Saturday, 11 January 2020

Synthesizer Build part-17: MIXER and PASSIVE ATTENUATOR in one.

It's a mixer and a passive attenuator in one and I recently also added a Clipping Indicator LED. Very useful module for the DIY Synthesizer.

This is a very simple project to build yourself and it will be a module you will use a lot in your synthesizer. I built two of them so far and they are in constant use. You can use this mixer for both Control Voltages (CV) and audio signals. I wanted to have a mixer in my synth but also a passive attenuator that I could use for signals that have no level control. So I decided to put both functions into one panel. I recently also added a clipping indicator. More about that at the bottom of this article.

Passive attenuator is nothing more than a fancy word for a volume knob. It's just a potmeter inbetween the in- and output of the signal. It's called 'passive' because it doesn't require any power source. But by flipping the switch from passive to mix, the signal is now also being led into the mixer and becomes part of the signal coming out of the 'Mix' output while still being available at the original output too. I used the super simple mixer circuit that Sam from LookMumNoComputer also uses and I added switches for the signal to be added to the mixer.

Here's the stripboard layout that I made of the complete Mixer/Attenuator. All potmeters viewed from the front. (There's a layout of the mixer with the clipping indicator, further down the article):

(Last revised: 05-Feb.-2020)

Print only:

Here's the schematic for this module:

In the panel wiring diagram, on the drawing, you can see how I combined the mixer with the passive attenuator function by simply adding switches that lead the output signal coming from the wiper of the potentiometer into the mixer. Only the top output jack carries the mixed signal out, and the other three always carry an attenuated version of their respective input signal. That line is never interrupted. Those other three outputs are all for passive attenuation. So you could get a mixed signal out of the top output jack, and 1 to 3 original signals on the other outputs. You can also split a signal into two parts by putting it on, for instance, input 2 and sending it into the mixer. Then you can tap the signal from the mixer output and from the output of channel 2. (Of course no other signal must go into the mixer otherwise the signal at the mix output will contain the mix of the multiple inputs but you can use the other passive attenuators at the same time.) 
So a very versatile mixer design! The mixer takes + and - 12 Volt but will work just as well on +/- 15 Volt. 
Because it's a tiny little circuitboard and weighs next to nothing, I glued the circuit board straight to the back of two of the potmeters in the panel with hot glue. Works great! :)
You can add as many channels to this mixer as you like by simply adding more potmeters with input jacks and 100K resistors to the input of the opamp. You can even add an inverted output by tapping off the signal from pin 1 of the chip (I've marked the place in the schematic on the drawing and the layout) with a 1K resistor going to an output jack. I didn't include that in my build because inversion doesn't do much for audio signals and my LFO already has an inversion option so there's really no need for it. There is also the possibility of adding a 'Gain Control' potmeter by changing the 100K resistor over pins 1 and 2 for a 500K potmeter in series with a 50K resistor. It is drawn in the schematic in dotted lines. This will give a gain of x 0.5 to x 5.5
The inputs of the 4 channels can never be shorted out because they are connected to pin 3 of the potmeters so the input impedance stays the same as the value of the potmeters you used. If one or more of the potmeters switched to the mixer is turned to zero, there still is a 100K resistor in series with the wiper(s) so there's never a short circuit possible. You won't even hear the slightest drop in volume, this is a very simple but very good working mixer. Like I mentioned earlier, I've built two of them so far and they are used all the time!
Sam Battle of LookMumNoComputer fame has a similar setup with the switches choosing between 'Mix' and 'Individual Output' on his quad VCA module, which you can see in this video (5m33 into the video). I noticed this only recently when I watched the linked video and when he talked about the VCAs it reminded me of this mixer.
You might wonder why the input signal is connected to the negative (inverting) input of the opamp. Why not just on the positive (non-inverting) input and do away with the second opamp? Well that's because the opamp's summing function only works on the negative (inverting) input. The way it's shown in the schematic is the right (and only) way to do it.
You can also ad a 'Mute' function by simply putting an extra single pole double throw switch into the mixer output. Some people find that a handy function to have but it's not included in this project.

Here's a picture of the finished panel (before I added the clipping indicator, so here the blue LED is always on.):

When I first made this mixer, the blue LED was just there so I knew the mixer had power but it served no real function other than that it looked cool. (I had actually drilled the 3mm hole by accident so I put a LED in it to fill it up.) But later I decided to give it a useful function and to use that LED as a clipping indicator. So I started looking for schematics of clipping detectors and I found some low resolution circuit images. I used one of those to draw my own schematic in a better readable higher resolution:

I made this stripboard layout for it which is verified, I used this for my build:

Because I had already built the mixer previously, I built the clipping indicator on a separate piece of stripboard and glued it to the mixer print with hot-glue, using a spacer in between so the prints wouldn't touch each others copper strips. Since then I have made a new layout combining the mixer with the indicator which is of course much more convenient. The layout below is verified, I recently built a 2nd mixer using this layout and it works fine. 
You can use a variety of (dual) opamp chips for this circuit. The TL072, TL082 or even a 4558 or an LM358 will work for both the clipping indicator and the mixer. It doesn't matter which you use where, they all work fine. As long as the dual opamps are suitable for audio circuits and are pin for pin compatible with the TL072 (see datasheets). Note the opamp in the clipping indicator circuit has it's minus pole (pin 4) connected to ground. It only needs a positive voltage source not a dual one. 
Btw, you can use other value potmeters for the attenuator/mixing pots. I used 1M but 100K or 50K will work too. I'd recommend not to go lower than 47K though.
All potmeters are viewed from the front with shaft facing you.

