Electronics > Beginners
Just how bad is it? Audio mixer with headphone amp.
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paulca:

--- Quote from: BrianHG on February 12, 2018, 11:14:07 am ---Don't go too crazy on the gain.  You'll begin to add hiss.  Also, having gain on you mixer stage means the primary input opamps wont have to go all the way to your rails to provide the same 2vrms output since their output is divided by 3 in the mixing.  Unless you go to Audioguru's negative scheme at the mix stage (when done right, it's used quite often in mixers), stick with my recommendation of having a gain of 2, or 4 if you want more at the input amps and the same gain again at your mixing middle amps (unless you choose 3x deliberately to compensate exactly for the mix) & don't forget to shrink those overly large sized 100k feedback & GND gain resistors.

Also remember, the higher you make the gain on your op-amp, the more lousy the high frequency response.  Another good reason to divide the gain between the input amps and you mid mix amp.

--- End quote ---

Thanks.  The plan is to maybe up the gain on the input amps a little.  I am still tempted to use feedback gain control there.

Hiss and over-gaining is why I want to avoid this pattern being suggested of attenuating the inputs then boosting them up at mix amp only to attenuate them back down again at the headphone amp.  I want a small amount of normalisation gain on the inputs, then use AudioGuru's inverting mixing amp with unity gain. 

In normal operating conditions with good inputs the input amps should be running in unity.  Another reason I'm considering adding variable gain feedback instead of amplify then attenuate.

Finally enough gain on the output amps to make my headphones clip....  I'll add a third 5532 is I need to :)  Or a quad of mosfets if I have to.

Ideally I would like the input level pots to be unity gain at centre. x2 or x3 fully clockwise and 0 fully anti-clockwise.  That might be tricky though but a log pot of the right size might work.  I have to review these gain figures in light of the DB gain calculations some placed above.

The mixing circuits seem to all have variable gain on the inputs and then mostly attenuate across the whole desk (except filters which allow positive and negative gain).  Having your channel faders about 0db is a mix smell.  Master faders usually don't even go about 0db.
paulca:

--- Quote from: BrianHG on February 12, 2018, 11:24:55 am ---Same goes for all my subsequent posts, I expect you have updated you schematic to my recommendation.

--- End quote ---

I'll try and make the updates tonight to clear up confusion.

What are the pros-cons of variable feedback?  I suppose finding the correct pot/resistors might get expensive?
BrianHG:
Do not use variable feedback.  The opamp feedback and gain resistor on the negative input is the most sensitive part of your circuit.  It responds to any RFI with ease.  Only use a fixed gain with 2 resistors as close as possible between the resistor and the - input of the opamp.

My directions were to place your series 10uf caps and volumes right at the RCA audio inputs.  This simultaneously protects your circuit from an audio source with a DC offset and makes a constant load on the RCA inputs matching the full series resistance, use 50kohm as this is standard, of the volume control.  The wiper goes to the + input of the opamp.  The opamp should have a fixed gain of 2 through 4 depending how much boost you want.

The opamp outputs of each channel should each have a series resistor feeding the center mix opamp which you can set a fixed gain of 3x to normalize the audio.

Doing things Audioguru's way means you need to invert the audio at each stage, and you still need now to further strategically place the mix resistors at the middle mixer stage right at that op-amp's - input to prevent noise.
Audioguru:
The Carvin mixer uses three opamps at the input to make an instrumentation amplifier instead of buying an IC that does it (they were not available years ago). The input of an instrumentation amplifier is balanced (microphones are balanced) and for lots of gain for a microphone. Your inputs are unbualanced line levels so you do not need the three input opamps for easch of your inputs. The Carvin circuit does have an input gain control that you need for each of your inputs.

The Carvin circuit has normal bass, mid and treble tone controls but does not have the high frequency cut filter that is in your circuit. All three tone controls and the Fader use variable feedback to adjust the gain.

You said that you have a headphones amplifier that you made from a kit but it does not have enough gain. It is simple to replace the feedback resistor with a higher value resistor for more gain.

EDIT: Here is an instrumentation amplifier:
paulca:

--- Quote from: Audioguru on February 12, 2018, 04:26:34 pm ---You said that you have a headphones amplifier that you made from a kit but it does not have enough gain. It is simple to replace the feedback resistor with a higher value resistor for more gain.

--- End quote ---

Yep.  I am borrowing the schematic for it into my design.  I believe the gain calculates as 3.13 but I could be wrong.  Opamps always surprise me.  It is the lower right corner of the schematic.

I can adjust the feedback resistor, but I need to check that I don't bump into the current limit of the parallel pair.  Of course, using 3 is an option or drop the parallel and use an output driver circuit.  I have to bear in mind that my headphones are fairly high impedance, around 34Ohms I believe, but if I connect a lower impedance set to the amps in the future I might bump into the current limiter of the 5532.

On the gain, it currently only has one input and with a good quality input I have enough gain, just about, it won't clip the headphones, but it comes close, distortion becomes present at full level, but barely noticable.  It is when there is a quiet source I don't have the gain.  In my experience in audio, yet not electronics, the place to add this gain is at the input, not the output and that becomes more important when there are multiple inputs.

EDIT:  Also if you note in the Carvin schematic both XLR and Line are amplified at the input.  Mixing desks have a "Gain" knob at the top of each channel.  It has 0db at the 12 o'clock position.  Anti CW attenuates, CW amplifies.  This is before the filters, channel fader etc.  They often have a +48V phantom voltage switch and a line level db selector..
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