A Not So Simple Headphone Amp Part 3

I realized that it may be helpful to give a basic layout of what is trying to be accomplished before moving on.

Conceptually this is very simple.  The important parts of the project are coming up with a solution that is clean for frequent use.  By removing the power supply from the amplifier, there will be less wires and parts involved, and it's operation becomes dictated by the mixer, which is ideal.  Also, the amp as well as mic and headphone connectors will apart be part of a single unit that mounts in an electrical gang box of the wall in the office where it will be used.  Let's get back to the design.

No More Op Amp
I previously said that the op amp didn't work well under phantom power.  There were issues that I might investigate later, but occasionally the supply voltage would dip, and the op amp would straight up shut off, resulting in some choppy sound.  My final design is really quite un-unique.  I actually saw it while looking into crossover distortion on Wikipedia.  When I did see it, I realized there was no reason to reinvent the wheel, just modify it to my needs.

Quite a bit different looking that the previous layout.  But nothing here that hasn't been around since the beginning of electronics.

Some of you may notice that the output is still very similar to the previous design.  However, instead of driving the point between diodes D3 & D4 we're driving transistor Q3 which is in a common emitter configuration.  This will accomplish the same thing in the end, be we get the advantage of Q3's gain to help keep the front end from being loaded down.

Others of you may have noticed that the "front end" is the basic "long tailed pair" (R6, R7, R12, Q5 and Q6).  It's a building block of your garden variety op amp.  Fancier versions replace the resistors with current mirrors and constant current sources, but that is not necessary for what we're doing.  The base of Q5 is our non-inverting input and the base of Q6 is the inverting input.  Incoming audio still goes thru a similar DC biasing scheme as the previous design.  The feedback (for those familiar with simple op amp circuits) is done the same way as an op amp.  The output of the amplifier is fed to Q6 thru R9, allowing our input to track the output.  The gain is given by $A_V = 1+\frac{R9}{R8||R13}=4$.  If you do a little more math you find gain in dB to be $20*log{\frac{4}{1}}\approx12dB$.  Put that on top of 18dB lost by plugging directly into the mixer and we're ahead 30dB.  Not a ton, but it doesn't need to be.

C4 might seem kind of random.  While listening to music thru the amp I was noticing distortion at lower volumes.  Almost like crossover distortion, which we already solved.  Except now, it would go away if the volume was turned down further (with crossover distortion, the audio would be distorted all the way down to zero).

This should be a nice sin wave....

Looking at the scope there appears to be some high frequency ringing oscillations.  The fix?  Capacitance.  Rather than analyze the circuit, I just used my finger as a test capacitor.  This only works for low voltage circuits, by the way.  By touching the various circuit nodes I could listen to any changes to find the problem.  And C4 came into being.  I could have done it to ground as well, but Vcc was easier to solder to as I already had everything built up.  When it comes to AC signals, DC sources act as AC grounds.... something about ideal DC sources having zero internal impedance.... and superposition of voltages... and I don't want to get into it.  But it does make sense that the problem is related to the feedback.  Anytime you have an amplifier, there is a potential for instability.

All said and done, this new circuit is essentially the same as the previous one.  Our "op amp" is just much simpler and it's operation more in our control (and it will work under 2.7V).  We still need power though.....