasons so well put by Benta in reply #1

Thanks mate. Yes, that's exactly the goal, to replicate the original preamp as faithfully as possible.

I think you're right, Benta hit the nail in the head and somehow I managed to miss the point badly. I'm exploring my options for a dual rail solution now. Maybe a mc34063 if I can get away with +9V/-9V by increasing  the value of R8 to attenuate the signal since all of the clipping is done in the second opamp. Problem is that would put the signal in between the opamps well bellow 0dB and I fear that might make it prone to picking up noise, not sure.

[spec]

Thanks for reply.

I think you quoted from my post while I was still editing it and the bit about the use of a battery is missing.

Could you please confirm that you will be using a 9V PP3 battery to power this pedal?

In any case, the approach that seems to be the most economical and lowest risk is to use a 9V to +-15V power supply module.

I havn't done a good search but something like this is what I have in mind: https://www.aliexpress.com/item/8W-2-5-15V-to-15V-Dual-Output-power-supply-DC-DC-Step-up-Converter-module/32789671683.html

just out of interest, what guitars do you have?

PS: if you run the opamps from +-9V, as you suggested, the sound would change, not due to headroom or noise considerations but because the opamp itself is  operating from lower rails. This effect is not so marked in newer opamps, but more so with the older opamps.

[dazz]

Could you please confirm that you will be using a 9V PP3 battery to power this pedal?

That's up to the user/builder of the pedal. I personally never use batteries, I have a cheap 9V PS that I slapped a linear regulator in front. I don't require much current at all for the few pedals I use.

In any case, the approach that seems to be the most economical and lowest risk is to use a 9V to +-15V power supply module.

I havn't done a good search but something like this is what I have in mind: https://www.aliexpress.com/item/8W-2-5-15V-to-15V-Dual-Output-power-supply-DC-DC-Step-up-Converter-module/32789671683.html

I ordered a couple of those just two days ago, hehe. In principle for testing purposes, I'd prefer to integrate the converter in the board if I can find a reasonable solution, but using one of those is certainly a possibility.

just out of interest, what guitars do you have?

Just a 90's Fender Mustang MIJ and a cheap chinese strat copy. I'm not a great guitar player at all and I only play at home these days, don't really need more than that.

PS: if you run the opamps from +-9V, as you suggested, the sound would change, not due to the headroom or noise considerations but because the opamp itself is  operating from lower rails. This effect is not so marked in newer opamps, but more so with the older opamps.

Damn, I see. Thanks for letting me know, that seals the deal for anything but a full +/-15V solution then

[spec]

I mentioned about capacitors affecting the sound. Just in case, the CDE 715 and 716p orange drop polypropylene capacitors are very much favored in the guitar amp world (without going crazy on price). Data sheet attached.

At the other end of the spectrum ceramic capacitors give a gritty sound which some players like. y5U dialectic in small leaded or surface mount versions have the greatest effect.

[dazz]

Damn it, look at this in the product description of those boards in aliexpress:

1 Due to the diode partial pressure, the voltage of -Vo is lower than +Vo
2 In order to obtain a more stable voltage, it is recommended that the output is greater than 15MA

I guess I could add a diode in series with +V to make it more symmetric, and a resistors in parallel with the preamp to increase load and go above 15mA?

I mentioned about capacitors affecting the sound. Just in case, the CDE 715 and 716p orange drop polypropylene capacitors are very much favored in the guitar amp word (without going crazy on price). Data sheet attached.

At the other end of the spectrum ceramic capacitors give a gritty sound which some players like. y5U dialectic in small leaded or surface mount versions have the greatest effect.

Thanks, I will include that info in the build instructions 

[spec]

I ordered a couple of those [9V to +-15V modules] just two days ago, hehe. In principle for testing purposes, I'd prefer to integrate the converter in the board if I can find a reasonable solution, but using one of those is certainly a possibility.

You can mount the PSU modules on the board, just like a component, or am I missing something?

Just a 90's Fender Mustang MIJ and a cheap chinese strat copy. I'm not a great guitar player at all and I only play at home these days, don't really need more than that.

Yamaha FG350W (1981), Washburn WLD20S, and  Crafter ST120 strat copy- can't play any of em 
Just sniped a Fender Deluxe 112+ combo for a song on ebay. The voice coil connecting wire has pulled of the cone and snapped under the dust cover- must get around to fixing it.

