Blacet Dual Filter: Does not seloscillate! :evil:

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e-grad
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Blacet Dual Filter: Does not seloscillate! :evil:

Post by e-grad » Mon Jun 08, 2009 6:55 am

I've build a Blacet Dual Filter and it works fine except: It will not self oscillate! :evil:

I've triple double checked but I can't find the prob. What puzzles me: The 2 filter cores are very distinctive blocks which do not share common parts (resistor networks, multiple op-amps), yet, both of the two VCFs share the same prob: No self oscilation.

Any idea what the culprit might be?

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Post by fonik » Mon Jun 08, 2009 7:16 am

there are filters out there that won't self oscillate, IIRC especially state variable filters won't self oscillate under certain circumstances (don't ask me about these circumstances).
AFAIK i.e. the SEM filter does not self oscillate. i read about that concerning the doepfer SEM VCF. i myself had the SEM VCF on my breadboard and it did not selfoscillate, too.

don't know about you blacet, though. schematics?
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Post by e-grad » Mon Jun 08, 2009 7:23 am

The DF is supposed to self oscillate! Yes, I've the schematics in front of me, yet, they're of no help to me. :huh:

BTW: I took advantage of John's offer here:
http://www.blacet.com/bargains.html
The circuit also needs 2 MAX365 which are hard to get!

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Post by Cybananna » Mon Jun 08, 2009 7:30 am

unfortunately, i don't know where your problem is, but the dual filter should and does self osc. :despair:

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Post by e-grad » Mon Jun 08, 2009 8:10 am

I'm coming closer!

AFAIK the DF is simplified version of Filthy Filter. Blacet's HP says about this one
If U13 [= 13600] is bad, the filter will not self oscillate
Without the 13600 my DF sounds as before: No self osc. but the rest works as supposed. [The input normalization does not work but this goes through the 13600 thus that was to be expected.]

Swapping the 13600 = no self osc. However the error should be some where within the Phase Shift circuit.

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Post by SynthBaron » Mon Jun 08, 2009 9:49 am

Yeah, my dual filter REALLY self oscillated, in a nasty good extreme way.

Probably new batch of 13600's and something is fucked with them? I'd contact Blacet.

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Post by Kwote » Mon Jun 08, 2009 11:23 am

yup. i've got two DF's and they both self oscillate. in fact that's the main reason they're worth anything to me. helps for good percussion sounds.

not sure if Blacet will help you on this but i'd still try. i think he put a disclaimer on the bargain basement stuff about not offering support.
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Post by consumed » Mon Jun 08, 2009 2:09 pm

i am going to mod my dual filter for *less* resonance since i seem to hit the self oscillating point about half way thru the pot travel. from the blacet tech page:
Just change R11 to 68K for the "A" channel and R21 to 68K for the "B" channel. It will still self oscillate just fine but at a point closer to the end of the pot travel.

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Post by e-grad » Mon Jun 08, 2009 5:26 pm

Lucky you. I wish I'd come to that point that "taming of the Q" would be on my mind. Of course I read that myself:
"Taming of the Q" The DF 2420 Dual Filter has an awful lot of Q; more than is reasonable for some folks. But who said the designer was "reasonable"!
I don't think that getting in touch with John will be of much use. This is the deal:
Bargain Basement [...] As-is. No technical help or warranty unless mentioned.
Don't get me wrong: I won't complain! I'm a big fanboy of John's work: Outstanding modules and kits which have introduced me to DIY at absolutely reasonable costs.

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Post by consumed » Mon Jun 08, 2009 6:15 pm

i dont mean to rub your nose in it, but rather to point out the resistors that are responsible for altering it. i assume from the bargain page that you sourced your own parts? could the resistor values be wrong in those spots?

if you are able to take a nice clear hi-resolution shot of your pcb i can look over it and compare it to mine?

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Post by scozbor » Mon Jun 08, 2009 8:21 pm

yeah gotta say i love the dual filter. surprisingly useful.
great as a mod source, or I use mine as a final stage EQ a lot of the time.

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Post by widdly » Mon Jun 08, 2009 10:43 pm

What sort of capacitors did you use in the filter block? I don't have the blacet but for something like the ms-20 filter, I needed to use decent poly caps to get the self oscillation.

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Post by e-grad » Tue Jun 09, 2009 2:53 am

consumed wrote:t could the resistor values be wrong in those spots?
I'm not in the position to rule out that I've done something wrong, I guess. I just can point out that I've multiple times checked the parts and I'm usually very careful on using the right part.
consumed wrote:if you are able to take a nice clear hi-resolution shot of your pcb i can look over it and compare it to mine?
Thanks! I've got no own cam but can borrow probably one later this week
widdly wrote:I don't have the blacet but for something like the ms-20 filter, I needed to use decent poly caps to get the self oscillation.
I've got the original BOM shipped with the PCB. The BOM does not ask for high quality but ceramic (X7R) and mylar so I don't expect prob.s due to the caps used.

However, interesting to learn that some cicuits depend on high quality caps to get into self oscillation!

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Post by valis » Tue Jun 09, 2009 4:04 pm

I'm in the process of one of these bargain basement builds myself. I'll probably be wiring it within the next few days and hoping to avoid any build errors. I'm curious as to how you wired the dual Frequency pots (R2 and R5). Did you buy the parts kit from Blacet as well or did you get a dual 50K pot (like I did). Could this be the source of the problem?

-d

Update: Sorry, I just read that you got the BOM from Blacet..

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Post by e-grad » Tue Jun 09, 2009 4:19 pm

valis wrote:Did you buy the parts kit from Blacet as well
Yes, bought the pots from Blacet.

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