Utility Mixer I

[Haralds:Werk]

This is an often used utility module. The mixer comes in handy for mixing CV sources. The mixer is DC coupled, so you can use it for DC and AC mixing. The input impedance is constant 1MOhm. The input signals are amplified by a factor of two. It is possible to offset every input with +/- 5V. The offset is signaled with diodes for every channel and for the summing output. The outputs are normalized, so you can remove selected channels from the output mix. The summed output has a -6dB switchable attenuator. There is an inverted summed output added as well. The added volume indicator us useful for finding the appropriate signal level.

Specs and features
• Three inputs, three outputs
• Inverted and non- inverted summed output
• 2x amplification
• +/- 5V offset for every channel
• -6dB switch
• Volume indicator
• Normalized outputs
• Runs on +/-12V and +/-15V
• Power consumption below 20mA each rail


The documentation and the Gerber files for download can be found in my website.

Utility_Mixer_I_schematic_01.png

Utility_Mixer_I_schematic_02.png

Utility_Mixer_I_front.JPG

Utility_Mixer_I_control_PCB.JPG

Utility_Mixer_I_main_PCB.JPG

Utility_Mixer_I_side.JPG

[cygmu]

Nicely done as usual.

Can I ask about the 5.6pF caps on some of the op amps please? Usually if I see those I think they are for stability but IC3A, 4A and 6A look like being the least likely to go unstable in the whole circuit. You don't have such caps on the output op amps for example. So I wonder if they are for something else -- are they there to suppress high frequency noise?

And, related: do you find there is no need for such caps on the outputs, for stability?

[Haralds:Werk]

Nicely done as usual.

Can I ask about the 5.6pF caps on some of the op amps please? Usually if I see those I think they are for stability but IC3A, 4A and 6A look like being the least likely to go unstable in the whole circuit. You don't have such caps on the output op amps for example. So I wonder if they are for something else -- are they there to suppress high frequency noise?

And, related: do you find there is no need for such caps on the outputs, for stability?

The utility mixer is mostly used for CV processing. So the 5p6 caps are for cutting the peak overshot from fast rising input signals. Say square or ramp down from a LFO. You are correct for the output caps. I am coming from my banana system. There is no need for those caps but they might be useful for Eurorack systems. I should have added them for stability reasons to stay on the save(r) side. But I didn't have any stability problems up to now though. If you run into stability problems here, you can easily add those caps on the backside of the PCB parallel to the feedback resistors.

[Haralds:Werk]

Just added the correct links in my first posts.

[Quinie]

Harald, Thanks again for these designs. Just build you Quad VCA, left my phone at work else I would have made some pictures (maybe tomorrow).

I enjoy building you're modules. The designs are quite easy to build, only some SMD for decoupling cap. This can build your confidence "I can do SMD". Your mixer and passive multi board just came in the mail today. So you starting to build me a backlog here.

[cygmu]

Heh, I had forgotten completely about overshoot. Thank you.

As for the euro/banana distinction, I’m a banana person too, but is it the case that shielded cables present more difficult (more capacitive?) loads than single cable banana leads?

[devinw1]

Nice one! Harald reigns as the productivity champ here on Muffs diy!

[Haralds:Werk]

Heh, I had forgotten completely about overshoot. Thank you.

As for the euro/banana distinction, I’m a banana person too, but is it the case that shielded cables present more difficult (more capacitive?) loads than single cable banana leads?

The shielded cables have some capacitance. You can look at them as a chain off R/C combination. Looking like a passive lowpass. The figures increases with length. There are OpAmps with problems driving capacitive loads. You can compensate for that with a cap in the feedback loop at the output OpAmp. Up to now I have not encountered any problems with capacitive loads in an Eurorack system though. Usually the cables are to short to cause problems.

[Haralds:Werk]

Nice one! Harald reigns as the productivity champ here on Muffs diy!

More to come. Still working on my publishing backlog. And some new stuff. I have a running prototype of a Quadrature trapezoid through zero LFO with all the usual wave forms at 90deg apart. And working on a scanning mixer and harmonic oscillator.

[Quinie]

Gerbers for panels (untested)