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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Steiner Synthacon VCF
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MirlitronOne



Joined: Nov 07, 2009
Posts: 77
Location: Surrey, UK
Audio files: 2

PostPosted: Mon Jun 14, 2010 3:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, I'm not usually a quitter by nature, but I've had it with this build. I now understand how the Q1/Q2 phase splitter works; I've built it on a breadboard and it behaves perfectly. On my filter board, I've changed both transistors; I've removed the PNP transistor as the only other component that is directly DC-coupled to the phase splitter; and still it doesn't work. The voltage difference across the diodes never gets positive enough to forward bias them.

Ah well, should've built the PCB in the first place, but now I've wasted too much time on this unit. Maybe the problem will be obvious in a year or two.

Thanks for the help.

dunno

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otherunicorn



Joined: May 11, 2008
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PostPosted: Mon Jun 14, 2010 5:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

It could be the Vero board itself - a stray whisker of copper left from cutting/drilling, or even the original etching.
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MirlitronOne



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PostPosted: Tue Jun 15, 2010 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Yup, could be.
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MirlitronOne



Joined: Nov 07, 2009
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PostPosted: Thu Jul 01, 2010 3:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

MirlitronOne wrote:
Maybe the problem will be obvious in a year or two.
dunno


...or maybe only a week or two. Very Happy Very Happy Very Happy Very Happy Very Happy

Like all the best Disney films, this story has a happy ending. It's amazing how when the pressure comes off at work, the brain can operate again. Last week, it occurred to me what a daft situation I was in. Thanks to Ken's help I now understand exactly how the circuit works. If I know how it works, I can fix it.

I decided to build it again in stages, testing it step-by-step.

As a result, after I got to the feedback system (Q3 and Q4), I found the fault. Yes, I admit, no track break where one was needed. I went back to the original build, cut the track, and I have a fully working Synthacon filter. Better still, I also have a half-built board for a second one. Best of all, I can now type those magic words, "what a great filter!".

The idea of having separate filter inputs instead of outputs is genius - the sounds that you can get by 'mixing' the inputs are fantastic.

Thanks again, Ken.

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otherunicorn



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PostPosted: Thu Jul 01, 2010 6:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

yay! good to know.
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MirlitronOne



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PostPosted: Tue Jul 06, 2010 12:50 am    Post subject: YMMV Reply with quote  Mark this post and the followings unread

As I stated above, I troubleshot (troubleshooted?) my original Synthacon build by building another and checking operation of the various parts stage-by-stage. Last night I completed the second build and ended up with a working, but completely different, filter.

This one oscillates! The first one doesn't. Furthermore, the oscillation isn't the nasty squawking that other people have reported, but instead a nice, smooth, stable sine wave. Very nice. Curious thing is, there is only one component change from the first build - instead of a BC557B transistor as the PNP device, I instead subbed a BC327 which demonstrated a significantly LOWER gain than the BC557 in the original. Sometimes, component tolerances and changes can have a much bigger effect than one might normally expect.

Still, this is cool, and fits perfectly with my plans - not a bland modules-off-the-shelf build, but instead a quirky, full-of-surprises DIY build. Hurrah for DIY!

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otherunicorn



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PostPosted: Tue Jul 06, 2010 2:25 am    Post subject: Reply with quote  Mark this post and the followings unread

I will sometimes reduce one of the 2k2 resistors in order to get oscillation. It is mentioned in the Euro version. It however, is usually a rude beast when oscillation starts. Lately I've been using 2N3906 and 2N3904 which supposedly have lower gain. As you say, this is a circuit that is rather sensitive to tolerance and component variations.
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MirlitronOne



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PostPosted: Tue Jul 06, 2010 1:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

"As you say, this is a circuit that is rather sensitive to tolerance and component variations."

...and all the better for it!

I knew of Nyle Steiner before I got into synth building - he has a great website of bizarre experiments - and this filter fits perfectly with the image he projects.

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arrmcbain



Joined: Dec 13, 2010
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PostPosted: Tue Dec 14, 2010 4:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi, I seem to have a problem with my build and have tried everything I can to fix it, without success..

