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Minimoog VCF problem
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LeBon



Joined: Mar 23, 2015
Posts: 5
Location: USA

PostPosted: Fri Aug 07, 2015 8:37 am    Post subject: Minimoog VCF problem
Subject description: Filter is not resonating
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Hi,
I have a minimoog VCF on a breadboard.
Everything seems to be working fine when feeding the VCF with audio signal, except that it is not oscillating, even with the emphasis pot and balance trim fully right.
When the transistor buffer is replaced for an opamp (like in the Roland SH3) the filter oscillates.
I really cannot understand what's going on here, since my circuit is identical to Bob's, except for the TIS97 transistors - I am using a CA3046 for the output buffer.
Can someone help me to figure out where's the fault?
Thanks.
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AlanP



Joined: Mar 11, 2014
Posts: 97
Location: New Zealand
Audio files: 3

PostPosted: Fri Aug 07, 2015 8:58 am    Post subject: Reply with quote  Mark this post and the followings unread

A brief google of "tis97 datasheet" and "ca3046 datasheet" tells me the typical hfe of the tis97 is roughly 800, and the typical hfe of the ca3046 is 100 to 54 (depending on current.)

My guess would be that there is your problem.
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LeBon



Joined: Mar 23, 2015
Posts: 5
Location: USA

PostPosted: Fri Aug 07, 2015 9:32 am    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
A brief google of "tis97 datasheet" and "ca3046 datasheet" tells me the typical hfe of the tis97 is roughly 800, and the typical hfe of the ca3046 is 100 to 54 (depending on current.)

My guess would be that there is your problem.


Maybe you're right.
However, TIS97 is unavailable in my area. Would you indicate a direct replacement? BC550 also didn't work.
Takeda uses the 2SC1815, which is easier to obtain.
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AlanP



Joined: Mar 11, 2014
Posts: 97
Location: New Zealand
Audio files: 3

PostPosted: Fri Aug 07, 2015 7:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

I suggest taking some looks at Yves' schematic (YUSYNTH), his Minimoog VCF works brilliantly.
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AndyR1960



Joined: Feb 16, 2007
Posts: 314
Location: Manchester UK
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PostPosted: Mon Aug 10, 2015 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

LeBon wrote:
However, TIS97 is unavailable in my area. Would you indicate a direct replacement? BC550 also didn't work.
Takeda uses the 2SC1815, which is easier to obtain.


I've built several Takeda's Moog VCF's and substituted the 2SC1815's with 2N3904's. Never had a problem with them.

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