I have migrated all libraries and projects to the Username “Documents” folder.
Please download and replace DO NOT OVERWRITE all previous v8 files.
This major edit is intended to allow simplified install of the Footprint and Symbol Libraries
and Custom 3D Models.
SEE THE HELP FILE AT END OF THREAD FO R DETAILED INSTRUCTIONS.
Design and Operation of the 4 Channel
Dual Operational Amplifier Audio Mixer
Because first stage amplification max is “50” (R1=Input * P5=Gain)
and the second stage capable of 10 for a total gain of 50*10 = 500
a 10mV input will produce 5V Out.
This is quite a boost of about +54dB (Decibels).
Enough to allow a low impedance mic (about a 10mV signal)
to be used without the usual expensive equipment needed to make one work.
The 4 potentiometers in parallel to the input results in Ohm’s Law impedance of
1 / (1 / 10k) + (1 / 10k) + (1 / 10k) + (1 / 10k +) = 2500 Ohms.
The ‘Goldlilocks Impedance’ just about right for high and low impedance
signal input devices.
If the input trimmer gain is set to max (50k) the roll off freq. is 3000 Hz.
Freq. cutoff = 1 / (2 * Pi * R * C) = 1 / (6.28 * 1e-9 * 50k) = approx. 3000 Hz…
Because the Low Pass capacitor is parallel strapped across the gain
potentiometer frequency will increase with decrease of gain.
This is useful for preventing high frequency feedback at high volume
and increasing bandwidth as the volume and feedback decrease.
There is quite a substantial amount of info on using KiCad within the download additional files.
I just posted this and just barely got the website up and running so I have not
had a chance to add more details.
This Audio Mixer is fully functional and I have even had it manufactured through JLPCB.
The project also contains a unique FacePlate that can be designed with KiCad PCB Editor.
Please move this post as best suited. I was not aware that it might be possible to
to submit it to the “made-with-kicad/categories” I will read up on submission guidelines.
I may wait for a few members to test the Audio Mixer Project out first and submit based
upon ratings.
I had no intentions of flagging. My thoughts were that this will disappear down the list in a week or so, probably never to be resurected. It is a completed project, with all the bells and whistles, and would have permanent exposure with the designer’s site links and whatever.
Maybe @invntefx will be able to get an Audio category organized also.
741’s.1458’s and I love the old LM386!
I tried some expensive op amps in this project too and it made little difference.
It works great on an inexpensive 8-pin dip op amp of almost any kind.
I built it so I could plug in low impedance mics that are 3pin to 2pin adapted.
If R1 is reduced to 1kOhm It will actually boost a Shure SM48 to squeal with a little feedback without too much noise level!
The secret is in the input channels design.
A decoupled potentiometer wiper as the input on all channels so that there is barely
any effect on the input impedance since the pot high is the out to the first stage op amp.
Then a good hefty 1000 picofarad (1nf) low pass on the first stage op amp gain/feedback loop
to keep noise level down and prevent oscillation if TL70x op amp or similar is used.
For high impedance mics, electric guitar or anything that will put out at least 100mv
to the input the trimmers can be dialed down to make it exceptionally quiet for use
with a DAW.
Wow that takes me back to the days when I had a metal punch for making tube socket holes in aluminium chassis.
A couple of years ago I bought a 50+50W stereo class D amplifier with Bluetooth input of about Altoid tin size for around $10. It’s powered by a ex-laptop power supply. How things have shrunk in 50 years.
I never ventured into assembly with tag strips. Single sided paper phenolic PCBs was my baptism.
I still have my Scope Irons for repairing tag strip stuff. Both in fully working condition and still occasionally used.
“Son, what’s that noise?”
