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Pehr
Joined: Aug 14, 2005
Posts: 1307
Location: Björkvik, Sweden
Audio files: 2
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 Posted: Tue Nov 06, 2007 7:37 am Post subject:
Portamento circuit question |
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A simple portamento circuit consists of a pot, a cap and an op amp configured as a voltage follower. Very often, I see that the pot has the value of 1MOhm. Why?
Is it possible to substitute the pot with a lower value one and the cap for one with higher value? To get the same portamento times... 
For example: I subsitute a 1M pot with 200k and a 0.1 uF Cap with 0.5 uF. Would I get the same portamento times?
Or does the pot need to be 1MOhm or higher?
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etaoin
Joined: Jun 30, 2005
Posts: 761
Location: Utrecht, NL
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| Posted: Tue Nov 06, 2007 8:36 am Post subject:
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A portamento cap needs to be very stable. Stable caps (polystyrene, polypropylene) usually only come in smaller values, requiring bigger resistor values. At least I think that's the reason.
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Tim Servo
Joined: Jul 16, 2006
Posts: 924
Location: Silicon Valley
Audio files: 11
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Posted: Tue Nov 06, 2007 8:51 am Post subject:
Re: Portamento circuit question
Subject description: Pesky Portamento Parts Proposal |
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| Pehr wrote: | A simple portamento circuit consists of a pot, a cap and an op amp configured as a voltage follower. Very often, I see that the pot has the value of 1MOhm. Why?
Is it possible to substitute the pot with a lower value one and the cap for one with higher value? To get the same portamento times...
For example: I subsitute a 1M pot with 200k and a 0.1 uF Cap with 0.5 uF. Would I get the same portamento times?
Or does the pot need to be 1MOhm or higher? |
The pot in a protamento circuit is usually configured as a variable resistor (as opposed to a voltage divider). This means that the pot is used to vary the current going to the cap. The large value pot gives you a wider range of current adjustment. You could probably get the same max time by subbing a lower value pot and a higher value cap, but you wouldn't get as much range out of the circuit (same max but different, slower min). With a lot of things like this, I'd recommend breadboarding it and see what happens when you change those values. It can be a great learning experience, and you're very unlikely to blow anything up with a portamento circuit.
Tim (different value crackpot) Servo |
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frijitz
Joined: May 04, 2007
Posts: 1734
Location: NM USA
Audio files: 54
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| Posted: Tue Nov 06, 2007 10:25 am Post subject:
Re: Portamento circuit question |
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| Pehr wrote: | A simple portamento circuit consists of a pot, a cap and an op amp configured as a voltage follower. Very often, I see that the pot has the value of 1MOhm. Why?
Is it possible to substitute the pot with a lower value one and the cap for one with higher value? To get the same portamento times... |
Yes, you can do that with no problem. The cap does not need to be super stable, so Mylar (polyester) works fine. The only drawback is the large physical size of the larger value caps. Hunt for a low voltage unit and you can get the large dimension under 8mm.
Ian |
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Pehr
Joined: Aug 14, 2005
Posts: 1307
Location: Björkvik, Sweden
Audio files: 2
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