Hey all, I am trying to design an all in one output module, I want it to consist of a CV Panner, a mixer, and outputs for headphones and speakers (might get crazy an add an equalizer if i have enough panel space).
It looks like it is going to be a cobbled together version of AO’s quad stereo output, and this Voltage Controlled Panner. I am trying to keep this relatively cheap, and it will almost certainly be a stripboard layout.
So my initial question is what should my signal chain be? Should I go Gain → Pan → mix or Pan → Gain → mix, and what are the reasons I should go for either one? Here is a VERY ROUGH mock-up of my idea, the top one has a 100k pot before the pan stage, the second one has a 100k dual pot after the pan stage.
I want panning to be cv and knob controlled, and mixing to be only knob controlled, though i may add cv if i have space (and energy).
While it does sound like a very interesting idea on paper, I would caution you on several points before trying to build a stereo line/heaphone/speaker output (with a panner!).
- Headphone output: @analogoutput based his design on Barton’s Stereo Output, but you might notice that he did not use headphones section. Barton’s headphone section is based on an TL072 with a 10Ω output resistor. I am pretty sure TL072 cannot deliver enough current to drive your typical 32Ω headphones even with such a low limit on the output current. Or maybe it can, but just barely. Analog Output used LM4808 instead, which is a SOIC part. Can you solder SMD parts? If not, you will probably need to buy one of these Aliexpress LM4808 ready made boards and kludge it in your design, or look for an op amp that can drive headphones (e.g. NJM4556, although it might be obsolete). Some people use two LM386 to drive headphones, which I wouldn’t do for other reasons. Their output is so big that you might damage your headphones and/or hearing if you’re not careful with the design.
- Speaker amplifier: Typical modular synthesizers do not have enough power for an amplifier driving a speaker that can deliver some decent output. I speak from experience because I’ve build an amplifier/speaker module for myself and I did not even attempt powering it with the modular supply. A class AB amplifier easily needs 2-3A to drive a decent speaker to reasonable volume. If you really must include an amplifier in your design be prepared to use an additional power supply (I used a 5A 12V “brick”) or switch to a class D amplifier (which again will be an SMD part to solder). Again, you can use an LM386 to drive a small speaker, but be prepared to be dissapointed by the quality and level of the output.
I am not saying you should not build this, but be prepared to face 10x challenges. It’s due to these challenges that several people who advise against output modules. Even Analog Output himself withdrew the design files of his module because he was not happy with how he put the module together! And while I did build a speaker module, I ended up using a small inexpensive mixer which gives me both a safe headphone output, a basic EQ, and a stereo output to send to a small external amplifier driving two decent bookshelf type speakers.
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Note that I only withheld the PCB design because of stupidities with its mechanics. I never had problems with the electrical design, which is why the schematic is available.
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OPA2134 might be good alternative to drive headphones, as it is used in the classic CMoy amp to drive headphones and can source/sink around 30mA. Or if you want to use TL072 you could increase the max output current with parallel opamps. I’d also consider adding a simple safety limiter/diode clipper just to be a bit more safe.
This is the case indeed. My comment referred to the likely problem of putting a mixer, headphone amplifier, and power amplifier in a single module. Apologies if the comment gave the impression that I implied otherwise.
I thought the CMoy used NE5534? (which is yet another alternative). Both of these offered in AE are likely to be fakes though, so there’s another thing to worry there.
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The original one actually used single-opamp chip OPA134 but OPA2134 is the current version available in DIP package. AliExpress is full of fake op amps, but also I’d be careful with all TI chips as many jellybean ICs are silently being worsened as they chage to smaller process node, for ex. current revision of NE5532s from 2025 december onwards has lower slew rate and max supply voltage than the ones they have made for 55 (!) years.
Oh perfect. As if things were not bad enough, we will be hunting NOS or vintage ICs that are still in production.
Well, I can’t check the example circuits you mentioned right now, but looking at your rough sketch, I see a few things you might want to consider:
Since you place the panpot in front of the LM13700 VCA, you’ll be limiting the signal amplitude already, and all the VCA will be able to do is reduce it even further. You won’t be able to bring the L or R channel volume up again with CV. If you want panning to be controllable with both a pot and CV, the best would be to sum the panpot center pins with the CV signal via a summing amp, and then feed that to the VCA CV input.
Regarding the overall volume control pot, placing it as the first element like your top sketch would work, but it wouldn’t work very well if you place it at the end as in your bottom sketch. The reason here is that an audio output to the outside world should ideally have very low impedance, so that most of the signal power can be correctly “sensed” without distortion by the receiving end. The inputs of mixers and other pro equipment will be relatively high impedance, around 20k. If you use a 10k pot as your output and dial it to the center position, the receiving end will be looking at a 5k impedance which is a bit too high relative to a 20k input. This means you’ll probably loose some high end clarity in the signal and make it more susceptible to noise. The best is to use the output of an OpAmp as your output, which is very low impedance. You could for example just buffer the output with an OpAmp, or use an active volume control circuit like Baxandall’s.
I’m about to send my own Output module for PCB fabrication, take a look there if you want: DMH-Output/DMH_Output_PCB_Main at main · DeMarco/DMH-Output · GitHub . It’s based on Meebilt’s module with various modifications that I learned from Douglas Self book “Small signal audio design”. But indeed it can’t drive amplifiers. It’s designed to just give me a Line Level balanced stereo output with a single volume knob, but it does also have a headphone output using NE5532 OpAmps.
I’ll put in a quick word in favour of the YuSynth panning mixer design. I built one of these (minus the tone controls) about 2½ years ago and have found it to be an excellent circuit. It has manual level controls, voltage controlled panning on each channel, a master level control and a built in headphone amplifier. It works extremely well.
I designed the circuit boards to make it easy to add extra channels and it now has six inputs in total. It’s at the bottom right of the cabinet in the picture here.
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Thanks for the response! I am using powered headphones (as that is what I have) but I will definitely look around for resources if I get some nicer headphones!
As for a speaker amplifier, I am mostly using it for R/L line outs, I have a stereo receiver with a couple of heresy II’s that i really wanna play with, but my current output is the super simple mixer design, so I wanted to make something with a little more intentionality. Any other setup I’d be playing with would already have speakers hooked to an amp of some sorts, so it is not the synth that is driving the speakers.
If you use the module just for a line out into an amplifier, then you will vastly simplify this. The amplifier is also likely to have a headphone output too, making it even easier for you.
