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Signal attenuation
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eamoex:
Hello All,
I'm an amateur electronic musician and I use a miniature modular synth. For those who don't know, this type of synth doesn't store particular settings for a creative sound; instead it relies on physical wire connections through a series of jacks using the many ports available on the machine itself, which mutally send and receive voltage-controlled signals, which in turn change frequencies of oscillators, or threshold of filters, etc... In other words, the control is analog, where, for instance, you have relations like octave-per-volt, or frequency-per-volt, etc...

https://cdn.korg.com/us/products/upload/7ff66deca06d37565b026ce984f059f2_pc.jpg

The thing is, the synth can receive and process voltage-controlled signals from external gear, but the standards to shape the signals are not very uniform. I have an external controller which sends signals oscillating between -10 and +10V, but the synth can only receive signals ranging between -5 and +5V.

Questions: could I easily modify a cable which would "attenuate" the signal, i.e. lower the potential from 20Vpp to 10Vpp, without losing the timing and integrity of it?

I know how to build a simple linear voltage regulator circuit, but that's for power, not signal. Not sure how to go about this one. Everything I found on the topic is not well informed and stem from people who, like me, don't really know how to do this right.

There's this: https://koma-elektronik.com/?product=koma-attenuator-cable, but it looks so sleek and simple that I can't help thinking I could build a dozen myself for a fraction of the price!

Any advice welcome!
johnwa:
Hi,

Yes, you should be able to just use a passive voltage divider - one resistor in series with the signal lead, and one across the output of the cable. For the voltages you have given, the values are easy - just make both resistors equal in value. The only issue you will have is with impedances: for best accuracy, the resistors need to be small compared to the input impedance of your synth, but large compared to the output impedance of your external controller. Try 2x 10kOhm, and experiment from there. You can overcome this problem using a buffer amplifier, but this will add complexity.
eamoex:
Thanks johnwa. Voltage divider, I know that. Haven't even thought about it. Plus it's real easy to test. Will do and report.
eamoex:
Okay, haven't been testing yet. But I've measured actual voltages and read the doc. Actually I need to go from 14V to 5V. I DuckDuckGoed this online calculator:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

I knew about voltage dividers but this is useful to understand the influence of RL and the sum of R1+R2 on Vout under load. Now, here is the question: is there a method to easily measure RL for a given device?

EDIT: oh yes and how do I know what power values I should remain under? I see it's pretty much possible to fry something here with too low R1+R2 values.
eamoex:
Voltage divider with 2x10K worked great. Synth is rocking and in tune. No need to explain about impedance (which I have a hard time understanding).

Thanks for your help!
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