Print only:

(Last revised: 27-April-2020: Corrected mistake with 10K resistor to pin 6 of IC2, it was connected to ground when it should be connected to V+. 30-March-2021: Cosmetic changes to make layout clearer.)

Here's the layout for if you want to add a 'Gain' potmeter. With a 500K potmeter the gain will be between 0.5 and 5.5 times and with a 1M pot it'll be between 0.5 and 10.5 times. 500K is really enough because with gain set to above 3 times the audio will start to clip anyway but it's up to you.
Note the 100K resistor over pins 1 and 2 of the mixer IC (to the left) is now changed for a 47K in series with the Gain potmeter. The 47K resistor is there to make sure the feedback resistance can't go all the way down to zero. I've highlighted the potmeter connection in the square on the layout. The rest is the same as the previous layout (although this is a somewhat older version but it's all correct.)

Calibrating the clipping circuit:
This clipping circuit works on anything from 9 to 15 Volt. It happily takes 10V peak-to-peak audio input signals (if powered from 12-15V). You can set the sensitivity or the 'clipping threshold' with the 5K trimmer potmeter. I just fed it the normal signals from the two VCO's on channels 1 and 2 and slightly mistuned one VCO so you get that frequency beating effect where the two signals amplify eachother when they are in phase and subtract when they are in opposite phase. With the mixer-level pots turned to maximum I set the trimmer in such a way that the LED would just come on when the combined signals from the two VCO's would be at their highest amplitude. That way any signal louder than the VCO's will trigger the clipping light and you also get a visual indication of the 'frequency beating effect' because the LED will blink in time with this effect. So at the level I set it, the audio isn't actually clipping yet but the volume is louder than the normal 10Vpp. You can of course choose any threshold level you like and calibrate the circuit with whatever method you wish.

Here's the mixer panel with the newly designated clipping LED:

Here you can see the two prints behind the Mixer panel. You can also see how I glued the two prints together with a plastic spacer inbetween to prevent them touching. It looks a bit messy with that big blob of glue but that doesn't matter to me. It needs to work and work it does! And it saved me from having to rebuild the whole mixer. (I might do that anyway though on a later date, using the layout above.)
If you decide to build this mixer you should of course use the layout with the clipping indicator included. It's still small enough to hot-glue it straight to the back of two or three potmeters in the panel, so no need for screws or brackets. I'm really happy that I was able to include this little visual aid to this mixer. Makes it just that little more appealing and useful.

Okay, that's an other one done. If you have any questions or remarks please put them in the comments below or post them in the special Facebook group for this website.

If you like what you see and would like to support my efforts and the upkeep of this website then you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can use this PAYPAL ME link. All donations go towards the purchase of new components for projects. Thank you very much!! 


  1. This is awesome! Love your DIY Synth design :D
    Thanks for sharing your Stripboard layouts <3
    Wish everyone did this!

    1. Thank you! I'm glad you're finding it useful. :)

  2. Hey Eddy, built this one on Carton to be on the safe side, but alas! it worked perfectly :)
    I also have a lot of Bassguitar related gear, and was thinking about making the "in" jacks 6,3mm and the "out" parts 3.5mm, kind of like a interface box between bass and eurorack. Anything special i should look out for ?
    Greets from Vienna, Ben

    1. Hi Ben, I'm glad it worked like it should =) It's a simple design so not much that can go wrong. You can easily replace the 3,5mm inputs for 6,3mm ones. The signal out from a bass guitar, if unamplified, is much lower than the output from synthesizer modules so the clipping indicator will be of little use I think but other than that it should work perfectly fine.

  3. Now having some more experience in building these synthesizers, I think you need logaritmic pots for auto and lineair for voltage control. It works using lineair for audio, but the controllable range is better with logaritmic.

    1. That is true but I always found the difference negligible but it's up to the builder.

  4. Hey Eddy, how relevant is the value of the 500K Pot for gain? Can I use, for instance, a 100K Poti and change the 50K Resistor that is in series? And, if possible, how would I have to change the resistor?

    1. The value of the potmeter determins the amount of gain you get. This is the formula with Rf being the resistance of the potmeter (500K) and R1 the resistance of the input resistor(s) (100K) Gain=-Rf/R1 So if you reduce the value of the potmeter to 100K then only the 50K would give a little bit of gain: -150/100=-1.5 times gain (them minus just means the signal is inverted. So it needs to be 500K to get 5.5 times gain or 1M for 10.5 times gain. Reducing the input resistors is not a good idea. You need high input impedance in a mixer.

    2. okay, i see. thanks for the lesson. ... my diy-synth is growing and so is my elektronic knowledge. thank you for sharing your awesome work!!

    3. That's good to hear. It's always so satisfying to see a modular synthesizer grow in size and all by your own work. In the end you'll have a system that'll sound awesome!! ^___^ Good luck and if you come across any more problems feel free to ask questions either here or on the Facebook group.

  5. hi wondering if the BC547 is lined up along row 43 or if the middle pin goes out to the next strip? thanks

    1. No they're all in line. Makes no difference anyway.