Simple Mixer

[theunrulychef]

I need to build a simple mixer in a stompbox for someone. He wants the ability to run guitar, vocals & sampler through it to a mono out - then through a bunch of effects to his amps. He doesn't want to run through a regular mixer because he'd like to have them footswitchable, in a rugged case, and he wants to keep the signal path as clean & simple as possible.

I found these schematics online: http://www.muzique.com/schem/mixer.gif & http://http://www.all-electric.com/s...c/simp_mix.gif.

I'm wondering if anyone here has built anything similar or if there's anything I should watch out for combing guitar with mic & line level. Of course, I'd love to just do a simple passive resistive mixer & be done with it, but I'm not sure that would do the trick given the different types of input sources expected. If I just used pots wired as resistors in line with each signal (100k?), would that be adjustable enough to do this? I know an active mixer with individual volume controls would be the most flexible, but I'm trying to keep from ordering any stinking parts for this. Any tips or recommendations would be great.

Thanks,
Jay in Philly

[cbarrow7625]

Chances are the Guitar, Microphone & sampler will all need different sensitivities and input impedances (i.e., have different output levels and source impedances). The microphone will likely need a lowish input impedance (to help keep it quiet) and the guitar will need a hi-ish one.

You might be better off going with a TL074 (4 op amps in one chip) or something similar. Each input would get it's own input op amp. Match the sensitivity of each device by setting the gain of each op amp so that the outputs are about equal, then use the 4th op amp as an output buffer (basically like it shows in the schematic). You can vary the gain of each op amp with a pot of the appropriate size in the feedback loop. This will work waaaay better than the simple mixer you posted and still be pretty simple itself.

[theunrulychef]

So for each "channel" I'd be going (from input): 1uf cap - resistor - op amp (with volume pot & cap in the feedback) - then into the buffer, master volume & out? Or does each op amp go before/between the cap & mixing resistor?

About the input impedance, I guess it would be best to have dedicated input channels (1 guitar, 1 mic, 1 line in). Would 100k on the guitar channel & 10k on the mic be about right? Any suggestion on what a line level input likes to see (from a sampler or cd player etc)?

Thanks for the quick reply,
Jay

[theunrulychef]

After finding this, I see a little better what you're talking about. I could use an inverting amp like this for the mic & line level, then use a non inverting one with a big ground reference on the guitar channel like this. Then I'd mix them each into the buffer op amp with parallel 10k's? Does this sound about right?

Thanks again for your help,
Jay

[cbarrow7625]

Jay,

I think you are going to have a lot more design flexibility using the non-inverting design for all 4 stages. The Rin Resistor for each input will set its input impedance while allowing you to us the same resistors for all 3 input stages in the feedback loop (i.e. same size volume pots). Using the non-inverting stages you would have to have different sized volume pots in the feedback loop to match the different input impedances.

Also, you shouldn't use inverting for some stages & non-inverting for others - they will sum out of polarity with each other & possibly cause big problems (or at least confusion). It is always good practice to make sure any device you make does NOT reverse polarity from input to output by default (phase reverse switches included in a product are a different case).

Mixing them into a buffer op amp using 10K series with a single 100k to ground (you will need a ground reference with a non-inverting op amp) should be just about right. Make sure to use a capacitor on the output of each op amp to block DC.

The input impedances will rally vary depending on exactly what the devices are that are plugging into it. Does the guitar have a magnetic pickup going right to this box (1Meg input resistor should do)? A piezo with no on board electronics (10 Meg is about right)? Piezo or magnetic with on-board active electronics (22k-100k should suffice)?

Does he expect the box to provide all of the gain for the microphone? If so, that input stage will have to be 20-40dB higher in gain than the guitar stage. Is it a high impedance / high output microphone (these are usually cheap mics) or an SM58 (or equivalent) with a relatively low output & low impedance (600 Ohms - balanced by the way, you will have to convert balanced to unbalance properly or build a balanced input for that stage).

The other electronics input is easy. 22k-100k input impedance with the lowest gain of the 3 stages (assuming it puts out a healthy driver level to start with in comparison to a guitar or microphone).

It can be simple to construct but it is never simple or easy to design it just right. Make sure you do your best to answer the impedance an output level questions above before you start to build. You'll be glad you did. Good luck. Keep us posted. I bet some others would like to build something similar.

[blckmynnse8]

unfortunately i'm at work right now, but at home i have a schematic for a mixer similar to the first example you gave that used two op amps with passive high and low pass filters. it also had adustable levels for each channel. i threw it together one day many moons ago and it worked quite nicely. that particular circuit used 741 op amps but you could find quite a few substitutes.

if still interested i can dig it out when i get home. i will have to painfully draw it using ms paint. our computer already has a tad too much software on it and i don't really want to download any schematic drawing software (plus it's a borrowed pc).

[theunrulychef]

Awesome, thanks so much for the replies! I think I'm understanding it much better. I neglected to think about how much more gain a mic might need. I'm pretty sure he has a tube mic pre-amp. I could have him use that & then I would just need 2 line level inputs & 1 guitar. Then he could jump around with his different mics.

blckmynnse8, thanks for the offer, but I think cbarrow7625 has me in the right direction for now. I wouldn't ask anyone to draw up a schematic in MS paint

Thanks again for the replies. I'll post my results once I get parts & wire it up.

-Jay

[jrfrond]

As much as I like to build things, and I do, I would just go purchase one of these. There are several companies who make this passive-summing amp mixer configuration in a snazzy, ready-made box for low $$$. Here's one:

http://www.fullcompass.com/product/315178.html

[theunrulychef]

Thanks for the link. I guess he could've bought that & I'd put some switches in there. So far though, he's probably got less than $20 in this, so that's a few beers difference. I do actually already have a few little mixers like that lying around, but he was looking for something a little more "stompy" - pedalboard friendly.

I know what you mean about not wasting time when there's something pre-made out there for cheap. Gotta factor in the cost of your time, especially when you're a pro (like yourself). I'm still a novice though, so I figure learning about op-amps on a project like this isn't the worst way to spend a few hours. It keeps me out of trouble at least.

I received the parts I was missing the other day, so I should be putting this together soon & I'll report back with my results.

Thanks,
Jay

[Aura]

I have just started a new thread "SIMPLE GUITAR EFFECTS MIXER" under "Guitar effects" after reading your request.I've also made some suggestions/improvements to the original design.Here is the original schematic.

[scopemaker]

Passive HI-Z ideas from an old guy

I am kind of butting in here, but I hear people trying to do what I did a while back: mix a mic with a guitar for cheap and reliable. Dead simple. I am not sure how to append the circuit diagram, but will give it a try. Should point out that if you mean to use any of the "best" harp mics, the mic signal will be louder than the guitar pickup.

OK, I have tried to append what I have and can't make it go. Tell me how to get my document to you and it shall be done.

[Enzo]

If you are using quick reply, click on Go Advanced. That gets you to the full reply window, if you are not there already.

Beneath that window you will find Manage Attachments. Click on that to open the attachments window. Click BROWSE and find your file from your computer and put it in the browse box. Now next to it click on the Upload button.

Wait for it to unpload.

Then close the attachment window and SUBMIT your post. the attachment should be at the bottom of your post then.

[scopemaker]

High Impedence Passive Mixer

High Impedence Passive Mixer

[scopemaker]

Did you get my circuit and description? I'm new to this !