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Just how bad is it? Audio mixer with headphone amp.
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paulca:
So tonight's mission was successful.

The input amp worked (mono).  Gain looked good.  Response, for as far as I could tell was grand on sine and "fair" on square.  I have waveform caps I can share later.  I tested at 1Khz, 20Khz and for some fun all the way up to 1.2Mhz.  Things go ropey above about 600Khz, but it wasn't that bad.

Square wave at 20Khz has quite a slow rise/fall time and a little overshoot, but I don't think it's that big an issue.

It was also successful in that the two RD Tech power supplies did a grand job of providing +12 0 -12V rails.

Also successful in revealing just how important decoupling caps on the power rails is.  I had huge noise ripples all over the place without them.

Also successful in confirming the cheap chinese kit build waveform generator is crap.  After decoupling the noise off the rails, I was depressed at the noise on the output until I hit "HOLD" a few times on the wave and confirmed 100% the noise was on the input and not being added by the amp.  Little oscillation pulses at a much higher frequency flowing long the waveform.  They existed on both input wave and output wave, so I ignored them.  (Signal gen was running on a completely separate battery, so clean power, though it did share the ground from the PSUs).

Finally, "successful" in finding out my laptop power supply has no damn discharge what-you-ma-call-ems on the main caps.  Bastard thing zapped me after I unplugged it and it hurt.  I do make an effort not to touch the pins when I pull a plug as I have been bitten before, but it got me.  Grrr.  Nothing like a little 120/240V zap to piss you off.  It has a proper UK plug with part insulation, so there is no way to touch the pins while they are connected to the mains, so it was definately the caps on the mains side discharging into my finger :(

paulca:

--- Quote from: BrianHG on February 13, 2018, 08:21:35 pm ---if you have 1w or 5w approx 12v zener diodes, you can make a second R/C filter stage at your power input caps and parallel the zeners in parallel to limit voltage to 12v on each cap.  That will suffice.

--- End quote ---

Not sure I follow, but... oh, voltage regulation.  It's fine the bench supplies stepped up to the task.

My finger hurts  and it twitches still :(
paulca:
So I had another session with the prototype.  Learnt a few more things.

My PC is foul for noise.  There is noise between 300mV and 1V peak to peak up around 40KHz though the scope was having trouble with it.  It's most likely the switch mode power supply or some other EMI generated in there.  I spend about 30 minutes ruling things out until I had to insight to connect my phone to the input.  Wham, no noise.

The PC noise was so intense it was impossible to determine what the amps where actually doing.

That sorted out, using the mobile phone as a source, with 2 of the preamp circuits above and an inverting sum amp it was working.  I was able to mix two channel together.

The gain structure is a bit off though.  What is the natural gain of a inverting amp?  I looked up the basics and it said if R1 = ... = Rn = Rf I would have a gain of 1.  But while I had a gain of 3 on the preamp the gain after the inverting amp was only about 30%.

I also had PSU noise to deal with, an oscillating ripple pulse appearing around 500Hz with the oscillation around 20Khz.  The waveform looked like a bass drum does, if that makes sense.  I expect it's the PSU's output mosfet ringing. 

Filters...

On the HF PC input noise.  I know it won't bother me as it's way outside of human hearing range, but is it worth trying to put a low pass filter in to kill anything over about 22Khz?

On the PSU noise, I had two switch mode bench supplies in series, probably not the best clean audio supply, but even with a 100uF and 100nF across both rails AND 100uF and 100nF across each rail to ground, I could not get rid of it.  Assuming the PSU I get for the project (probably an isolated encapulated PSU) has similar noise, how do I kill it? 
Audioguru:
Since your pc generates so much noise then it might cause the opamps to amplify it and produce clipping distortion of the audio. Filter out the ultrasonic noise.
The gain of an inverting opamp is Rf/Ri. If the two resistors have the same value then the gain is exactly 1 regardless of how many input resistors there are. The opamp must be able to drive the value of Rf and most opamps have trouble driving less than 2k ohms including the output load. A non-inverting opamp is a passive mixer where the inputs affect each other.

Why aren't you using logarithmic level controls? If a linear control is turned down from max to halfway then the signal level drops only a little, then drops more and more (like an on-off switch) as it is turned down to zero. A log pot drops the level properly and evenly.
paulca:
Thanks.

I haven't decided on pots yet.  I expect I will end up going for some 1% Alps log pots from RS.  They may not play on the breadboard.

At the moment I am using stuff out of my component stock and as I only have 1 50K pot, I was using a 100K 10 turn trimmer on the second input channel.  It's a bit rough and ready.

Also it's on a breadboard... my feedback resistors are like noise antennas.  I can get all manor of pretty scope pictures by touching them.

That's why I'm not being too fussy.  I just want to prove a few concepts to myself before I consider actually building it properly on possibly strip board.

I expect I need a bit more practice and study on PCB layout to make the end device I want to build.  So I'm spending time doing digital circuits on PCB to learn before I move to an analogue board and all it's sensitivities.
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