The issue is that both the frequency pot and the cv in do absolutely nothing. A saw wave into each input gets the expected result - lp in is definitely passing low freq signal and cutting the highs; bp gives midrange; hp in attenuates low frequencies nicely - but there is no modulation with cv! Also, I initially thought that the res pot was doing the right thing but on reading Ken Stone's info again it seems that it's supposed to boost signal - mine is attenuating it slightly as I turn up the the pot.

I checked the dc voltages per Ken's previous post and the points I can find easily seem within range - although my testing was less than comprehensive - are there critical voltages I can check to see what the problem might be?

I have checked every solder joint and reflowed a few. I have tried replacing the IC (twice). Shorting the +15 to either cv pad does nothing. I turned the trim pot all the way both ways - nothing. I removed the trim pot - no change (does the trim pot need to be there for the circuit to work - as far a I can see it doesn't but I am relatively new to this so..?)

My next plan is to replace all the transistors. Any other suggestions would be greatly appreciated.
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otherunicorn



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PostPosted: Tue Dec 14, 2010 5:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Check all your diodes are facing the right way. Adjusting the CV will make the collector of one of the two NPNs at the ends of the diode strings rise, while the collector of the other will fall.
There WILL be a construction error somewhere. It is very rare for there to be component failure.

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arrmcbain



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PostPosted: Tue Dec 14, 2010 5:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Many thanks, Ken, for the quick reply. I don't doubt that I've made a mistake - just having trouble finding it..

The diodes look right. *edit* yep., past my bedime- I measured the emitters instead of the collectors.. Will check collectors tomorrow..* The emitter on the left transistor changes from 7.43V to 7.11V and the one on the right goes from 5.43 to 5.65. Is this a smaller than expected range?
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otherunicorn



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PostPosted: Tue Dec 14, 2010 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

The emitters are tied together, so if you really were getting those voltages, I'd be worried. You probably did measure the collectors. I would expect to see a greater difference from one side to the other. If you don't get current flowing through the diode chain, nothing will happen.
If you are running on 12 volts, did you omit the required number of diodes?

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arrmcbain



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PostPosted: Tue Dec 14, 2010 2:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Learning all the time Smile
I'll have to double check what I actually measured.. is there a difference between 2222 and 2222A? The transistors I have are KSP2222A. Is there a chance they are in backwards? I'll read up on it and check the board when I get home - I know I just put them in as per the overlay.
I'm running a 15V setup.
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arrmcbain



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PostPosted: Wed Dec 15, 2010 12:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, so I checked the orientation of the transistors and confirmed that I was measuring the collectors - so they aren't changing as expected with cv. Still behaving the same as the measurements from my above post.

I double checked that the cv from the freq pot is going into the board and is modulated from 0 to +15V as I turn up the pot.

Does this point to a wrong resistor somewhere? I checked them all with a DMM before placing them, but if the symptoms point to that..
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otherunicorn



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PostPosted: Wed Dec 15, 2010 12:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Increase the value of the 1k that is connected between 0V and the base of the first NPN. Go to 4k7. Also remove the 1uF across it.
If that still doesn't give you any voltage control, start looking elsewhere.

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arrmcbain



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PostPosted: Wed Dec 15, 2010 1:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Done, and no change - still the same voltages as before at the collectors, and no audible change in frequency as I turn the pot.

I have to say that your help is much appreciated. I am obviously out of my depth here.
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otherunicorn



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PostPosted: Wed Dec 15, 2010 3:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Upload photos of both sides of the board if you can. I'll try to see if anything looks odd.
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arrmcbain



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PostPosted: Wed Dec 15, 2010 3:44 am    Post subject: Reply with quote  Mark this post and the followings unread

A minor breakthrough.. on a whim I swapped 2222A on the left of the diode chain to a bc547, and lo and behold, the frequency cv works! Although all is not right - the lp in sounds a lot like a bp filter and the bp gives a fairly convincing impression of a low pass filter. The hp in sounds right. And yes, the jacks are connected to the correct pads.

Also, the resonance pot attenuates the signal as I turn up the pot and there is no evidence of ringing/self oscillation at any point, even driven with a pretty powerful saw wave.

