I have seen many mixer schematics. They are pretty much always the same - a stack of resistors.

Even some DJ mixers using the SSM2164 simply joined outputs followed by an opamp. The same thing always happens - the amplitude gets bigger after combining 2 or more signals.

Then I found this question.

I have read a little on the LM3900 and the application note, page 41, not sure about that one.

The guy mentioned something about using current instead of voltage, so I figured, let's try with an LM13700, since they output current.

The same thing still happens: remove one of the 1 ohm resistors in the summing bus and the amplitude drops to to 50%.

enter image description here
Link to simulated circuit @ falstad

Does a (simple) fix exist for this, while keeping the LM3700, not using microcontrollers?

The inputs are 5 V.p.p., directly followed by a 100K pot, before the 100K resistors.

(I didn't do the AC coupling so far.)

  • 1
    \$\begingroup\$ Please use standard English capitalisation. It affects legibility and general tidyness of the site. See Write to the best of your ability on the site's help pages. \$\endgroup\$
    – Transistor
    Sep 18 '21 at 17:08
  • \$\begingroup\$ What are you after exactly? If you're summing, of course it is going to come out bigger. If you don't want it to come out bigger than you need to think about what amplitude it is that you want it to come out as (which you never mentioned). As written, we can only assume that the input amplitudes and phase are different (if they are even pure sine waves at all) so "getting the same amplitude on the output" has no meaning. Either you're omitting some information about the input signals, or you need to sit back and think about what "the same amplitude" actually means. \$\endgroup\$
    – DKNguyen
    Sep 18 '21 at 17:14
  • 2
    \$\begingroup\$ 1+1=2. If you want to to be same level, then (1+1)/2=1 or 1/2+1/2=1. \$\endgroup\$ Sep 18 '21 at 17:17
  • \$\begingroup\$ I realize this may come off as insulting, but we need to know where you're starting from in order to help: Do you understand what you're doing with circuits here, or are you just assembling schematics you found on the internet? (Please don't take this the wrong way! I started out just assembling schematics I found on the internet, and now I've got a master's degree in EE--nothing wrong with being a beginner if you are!) \$\endgroup\$
    – Hearth
    Sep 18 '21 at 17:28
  • \$\begingroup\$ Compare the simplified schematic of the LT1228 with your earlier post (now deleted.) See if you find anything familiar there. I think the problem isn't in the availability of devices to help out. The problem is that you don't yet understand how they can be applied to your specific case. So I believe you use them in ways that remove their benefits. I wish I had enough time to help more. But I don't. \$\endgroup\$
    – jonk
    Sep 19 '21 at 5:44

The same thing always happens - the amplitude gets bigger after combining 2 or more signals.

...unless you reduce the amplitude before, during, or after mixing the signals. The simplest possible mixer (a resistor in series with each input, joined together at the output) does this automatically because it halves the level of each input.

But you linked to a question which asks about 'Combining 1, 2 or 3 audio signals so the output always has the same amplitude'. If this is what you really want then you have a dilemma. When combining two linear signals without distortion the peak output amplitude could be up to double the value of a single input. If that is not acceptable (eg. because it would overload a following power amplifier or speakers) then you must reduce the output level, and then each input will be weaker at the output.

So consider what you are asking for. A linear system must be able to handle the combined instantaneous levels of each input without distortion. If the combined signals may exceed the system's capability then you must reduce them, but if they won't then you don't want to reduce them. That means you need some kind of level control that adjusts the gain or attenuation to keep the output level constant no matter what the inputs do.

But with an AC signal the instantaneous level is constantly changing, and the circuit cannot predict what a 'random' waveform will do, so it cannot know what the peak level is until it has already occurred (by which time it is too late to adjust the gain). The upshot of this is that you cannot do what you want with a purely linear circuit.

So you have two choices:-

  1. Accept a higher peak output amplitude or reduced individual output levels.

You could use manual level adjustment (eg. a volume control on each input) to suit different situations, or simply not worry about it. When two signals are sent through the same medium it is often expected that the combined level will be higher, and the system should be designed to handle it.

  1. Create a circuit that measures the peak amplitude and adjusts the mixer gain accordingly to keep the peak output level constant (or at least below the maximum permitted level).

This 'automatic' gain control will necessarily introduce distortion while it is changing the gain. To act fast enough to catch transients the signal must be delayed so the gain control has enough time to act on it. But acting to fast causes more distortion. To avoid this the gain must be changed slowly - which produces 'breathing'.

Designing an AGC circuit that minimizes undesirable effects is not easy. It is generally only used in applications where manual gain control is impractical (eg. radio receivers, remote microphones), when some distortion is acceptable, or when the level is normally relatively constant.

The guy mentioned something about using current instead of voltage, so I figured, let's try with an LM13700, since they output current.

The same thing still happens:

There is no advantage using an LM13700 over a standard op amp because the circuits are equivalent. With negative feedback the inverting input of the mixing op amp is a 'virtual earth' which the op amp keeps close to ground. That means the mixer resisters already convert the voltages into currents.

  • \$\begingroup\$ i did made something with an opamp, resistors and diodes to control the the lm13700, a bit earlier, it looked good in Falstad simulator, but couldn't automatically account for the amplitude at the output. it just looked what was at the input and adjust the lm13700 accordingly. i have already figured out a work around. just wanted to know if someone knows another way. \$\endgroup\$ Sep 19 '21 at 10:26

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