# Where to add equalization in active audio mixer?

I want a 2-input summing amplifier where I can adjust the amount of signal from each source with a single potentiometer. I intend to hook up two microphones (Shure SM57 and Audio Technica ATR 1500, both with impedance matching transformers) to a single Orange guitar amplifier. The circuit I have currently built looks like this:

simulate this circuit – Schematic created using CircuitLab

The single pot allows me to add some of one signal and simultaneously subtract from the other. The 330 $$\\Omega\$$ resistors are added so neither mic is ever completely silent. I am using unity buffers because each microphone has slightly different impedance and output levels, so it's much easier than trying to create some sort of impedance matching network for the two different microphones.

The two buffers are on the same LM358 chip while the summing amp is on a separate chip. I'm currently running the op amps off of a DC power supply, +7.5V, 0V, -7.5V, although I intend to ultimately run the circuit off either one or two 9V batteries.

The problem is that I'm getting some low frequency hum that I don't get when I just run a single mic into the guitar amp. So I figure I need to add a high pass filter somewhere but I'm not sure whether its better to put it before the buffers, before the summing amp, or after the summing amp. Any advice?

• I see now that the 330 $\Omega$ resistors actually keep the gain for each signal finite, rather than never letting them go silent. So I should probably adjust my component values so that I won't saturate my second op amp so easily. Oct 20, 2021 at 2:45
• Does this hum change in intensity when you rotate your contraption or move wires around? What are you using for capacitors and how are they connected? (Your schematic shows "none" on the power rails which would be quite "wrong") Did you use a 'star ground' scheme, or just tie all your grounds however it was physically convenient? Oct 20, 2021 at 5:52
• As Kyle suggests, find and fix the problem rather than adding EQ to hide it. Oct 20, 2021 at 14:15
• Since you've got the buffers anyway, why not omit the transformers and use a differential input for each mic (and a balanced line)? Either way, the hum should not be there so try to solve it rather than try filtering it. Oct 20, 2021 at 21:05
• As shown, the voltage gain is 15.5. With all other connections and cables unchanged, power the circuit with two 9 V batteries and see if the hum changes. Also (off-topic), I recommend changing to a different opamp. The 358 is cheap and indestructible, but it has problems with high noise and crossover distortion, and low bandwidth and slew rate. The Burr-Brown (now part of TI) OPA134-2134-4134 is very well regarded for precision audio work. Oct 20, 2021 at 21:17

I agree with the comments better to fix it than filter it. Unless you build something rather complicated you'll have to wipe out a lot of bandwidth and that will sound poor.

The hum you referring to is highly likely due to AC mains pickup. This type of issue is usually due to stray magnetic fields inducing currents in conductor loops, the bigger the loop the worse the issue - think loops involving separate pieces of equipment.

How is the +/- 7.5 VDC sourced? A mains powered bench supply? Is the output of this source floating or connected to mains earth? If it is earthed and so is the amp there is a big earth loop through through your circuit and the amp and then the building earthing.

Some tests you can do. Does the hum occur only when the above circuit is plugged into the amp and no microphones connected. (On this the inputs to the op-amps should have bias resistors, maybe 10-100k to ensure the op-amps are well behaved without a mic plugged). As is the op-amp will saturate. Or does it occur when the mics are plugged into the circuit.

Try running from battery and disconnect the PSU. The will remove two potential sources, earth loops or voltage ripple in the supply (less likely).

Another option: does the amp have higher level input, ie line level. If so you could change the pre-amp to have gain and feed into the higher level input.

You might want to use the well known Baxandall's equalizer at PS3 input:

https://www.petervis.com/record_players_and_turntables/baxandall-bass-and-treble-control/baxandall-eq-circuit-design.html