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 Forum index » DIY Hardware and Software » YuSynth
Minimoog VCF (tracking problem)!
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otherdreams



Joined: Mar 19, 2009
Posts: 3
Location: Virginia, USA

PostPosted: Fri Mar 20, 2009 7:15 am    Post subject: Minimoog VCF (tracking problem)! Reply with quote  Mark this post and the followings unread

Hello!

I've recently built my first Yusynth module- the Minimoog VCF, which sounds beautiful! But for some reason I cannot get it to track linearly.

I've checked the CV from the keyboard and its output is perfect. I also measured the voltages at the base of Q1, which perfectly match the tuning details on the Yusynth project page (http://yusynth.net/Modular/EN/MOOGVCF/index.html).

Instead of a linear frequency response, the intervals are micro-tonal on the bass end of the keyboard, almost right near the center of the keyboard, and macro-tonal (wide intervals) on the treble end.


Looking at the schematics, does anyone see a mistake that could potentially be causing this?


Thank you!

-robert
[ quietchannel.blogspot.com ]
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Fri Mar 20, 2009 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

A question : the instruction on my pages are guidelines, for example the values of voltage that are given are to be used as STARTING point for adjusting the V/Octave tracking, if you set the trimmer to the value indicated and leave it as that, your V/oct won't be perfect. From this starting point you have to adjust the trimmer until you get the v/oct tracking. Once done you may achieve a quite good tracking over at least 4 octaves and more.
If this not the case you may think of changing Q1...

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Yves
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otherdreams



Joined: Mar 19, 2009
Posts: 3
Location: Virginia, USA

PostPosted: Fri Mar 20, 2009 7:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

The transistor I purchased was the 'bc557', not '557a', '557b', or '557c'.

Should it be the 'bc557b'?

I purchased this exact part from Mouser-
http://mouser.com/Search/ProductDetail.aspx?R=BC557virtualkey51210000virtualkey512-BC557


No matter how I adjust T1, the intervals are still smaller on the low-end of the keyboard than on the high end.

Could this transistor be the cause of the non-linearity?
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Sat Mar 21, 2009 12:40 am    Post subject: Reply with quote  Mark this post and the followings unread

On those I built I used a BC557B.
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Yves
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Sebo



Joined: Apr 27, 2007
Posts: 525
Location: Argentina

PostPosted: Sat Mar 21, 2009 4:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi:
I can't get more of 2 octaves out of mine.
Also the resonance decreases in the low frequency range, and stops oscillating
below 100 Hz (I mentioned this problem earlier).
In the previous Minimoog VCF I built, I didn't check the tracking, but the
resonance behaves the same, except that stops oscillating at a lower frequency
(about 60 Hz).
I used to blame bad matched transistors for those problems, but now I think
the problem is the CA3046, I read that the differential pair is poorly matched
in CA3046s made in Thailand, and that affect the self oscillation.
I have a 3046 with a different printing, I will try to swap it to see if things
gets better (or worse).

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---------------------------------------
My Music:
www.cosaquitosenglobo.com.ar
www.myspace.com/cosaquitosenglobo
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Sat Mar 21, 2009 5:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sebo wrote:

below 100 Hz (I mentioned this problem earlier).
In the previous Minimoog VCF I built, I didn't check the tracking, but the
resonance behaves the same, except that stops oscillating at a lower frequency
(about 60 Hz).

that's what one can expect, in a genuine minimoog the filter hardly oscillates under 100Hz, this is mainly due to the original design that uses two 220n caps to connect the upper stage of the ladder to the differential discrete OPA. If you wish to improve the resonance (and beef-up bass too) toward the lower range (but you must realize that this won't be a Minmoog filter clone anymore !) you may increase the value of these two capacitors up to 1uF (C9 and C10). you may also increase the value of the two electrolytic caps (C3 and C11) to 22 or 47uF.

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Yves
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Sebo



Joined: Apr 27, 2007
Posts: 525
Location: Argentina

PostPosted: Sun Mar 22, 2009 6:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Yves:
That's great info!!!
I didn't use a real Minimoog, I was comparing it to my Moogerfooger VCF.
I didn't have the time to play much with my modular, but I think I like the
filter this way. May be I will build another one, moded to oscillate in the sub 100 Hz
range, in the future.
Thanks.

