[IN STOCK] Eurorack: Discrete State-Variable VCF v1.4

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negativspace
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Post by negativspace » Thu Oct 15, 2015 3:03 am

Thanks guys... looking forward to taking better care of these threads now that my season of travel is past. (And my taxes are done!)

I have a few boards left, going to order more but I need to finish and bill another order before I can do that. If they go out of stock it will only be for a couple of weeks. :tu:

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Post by enj_music » Wed Oct 21, 2015 3:29 am

Just built the filter - it sounds beautiful, but like a lot of people, my notch output is very very quiet/not working...

The lowpass and highpass outputs work really nicely, so it must be the SMT section on the control board, but everything looks good there. Should I just assume I fried the dual opamp chip and get a new one?
Last edited by enj_music on Wed Oct 21, 2015 4:13 pm, edited 1 time in total.

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Post by shiftr » Wed Oct 21, 2015 3:50 pm

I'm about to start my build and I have two questions about the BOM:

Would it be ok to use A100k pots on the audio inputs?
I can't find the 2.7nF film cap at my regular suppliers Does anybody have reference for a that (preferably in EU).

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Post by negativspace » Wed Oct 21, 2015 4:28 pm

Yeah, that'll be ok on the pots although the closer you can get to 25k the better. (10k would be better than 50k which is better than 100k.) The higher the pot value in relationship to the input resistor (22k), the more exaggerated the 'log' curve will be. Other than that they're wired as voltage dividers so the value isn't terribly important.

I can give you the mouser # for the 2.7n cap if you can't find one closer to home.

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Post by enj_music » Wed Oct 21, 2015 4:51 pm

By any chance, do you have a Mouser part number for the spacer/screw/washer pair you use? The ones I got were too long and are unusable

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Post by negativspace » Wed Oct 21, 2015 5:34 pm

I use spacers from Fastenal, no idea on a Mouser number. They're 7/16" which is ~11.25mm so 11 or 12mm would work fine.

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Post by enj_music » Wed Oct 21, 2015 11:38 pm

While trying to fix the module, I stupidly put some needle nose pliers in between the two boards to grab a test component and probably shorted something out. The filter outputs are all at various levels, and no longer filter. The lowpass out is dead. The mixer still works. What did I most likely break?

I am losing so much sleep trying to figure this out :hail:

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Post by negativspace » Thu Oct 22, 2015 5:45 am

Ugh, I hate it when I do that. (We all do that.) :doh:

How's the 5V rail?

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Post by enj_music » Thu Oct 22, 2015 2:07 pm

negativspace wrote:Ugh, I hate it when I do that. (We all do that.) :doh:

How's the 5V rail?
5V rail measures in at 5.03V, -6 is still at a beautiful -6.01V. I tried swapping out the TL072s one at a time and that didn't seem to fix the problem, unless more than one of 'em died (unlikely). Q19 is probing a little suspiciously.

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Post by fragletrollet » Thu Oct 22, 2015 3:10 pm

Are both cv inputs wired to affect the filter? Is the state moduletable? (LP-Notch-HP)

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Post by negativspace » Fri Oct 23, 2015 4:32 am

enj_music wrote:5V rail measures in at 5.03V, -6 is still at a beautiful -6.01V. I tried swapping out the TL072s one at a time and that didn't seem to fix the problem, unless more than one of 'em died (unlikely). Q19 is probing a little suspiciously.
I'd try replacing Q19 then, esp. if that's the direction from which you entered with the pliers. With a working 5V rail the problem is most certainly in the core transistors, Q19 or any of the cluster on the left side of the board. (The 4 near the LM337 are in the mixer.)

FYI the opamps are there as output buffers for LP/BP/HP and also as a summing mixer on the CV side.
fragletrollet wrote:Are both cv inputs wired to affect the filter? Is the state moduletable? (LP-Notch-HP)
Yes, CV1 and 2 + v/oct are all wired to cutoff freq. No, the notch is not CV-modulatable.

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Post by fragletrollet » Fri Oct 23, 2015 4:40 am

Thanks! :party:

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Post by negativspace » Fri Oct 23, 2015 3:40 pm

You bet! :sb:

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Post by enj_music » Fri Oct 23, 2015 4:07 pm

negativspace wrote:I'd try replacing Q19 then, esp. if that's the direction from which you entered with the pliers. With a working 5V rail the problem is most certainly in the core transistors, Q19 or any of the cluster on the left side of the board. (The 4 near the LM337 are in the mixer.)

FYI the opamps are there as output buffers for LP/BP/HP and also as a summing mixer on the CV side.
So the opamps aren't actually part of the filter core? Also, is there a resistor/capacitor wired up in parallel with the C/E pins of Q19? Just wanna make sure that my on-board diode test results were okay

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Post by negativspace » Fri Oct 23, 2015 5:12 pm

Correct, they're only there to provide a bit of a buffer and level adjustment on the way to the outside world. (And the CV mixer.) The filter itself is discrete, as advertised.

