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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS State variable filter inputs
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prgdeltablues



Joined: Sep 25, 2006
Posts: 180
Location: UK
Audio files: 11

PostPosted: Mon Oct 31, 2011 4:15 pm    Post subject: MFOS State variable filter inputs Reply with quote  Mark this post and the followings unread

I was wondering whether the 100n / 1MEG resistor combination that Ray uses for the audio inputs to the state variable filter (which as he says, form a high pass filter) could be replaced with a 1u / 100k combination. (I know that would mean using electrolytic caps, but...) The feedback resistor R19 around U3-A would then be reduced to 30k to maintain the gain at 0.3, and I guess R32 would also need scaling to 4k7. That would draw more current from U4-A, but nothing excessive?

In fact, I wondered whether the input caps were needed at all - just audio input into 100k resistors into the inverting summer amp (any DC offset in the incoming signal would be reduced by a factor of 0.3 anyway).

Anyone tried this? (before I breadboard!)

Peter
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Tue Nov 01, 2011 10:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I was wondering whether the 100n / 1MEG resistor combination that Ray uses for the audio inputs to the state variable filter (which as he says, form a high pass filter) could be replaced with a 1u / 100k combination.


At minimum, doing that would change the input impedence of the filter, which will present a higher load to whatever you feed into it. Since that circuitry is part of the high-pass filter loop, you also need to consider what the changes might mean to the overall response. I'm not sure what the effect would be.

Quote:
In fact, I wondered whether the input caps were needed at all


While those caps prevent any DC voltage from being present on the inputs, they also prevent any DC voltage created by the filter from feeding back into whatever you plug into the input. I'd suggest leaving them in place.

Gary
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