Intriguing? or just plain wrong? Confused
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arrmcbain



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PostPosted: Wed Dec 15, 2010 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes well I know it looks like a pretty bad job, and it is (my first build since I was about 14.. since this build I have successfully put together about half a dozen modules) - the resistors are in either way (I didn't realise the convention of keeping them left to right) and I was probably a bit heavy on the solder.
Also, one of the pads lifted when I gave it too much heat so there is a blobby direct solder link between the leads of a resistor and a diode. Embarassed The multimeter says it's ok, and I have gone over the rest of the board thoroughly to check for bad joints or bridges. None found.
Also, since I wired it from the top it might be a bit hard to see all the components. Anyway..
Also note that I have taken out the trim pot and made the changes you suggested earlier.


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otherunicorn



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PostPosted: Wed Dec 15, 2010 12:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is a limit to what I can tell with these photos, but considering that, I can't see any problems. If you want to, you could substitute 2N3904 and 2N3906 for the transistors, though they would need to be installed backwards to the ones you have there (though I would check the transistors in a meter before installing , because there are some that have reversed pinouts).
If you have a friend that knows anything about electronics, have them give it a look-over. They may see something that you miss every time you look.

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arrmcbain



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PostPosted: Thu Dec 16, 2010 1:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
you could substitute 2N3904 and 2N3906 for the transistors


Did that, and.. Great Success!!

Sounds just like it should! Very nice! Just needs a minor mod as you have previously suggested, to get oscillation with the new transistors.

So I guess there was something up with the 2222As I originally had in there..? Whatever the case, I'm just glad it's working now!

Thanks again for all your help Ken! cheers
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arrmcbain



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PostPosted: Thu Dec 16, 2010 4:05 am    Post subject: Reply with quote  Mark this post and the followings unread

and just to follow-up on the resonance issue..
.. an additional resistor across the 2k2 connected to the emitter of the npn in the resonance circuit did the trick (as recommended by Ken).
I used 8k2 (I don't have any of the recommended 15k lying around).
*edit* the range from the resonance pot is pretty spot-on, just self-oscillating when turned up full.
A bit harsh and squealy when self-oscillating, like many report, but sounds great just shy of that point Smile

It has been said before, and I'll happily corroborate.. awesome filter!
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otherunicorn



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PostPosted: Thu Dec 16, 2010 7:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great stuff Smile Glad to have been of assistance.
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-minus-



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PostPosted: Wed Jul 20, 2011 6:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've just done a 24 hour straight shift with this filter! Shocked Laughing

For a while there I thought I had the Synthacon Curse too. My hairline may have receded further than it already has, but after breadboarding this 5 times, it is working!!!

I'm going to continue with more transistor experiments and I shall report further findings... but what finally got this going was changing the transistor on the far left from a 2N2222 to a bc547 as arrmcbain mentioned. It was an act of sheer desperation! I also tried a resonance pot with no cap across it as I saw in a previous version of Ken's. This is what is on the breadboard as I type. I have the trimmer in there too.

I just tried pulling these two jumpers from the breadboard which I was using to reduce the diode string to 6. I am running this all off 12V and it seems to work fine with the 8 diodes.

The PNP is a 2N3906 as it was all I had on hand after my order was stuffed up last week.... but it sounds pretty good to me! Very Happy

A really wicked filter this one! Twisted Evil

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-minus-



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PostPosted: Mon Aug 15, 2011 11:26 am    Post subject: Reply with quote  Mark this post and the followings unread

hmmm... yes, it appears the curse is real. I stripboarded this and had no luck. I then out of frustration tried redesigning my stripboard layout without trying to be smart with cramming components into as little space as possible... and still no luck. I'm not sure where I have gone wrong. I'm going over my diagram and checking my board. The curse is not so much the filter not working. It is refusing to admit defeat. I cant get much else in my life done until I sort this out. It is strange that I have had great results on the breadboard, but complete failure when I attempt to solder it. I am now faced with having to mount the breadboard behind a panel Embarassed . THAT is exactly what the Steiner curse is all about!

EDIT: Still cursed! That's 3 different stripboard layouts not working. I'm doing something wrong in all 3.... at least I am consistent! It's weird. I have this working fine on the breadboard. I may just have to cut a piece of stripboard to look EXACTLY like the breadboard, and solder EXACTLY as I have placed components in the breadboard. Getting a bit desperate here! I guess once I work this out and fix up the failures, I'll have several of these filters!
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