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. 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)

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. So very versatile! The mixer takes + and - 12 Volt but will work just as well on +/- 15 Volt. 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. I've built two of them so far and they are used all the time!

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. 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 indicators and I found some low resolution circuit images. I used one of those to draw my own schematic:

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

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 either a TL072 or TL082 for both the clipping indicator and the mixer. It doesn't matter which you use where, they both work fine.

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+.)

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.

Here's a few pictures of how the complete synthesizer, described in these 17 blog-posts, now looks. (These pictures were taken before I changed out the two VCO's for the Digisound 80 VCO's):

It may not look very professional, especially the hand written labeling of the controls, but I love it and I must say I'm really proud of it too. I even made the local newspaper with this synthesizer. A journalist came and interviewed me and wrote a cool little article about it. Let me know in the comments what you think of it please! I would love to hear your input and feedback, ideas or even criticism.

There's at least one more item that will be added and that's a little oscilloscope. I have the scope here but I'm still waiting for the acrylic case.  I also want to change the VCO's because I can not get them in tune over the whole 4 octaves. So I'm looking with great interest at the modules Sam Battle is putting out at the moment especially the VCO with build in tuner. In the mean time I'm building two VCO's using the Digisound 80 design from 1980. More on that in the next article.

Okay, that's an other one done. Stay tuned (pun intended) there's more to come!  :)


  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.