[sdiy] Something better than CD4066 to switch CVs?

Tom Wiltshire tom at electricdruid.net
Tue Feb 19 21:55:39 CET 2019

In this case though, we were discussing switching the *timng cap*, so the extra resistance is not “in series with the V/oct input” at all.

I’m not saying that it wouldn’t mess stuff up, just that that’s not where we were suggesting putting it! And as Christian pointed out, it’s actually more the *off* resistance that’s important, since if things go a bit out when the switch is on for LFO mode, we probably don’t care too much.

       Electric Druid
Synth & Stompbox DIY

> On 19 Feb 2019, at 19:16, Tim Ressel <timr at circuitabbey.com> wrote:
> 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.
> --TimR
> 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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