Steiner Synthacon VCF

-minus- · Joined: Oct 26, 2008 Posts: 718 Audio files: 13

well I am now on the 5th board layout and I have had no luck Crying or Very sad

. I'm unsure what I am doing wrong here. I have literally copied onto stripboard, the schematic exactly as it is on the CGS site. Are these transistors so hit and miss? Do I need a bunch of them to test and see which combination works best? I'm getting very little sound from the output, and I only get sound if I bypass the 47K resistors on the HP BP LP inputs. I just cant work this out... It IS haunting me now and I can't let it go! I have checked for solder bridges etc. I have reflowed all solder on the reverse of the board. scratch

Last night I had a go at building this on perf. I had the opamp section working fine. Then I built the transistor section to the right of the ladder. I hooked up the ladder and CV section on my breadboard to this and it worked fine. Then I soldered the rest of the circuit and it wouldn't work. Confused

I've ordered some more 2N2222's but they wont be in until next week. If this keeps failing I'll get a couple of boards from Ken and try that. It's not that I don't want to support CGS! I just like the challenge of stripboarding and would like to build my synth that way. I'll continue with this... I cant give up on this now!

-minus- · Joined: Oct 26, 2008 Posts: 718 Audio files: 13

Finally! After one more go at this, I have a working board! Very Happy

I had to basically cut a piece of stripboard EXACTLY in the pattern of my breadboard: 8 columns side by side of 5 hole strips, 20 strips deep. Then it was just a matter of replicating my working breadboard....

I'm a bit annoyed I couldn't figure out why the other ones wouldn't work, but I do have a working filter so I should be happy.

The sun is rising again and I'm fully tanked up on coffee.... so I might just push my luck and solder another one for good measure!

-minus- · Joined: Oct 26, 2008 Posts: 718 Audio files: 13

Just got a second filter working! Very Happy

Had a bit of trouble at first. Was not getting any sound in low pass mode.... BUT, I reheated the solder around that area and success!

See, this is all part of the curse... talking to oneself! Laughing

You have been warned!

otherunicorn · Joined: May 11, 2008 Posts: 135 Location: Australia

Talking to your self is one thing - expecting an answer is another!

Mongo1 · Joined: Aug 11, 2011 Posts: 392 Location: Raleigh NC

Well, it appears another woolly mammoth has wandered too close to the tar pit.

I'm stuck in Synthacon hell, believing with all my heart that these two miserable little transistors have everything they need to thrive, but no dice.

Symptoms- sorta filtery response on lowpass and highpass, but very low output. Insert a signal into BP, and its about 10 times louder. "Q" pot does absolutely nothing. Op Amp stage seems to function ok. CVs seem to do about what I'd expect.

I've put a 2K trimmer in place of the 2K2. I'm using the 3906 and 3904 transistors in the amplifier, and 2n2222s in the filter core. I've used trannies from these batches in other projects with the no problem, so I don't think it's the reverse pin thing. I did insert the 04 and 06 into the board backwards.

DC voltages look reasonable according to the diagram in this thread, but I'll take another look at that tomorrow.

Here's a couple of weird symptoms:
I put a signal in the LP input, I touch a scope probe to trace that connects the collector of the first tranny to the base of the other. When I do that any sound I have coming out stops, and stays off as long as I have the probe there.

Also - when I first turn the system on, I can hear something that sounds much more 'Q'y and but that fades in to the dull sound I normally have.

I just can't believe that a board this simple is causing me so much grief.

If anyone has any ideas I'd sure appreciate it.

Thanks
Gary

otherunicorn · Joined: May 11, 2008 Posts: 135 Location: Australia

The "fadey" business suggests an electrolytic is involved. Check that your 0 volt line isn't broken somewhere. I've had similar sorts of things happen in other modules with a broken 0V line.
You may have broken a track or lifted a pad on the PCB. Single sided boards do not like a lot of heat.

Ken

Mongo1 · Joined: Aug 11, 2011 Posts: 392 Location: Raleigh NC

Thanks - I'll check that....

Gary

Paradigm X · Joined: Feb 15, 2011 Posts: 252 Location: Null and void

roglok · Joined: Aug 28, 2010 Posts: 125 Location: uptown

Finally got my Synthacon stripboard working. I had to substitute the 2N2222A/2N2907 with 2N3904/2N3906 to get an active full range on the frequency pot. Filter sounds great!

But... I have one problem:

On the last quarter towards 15V end (filter open), the frequency pot becomes really noisy, as if there's dust on the carbon track. This happens only when actually moving the wiper. I'm using 16mm Alpha pots and have tried several of those with the same result.

Not sure what this issue related to... Are 15V too hot for my pots or is it something else?

otherunicorn · Joined: May 11, 2008 Posts: 135 Location: Australia

I've never heard of the frequency pot doing that. The resonance pot is known to do that in the earlier designs.
15V is not too hot for the pot. If you have built it as per the diagram, there is a low pass filter (220k and 1uF) that should prevent any such pot noise from getting through.

roglok · Joined: Aug 28, 2010 Posts: 125 Location: uptown

aaah, thanks ken! that was the hint i needed. i accidentially put a 33k instead of a 220k after the cv input. now the noise is gone! i only have to recalibrate the thing. Smile

stefanosdan · Joined: Jul 29, 2012 Posts: 1 Location: Athens Greece

chguigoz · Joined: Mar 02, 2012 Posts: 13 Location: lille france

Hi!
I have the same problem of hum. My eurorack have two power supply, one well filtered and the other not. The hum disapperead when my filter is connected to the good one. Check your power supply and be careful to avoid ground loop, this filter is very sensible.

MuBase

Just had a careful look at this forum post on Ken stone's Steiner VCF. After having one for 2 years now to put inside my WSG Redux I haven't been able to get the resonance control doing anything. It was changing the gain slighty but that is all.
I had another look today following the idea there might be a bad trace on the PCB. There was!
In between the middle resonance pot connection and the 10uF capacitor's - terminal . after having connected the cap's - terminal straight up to the pot connection I got a very lovely resonance. Smile

I am using a 2.2K resistor for the 'q' which barely rings but I might try a lower value for oscillation. absolutely beautiful filter. If I plug 3 of the WSG oscillators into the three different filter inputs (LP,HP,BP) I get some very nice panning effects when the cutoff is VC'd by the LFO...
If you want to see the WSG redux in action theres a short vid here:
http://www.youtube.com/watch?v=z22abVAWuSw
but it was filmed before the Steiner VCF fix..
All the best . Razz

Steve.