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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
State Variable Filter 1V/Oct trimming
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prgdeltablues



Joined: Sep 25, 2006
Posts: 162
Location: UK
Audio files: 10

PostPosted: Sat Dec 10, 2011 4:10 am    Post subject: State Variable Filter 1V/Oct trimming Reply with quote  Mark this post and the followings unread

Hi, I've just built two MFOS State Variable Filters. Both are working fine, except that I can't get them to track at 1V/Oct. They're pretty close, but I've reached the limits of the CV input trimmer, which is puzzling. Any suggestions would be welcome.

Here are some measurements:
With 0V on a CV input, I measure +5.4mV at Pin7 of U1-A on Ray's schematic (I forgot to zero the initial cutoff pot, so that probably accounts for the reading not being 0mV).
With 1V on the CV input, I read -14.5mV, which is 19.9mV difference
With 2V, I read -34.4mV, and with 3V, -54.4mV. So U1-A would appear to be working, producing 20mV difference per 1V input.

With the trimmer centred, and the resonance turned up so the VCF oscillates, for 0V, 1V, 2V, 3V Cv input, I get frequencies (nice sine waves too) of 215, 448, 956 and 2100Hz - rather sharp. Even adjusting the trimpot so the feed into the base of Q1 is taken from the top of R7 (that is the trimpot and R7 divide the voltage in the ratio 100/475), I still get 223, 454, 948 and 2050 Hz. Not bad, but I'm puzzled as to why I can't get closer. Or is this as good as the expo will get?

The trimpot measures at 93R, rather than 100R, and R7 at 463R, but that shouldn't be a problem?

The only other circuit changes I've made that I can think might be relevant are using 47k resistors instead of 49.9k for R30 and R32.

HA! Comparing my schematic with Ray's as I write, I've just seen that I've used 100k for R8 instead of 10k. I don't quite understand that part of the circuit, but out with the soldering iron might be the answer?

Will test and report back, but any thoughts still welcome.

Peter
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prgdeltablues



Joined: Sep 25, 2006
Posts: 162
Location: UK
Audio files: 10

PostPosted: Sat Dec 10, 2011 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes! Putting the proper value in R8 did indeed solve the problem. Now tracks very well over four octaves, and not badly over 7. Nice filter!

Peter
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Mongo1



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

PostPosted: Sat Dec 10, 2011 5:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm glad you like it - I've got one sitting in my backlog waiting to be built.
I'm looking forward to completing it.

Gary
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