# Automatic gain summing amplifier

Discussion in 'The Projects Forum' started by novastorm17, Aug 31, 2013.

1. ### novastorm17 Thread Starter New Member

Aug 5, 2013
2
0
Hey,

Im working on a summing amplifier for a small course project.
It will be summing ~26 signals coming from oscillators. All oscillator outputs will be the same voltage input (5v), but different frequencies.
What I want to do is have a constant voltage out of the summing amplifier, say 5v. However, not all oscillators will be outputting a voltage at all times, they're essentially notes.

So, my issue is that I can vary the gain of the summing amplifier to achieve this constant voltage out, but Ill have to change the gain every time I change the number of active oscillators. I could just clip the voltage, but then Ill lose those high peaks.

I was wondering if there is a configuration that can detect an active input and change the gain accordingly?

Cheers

NOTE: As per the design specs, I cant use digital circuitry (unfortunately)

2. ### Shagas Active Member

May 13, 2013
802
74
I was trying to do something similar a few months ago , haven't been able to do it yet :/

3. ### WBahn Moderator

Mar 31, 2012
17,457
4,701
If you want a constant 5V out of an amplifier, then just 5V into it and configure it as a unity gain voltage follower.

If you mean something other than a constant voltage out of your amplifier, please clarify.

Exactly what is to be constant and what does it mean for it to be constant? For instance, you might want the RMS voltage to be servo'ed to a constant level with a 2s time constant.

4. ### Shagas Active Member

May 13, 2013
802
74
I think what he means (and what I wanted to do ) is :

Say you have an input sinusoid of 1v pk-pk and you have a gain of 5 in the op amp resulting in an output of 5v pk-pk . Now let's say that your input signal amplitude fluctuates from 1volt to 2 volts , how would you make an automatic gain adjust so that the op amp always outputs a 5volt version of the signal regardless of the input amplitude

5. ### crutschow Expert

Mar 14, 2008
12,552
3,077
You could add an AGC circuit at the summing amp output.

Shagas likes this.
6. ### WBahn Moderator

Mar 31, 2012
17,457
4,701
Oh, I have little doubt that he is looking for an AGC (automatic gain control) circuit. But it will be good for him to learn to be more specific and precise in his descriptions.

And there is no way to hold the output amplitude fixed (short of just using enough gain so that it always clips, which he has stated he doesn't want to do) at all times. What you do is use a circuit that slowly increases or decreases the gain according to whether the signal has bee, on average, two large or too small over the previous time interval. That time interval and how fast the circuit responds to it are key design specs and depend on the intended application.

Aug 17, 2013
858
39
I'm also trying to do the same; keep the Vpp constant on a signal that changes the frequency.

For example, on the circuit I have so far I can adjust the values of the top resistors for a certain Vpp, but as soon as I change the input frequency (when it's increased) the Vpp goes down.

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8. ### #12 Expert

Nov 30, 2010
16,031
6,542
I suspect a wild goose chase. Nova...do you think that activating 26 signals will make the output 26 times as loud? Decibels don't work like that. Please explain more about your goals.

Edit: is this school work? I can't imagine a practical person that would demand you not use any digital techniques.

9. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Do you have much experience with synths or making electronic musical instruments?

I think your approach is wrong. If you just add multiple notes much of their energy cancels each other and they "mix" very naturally, the same way that different sounds in a room mix naturally (like from an orchestra).

If you try to attenuate notes when more than one note is playing it will sound very unnatural, quiet and muddy. I've personally heard the result and didn't like it, especially that transition from one loud note to two quiet mixed notes.

My suggestion is to sum the multiple sounds through a traditional way like a resistor mixer or an opamp mixer, as it is done in commercial synths and instruments.