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Pascal
Joined: Feb 25, 2007 Posts: 23 Location: Belgium
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Posted: Wed Jul 04, 2007 5:45 am Post subject:
Extra CV-inputs - 100K or 220K ? |
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I have a question about the SL modification-page:
http://www.musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/SoundLabMods.html#ADDMORECVINPUTS
In the 'Adding more CV-inputs'-modification they use for each extra CV-input a 100K ohm resistor.
If you go down on the page to the VCO patch-it out scheme, the original CV-in has a 100K ohm resistor (R62 and R63), but the new CV-in resistors (R77 - R78 - R82 - R83) have 220K resistors.
Which resistor do I need foor extra CV-inputs 100K or 220K ? _________________ www.groeneridder.net |
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Pehr
Joined: Aug 14, 2005 Posts: 1307 Location: Björkvik, Sweden
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Pascal
Joined: Feb 25, 2007 Posts: 23 Location: Belgium
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Posted: Wed Jul 04, 2007 9:33 am Post subject:
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Can I use for CV 1 a 100K resistor and for CV 2 and 3 a 220K resistor? _________________ www.groeneridder.net |
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Pehr
Joined: Aug 14, 2005 Posts: 1307 Location: Björkvik, Sweden
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Pascal
Joined: Feb 25, 2007 Posts: 23 Location: Belgium
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Pehr
Joined: Aug 14, 2005 Posts: 1307 Location: Björkvik, Sweden
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Randaleem
Joined: May 17, 2007 Posts: 456 Location: Northern CA, USA
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Posted: Thu Jul 05, 2007 7:14 pm Post subject:
Re: Extra CV-inputs - 100K or 220K ? |
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Hi,
Yes, that's confusing. You might want to ask Ray what his reasons were/are for having the 220K's?
(They're probably left over from when all parts of the SL were internally connected together.)
FWIW, The 220K's are going to have a little less than half the effect on the VCO frequency compared to the 100K's. (when both are supplied with the same control voltage.)
The important point<pun> is that pins 6 of the VCO IC's are a "virtual GND" summing point for the voltages coming into them through the resistors. If you use evenly matched resistors, each input will contribute the same amount (given each has the same applied voltage).
You will often see 1% or even .1% resistors specified here in precision VCO designs, so that each CV input causes the same frequency output from the VCO. Another method using less expensive R's is: One input is labeled as 1v/oct, and the VCO is tuned using this input. The others will be close; but this is the only CV input guaranteed to have 1v/0ctave scaling.
When you use different size resistors going to the summing point (or node); the inputs with the larger R's will contribute less to the overall sum than the paths with smaller resistance values. Again, based on all applied voltages at the CV inputs being the same.
Think of the resistors as pots in a mixer. If you want less of a given input, you increase the resistance in that "channel". Lower resistor values will contribute more "to the mix" (for a given voltage) .
The reason you see 100K is that this has become the "standard" input impedance for synthesizer module inputs.
(BTW, If you look at synth module outputs you'll see a lot of cases with 1K resistors; because that is the output impedance "standard" for synthesisers. See the 1K R's inline with the SL VCO outputs?)
To answer your question: In this situation, I'd use 100K for any additional CV inputs you want to add.
If you know an input control voltage is going to be higher or lower than what is "typical", THEN you'd add a specially sized resistor and associated CV input leg to the Pin 6 summer to handle that particular type of input. Otherwise you just use the standard 100K's. (As mentioned above; this may be why Ray put the 220's in there: he was feeding from some known CV source that was higher than normal. (But ask him; I'm just guessing.)
One last point; let's say you want to have a coarse and fine frequency setting on the input of your VCO's. One way is to use two different size pots and feed the signal from each to this summing point.
But by using the principles described above you can use the same size pot for both Coarse and Fine tuning. You simply use a larger input resistor with the fine tuning pot so that it will contribute less overall to the summing point.
You can see an example of this in Thomas Henry's VCO-1, which is available from a link in a thread over at the Thomas Henry Designs part of this website.
Randal
Pascal wrote: | I have a question about the SL modification-page:
http://www.musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/SoundLabMods.html#ADDMORECVINPUTS
In the 'Adding more CV-inputs'-modification they use for each extra CV-input a 100K ohm resistor.
If you go down on the page to the VCO patch-it out scheme, the original CV-in has a 100K ohm resistor (R62 and R63), but the new CV-in resistors (R77 - R78 - R82 - R83) have 220K resistors.
Which resistor do I need foor extra CV-inputs 100K or 220K ? |
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