Just how bad is it? Audio mixer with headphone amp.

[BrianHG]

heard them clip or distort very often.  Too soft.

They wont be soft anymore with all your gain everywhere + a 30v supply range.  Also, sometimes, there is clipping in your source as well.

[paulca]

BTW... the general pattern in the book is to present the basic "default" textbook solution and the discuss it's problems.  Then present known designs by some amazingly intelligent people, sometimes the author which try to "have their cake and eat it" with little draw backs.

From my main background in software engineering, I see this happen a lot.  And... I agree... sledgehammer to crack a nut often actually makes things worse!

Back on topic, the second iteration of design in the book on headphone amps is this, which addresses a few problems with the first (a) posted above.



I haven't read all my way between the sections, but it ends explaining that you might as well just use parallel 5532s.  I think he owns shares in TI.

[paulca]

heard them clip or distort very often.  Too soft.

They wont be soft anymore with all your gain everywhere + a 30v supply range.  Also, sometimes, there is clipping in your source as well.

I have +17db MAX on the input and only if the gain knob is maxed out.  At "half way", it will have 0db gain.  (remember active gain).

I mentioned i have not decided on the master volume and might not use an active gain "block", but if I do and it's at "nominal" it will be 0db gain.

So a full path through the mixer might have absolutely zero gain until the power amp.  Zero gain and zero attenuation mind.  Flat gain structure, so the only noise added is the op amp inherent noise which is tiny.

Of course I can have at least the +17db on the input preamp and ... unless I change it another +17db on the output volume.

Actually, 17+17db is probably not = 34db, but there might be a risk of actually killing headphones with an accidental volume setting which could be an issue.  I want driven hard, not release smoke.

[Audioguru]

Of course opamps and most power amps have a current limiter for protection against idiots who short the output to ground. But the current limiter usually does absolutely nothing when the current is not too high for it. Why do cars have brakes? They are not used all the time (but idiots drive with both feet on the pedals).

[paulca]

Of course opamps and most power amps have a current limiter for protection against idiots who short the output to ground. But the current limiter usually does absolutely nothing when the current is not too high for it. Why do cars have brakes? They are not used all the time (but idiots drive with both feet on the pedals).

Fair enough.  I'm probably mixing two different things.  Somewhere in the early 90s after "Sony Walkman" devices started to put out some serious headphone volume a group of nannies with pitch forks forced some sort of regulation on such devices and Sony and most major brands adapted these... To limit damaging teenagers ears.  It is hard today to find a device without such a limit.

My experiences with the NE5532 have pretty much told me that by using similar chips for headphone amps their inherent current limit almost matches or comes in under those soft regulations on output.  Which is why almost all headphone out devices today are effectively limited to "Nanny state" mode.

I don't want to cook the headphones or my ears, but I do want to know where the ceiling is.  I want to be able to overdrive them, I want to know where the limit is and the headphones clip.   I don't want it imposed upon me.

Which loops back to one of the first design criteria I selected.  When you have a limited output and a quiet signal, you are incapable of addressing that issue.  You need pre-amp gain to lift it to a normal level to do that.

[paulca]

So the pre-amp board got finished.  Works a treat.  Through my existing setup I can push the headphone amp into distortion which is most likely the pair of 5532s in it limiting.  Sounds clean, but I haven't done much scoping for noise.

It is still current hungry which has me a little perlexed.  My first thought was the 100R outputs.  Which should be much higher.  However it is still current hungry when not driving a load.

I need to get a proper measure of the current as I'm only going by the PSU's consumption which is 30mA before connecting the circuit and 76mA when connected.  Driving a load with it barely moves the current consumption, maybe 1 or 2 mA.

The other thing it could be is ultrasonics making the opamps work hard, no idea.

[BrianHG]

Any scope shots?  From input to output... Feed a 20Khz square wave at different gains and different source amplitudes to verify flat response.

[paulca]

So nothing I have will generate anything but a slightly mangled sqaure wave over about 5kHz even at 5K it's barely square.  I tried both my phone and my PC.  The mickey mouse chinese sig gen kit I didn't even bother with as it's square output is unattenutated and about 10V peak to peak with a 5V virtual ground.

The main issue I seem to have is noise from the PSU.  Something around 80kHz but the scope is having trouble with locking onto it at times.

Images here some are worth a laugh, like my phone trying to produce 20kHz:
https://imgur.com/a/owSEF

Sample of PSU noise, straight off the negative power rail:


This was after putting 470uF caps and 100nF caps from both rails to common.

So as this appears on all op amp output I need to get rid of it somehow.  It's at such a high frequency it shouldn't bother the audible range, but it does bother me and it probably isn't helping!

Max gain, 2k sine:


Min gain, 2k sine:


Just noise on the output.  EDIT:  I fixed the none zero output, was a miss connection.  It now mutes at minimum gain... or near enough.

EDIT2:  Yes, it's annoying that the scope does not capture the right hand side with the time division read out and sample rate

[BrianHG]

What do you have for your PC?  What's the sample rate set to in the 'Speaker Properties' on the 'Advanced Tab'.  Also, in the 'Enhancements Tab',  make sue the 'Disable all enhancements' is selected.

