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VCF matched transistor pair workings
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VA1



Joined: Aug 20, 2018
Posts: 73
Location: Nederland

PostPosted: Wed Apr 10, 2019 5:01 am    Post subject: VCF matched transistor pair workings
Subject description: VCF matched transistor workings
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Hi, i build many transistor/diode ladder filters,
each time it will cost me 4 matched NPN transistor pairs :

For the exponential converter :
This is important for the keyfollow so the notes wont sound false.

Then the input/reso input pair :
Why is this important ?

Then the top of the ladder :
Why is this important ?

Then the differential amplifier pair :
Why is this important ?

Are these pairs to level the reso to exact selfresonating for example ?,
or how does it change sound wise ?

I measure these pairs, it cost a lot of time, so i like to eliminate the not so important ones that dont change the sound, only the volume.

Can you really hear a difference ?
thanks
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AlanP



Joined: Mar 11, 2014
Posts: 575
Location: New Zealand
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PostPosted: Wed Apr 10, 2019 9:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

I heard somewhere that matching the capacitors in the ladder is more important.

I socketed all the transistors in my first couple of Minimoog ladder VCFs. First go around, I just put random transistors in every place, out of sheer laziness.

Then I spent a couple hours measuring a bunch of transistors, and replaced the lot with matched pairs.

Couldn't hear that much of a difference at all.
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Grumble



Joined: Nov 23, 2015
Posts: 956
Location: Netherlands
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PostPosted: Wed Apr 10, 2019 10:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think that is because the fabrication process is so refined and controlled that transistors from the same batch barely have any differences, especially when compared with transistors fabricated in the 70’s
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ixtern



Joined: Jun 25, 2018
Posts: 76
Location: Poland

PostPosted: Thu Apr 11, 2019 12:46 am    Post subject: Reply with quote  Mark this post and the followings unread

There is a document: "Analysis of the Moog Transistor Ladder and Derivative Filters" by Timothy E. Stinchcombe on:
http://www.timstinchcombe.co.uk/synth/Moog_ladder_tf.pdf
but is packed with math. Some decades ago I could do such analysis but not today Smile
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VA1



Joined: Aug 20, 2018
Posts: 73
Location: Nederland

PostPosted: Thu Apr 11, 2019 7:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Its a good document, i seen it before.
No farmer knowledge, i just look at the pictures.

When measuring transistors you could find the oddest pair to test in your sockets.
Or taking 2 different transistors.
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