[spec]

Damn it, look at this in the product description of those boards in aliexpress:

1 Due to the diode partial pressure, the voltage of -Vo is lower than +Vo
2 In order to obtain a more stable voltage, it is recommended that the output is greater than 15MA

I guess I could add a diode in series with +V to make it more symmetric, and a resistors in parallel with the preamp to increase load and go above 15mA?

That module is just the first one in Google. It is just an example. But all the same, the minimum current draw is no big deal and is a requirement for many PSUs. (even the LM317, LM337 three-terminal regulators).

Minor differences in the PSU rail voltage should make little difference- you will see more variation in sound between different samples of the opamps, especially different ages and suppliers.

[dazz]

You can mount the PSU modules on the board, just like a component, or am I missing something?

Yeah, sure. In fact I have an extra pad in the pcb design to connect the boosted voltage from an external converter to give that option to the user in case s/he doesn't want to use the charge pump. Yes, I'm so stupid that I couldn't wait to make the pcb until I had a closed design  . Now I'll need to modify it to use dual rails and whatever I put there to implement them

Yamaha FG350W (1981), Washburn WLD20S, and  Crafter ST120 strat copy- can't play any of em 
Just snagged a Fender Deluxe 112+ combo for a song on ebay. The voice coil connecting wire has pulled of the cone and snapped under the dust cover- must get around to fixing it. 

Ouch! I wouldn't know how to fix that. Congrats on the bargain!

[dazz]

That module is just the first one in Google. It is just an example. But all the same, the minimum current draw is no big deal and is a requirement for many PSUs. (even the LM317, LM337 three-terminal regulators).

So if my preamp draws say 5mA, I need to add a dummy load to increase the current, right?

Minor differences in the PSU rail voltage should make little difference- you will see more variation in sound between different samples of the opamps, especially different ages and suppliers.

Ok, I was concerned that an imbalance in the rails might induce asymmetrical clipping and change the tone.
Apparently people are using LT1054 based charge pumps for the dual rails with good results
I might just put that in my pcb and then the V+/V- pads to feed +15/-15V from an external board

[spec]

Ouch! I wouldn't know how to fix that. Congrats on the bargain!

Thanks- it was a bargain. Apart from some work by a gorilla inside, the amp is in good condition- I was expecting some beat-up wreck.

Fixing the speaker is fairly simple:

• Heat the dust cover to melt the glue and pull it off.
• Remove broken tail wire.
• Solder new tail wire to protruding voice coil wire.
• Araldite voice coil wire, solder joint, and tail wire to cone
• Re-glue dust cover.

Of course, that is in theory- I changed the car battery yesterday- took four hours. 
But I have done steps 1, and 2 already, mainly to establish the fault.

[spec]

So if my preamp draws say 5mA, I need to add a dummy load to increase the current, right?

Yes, spot on. But if you were using batteries it would be benificial to get a PSU that had no minimum drain requirement.
By the way, having a constant resistive load is good design practice- it helps all PSUs.

Ok, I was concerned that an imbalance in the rails might induce asymmetrical clipping and change the tone.

With non RRO opamps, like the TL072, you get non-symetrical clipping even if the supply rails are exactly equal in magnitude.

Apparently people are using LT1054 based charge pumps for the dual rails with good results
I might just put that in my pcb and then the V+/V- pads to feed +15/-15V from an external board

Sounds good to me.

[schmitt trigger]

1 Due to the diode partial pressure, the voltage of -Vo is lower than +Vo

Chinglish translations at its best.

[dazz]

OK, how about this for a dual rail supply? It's based on two MC34063. The first one boosts the voltage to 18V, the second inverts it to -18V, then I can regulate them with some 7815 + 7915.
Or maybe I can do what b_force suggested here and get rid of the inverting 34063?

Another idea i to get a basic 9-12 or 9-15 boost converter and use a flyback or 1:1 coupled inductor to form the negative voltage.

I have no idea what a flyback or 1:1 coupled inductors are, but that's what google is for, isn't it?

Pros and cons of this last solution seem to be:

-PROS-
1. MC34063 & 220uH inductors and of course the obiquitous 7815 & 7915 are pretty cheap and readily available

2. Noise free regulated rails

-CONS-
1. Component count increases. I still have two DIP8 IC's, and i don't need the voltage divides and the coupling caps for the opamp bias, but the power supply section gets significantly more complex with 7 resistors, 10 caps (including the ones in the regulators) 2 diodes, 2 inductors and two linear regulators. If I could do the flyback thing this may not be an issue

2. What the hell are those 0.22R & 0.24R resistors all about? Apparently they need to be that precise at those ridiculously low values or else the converter doesn't work.