By default, if you never set this, usually it is set to the crummiest weakest setting.

[paulca]

What do you have for your PC?  What's the sample rate set to in the 'Speaker Properties' on the 'Advanced Tab'.  Also, in the 'Enhancements Tab',  make sue the 'Disable all enhancements' is selected.

By default, if you never set this, usually it is set to the crummiest weakest setting.

The PC output is a Texas Instruments PCM2707 USB DAC, but I was trying to use a YouTube video    I might install audacity again at some point.  But I have a signal gen in the post, though it's an FY6600 so it remains to be seen if it will need modified before it's safe to use.

However the phone didn't do a very good job either.

On the PSU noise.  A quick google suggests that meerly decoupling caps will not really help and I need to "bypass" with LC circuits.  I'll do some more reading.

[paulca]

I do have the 1Khz square scope calibration signal...  It might at least be square.

[paulca]

Also it's slightly odd that the amps do not seem to be amplifying the noise much, they seem to just be modulating their output with it.  Maybe that makes sense as the noise is not on the inputs, but the power rail.

[BrianHG]

The PC output is a Texas Instruments PCM2707 USB DAC, but I was trying to use a YouTube video    I might install audacity again at some point.

Arrrggg, 48khz 16bit as a reference.  Even bottom of the line motherboard/laptop audio chipsets usually are better quality as long as you have a good grounding on you cable.  These bottom end USB Dacs are only good if you need another sound card source.  There do exist high quality USB dacs, but, the PCM2707 isn't one of them.

Here is 2 samples, 24khz square which will come out as sine and 12khz square sampled at 48khz.

[paulca]

Scoped them from the audio out and they both show as sine waves.  Slight wobbly, but I was holding the probe to the jack plug.

Come to think... you can't really do a 20Khz square wave with a 48Khz digital source, not very well anyway, it's going to filter down to a sine anyway isn't it?

Hang on a moment.  I doubted my little chinese friend.  I hooked it up and got these:


and...  pushing it a bit.



The output is not attenuatable though and it's a bit much to be putting into the pre-amp.  I can try tomorrow evening, can't tonight, I also need to use the little generator up to a jack plug.

[paulca]

Even bottom of the line motherboard/laptop audio chipsets usually are better quality as long as you have a good grounding on you cable.  These bottom end USB Dacs are only good if you need another sound card source.  There do exist high quality USB dacs, but, the PCM2707 isn't one of them.

That PCM2707  sounds a thousand times better than my Intel HDA on my motherboard.  Maybe not in it's sound reproduction, but in the fact that the PC onboard sound is completely saturated in video card noise.  Was sending me nuts until I happened to hook up that DAC which is a £3 board from Alice on ebay.  No more noise.

My my audio project, which this thread's amp is the heart of I hope to use a PCM2902 or similar.  I'm open to suggestions.

[BrianHG]

Scoped them from the audio out and they both show as sine waves.  Slight wobbly, but I was holding the probe to the jack plug.

Come to think... you can't really do a 20Khz square wave with a 48Khz digital source, not very well anyway, it's going to filter down to a sine anyway isn't it?

If my samples are wobbly, your sound card must be set to 44.1Khz, not 48Khz.
Give these 2 a try, they have a native 44100hz sample rate:

[BrianHG]

That PCM2707  sounds a thousand times better than my Intel HDA on my motherboard.  Maybe not in it's sound reproduction, but in the fact that the PC onboard sound is completely saturated in video card noise.  Was sending me nuts until I happened to hook up that DAC which is a £3 board from Alice on ebay.  No more noise.

My my audio project, which this thread's amp is the heart of I hope to use a PCM2902 or similar.  I'm open to suggestions.

I must have gotten lucky with my Asus motherboard and HP laptop.  Full flat to 96Khz completely noise free even wired direct to a 500 watt amp, using the desktop volume settings.  Note I use the plugs in the back of my motherboard and never wire those front panel unshielded audio jacks.  Also read reviews on the audio performance of the PC motherboard I chose.

[paulca]

Try a pair of headphones on max gain, don't play any audio.  Now move the mouse around.  If you can't hear the buzzt-t-t-t-tbuzzzt-t-t-t you are lucky.

Now bear in mind the video card in my PC, which regrettably sits smack bang on top of the chipset more or less and right beside the back plane connectors, not 2 cm from the audio jacks, has dual 10A 12V rails, dedicated to it alone.  NVideo GTX 980.  (It's so big it doesn't quite fit in the case and pushes one of the HD bays out the front.

[Bassman59]

Back on topic, the second iteration of design in the book on headphone amps is this, which addresses a few problems with the first (a) posted above.



I haven't read all my way between the sections, but it ends explaining that you might as well just use parallel 5532s.  I think he owns shares in TI.

