Op-amp summing mixer resistor values?

[Buffalo Tom]

Hi. I wanna build a simple mixer thats summing two inputs (wet and dry) to one output. I found a circuit with LM741 that seems very interesting for my application. The mixer is going to be placed on a pedalboard after buffered pedals.

What values should I have for the resistors??. My buffer (pedal before the mixer) has 150 ohm output impedance.
And how do I power the mixer? Is it just +15VDC to pin7 on the Op-amp?

[robthequiet]

I've seen anywhere from 10K to 100K and probably more. I've also seen that theoretically you would make your feedback resistor equal to your input resistors for unity gain. In your case, you may want to build out your circuit with input potentiometers and a buffer on the output to re-invert the signal back to proper phase if needed. There have been a number of projects related to this on the forum, some more complex than others.

But, it's a well-regarded circuit, so a good basis to proceed. [edit: see PRR's comment below on power supply]. Are you using a separate power supply from the rest of your pedals?

Here is a related topic

[Buffalo Tom]

Thanks for your reply! Everything on the pedalboard (including the mixer) will be powered from the same power supply with isolated outputs. So I also be needing a dc boost circuit that boost 9VDC to 15VDC. But thats the next "problem".. First I need the mixing to work.. I have a bench power supply that can give me 15 volt. I will look at the topic link you posted!

I've seen anywhere from 10K to 100K and probably more. I've also seen that theoretically you would make your feedback resistor equal to your input resistors for unity gain. In your case, you may want to build out your circuit with input potentiometers and a buffer on the output to re-invert the signal back to proper phase if needed. There have been a number of projects related to this on the forum, some more complex than others.

But, it's a well-regarded circuit, so a good basis to proceed. And yes, 15V on pin 7 and pin 4 to ground, on a 741. Are you using a separate power supply from the rest of your pedals?

Here is a related topic

[PRR]

> What values should I have for the resistors??

High enough to not-load what feeds it. Low enough to not add hiss.

In studio gear, 10K or 22K are fine.

Coming out of pedals, 50K-100K is probably better.

> Is it just +15VDC to pin7 on the Op-amp?

NO!

This diagram assumes bi-polar + and - supplies! Two power supplies, one each side of ground.

In almost all stomp-world applications, you want to re-rig for +9V and 0V power, but +4.5V "audio ground". (Yes, '741 will work on single 9V power, though TL072 is more available and generally a better choice.)

You also tend to want input/output blocking caps, output resistor, power filtering and protection. AMZ will have more-detailed stomp-world plans. RG's site has a paper "what is all this extra stuff?" which explains why simple concepts need real-world added-parts.

The inverting summer mixer has neat math, but for most small-mix applications you can use passive mixing, with perhaps a slight boost behind to make-up mix-loss.

[PRR]

me> AMZ will have more-detailed stomp-world plans.

http://www.muzique.com/schem/projects.htm
About 2/3 down the page.

[ashcat_lt]

Ultimately, you'd like them to be as small as you can get away with in order to minimize noise.  If you can actually trust all of your sources to be 150 ohms, you can go down to like 1.5K and still meet the 10:1 rule of thumb.  But then, you've only described the buffer.  With what will you be mixing that signal?

[Transmogrifox]

http://www.muzique.com/schem/projects.htm
About 2/3 down the page. 

If you have volume control on each "device" and a system master volume control then mixing by means of a bunch of 10k resistors connected together is a very simple way to do it.

[Buffalo Tom]

This is where I'm at right now.. Its RGs mixer with some minor edits.. Gonna buy parts today and try it. Does it seem like a good circuit for what I wanna do? Summing stereo to mono. Can something be improved? The signal thats feeds the mixer is the stereo output from a Strymon pedal.

[Cozybuilder]

This is where I'm at right now.. Its RGs mixer with some minor edits.. Gonna buy parts today and try it. Does it seem like a good circuit for what I wanna do? Summing stereo to mono. Can something be improved? The signal thats feeds the mixer is the stereo output from a Strymon pedal.

You might consider feeding R3 and R4 to lugs 1 and 3 of a pot, with lug 2 feeding your op-amp. This would be a "balance" control.

[anotherjim]

I don't know what your Strymon pedal does, but do be aware that some effects create artificial stereo from a mono source by messing with the channel phases. Merging the 2 channels back together can change the sound drastically, giving you less than if you simply use only one channel feeding both left and right.

[robthequiet]

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