Electronics > Beginners
Just how bad is it? Audio mixer with headphone amp.
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paulca:
The original circuit was wrong.  It was a best effort with a few bits of critical information missing from my concept.  I'm learning.

The Carvin and Rane as well as the mixers I have used have gain on the inputs to normalize them to the same level before mixing them.  The only exception was a £35 DJ mixer which assumed identical turntable decks.  Even that one (actually a 3 channel with 1 line input) was fairly useless as the line input was too low and needed the mix fader planted to MAX and the turntables turned down and then master raised to get a decent mix.  Bad gain structure = noise. 

If you search on ebay for "Audio mixer" all but the cheapest and dodgy-ist have pre-fader input gain on all channels.  Including line only channels.

I could go for more gain at the input, a mixing desk often has -10db / +60db and will clip.  The channel faders often have gain too, into the mix, though less than the input gain.  The master rarely has gain and usually 0db is the max stop.

I know your way will work AudioGuru, but I would like to aim for sending already gained or attenuated channels into the mix amp.  To be more honest to the mixing desk idea I should have another pot after the input amps.  But it's overkill for the purpose of a desktop audio box.  Though I expect you believe the input gain/attenuation is overkill too.

I accept your point on using log pots.  I also intend to not get caught into audiophoolery but I hope to buy branded 1% resistors and decent caps and log pots.  This is why I'm also trying to limit the number of resistor and cap values.

I'm still unsure of whether I should go with the inverting summing amp, post input opamp or not.

Anyway, I'm going to try and clear a breadboard tonight and start testing these things with the scope and some waveforms.  I only have a cheap signal generator kit thing and a soundcard, but it will do.

The only hurdle is not having a +-12V or +-15V supply.  I'm wondering if I can series my two RD Tech PSUs.  Can't find anything with google, but they pass they are definately not earth referenced as one is running off +- DC and the other off a battery.
BrianHG:
For an op-amp based mixer, you need nothing more than a wall-wart 15v/12v AC out adapter.  2 1N400x diodes, an optional series 1w resistor to soften the rectification current peaks, 6 caps and a 7815/12 and 7915/12 regulators for your power supply.  This project wont suck more than 200ma total.  The AC out wall wart guarantees transformer isolation from the mains.  No stinking Y capacitor means no looping ground hum and the low power means you pretty much will survive an accidental short-circuit.  That is unless for some reason you use an above 500ma wall-wart.  I wouldn't go above 1 amp.

Makes the device safe to the touch as well, with approved wall-wart of course.
Stay away from switching supplies here.  You just don't need em.

paulca:
I know I can do that and probably will for the breadboard.  I still think I can series my PSUs and take the mid point as ground to get true +/-15V.

I want to avoid a virtual ground in the real thing.  It tends to cause issues when connecting to other equipment, especially hot plugging.  Suddenly you are raising a piece of equipment ground by 6V, they tend to get upset, especially 5V devices which I found brown out and crash.  Not a bit deal for headphones though, but connecting it to an Arduino to display an LED level meter... brown out crash.  A bigger coupling cap would raise the ground slower, but maybe not slow enough. 

I also crashed the USB Host in the PC doing similar.  It did not like it's ground being ramped up to 6V at all.  "dmesg" output showed the OS considered the host as "dying" and disabled it.  I had to reboot to get it back after 20 minutes of trying to send the reset code to the device /sys
BrianHG:

--- Quote from: paulca on February 13, 2018, 09:39:00 am ---I want to avoid a virtual ground in the real thing.

--- End quote ---
:-+
paulca:
Oh, hang on, I think I follow you now.  You mean to open the AC/DC wall wart and tap the transformer to get 15VAC then filter it to get +/- 15V DC?  EDIT: or more likely +- 6V or 9V or something similar after caps filters the half sines.

I actually bought a 12V wall wart off RS destined for that experiment exactly.
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