Here's the thing -- that book (which I have) was written a long time ago. Not that the principles have changed since it was published, but the devices available for our use have improved greatly. Back then, the jellybean op-amps couldn't drive headphones, hence the designs using discrete transistor buffers or specialist buffer amplifiers. Nothing wrong with using them. But you can save yourself some aggravation and simplify your design by using something like the OPA551 op-amp. Low distortion and enough drive to clean the wax outta your ears.

[Bassman59]

I think we agree, however do not be fooled into thinking that "everything is digital today".

In the past few years I have probably seen about 20 mixing desks in person,  I have yet to see a digital one (that wasn't a PC or Mac based thing).

Small 8 to 16 channel mixing desks you would use as an audio semi-pro running a few gigs and recording a few demo tapes would be like buying a higher grade oscilloscope.  Priced, today, around £1000.  A fully digital mixer would cost about ten times that.

I'm a mostly-retired soundguy, but several times a year I still get on the bus to mix front-of-house on tour for old friends who are a well-regarded college/indie rock band. We play venues like the 9:30 Club in Washington, DC (the best club in the country), the Sinclair in Cambridge, MA, Rough Trade and Williamsburg Hall Of Music in Brooklyn, NY, World Cafe Live and Union Transfer in Philadephia, PA. Plus we do the occasional festival and outdoor gig (Central Park Summerstage).

Over all of the shows in the last, oh, ten years, I can think of exactly one venue that still has an analog console: the Met in Pawtucket, RI, and that piece of crap needs to get gone. Everywhere else has digital. 9:30 has DiGiCo (which I love), Rough Trade and WCL have Midas Pro 2, Sinclair has Avid SC-48, and the festivals bring out Avid Profile or SC-48 because everyone who'll mix a band on those shows can walk up to it and get right to work.

The big theater here in my city has a Yamaha M7CL. The 400-cap club across the street from it has a Soundcraft Si Performer 3, and the other 400-cap club up the street has a Midas M32. The Behringer X-32 has taken over the small-club market. A friend mixes at a hall in NJ on a Mackie DL32R, which sounds really good but I hate iPad mixing (I've tried, and I can't do it, I need tactile feel.).

Analog consoles for live sound have gone the way of the dinosaur. The last time I mixed on a Midas H3000 was nearly ten years ago. I did see one last summer at Pitchfork in Chicago; PJ Harvey headlined my stage and her FOH guy had one. But he was the exception, and her monitor guy used a Yamaha PM-5D. The reasons are obvious: racks of outboard gear (effects, dynamics, EQs) are unnecessary, it's all at your fingers, and it saves truck pack space. FOH area can be smaller, reducing seat-kill for theaters. Recall of settings makes switchover between acts as simple as a button press. Wireless control means you can stand at a vocal position and ring out a monitor wedge and make adjustments right there, instead of yelling over to the person at the desk.

Anyway, back to the design.

[BrianHG]

and enough drive to clean the wax outta your ears.

 

[paulca]

Apparently you can buy cheap digital 8 channel desks for £600, but most of the stuff I have seen is pubs, clubs, bands and bedroom musicians are using things like Mackie's and Soundcraft spirits.  So maybe they are getting cheap enough for smaller venues, but the only digital mixing I have seen has been using PC/Mac sound card style solutions with USB remotes.  Those are just the electronic musicians though.

Anywhere big enough to have an installed sound system and proper desk/console is a lot larger than the places I have been into. 

Besides I think my info is a few years out of date.  I forget how quickly time passes the older you get   What seemed like a few years ago turns out to be 5 or 10.

[BrianHG]

By the way, Bassman59's suggestion of using an OPA551 op-amp is an excellent idea.  Buy a few in the DIP package so you may swap them if you burn one by accident...  Use gold machine screw alloy IC sockets for this one in your design...

Damn, I now have an app for the little 1/2 watt buggers....

Now, it's time to get serious, try the TCA0372BDP1G at 0.48$ each, that's 1/5th the price of the OP551.   1 Amp out.  Driving an 8 ohm speaker at +/-8v, you will get 8 watts out of this opamp.  And it's a dual, so 1 8 pin device for stereo...

Wow, an 8 watt bugger for 48 cents, stereo.  With headphones, this guy will remove your eardrum from your ears!
You can drive small bookshelf speakers with your pre-amp directly through the headphone output.

[paulca]

Light weights.  I had a TRIO amplifier.  I'm fairly sure they just connected the speaker outs to the headphone jack

I should have had little headphone stickers on that amp with an X through them.  It would have been an Ace in WWII.

[Bassman59]

Now, it's time to get serious, try the TCA0372BDP1G at 0.48$ each, that's 1/5th the price of the OP551.   1 Amp out.  Driving an 8 ohm speaker at +/-8v, you will get 8 watts out of this opamp.  And it's a dual, so 1 8 pin device for stereo...

Wow, an 8 watt bugger for 48 cents, stereo.  With headphones, this guy will remove your eardrum from your ears!
You can drive small bookshelf speakers with your pre-amp directly through the headphone output.

Wow, I had no idea that guy even existed. The data sheet advertises “No deadband crossover distortion provides better performance for driving coils,” and of course headphones are coils. Sounds (pun intended) good to me. Gonna order a few and test.