Electronics > Beginners
Just how bad is it? Audio mixer with headphone amp.
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Audioguru:
Why does a volume control need two opamps??

The high stray capacitance of your solderless breadboard probably causes the 10MHz opamps to oscillate and draw extra current.
The linear volume control is an on-off switch that does not match the logarithmic sensitivity of our hearing to loudness.
paulca:
I believe the Baxandall configuration causes the volume control to be an approximation of logarithmic with a steep drop at lower settings.  It forms a nearly straight line on a log graph.



I'm reading Small Signal Audio Design book and he does not favour log pots very well and while he mentions them, points out their issues, he then avoids using them altogether.   There is a large section on using circuitry around the pot to emulate the log law.  One of which is the Baxandall design. The main problems with them are their tolerances are poor, usually 20% and for stereo pairs they are never matched very well resulting in perceptible balance offsets.

Another reference for this configuration, including a HF filter, that uses 4 op amps is here:
http://www.ti.com/lit/ug/tidu034/tidu034.pdf

However using this configuration with 2 opamps and no filter stage I get very little noise on the scope.  It's down around 70mV which I get anyway no matter what I connect the probes to.
paulca:
If I get a working module in this prototyping, is it worth perf boarding or strip boarding it and then hooking them together rather than using the breadboard in one mass of mess?

I find it's too easy to nudge a wire or a resistor and upset things... or fry an amp apparently.  I'm fairly sure they are dead.  Definately the one that howls at a 13.5V DC offset.
Audioguru:
I make all my prototypes soldered together on a carefully planned compact stripboard layout. If a part needs to be changed then unsoldering and replacing it is simple. No stray capacitance to cause oscillation and no messy antenna wires to pickup interference. I cut each copper strip with a drill bit so that each strip is used for a few functions to be compact and a few short jumper wires make the remaining connections. Many of my stripboard prototypes looked good enough to be sold as the finished product when only one at a couple were custom designed by me. 
paulca:
Thanks.  I think I might made a prototype of a stereo baxandall.  If I create that I can put it inline with my current system and test the extra gain on the pre-amp works as expected.  Then I just need to replicate it twice.

The only thing I don't have is a dual pot. :(  I suppose that's another RS order then :)  I can hack in 2 trim pots for now they should match the footprint.

I am going to try and use KiCad for planning the strip board.  I'm also low on strip board, might have a bit big enough, might not.  After that I have IC footprint board (groups of 3) that I could use and if that fails it would be dual sided pad board which works, but is messy creating tracks with fuse wire grid style. :)
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