LM3900 Audio Mixer questions


I was wondering if anyone here has made an Audio Mixer using the LM3900?

I found this schematic and gave it a try:
Muti-channel audio mixer circuit based on LM3900 IC. Four channels. (circuitstoday.com)

I changed the circuit slightly so that instead of 2 MIC ins, we have 4 LINE ins total (no mics). However I am not able to get this to work, since I am not receiving any audio at the output. Here are my concerns:

  • The circuit seems to power correctly with USB. Could 5V be too low for this?
  • I can hear some low noise on the output there doesnt seem to be any audio signal there.

If anyone has build this audio mixer I would like to know and share some comments.



That's a screwy design...

I don't see any DC bias/reference. With a single supply, the input needs to be biased at half the supply voltage. I'm guessing you have constant 0V or 5VDC at the amplifier outputs, before the capacitor and that's "killing" your signal. (The input & output capacitors block/isolate the bias from the audio circuitry.)

I've never used a Norton amplifier. I'm sure it can be used in a regular voltage-amplification application but I don't know the difference.

Normally, mixers are built-around a summing amplifier, which only requires one op-amp There's usually a volume pot on each input and microphone mixers have a separate amplifier stage for each mic. Again, if you have a single supply the +input on the op-amp goes to a 2:1 voltage divider.

Could 5V be too low for this?

You can get a rail-to-rail op-amp which goes from (nearly) 0V to (nearly) 5V (with a single 5V power supply). I don't know how close the LM3900 goes to the power supply rails. A regular op-amp will probably work but you might get clipping (distortion) depending on the actual signal levels.

USB power (from a computer) tends to be noisy but it might be OK for line-level signals.

...Depending on the application you might be able to use a passive (resistive) mixer. The circuit you have is actually a 4-channel passive mixer, which is the four 470 Ohm resistors. If you want to mix 2 line-level signals, you could use two 4.7K resistors, and that's it! No other components needed! That's actually an "averaging" circuit so if there is only one signal it will be cut in half (assuming 2 mixed channels) and there can be some "impedance related " issues but in many cases it works fine and there is no noise or distortion.

Thank you for the reply. I found this circuit online on many webpages so I assumed it was correct, but after trying it and testing it I assume it is not. I have been trying to get it to work on a breadboard and noticed that removing the resistor that connects from an inverting input to ground makes the signal go through, but with a great deal of distortion.

Since you mentioned the passive mixer, is there actually any advantage using the active mixer instead of a passive?

I want to build a mixer for general use, with 4 inputs. My inputs would mostly be a computer, phone, or some of my arduino / raspberry pi audio synthesizers. I have a pair of USB powered desktop speakers that will be the output, or just a headphones. Do you simple passive mixer, like the one you mentioned is enough? All this is for low powered speakers (3W max) so I dont need a lot of volume.


The LM3900 is a quad Norton amp, not a standard opamp as it operates in current mode.
It certainly can't be used in a standard opamp circuit as the voltage difference between the inputs
is not amplified and the biasing arrangement is completely different.

However I'd recommend finding a more standard circuit, as you won't get decent performance
with this amp (especially for the microphone noise level).

Norton amps were fashionable for a while when they were new, then people realized for
most purposes they are beaten by voltage amps - they have a niche use for AGC's and VGA's

Since you mentioned the passive mixer, is there actually any advantage using the active mixer instead of a passive?

Go ahead and try it. This page shows a schematic with one 4.7K resistor for each channel (so 4 resistors to mix 2 stereo signals.)

You'll get signal loss with the passive circuit. And, if one of the outputs has higher impedance than the other the signal might not be equal in both channels. (With normal line connections and 4.7K resistors the signal loss should be equal in both channels.)

If you mix two signals you loose 6dB. You can probably live with that. If you mix 4 signals you'll loose 12dB and that's probably too much. (Two stereo inputs and one stereo output is probably OK at -6dB in each channel.)

Note that you'll only get that attenuation with multiple inputs plugged-in. If you unplug one connection the signal will go up.

If the input impedance on the amplifier (or headphones) is too low, you'll get more attenuation. In fact a passive mixer won't work at all with headphones. The 470 Ohm resistors in the circuit you're using will also kill most of the signal into headphones (usually less than 100 Ohms).

With headphones you need a "real" headphone amplifier. A regular op-amp won't work. The voltage is about the same as line level, but headphones are lower impedance (they require more current & power) so you need an amplifier designed to drive the lower impedance.

With an active mixer you can get gain or unity-gain and it's independent of impedance.

All this is for low powered speakers (3W max) so I dont need a lot of volume.

In order to get the full 3W you need enough signal-in and enough gain from the amplifier, so you'll just have to try it to see if it's loud enough.