Schematic is posted a few pages up... I went ahead and stuck it in one of the posts on the first page of the thread to make it easy to find. Have at it. :party:

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Post by enj_music » Fri Oct 23, 2015 5:52 pm

Thank you so much for the schematic! It is a little overwhelming however. Is Q19 part of the CV section, and would a broken Q19 really cause the filter to completely stop filtering/have weird level discrepancies between channels?

Also, there are a couple points in the circuit where there's just a random BJT with the base shorted to ground. Is that just you exploiting the properties of these transistors for a sonic quality, or is it doing something else?

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Post by enj_music » Thu Oct 29, 2015 12:05 am

Just finished repairing the filter - switched out all the transistors and it works again. However, I still am having a couple problems. First of all, the notch output is getting the highpass signal fine, but does not have the lowpass signal. Also, the filter range seems skewed. It does not open very far when the knob is fully clockwise, and the filter closes completely before even reaching 10 o'clock. It is calibrated to track 1v/oct, and the -6 and +5 rails look good.

With filter cutoff fully clockwise, no CV input, resonant peak occurs around 7kHz:

Image

Here is a recording: (The comments talk about the bugs)

http://soundcloud.com/enj_hoffman/svvcf-bugs

Also, is there a teensy schematic for the SMT section?

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sromanel
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Post by sromanel » Thu Oct 29, 2015 2:33 pm

Still haven't figured out the cause of the notch issue with mine... :hmm:

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Post by james2098 » Thu Nov 26, 2015 8:08 am

yes please!

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Post by negativspace » Thu Nov 26, 2015 3:14 pm

sromanel wrote:Still haven't figured out the cause of the notch issue with mine... :hmm:
Based on your soundcloud clip, it still sounds to me like the HP signal simply isn't getting to the notch balance pot... unless I'm totally mis-hearing things.

If you like, PM me and I'll give you my shipping address. You can send it over (I know it's international, but...) and I'll take a look.

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Post by DREIPOLAR » Thu Nov 26, 2015 4:14 pm

just want to announce that next week we are finally doing the 5-SVVCF group build in berlin with the pcbs/panels i got from jason. before i read through all the posts, is there anything that we have to change or look out for?

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Post by sromanel » Sat Dec 05, 2015 11:32 am

negativspace wrote:
sromanel wrote:Still haven't figured out the cause of the notch issue with mine... :hmm:
Based on your soundcloud clip, it still sounds to me like the HP signal simply isn't getting to the notch balance pot... unless I'm totally mis-hearing things.

If you like, PM me and I'll give you my shipping address. You can send it over (I know it's international, but...) and I'll take a look.
I don't think it'll be worth the shipment price, we're talking about almost a hundred euros send+return. I've had it checked by a synth repairman, there was a faulty trimmer (6V IIRC) but that didn't change much. HP works just fine, the only problem is with notch out. He even changed all SMD components, but with no luck. Is there any measurements I should do in order to understand a little bit of what's wrong?

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Post by DREIPOLAR » Mon Dec 07, 2015 5:58 am

here is our lötsportgruppe berlin (soldering sport group, berlin chapter) after completion of 5x SVVCF. final result are: 3 units working, 1 unit missing the highpass and 1 unit still missing in action. not bad for the first time.

assembly is no problem at all, except maybe the pots being fumbly to connect. the adjustments via the trimpots are mysteriously confusing but good thing a filter sounds even without being correctly adjusted ... if its not outright broken call it a feature ;-)

thanks jason for such a good module!

ps: yes that image is large but i dont know how to resize it

Image
Last edited by DREIPOLAR on Mon Dec 07, 2015 6:09 am, edited 1 time in total.

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Post by negativspace » Mon Dec 07, 2015 6:09 am

Nice work, guys! :party:

Calibration is not absolutely necessary, but also not difficult. It takes me about 2 minutes to calibrate each one now, but that's with a hundred or so builds under my belt.

Would be good to set the -6V rail correctly, though, as the clipping behavior of the CP3 stage depends on it.

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Post by negativspace » Mon Dec 07, 2015 6:16 am

sromanel wrote:I don't think it'll be worth the shipment price, we're talking about almost a hundred euros send+return. I've had it checked by a synth repairman, there was a faulty trimmer (6V IIRC) but that didn't change much. HP works just fine, the only problem is with notch out. He even changed all SMD components, but with no luck. Is there any measurements I should do in order to understand a little bit of what's wrong?
It's ~$15 to ship one of these things from here to Italy, I've done it more than once. Surely Italian airmail isn't $85 for a 12oz package.

Anyway... if you *do* have a HP out then there is only one possibility for the fault and that is somewhere in the SMT/notch balance pot/jack "subsystem." Try probing the CW and CCW pins of the balance pot with a patch cable just to see if there's a signal on either, and we can work backwards from there. A photo might also help, otherwise I'm just taking blind guesses. :hihi:

6V trimmer will only have an effect on the mixer section, that rail is not used on the filter side.

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