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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS77 1973 VCF ~ output levels
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gddfp



Joined: Jan 11, 2009
Posts: 21
Location: Belgium

PostPosted: Sat Jun 12, 2010 2:06 am    Post subject: CGS77 1973 VCF ~ output levels Reply with quote  Mark this post and the followings unread

I've used the buffer section feeding the pots for the LP/HP/BP outputs.
Initially I left off the RGx resistors, but found the output way too low.
After using 100k's for RGx (which should double the output level), I thought the outputs were still too low; comparing the unfiltered signal with the one going through the VCF...
Finally used a 18k resistor in place of one RGx -- thinking "this can't be" -- and still finding the output too low (a tad better than no RGx at all, but not what I think it should be).

Then I had the brilliant idea of actually measuring the outputs. They all sit at almost-rail level !

No idea what's going on, or where I should start to look.

It would be a tremendous help if any of you can provide me with some voltages at various points, because I'm not sure anymore what I should expect ?


Many thanks !
_g
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gddfp



Joined: Jan 11, 2009
Posts: 21
Location: Belgium

PostPosted: Wed Jul 07, 2010 2:11 pm    Post subject:
Subject description: CGS77 1973 VCF ~ succes !
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To answer my own post:

Apparently I've blown up one of the JFet's (and probably also the nearby 2N3906), most likely by messing too much with the buffer section...

After replacing those, I junked the whole buffer section and put a simpler design in place (straightforward inverting opamp). The output levels coming off the board are sitting around 0.4V, so I implemented a 10x gain stage.

Filter works perfectly smooth now, with the output being the same as the original input signal. Yay.

_g
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