Signal Mixer/Summing circuit

[LukeeeeBennett]

I was wondering if any of you could confirm that this would work as a summing circuit/signal mixer?

The second OpAmp I believe is used to phase invert again after the first one and the first I'm assuming is a buffer.



Any help is appreciated.

[km6xz]

The first stage is an inverting adder or now usually called a summing amplifier where the inverting input becomes a virtual ground point. The output is proportional to the sum of the voltages at the inputs. The second stage is an inverting amp so the output signal is in-phase with the inputs. The second stage inverts the inverted output of the first stage. The switch bypasses the second inverting amp so the output of the circuit is inverted sum of the inputs.

[jrfrond]

Not only will it work, but you don't need to use the oddball component values listed. 10K resistors throughout work just fine. As a matter of fact, you really don't need any components in the opamp feedback networks either. The outputs can just be tied back to the inverting inputs. Those components are there for high-frequency limiting/stabilization, which shouldn't really be an issue because there's no voltage gain. If you choose to go with the feedback networks, the caps can be anywhere between 20-100pF, the smaller the better, or you can just go with ISO standard values of 27pF and 47pF.

[LukeeeeBennett]

The first stage is an inverting adder or now usually called a s.....

Not only will it work, b.....

Awesome, thanks a lot.

[Brad1]

Luke,

Just wondering....what will that be used for? I notice it splits to, and sums, "pedals". Which pedals, exactly? There's a reason for my curiousity.

Thanks,

Brad1

[jrfrond]

Actually, you COULD dumb this down even more by using a single JFET source follower buffer.

[J M Fahey]

*Almost* there.
As drawn, the inverted signal (fom OA3 output) gets attenuated 20dB (10X) respective to the non inverted one (taken from OA4 out).
You must move the switch to the output jack, selecting between OA3 and OA4 outputs.
For stability use a series resistor, 100 ohms up; the 1k you have is fine.
Also for stability in the real world, although being unity gain the 22 to 100pF feedback caps seem unnecessary, sometimes they help.
They definitely do not hurt.

[picbuck]

Jumping in here because I speak newbie (being one), so I might be able to clarify.

The previous comments are right, of course. The circuit is generally a standard one. There are jillions in use, many of them on mixer boards that studios paid thousands for.

However, there are some things that catch the eye.

First is jrfrond's correct comment about the bizarre component values. Just use 10K resistors instead of 11K. R24 becomes 1K, and C6 becomes a standard 10uF.

Capacitors C4 and C5 are for very high frequency rolloff. You're unlikely to need them in an audio circuit. If you decide to include them, just use the closest standard value. Note that their values are in picofarads, pF, which is one-one-millioneth of a microfarad. The values go micro - nano - pico (but you never say nano, you say .00x pico).

Moving along, switch SPDT2 is in a funny place.

In the circuit, OA3 does the mixing, but in doing so it inverts the signal. OA4 is there simply to re-invert the signal, which avoids possible problems with phase. So OA4 is commonly an inverting buffer, "buffer" implying unity gain (no gain at all, what comes out is what went in).

Both R22 and R23 are part of the OA4 circuit, the un-valued R22 assumed to be the same value as R23 (for unity gain), and this puts SPDT2 in a funny place. Commonly, it would be on the other side of R22, the input side.

At the same time, When SPDT2 in in the bypass position, then the input of OA4 floats, and this is not generally done. I think it would be better to use a DPDT switch, and use the other pole to ground the input of OA4. This can prevent problems.

But if it were me, I'd leave the thing out entirely. See following.

All of which brings us to the output, which is kinda weird. The output is sometimes through a capacitor (C6), sometimes not. More disturbing, With SPDT2 (or DPDT2) in the bypass position, the output is reverse-phased, while in non-bypass the output is as expected, in-phase.

This is downright strange, because OA4 is, as said, simply an inverting buffer. It adds or subtracts nothing, it only puts the phase back where it should be. This makes bypassing it pointless.

Therefore, SPDT2 (or DPDT2) seems to be some kind of effect. But unless there's good reason to have it in, it seems to me it could be simply left out. Connect the output side of R22 directly to the inverting input of OA4, omit the bypass wire entirely, and forget that anybody ever mentioned a bypass switch.

If you do this, move R24 to the other side (the output side) of C6, and change its value to 100 ohms. This is recommended by the engineers in the case of long output lines. If everything is in one chassis just forget about R24.

For a final note, you could just as well use a TL07x. A single TL072 would do the whole job, and they cost under a buck. If you don't have a supplier, you can get them on eBay.

And for another final note, I notice there are no capacitors on the input side of the mixer. Unless you're sure you don't need any, this might be something to consider.

Hope this might be of some interest.

-al-