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My Music:
www.cosaquitosenglobo.com.ar
www.myspace.com/cosaquitosenglobo
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otherdreams



Joined: Mar 19, 2009
Posts: 3
Location: Virginia, USA

PostPosted: Wed Apr 22, 2009 9:47 pm    Post subject: Fixed! Reply with quote  Mark this post and the followings unread

It turns out that the previous owner of my synth had replaced the linear CV output from my synthesizer with the exponential output! That really explains it!

Anyway, the Minimoog VCF works perfectly now. Except that I can't find a 24mm 50k reverse log potentiometer. Is it possible to make a rev log pot with a tapering resistor? or somehow?
Please let me know if there's any way.


Also, the VCF only tracks linearly when the CV input is less than about 7 or 8 volts.. Unfortunately I'll need to build a little offset, as my EML101 outputs 4.48-8.78V (1.2V/Oct). With the offset the output should be 0-4.38V.

By the way, is there anything that can allow for a wider range of effect from the Cutoff control? As it is now, I can get 0Hz at the minimum of pot rotation.. but I need external CV to reach the highest audible cutoff frequencies.

Thanks for your help!

-Robert
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Thu Apr 23, 2009 4:41 am    Post subject: Re: Fixed! Reply with quote  Mark this post and the followings unread

otherdreams wrote:
It turns out that the previous owner of my synth had replaced the linear CV output from my synthesizer with the exponential output! That really explains it!


Sure it does !

otherdreams wrote:
Anyway, the Minimoog VCF works perfectly now. Except that I can't find a 24mm 50k reverse log potentiometer. Is it possible to make a rev log pot with a tapering resistor? or somehow?
Please let me know if there's any way.

Unfortunately no. Tapping trick works fine for creating log response not for the rev log response. But REV LOG pots are not that hard to find nowadays,
you can find them at banzaieffects or musikding :
http://www.banzaieffects.com/Alpha-24mm-US-50k-REV-log-pr-30840.html

http://www.musikding.de/forums.html/cat/c100_rev-log-pots.html


otherdreams wrote:
By the way, is there anything that can allow for a wider range of effect from the Cutoff control? As it is now, I can get 0Hz at the minimum of pot rotation.. but I need external CV to reach the highest audible cutoff frequencies.

Thanks for your help!

-Robert
Change (reduce) the value of the input resistor for the CV pot
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Yves
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Thu Jun 04, 2009 12:25 pm    Post subject: Re: Fixed! Reply with quote  Mark this post and the followings unread

yusynth wrote:
otherdreams wrote:
Anyway, the Minimoog VCF works perfectly now. Except that I can't find a 24mm 50k reverse log potentiometer. Is it possible to make a rev log pot with a tapering resistor? or somehow?
Please let me know if there's any way.

Unfortunately no. Tapping trick works fine for creating log response not for the rev log response. But REV LOG pots are not that hard to find nowadays,
you can find them at banzaieffects or musikding :
http://www.banzaieffects.com/Alpha-24mm-US-50k-REV-log-pr-30840.html

http://www.musikding.de/forums.html/cat/c100_rev-log-pots.html


Unfortunately I can't find a reverse tapered 50K pot with a knurled shaft. This article says that it is possible to make a reverse log pot. Why do you think it's not possible? I'm not doubting your electronic expertise! I just want to know if it's because of something particular to your circuit....

Thanks a lot!
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Thu Jun 04, 2009 1:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

This article is right but using taper resistor works only for a voltage divider circuit not for a serial variable resistor as in the moog circuit. If you try it you will notice that you'll get a log response not an antilog response when turning the pot clockwise. Should this work for a Moog filter it would have been used for decades and that's not the case ...
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Thu Jun 04, 2009 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Aha, of course! I knew there had to be a easy explanation for it!

Do rev log 50K pots with knurled shafts exist at all?
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yusynth



Joined: Nov 24, 2005
Posts: 1214
Location: France

PostPosted: Sun Jun 07, 2009 1:15 am    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
Do rev log 50K pots with knurled shafts exist at all?

Yes, here :

http://www.banzaieffects.com/CTS-Pot-50k-REVERSE-log-pr-17245.html
Most commonly used in American amps and guitars.
Smooth action - top quality!
1/2 watt, 3/8" mounting, 1/4" knurled plastic shaft.
Details
Product ID: SKU17245
Weight 12.78 g
Price: €2.86
including VAT 19% (€0.46)
[plus shipping]


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Yves
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Sun Jun 07, 2009 2:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Excellent, thanks very much! I thought I'd scoured Banzai Effects from top to bottom, but there you go!
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