[sdiy] Something better than CD4066 to switch CVs?

Tim Ressel timr at circuitabbey.com
Tue Feb 19 20:16:10 CET 2019

This is a good point. In general any resistance in series with a v/oct 
vco input will introduce a tracking error. This is why things that drive 
vco inputs have to be designed carefully. Assuming 100K input resistance 
on the vco input (seems to be the standard) and 3 cent max error allowed 
(@ 10 volts), 25 ohms is the max added resistance allowed. This can 
loosen up if tracking to 10 volts isn't important.


On 2/19/2019 10:09 AM, chris wrote:
> On Tue, 19 Feb 2019 18:35:44 +0100 Florian Anwander <fanwander at mnet-online.de> wrote:
>> Am 19.02.19 um 11:06 schrieb Tom Wiltshire:
>>> The Prophet 5 managed to switch Oscillator B into low frequency mode about thirty years ago, so it must be possible.
>>> I had a look at the schematic and it doesn’t look like they switch an extra cap in though. Instead they switch an extra -15V into the CV. The Pro-1 uses the same trick.
>> Someone more knowledged may correct me, but I think this may work with a
>> coarse parameter change, but with tracking sensitive controls, the inner
>> resistance of the 4066 might become an influence.
>> Florian
> But once you're entering LFO range, IMHO tracking becomes less important.
> And I don't think an *open* switch will affect audio tracking.
> I agree that using a CMOS switch in tracking relevant areas requires at
> least some consideration. Switch resistance may be specified as foggy as
> "yes, it's there, and it's something around ^&@&%* ohms at room
> temperature".
> Christian
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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