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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
SynthDIY - Filter debugging help!?
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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Tue Aug 12, 2014 2:39 am    Post subject:  SynthDIY - Filter debugging help!? Reply with quote  Mark this post and the followings unread

Hi All,

Im building the synth DIY, the VCOs and the LFO are ok but it seems that my VCF is not working.

Im feeding 10khz Square 2 to 4Vpp and I dont see any action in the output.. maybe minor square noise in the 20mV div on the scope.

I even feed the in-signal over the 10M resistor to clearly over feed it but nothing. The trimmer pins are checked to -7.5V.

Can someone give me the Volt values of the IC?
Im using the TL074ACN variant by the way)
Any other ideas welcome!

Thanks
Tasos
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m.o



Joined: Jul 05, 2014
Posts: 44
Location: Sweden

PostPosted: Tue Aug 12, 2014 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well for starters you should see +12 on pin 4 and -12 on pin 11.

Which trimmer pins are 7.5V? Is it 7.5 on all pins?

On Pin 5 and 10 you should see your CV for respective filter (you should see it vary while turning the filter cutoff pots).

On Pin 1 & 14 are your outputs.

Pin 3 & 12 should be zero volts.

I would start by also feeding different, lower frequencies through the filter (it's possible it is actually filtering out all of your 10kHz) - if lower frequencies get through at higher volume I'd start examining the whole CV part, the FET transistor, the filter CV bias trimmer and surrounding resistors.

If not I think I'd start by seeing if a signal can get through at all without filtering - to do this maybe try lifting R152 and C7 maybe also filter resonance pot R46.
With all that disconnected you should have just an inverting op-amp with a 1/3 implification (ie the output should be 1/3)...
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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Tue Aug 12, 2014 1:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

m.o wrote:

Which trimmer pins are 7.5V? Is it 7.5 on all pins?


Hi and thanks for helping.

Im not sure I know what you mean there.

With -12V on X20 (the mid of the Cutoff Pot) I trimmed the R66 to -7.5V.

Pin 10 goes from 0.08 to -0.08 if I move the cutoff end to end.

With 30Hz 4Vpp like the MFOS web I only get this pic on the scope.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I move everything but but nothing really changes.. weird!


PS: image is not mine. but it the same thing Smile
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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Thu Aug 14, 2014 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Rebuilding this again I end up in the same result...
I have measured all resistors and pots. and the 2.2nF caps.

I got some 2n4391 to test.

Images below are from the circuit and scope. thats the only scope I get by changing all pots around...


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IMG_5507.JPG
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IMG_5507.JPG


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m.o



Joined: Jul 05, 2014
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PostPosted: Thu Aug 14, 2014 1:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

I just tried to simulate this circuit in LTSpice, and I got pretty much the same results as you (<20mV output from a 1V input).
Probably something we're missing..

What I tried in LTSpice that did (kindof) make it work like a lowpass filter was to simply put a pot wired as a "variable resistor" (like the filter res one) instead of the whole CV bit with the transistor.

This points towards the transistor + CV parts being the possible problem.

Maybe you can try something like this as well and see if you get the same results (the filter seemed to be able to sweep up to about 5-8kHz max in my simulation)?


MFOS LPF test.jpg
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MFOS LPF test.jpg


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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Fri Aug 15, 2014 12:44 am    Post subject: Reply with quote  Mark this post and the followings unread

m.o wrote:
I just tried to simulate this circuit in LTSpice, and I got pretty much the same results as you (<20mV output from a 1V input).
Probably something we're missing..

What I tried in LTSpice that did (kindof) make it work like a lowpass filter was to simply put a pot wired as a "variable resistor" (like the filter res one) instead of the whole CV bit with the transistor.

This points towards the transistor + CV parts being the possible problem.

Maybe you can try something like this as well and see if you get the same results (the filter seemed to be able to sweep up to about 5-8kHz max in my simulation)?


Excellent work, thanks!
I think the transistor has something to do too. mpf102 had different result from the 2n4391.
Ill test your suggestion!

I think in the end it will easier if I pick another simple VCF to build alongside the SynthDIY.

Thanks a lot!
Tasos
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NickD



Joined: Sep 14, 2014
Posts: 2
Location: Staffordshire UK

PostPosted: Sun Sep 14, 2014 11:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi ! I had similar problems with the VCF . The specified FET is like Unobtanium in UK , so I tried 2n5457 , 2n5458 and 2n5485 , no luck all round . Very faint audio output , all voltages seemed sensible .
As all the other modules were good , I built a pair of filters using LM 13700 , based on one of the many " MS20 " clones floating around the net . The end result , lots of extremely silly but wonderful noises !
Other than the VCF snags , an excellent project .
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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Mon Sep 15, 2014 2:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

NickD wrote:
Hi ! I had similar problems with the VCF . The specified FET is like Unobtanium in UK , so I tried 2n5457 , 2n5458 and 2n5485 , no luck all round . Very faint audio output , all voltages seemed sensible .
As all the other modules were good , I built a pair of filters using LM 13700 , based on one of the many " MS20 " clones floating around the net . The end result , lots of extremely silly but wonderful noises !
Other than the VCF snags , an excellent project .


I think ill do the same thing!

I found one of the specified FETs from the states.
Any pointer to the schematic you used?
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NickD



Joined: Sep 14, 2014
Posts: 2
Location: Staffordshire UK

PostPosted: Tue Sep 23, 2014 12:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

I found the EFM Wildcat on SynthDIY.com , I used VCF2 . Some of the schematics had component values missing , had to do a bit of trial-and-error to get it working , mostly around the resonance control . As always , it pays to have a breadboard .
There are quite a few other variations of the " MS20 " circuit around , it's worth seeking them out for comparison .
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Varoudis



Joined: Dec 03, 2009
Posts: 31
Location: London

PostPosted: Sat Oct 18, 2014 8:25 am    Post subject: Reply with quote  Mark this post and the followings unread

NickD wrote:
I found the EFM Wildcat on SynthDIY.com , I used VCF2 . Some of the schematics had component values missing , had to do a bit of trial-and-error to get it working , mostly around the resonance control . As always , it pays to have a breadboard .
There are quite a few other variations of the " MS20 " circuit around , it's worth seeking them out for comparison .


thanks a lot for the pointers!
Ill report results when I get something nice.
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