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Voltage Divider on output. Bad Design? Ok?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Voltage Divider on output. Bad Design? Ok?
SoManyQuestions
Hey friends.
Working on building some simple AD envelopes. In the normal state the output swings from about 0v to 10v. I would like to possibly use these with a eurorack system and would like to restrict that range to about 0 to 5v.

I couldn't find much information on "best practices" for this sort of thing.
From looking at other schematics, there seem to be 3 main ways of setting output level.

1. Simple voltage divider, usually post op amp. (this is what I would be doing).
2. Op amp with gain set via feedback resistor
3. Transistor set as a constant current source with a voltage divider following.

I understand that Option 2 is probably the best, but I don't have room in my circuit (or routing) to add another op amp.

Is the following circuit a terrible idea?
Will there be terrible consequences that I don't fully understand?
Thank you.

(P.S. Just assume that the output of the op amp will be at 10v. This is necessary for the A to D switching logic.
Oh also the trimmer would be 10k)


guest
there is probably some way you can feed a signal back elsewhere in your circuit so that the opamp only does 0-5V. but, assuming thats not the case, a divider on the output is ok. the downside is that the output impedance varies as you turn the knob. in your case, youd want the pot on the bottom, and the resistor on the top. with a 10k pot, the max output impedance would be 6k (5k from the pot/resistor in parrallel, and 1k from your output resistor). this would drop to 1k with the pot turned all the way down. most modules have a 100k input impedance, so youll get some unwanted attenuation, but seeing as youve got a pot there anyways, you can just turn it to whatever attenuation you want. this sort of thing would be problematic for a VCO CV, but for an envelope, its probably fine.
SoManyQuestions
Hi Guest thanks for your answer.
This is sort of what I expected, I was just hoping to have someone with a better understanding of this take a look. So thank you.

I'm not sure that I entirely understand the reason (or routing) to have the pot on the bottom. Do you mean running the 10k resistor and the 10k pot in parallel between the opamp output and ground? And then the 1k output resistor between pot wiper and output jack?

This works but it means that I can reduce the envelope output down to essentially 0. I would like to keep the env output (maximum) between about 5v to 10v. So 0 to 5v envelopes with pot at it's minimum, and 0 to 10v envelopes with pot at its maximum. Is there a downside to keeping it setup as shown in the initial post?

Also, (showing my ignorance here) if the goal is to keep the output impedence near 1k (eurorack "standard"?) using a 1k resistor and 1k pot (instead of 10k) would get closer to that, and in my understanding result in less unwanted attenuation. Are there negative tradeoffs associated with that? I'm guessing that the power consumption goes up, but is there anything else that it negatively effects?
guest
sorry, i misinterpreted your question. keep the pot where it is if you want the level to be 5V to 10V. i thought you wanted it variable from 0V to 5V. a 1k pot is a small load for your opamp, so id stick with the 10k pot. the impedance variation wont be a big issue here.
Borogove
What's the op-amp on the left side of your circuit doing?
SoManyQuestions
Thank you Guest
I just noticed that you answered almost the same question from another poster today. thumbs up
We appreciate your sharing your knowledge.

Borogove
The op amp on the left in that pic is not really doing anything. I just made that quick setup to illustrate the question that I was asking.

The actual circuit that I am working on is a Serge/Buchla style function generator. There is an Lm13700 that is pushing current into capacitor in parallel with the op amp. The op amp output increases with the capacitor charge until it reaches a threshold voltage where it triggers a flipflop and everything flips and it ramps back down to 0v
Navs
Could you set a lower threshold voltage for the reset? That's how it is done on the Serge VCS (or at least, it's possible to tweak this - the trimmer is actually used to set fastest rate but the consequence is a changed output amplitude).

Edit: TBH I would leave it at 0-10V and just attenuate at the destination ...
BARD
If you want to attenuate output to 0V, then dropping resistor should be placed before output pot, just like this:

Potentiometer is highlighted with dashed rectangle. Resistor values are exemplary razz
Also, make sure you don't exceed potentiometer power rating (anyway >1k resistor on the output will solve that problem for most of the pots)

I think it's fine for module to have non-zero impedance at output. It's gonna be OK in most use cases.
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