euro-SVF is self oscillating duh! -solved

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Pav
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euro-SVF is self oscillating duh! -solved

Post by Pav » Mon Mar 23, 2020 1:12 pm

Hi again,
the euro-SVF is built and really happy with it. All working fine.
In passing i was talking to eljay , he reminded me it is not supposed to self oscillate.
Mine appears to do just that at high resonance (past 2 oClock.)

Now I usually leave Tune trimmer as centered and i still do not have a plug in for spectrum analyser so leave that alone.
Could it be I just need to adjust the the frequency cutoff a bit lower, or maybe a build issue?

I was asking the overdrive question, so thought id run this by you too.

just need to complete my euro-vcas before i show off my 6U oakley eurorack case. :hihi:
Cheers
Pav

ps, i still have space for a lfo + sub osc + sample and hold utility if you could knock one up :sb:
Last edited by Pav on Tue Mar 24, 2020 8:18 am, edited 1 time in total.
rgds Pav

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Synthbuilder
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Re: euro-SVF is self oscillating duh!

Post by Synthbuilder » Mon Mar 23, 2020 4:11 pm

Pav wrote:
Mon Mar 23, 2020 1:12 pm
he reminded me it is not supposed to self oscillate.
Mine appears to do just that at high resonance (past 2 oClock.)
Possibly some resistors are the wrong value. Check that R25 and R26 are both 33K, and not something like 330K. You may even be able to simply measure them in situ with a multimeter. It should measure 33K or less, if it's more than that, R25 and R26 could be wrong.
i still have space for a lfo + sub osc + sample and hold utility if you could knock one up...
There will be a version of the sample/slew coming at some point. And there may be some sort of octave divider thing. Some older readers may well remember the 'two up, two down' Oakley module that never happened in 5U. That may see the light of day as a separate Eurorack module.

Next up though is the DLF as a 6HP module.

Tony

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Pav
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Re: euro-SVF is self oscillating duh!

Post by Pav » Tue Mar 24, 2020 6:34 am

Checked r25,26 are readin 30k in situ, so that looks ok. R27,28 have right values too and diodes d2,3 are correctly oriented. R21-24 also associated with u3 have correct values. C9 is marked .47k which should be 470nf .
rgds Pav

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Re: euro-SVF is self oscillating duh!

Post by Synthbuilder » Tue Mar 24, 2020 6:51 am

Not sure then. How about R19 and R20? Maybe even C8.

Tony

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Pav
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Re: euro-SVF is self oscillating duh! (solved)

Post by Pav » Tue Mar 24, 2020 8:17 am

Well values are correct - I did reflow all of the named components and im now thinking all is ok.
I could well have been confusing the harmonics with oscillation, or the reflow solved it.
Thanks for your time. with 100% resonance , the scope flatlines at 9oc position of frequency, and loses all harmonics at 3ock.
This is normal i guess.
Time to go sit in the garden and catch some rays..or paint the shed..not decided yet.
rgds Pav

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Re: euro-SVF is self oscillating duh! -solved

Post by Synthbuilder » Tue Mar 24, 2020 10:27 am

Ah right. The thing about the Euro SVF is that it will hugely amplify any input signal at the resonant frequency when the resonance is turned up. To test whether it self oscillates will be to listen to the output of the filter without any input. If you can hear it squealing it's self oscillating. If all you can hear is a faint noisy whistle it's probably working fine.

BTW the Euro SVF is more or less the same filter as the one in the ASV. It's not the same as the 5U SVF which does self oscillate.

Tony

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