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Question about Buchla Timbre circuit
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Tenine



Joined: Sep 13, 2006
Posts: 44
Location: Coventry

PostPosted: Mon Feb 28, 2011 12:34 pm    Post subject: Question about Buchla Timbre circuit Reply with quote  Mark this post and the followings unread

Hi all!

Just curious...
Looking at the schematic on these pages:
http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/
http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/mea_timbre_schem.png

What exactly does everything to the left of (or, in terms of signal path, before) Pin 5 of IC1B do?
I can see the two pots on the left generate CV / attenuate CV input,
but I'm confused what the PNP transistor does and especially what the JFET does. Do they form some sort of Voltage-Controlled Attenuator on the input of the following circuitry?
Could this be used on other circuits perhaps?

Cheers!
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TekniK



Joined: Aug 10, 2008
Posts: 1059

PostPosted: Wed Mar 02, 2011 4:25 am    Post subject: Reply with quote  Mark this post and the followings unread

http://electro-music.com/forum/topic-21752.html

http://electro-music.com/forum/viewtopic.php?highlight=buchla&t=40614


I did ask the admin if we could get a Buchla DiY subforum as manny topics are started with questions or projects concerning buchla circuitry.

I hope they take positive decision so we don't have to use the search function each time.
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JovianPyx



Joined: Nov 20, 2007
Posts: 1693
Location: West Red Spot, Jupiter
Audio files: 194

PostPosted: Thu Mar 03, 2011 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Professor Aaron Lanterman is on the Synth-DIY email forum and he is an awesome nice guy, very helpful. You might want to subscribe to the list and ask there, he has studied Buchla quite a bit and is very knowledgable.

And that bit of circuitry is a interestingly odd! Buchla is "famous" for that sort of thing. The weird FET thing is just, well, weird.

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abovenyquist



Joined: Aug 31, 2009
Posts: 21
Location: Atlanta

PostPosted: Wed Apr 06, 2016 12:28 am    Post subject: Re: Question about Buchla Timbre circuit Reply with quote  Mark this post and the followings unread

Tenine wrote:
Hi all!

Just curious...
Looking at the schematic on these pages:
http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/
http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/mea_timbre_schem.png

What exactly does everything to the left of (or, in terms of signal path, before) Pin 5 of IC1B do?
I can see the two pots on the left generate CV / attenuate CV input,
but I'm confused what the PNP transistor does and especially what the JFET does. Do they form some sort of Voltage-Controlled Attenuator on the input of the following circuitry?
Could this be used on other circuits perhaps?


Ooops, sorry... be sure you are looking at the latest version:

http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/mea_timbre_rev1_schem.png

Yeah, the FET is operating like a voltage-controlled resistor. Think about it like this:

R17 (68K) and R19 (33K) form a voltage divider that cuts down the audio input a bit. Assume for a moment that C2 is shorted (it's just a DC blocking cap) and the JFET isn't there. IC1B is acting like a non-inverting amplifier, with a gain of (1 + R20/R21) = (1 + 33K/330K). Now, putting the FET in parallel with R21 means that as the FET starts conducting, as controlled by the voltage put on the gate through R22, that 330K in the formula decreases and the gain goes up.

R18 is feeds some of the audio signal into the gate as part of a trick to help linearize the behavior of the FET. You can read about it here:

http://www.vishay.com/docs/70598/70598.pdf (see Figure 6)

This trick of using a JFET as a voltage-controlled resistor shows up a lot. Compressor like the 1176 use a voltage divider with the JFET as the "lower leg" of the divider (the "shunt" part of the divider). Using the JFET in the way it is in the Buchla circuit is less common, I think. The only compressor I know of that uses a JFET that way is the API 525:

http://danalexanderaudio.com/ApiInfo/525Cschem.jpg

- Aaron
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thermionicjunky



Joined: Dec 07, 2006
Posts: 90
Location: san francisco

PostPosted: Wed Apr 06, 2016 7:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Since this has been resurrected, I'd like to suggest a correction. Dave Brown pointed out on his build page that R19 appears to be 3k3 rather than 33k in the original schematic. I agree with him. It is much easier to get the gain structure correct with more severe attenuation at this node. It could even be more severe, considering how hot most VCO outputs are compared to Buchla's.
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thermionicjunky



Joined: Dec 07, 2006
Posts: 90
Location: san francisco

PostPosted: Sun Apr 24, 2016 8:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Another builder, with the aid of a circuit simulator, discovered that the gate-source cutoff voltage of the J201 does not match the original 2n4341 well enough. The J112 is closer, and will provide a better parameter range.
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jurekprzezdziecki



Joined: Mar 22, 2016
Posts: 63
Location: warsaw

PostPosted: Tue Sep 13, 2016 6:32 am    Post subject: Re: Question about Buchla Timbre circuit Reply with quote  Mark this post and the followings unread

abovenyquist wrote:


Ooops, sorry... be sure you are looking at the latest version:

http://users.ece.gatech.edu/~lanterma/buchla/boards/mea_timbre/mea_timbre_rev1_schem.png

- Aaron


Thanks for this great circuit. So the REV 1 is the latest schematics version including all those errors mentioned on your site? Or it's already fixed?

Thanks Jurek
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