Fun with DIY analog summing

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mshilarious

mshilarious

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I thought I'd post a clip of something I've been playing with, a simple phantom-powered analog active summing mixer (no power supply, no wall wart!). This is a fairly clever device that uses a single opamp, I use an OPA134, run by phantom power into a mic preamp. It's a circuit that could be built in stereo for about $10 plus cost of connectors and a box, which depends on how many channels you want. I think 8 stereo channels would be reasonable.

A couple of notes on the clips; first this is just a test of two vocal clips summed to mono from a Christmas carol I recorded last year, using B57s through an A&H MixWiz into an HD24. The digital mix was straight summed with plenty of headroom both in tracking and mixing, and the analog mix was kept down similarly. After an 80Hz bass cut (mainly to help out the 60Hz hum on the analog; see below) for both, they were normalized. No other processing on the digital mix.

This is not intended as fuel for the analog vs. digital debate; there are some obvious nonlinearities in the analog mix that make an academic comparison moot. First off, this circuit is sitting on a breadboard at the moment, so there is audible hiss and 60Hz interference. Properly shielded, that should be greatly minimized. Second, for my quick & dirty setup, it turned out to be handiest to pull the inputs from a headphone amp (Oz Audio HR4), since I haven't wired up my patchbay yet :o Third, the output is fed into an ART Digital MPA tube amp--a nice unit I like a lot, much better than their starved plate stuff, but certainly not highly accurate.

I *think* headroom is about +7dBV for this mixer, so it would behoove one to run it at -10dBV and watch levels. I'm still testing and I'm working to see if I can up this a bit, but if anybody wants to see the current schematic, just let me know, I'll clean it up and post it :)

So, treat this as a simple example of what might be attained with a basic, cheap DIY summing box.

24 bit 44.1kHz wav files, about 4MB each:

Analog

Digital
 
On the off chance anybody saw this thread :( I have updated the Analog clip; after a morning's work I believe I have eliminated all the "obvious nonlinearities" :) Use the link above again, the new file is there.

I have one last issue I'm working with, which is output gain attenuation; since this has to go back into a mic preamp it needs to be trimmed a bit from line level , depending on the mic pre . . . that's fairly simple though, I'll do that later today.
 
I plan to listen, but I'm at work now. I'll give a listen at home. Thanks for sharing the clips.
 
I should add that of course this can be powered with a wall wart, I just have a bizarre belief that everything should be phantom powered :confused: Except phantom power supplies :o ;)
 
Just curious ... why would you run it in to a mic pre instead of a line-level device ....like a line amp, active di w/ gain or similar ?

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chessrock said:
Just curious ... why would you run it in to a mic pre instead of a line-level device ....like a line amp, active di w/ gain or similar ?

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Click to expand...

Depends on what you have laying around, I guess. The typical DIY summing mixer is passive and thus needs lots of gain, so a pre stage is a necessity. An active summing mixer doesn't have that problem, but the concept sort of built from there.

I just look at phantom power as a free power source. If you have a tube channel strip or a stereo tube pre or something, you might want the color. Otherwise, grab a wall wart and run into whatever else you want to use.
 
I'm hoping this will be useful as a good field powerable mixer. Being a Naiant product, I suspect it will be inexpensive, and high quality. The current choice of inexpensive field powerable line mixers is not good. I also question the usefulness and decision to make it input into a microphone pre-amp input, but then, I'm not an engineer or a designer. I do trust MSHilarious and assume he has it all figured out. Anyway, for my application, I'd like to use it with 12v DC power and run it line-in to a recorder.

Oh, and I would like to see the circuit too... when you get it finalized.
 
Well phantom does one clever trick, which is that it's relatively high voltage so it offers headroom equivalent to about a 15V wall wart. The issue is that most wall warts are designed to provide at least 250mA, which is totally unnecessary for a single channel mixer (technically two channels, but each mix would have its own phantom supply). So to match the performance of phantom, you're talking about 2 9V batteries, or a honkin' big wall wart, or an internal power supply.

For myself, I need a 24 input summing mixer for a live HD24 rig, just for monitoring purposes. I already have an 8 channel mic pre, so the full-on mixer route doesn't seem too appealing, given the cost (and weight!). My project will use an internal power supply, because I detest wall warts and I can build a supply for myself that is +-18V.

I will add pan and level controls, but that is completely optional. Really all you should need for a DAW interface is a master gain control so you can output full scale digital audio and avoid clipping a mic pre. But if your mic pre has a pad, you might not even need that.

Right now my fondest hope is maybe for a group PCB order, maybe partially stuffed, again with a flexible power supply option . . . I thought there seemed a lot of interest in analog summing here on the cheap :confused:
 
I still think this would be useful for field recordists. ;)
Maybe you could get some interest with that group?
I'm certainly interested.
 
Yes, 24-track summing for field recording with an HD24 - count me in.
 
if you're running it into a preamp for makeup gain, what does the opamp do? is it a buffer in front of the mix bus?
 
FALKEN said:
if you're running it into a preamp for makeup gain, what does the opamp do? is it a buffer in front of the mix bus?
Click to expand...

No, it is a summing amplifier. With active summing, you run each input across a resistor into the - input of the opamp, and you ground the +. That way, each input effectively does not see the impedance of the other inputs, and thus there is no loss on summing. Therefore the circuit doesn't need makeup gain. The circuit is designed to have a variable attenuator, if needed, to prevent preamps from clipping due to minimum preamp gain of greater than zero.

That attenuation should be much less than a large passive summing network. However, the same concept works as a makeup gain stage for a passive summing network (in which case a preamp would be mandatory); I believe it's possible to obtain somewhat better noise performance from an active summing amp though.

The mic preamp option is intended as an easily available power supply, and where the mic preamp has desirable tonal characteristics or onboard EQ or compression, a signal processor. The other option is to hook up a wall wart and run the output straight into a line-level input.

For the HD24 application, my thought was it would be convenient to carry around a 10 or 12 channel mixer, route inputs 1-8 to the HD24 (which can be internally rerouted in the HD24 to all 24 channels), and use two spare channels as a submix return, with the DIY summing mixer creating a simple monitor mix. Many mixers have pad switches, so the summing mixer might not need a master level control.

While that would be very portable, you do have to be careful as if you add level and pan controls for all 24 channels, the cost of the project would increase significantly. For my box, I'm going to skip the pan controls and live with mono monitor mixes.

For anybody that wants to get in on the first PCB run, let me know via email what you are looking for, I will sell parts at cost :) and if you want more than just the PCB I have a lot of those parts (opamps etc., pots, knobs, even a few boxes) too.
 
is the signal actually going through the preamp or are you just siphoning the power? that is pretty ingenious actually. hey I have an idea for you though...24 channels is a lot without mute switches...you'll never be able to "solo" any of the channels without killing your mix level settings.
 
FALKEN said:
is the signal actually going through the preamp or are you just siphoning the power? that is pretty ingenious actually. hey I have an idea for you though...24 channels is a lot without mute switches...you'll never be able to "solo" any of the channels without killing your mix level settings.
Click to expand...

I can mute channels via the HD24, although soloing isn't too easy. I like your idea about merely siphoning power, technically it isn't necessary to send the signal back to the preamp, it could go anywhere you like . . . however it would require another pair of connectors.
 
otherwise I would think you would need to pad the signal before the preamp, no?
 
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