ETA: Of course I wasn't the first one to come up with this idea: https://www.diyaudio.com/forums/chip-amps/206788-using-2-mc34063-cmoy-virtual-ground.html
Dave even has a video on it apparently:

[dazz]

then I can regulate them with some 7815 + 7915.

Wait a minute, there's feedback in those MC34063, so they're regulated, and if I calculate my values to have a high enough ripple frequency that it's not audible it won't matter for my application and I won't need those linear regulators, is that right? Because that would significantly simplify my circuit

[Zero999]

You could build your own power supply but it's a pain. Just use a DC:DC converter module. Some examples are linked below, which have a regulated, isolated output. Additional filtering will probably be necessary to prevent audible noise, due to rectification of the RF in the op-amps. No doubt there are cheaper modules available off ebay, Amazon etc.

https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1714747/
https://docs-emea.rs-online.com/webdocs/168f/0900766b8168f657.pdf

https://uk.farnell.com/xp-power/itw0515s/dc-dc-converter-1w-15v-0-033a/dp/2422847
http://www.farnell.com/datasheets/1847138.pdf?_ga=2.154705917.435636270.1543350785-1037805362.1481052103

[John B]

Don't use the MC34063, far too much quiescent power wasted.

I'm in the middle of some similar experiments with the 7660s, to see if I can reduce the switching noise. Probably a mix of both input and output filtering.

It's going to be far more complicated than anything implemented in a commercial musical product, like pedals or guitar preamps, but for a DIY application, parts count isn't the problem, its noise and efficiency.

You're going to have to filter the MC34063 anyways.

I'm a little confused with your input power requirements. So you're not going to run it off batteries ever? Always an adapter? If that is the case, simply supply your pedal with a wide enough +/- range in the first place. Either with 2 floating DC adapters or an AC transformer supply. The input can be filtered and linear regulated, both on the + and - rails.

Your dual power supply idea seems like the way to go at first, but you end up spending far too much time, effort and space trying to make it work, not to mention probably about 30% efficient 

[dazz]

You could build your own power supply but it's a pain. Just use a DC:DC converter module. Some examples are linked below, which have a regulated, isolated output. Additional filtering will probably be necessary to prevent audible noise, due to rectification of the RF in the op-amps. No doubt there are cheaper modules available off ebay, Amazon etc.

https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1714747/
https://docs-emea.rs-online.com/webdocs/168f/0900766b8168f657.pdf

https://uk.farnell.com/xp-power/itw0515s/dc-dc-converter-1w-15v-0-033a/dp/2422847
http://www.farnell.com/datasheets/1847138.pdf?_ga=2.154705917.435636270.1543350785-1037805362.1481052103

Those are just great simple, compact solutions, than you. If I can find those for cheap it's a done deal

Don't use the MC34063, far too much quiescent power wasted.

According to the datasheet the quiescent current is 3mA. 6mA if I was to use two chips... just as much as the opamps themselves, so yeah, not ideal at all I guess.

I'm in the middle of some similar experiments with the 7660s, to see if I can reduce the switching noise. Probably a mix of both input and output filtering.

It's going to be far more complicated than anything implemented in a commercial musical product, like pedals or guitar preamps, but for a DIY application, parts count isn't the problem, its noise and efficiency.

You're going to have to filter the MC34063 anyways.

I'm a little confused with your input power requirements. So you're not going to run it off batteries ever? Always an adapter? If that is the case, simply supply your pedal with a wide enough +/- range in the first place. Either with 2 floating DC adapters or an AC transformer supply. The input can be filtered and linear regulated, both on the + and - rails.

Your dual power supply idea seems like the way to go at first, but you end up spending far too much time, effort and space trying to make it work, not to mention probably about 30% efficient 

Well, I have an adapter cable to plug a 9v battery or two Li-Ion cells to the 3.5mm socket in case I need it but what I was referring to is that I personally hardly ever use anything but a wall wart 9V supply. There's no requirement in the design for that though, If someone else wants to build the pedal and power it with batteries, that should be an option. I'll just provide a 9V/DC input in my board, at least that's the initial plan.

Are you having trouble with noise with those 7660S's? Even using the boost to 30KHz pin? Bummer. I was hoping I wouldn't need to filter my supply if I could get the ripple frequency above 30KHz or so

BTW, I said before that for my application (dual opamp) I would have an almost constant current draw from the supply, which I now think is BS. Wouldn't that be a problem with something like a 7660S since it would sag pretty bad (a couple of volts) if you get a 5mA spike in your current draw?