Parallel Inverting Summing Amplifiers

Thread Starter


Joined Jan 28, 2010
Hello Everyone,

To add on to my newbness from yesterday, I have one other part of my design that I am not sure what's going on. I have some suspicions but thought you guys probably could spot it pretty quickly or at least probe at me to look at it differently. I have again attached a setup below. I have multiple audio signals that I have coming into multiple summing amplifiers. Application wise, you can think about headsets. You want your audio to go to everyone else but not to your own channel. I found some information online about summing amplifiers and I thought that I could just leverage that to work the way I want. Probing the design, I fear that what I am seeing is a cascading effect from each amplifier but maybe its something else.

Let's assume all the resistor are the same value so everything is equal for the signals and the feedback. The thought process was to map the signals to combine them into the other channels except its own headset channel.

I put this into LTSpice to get an idea on what should happen. Once I received the circuit I just put in a simple sinewave into one channel to see if the mixers were working for the output. I measured the output and no sine wave. The mixer essentially made the signal disappear. The sinewave comes from a function generator and goes through some preamp and filters before it gets to here. After the filters, I have a sinewave that is about 230mV peak to peak at a frequency of 300Hz. This is not a DC biased signal. That would be the audio input signal in this case. In the simulation world, I would suspect the inverse of that waveform on the output.

Each amplifier should be getting the same signal but since these signals are tied to other amplifiers, is this a no no? That is what I am thinking. If so, what is the best way to isolate the signals and amplifiers to sum these channels like this?



Joined Jun 19, 2012
Add 3 more resistors and make 3 separate summers that are independent, each with 2 inputs.

Connecting the opamp summing junctions together = fail.

Thread Starter


Joined Jan 28, 2010
Arg. I was afraid of that. You're suggesting adding a parallel resistor path for each input and then seperating out the summer correct?

Thread Starter


Joined Jan 28, 2010
Would the parallel resistors have an effect on the input audio signal since its almost splitting it or is the mitigated by the amplifier?

Deleted member 115935

Joined Dec 31, 1969
Would the parallel resistors have an effect on the input audio signal since its almost splitting it or is the mitigated by the amplifier?
Quick rule of op amps, ( very basic )

the -ve input is the same voltage as the +ve input,
op amps have infinite gain,
op amp input impedance is infinite.

The way you have the op amp connected, the +ve input is at zero, so the -ve input is also at zero,
Its called a virtual earth mixer.


Joined Nov 6, 2012
I can't figure out what You are trying to accomplish or why.

It would seem that what you are trying to achieve is .........
everyone around a table has their own Headphone-Amplifier-Channel,
with everybody's Mic fed to their Amp, except for their own Mic.

If this is a correct assumption, it can be done, but it's a bad idea.
Always remember, just because you CAN do something, doesn't make it a good idea.

Everyone needs to hear every Mic, including their own, in their Headphones.
It's very disconcerting to not have any indication of whether or not you are being heard,
and how loud your voice is, relative to all of the other voices speaking.

Unless You are mixing a Band on a Stage with "Floor-Monitors" and a
separate FOH ( Front of House ), PA System,
there is no need for a special "Mix" going into each Headphone-Amp.

If I have made an incorrect assumption, please correct me,
and in any case provide a detailed description of the
types, and number, of Mics,
types, and number, of Headphones,
a description of the expected environment,
and the purpose of needing Microphones in the first place.
Are you having a big business meeting ?, making a Video-Pod-Cast ?
Will this Audio be run into a PA-System ?
Or are You just trying to figure out how it could be done ?