Summing amps

[saison94]

I've been working with this summing amp schematic to blend a guitar signal and a theremin signal, and would greatly appreciate any advice:

http://www.play-hookey.com/analog/experiments/images/two_input_summing_opamp_sch.gif

I have three separate circuit boards, a fuzz circuit, a simple IC 555 theremin circuit and the above summing amp circuit. I have the guitar signal running off the input jack tip into the first input on the summing amp, and the theremin output coming in on the second summing amp input. Pin 7 is going to +9v, and Pin 4 is going to -9V. The two independent signals are going into Pin 2, and Pin 3 is connected to ground with a resistor, which is 2.5k rather than the 3.3k in the diagram. Pin 6 (the output) is connected to the resistor looped back to the two signals, as well as the input of the fuzz circuit. The output of the fuzz circuit is connected to the tip on the output jack.

So the idea is to sum both signals before they reach the fuzz circuit, so they can be fuzz blended independently or together for awesome noise.

Each circuit has the same common +9v and ground.

Does anyone here have any experience with something like this? I can't seem to get it to work right. Another problem is the theremin sound/signal is somehow getting through to the speaker on my amp without going through the summing amp, like its radio interference carrying it or something. It ceases to do this when I give it a separate power and ground source. Why is this? Sorry, still kinda new to all this...

Any ideas?

[Ripthorn]

What exactly are you trying to accomplish with summing the guitar signal and the theremin signal?  Are you trying to somehow modulate your guitar signal prior to going into the fuzz circuit? 

[saison94]

It's an optical theremin with a photoresistor which is mounted on the top of the enclosure, so I want to be able to play fuzz guitar in one mode, and then step on a separate switch to bring in the optical theremin as a sort of theremin-wah that can create a whole separate sound/signal independent of the guitar signal within the mix. Hope that makes sense, thanks for any advice!

[chi_boy]

Would something like the mini-mixer work??

http://www.generalguitargadgets.com/content/view/68/26/

[saison94]

That looks like a really cool idea, but shouldn't I be able to do what I'm trying to do with a simple summing amp circuit inside the box I'm already building? My understanding of summing amps is to blend two or more voltages or signals together in a way so that variations in input voltages do not effect each independent output signal. In other words, if the project I have in mind works like it should, I would be able to play my guitar and have that signal run through the fuzz circuit, and then be able to hit a separate switch and engage the theremin signal, which would go through the summing amp, through the fuzz circuit and out. Isn't this the most effective way to have two separate audio effects/signals tied together and have each sound clearly distinguishable from the other one?

[sean k]

I built a small mixer to do what your doing because with my music, well sound making, I've used up to four instruments at once, but not all at the same time, though occasionally I might keep a drone going so I get where your coming from. I just bypassed the opamp approach and went straight to a single transistor but I did go to a 100k pots and 10k summing resistors.

If you're going to use the instruments independantly then you don't really need to sum anything but if you are, like having a guitar signal going through a delay thats got the feedback set high, then youll want to sum two signals... otherwise just have parallel input pots that will ground one incoming signal without grounding the other... maybe even one linear dual ganged slider and just flick it from one end to the other. Just have the incoming signals on the wipers. I'll be doing something like this soon with two different microphones... they'll never be used together as I have only one mouth, so they don't need summing... just click free... switching!

[daverdave]

What opamp are you using for the summing amp? Are you using a dual power supply (+9v and -9v) or a single?

[saison94]

I'm using a 741, like the one in the summing amp schematic above. I'm powering with a single 9v battery where everything in the box (fuzz, theremin and summing) is getting it's power and ground from.

[saison94]

The idea is to have two separate stomp switches, the first being the fuzz effect, the second being the theremin. By summing the signals before they reach the fuzz circuit, I would be able to bypass the summing amp completely and just play by-pass clean guitar, or hit the fuzz stomp and have it directed through the summer and then into the fuzz. Then, if I wanted to add the theremin into the mix, I would hit the other stomp switch and send the theremin into the fuzz through the summer. I'd do this by placing a basic on/off switch between the power and theremin circuit, since the theremin begins making sound whenever it's powered up. So when the power was switched on with the theremin, the signal would be sent separately into the input on the summer. Both signals are going into Pin 2 on the 741, as shown in the schematic linked above.

[daverdave]

That circuit you posted won't work as shown with a single power supply. You need to create a midpoint voltage, using a divider, as the 'ground' point. So the resistor from the non inverting input would go to 4.5V, you might want to put the pots to +4.5v as well I think, other wise you might pull the signal to ground, and you'll need a dc blocking cap on the output, 100nF or so.

[saison94]

Thanks so much! Could you perhaps explain a little more?

I'm sure you already realize this, but the schematic linked was one I took from a voltage summing experiment, where they were running various voltages to see if the sum of each would stay constant on the output when each one was varied slightly. As I understood it you could substitutate straight-up voltage for two audio signals and sum them together and have the level of each within the mix determined by the 10k pots.

So what you're saying is I need to run the resistor from the non-inverting input (pin 3 on a 741) into +4.5v from a voltage divider rather than to ground as it currently is? And the dc filter cap would be placed between pin 6 (or output) on the 741 and the fuzz circuit?

I really appreciate your help, if you could just put it to me a little more in laymen's terms, I'm new to a lot of this so I guess I need it spelled out for me

Thanks again!~

[saison94]

So what you're saying is there would be no actual "ground" anywhere in the summing amp circuit other than Lug 1 on the pots? How would I create the voltage divider to have the one half +4.5v act as ground and the other as power? What about pins 4 and 7 on the 741, where +9v and -9v are normally supposed to go?

[daverdave]

Generally opamps use 2 power supplies, a positive voltage and a negative voltage, and your ground is the midpoint or reference point. When you're using a single power supply, like in alot of effects circuits, you need to create a mid voltage point that is like a VIRTUAL ground. So you could say that your power supply is +4.5V and -4.5V. Basically it's so the positive and negative cycles of the incoming ac signal can pass through without hitting the power supply limit, either lower or upper. The DC cap then blocks the +4.5V signal from the output.

The idea of the summing amp is that you're taking 2 signal's and mixing them together without increasing the overall voltage level of the individual signals, as they would if they were just mixed together. Like if you took two 5 volt ac signals at the same frequency and in phase, they would produce a 10V signal for example, if they were passed into a summing amp there would only be a 5V signal I think. You also get the opamps high input impedance and low output impedance.

This might not all be exactly right, I've nearly finished my first year of uni doing electronics, but there's still some stuff I'm not that clear on. Hopefully someone else will chirp in if I'm wrong on somet.

[daverdave]

Pin 7 of the opamp would go to +9V and pin 4 to ground. A voltage divider is just 2 resistors, one from +V and one from ground, the output from the middle of those resistors would be a divided voltage:

Vbias = V.(r2/(r1+r2))

That's assuming that r1 is the resistor coming from +9V and r2 is the resistor coming from ground. If you used the same value for the resistors you'd get half the supplu voltage, I'd use at least 10k resistors, and I think you'd have lugs-1 of the pots going to the midpoint voltage as well, you might even want to include decoupling caps between the different voltage points.

Hope this helps.