[sdiy] summing amp v/oct trim

[Happy Harry]

Hi Merv


>From: "THOMAS, Mervyn" <mervyn.thomas at astrium-space.com>
>To: "'Synth DIY List'" <synth-diy at dropmix.xs4all.nl>
>Subject: [sdiy] summing amp v/oct trim
>Date: Thu, 18 Oct 2001 13:44:49 +0100
>
>OK Obe-Wans, help me understand!
>
>I've just got my head round how a summing amp works (with a little help 
>from
>Walt Jung's fine book) but now I'm confused over v/oct trimming.
>
>In the ASM-1 vco there's a trim pot in series with the feedback resistor of
>the summing amp. This is tweaked to get 18mV/V to the base of the first 
>expo
>tranny.

You can do it this way... Remember in this case that the control voltages 
are going to be inverted by the summing amplifier. In some cases this may 
mean you need an additional inverting amp after the
summing amp... with extra components, errors, etc.

>
>In other circuits I've seen a divider to ground on the output of the 
>summing
>amp feeding the expo.

or this way...

Another way to go. Often the choice is made by how the temperature
compensation will be done. You can compensate in the feedback loop of
the summing amp , or at the base of the expo tranny. The first way uses 
(crosses fingers) a negative temperature coefficient sensor...the
second uses a positive temperature coefficient sensor.

Designer's choice.

>
>and in other circuits I've seen the summing done without an op amp at all
>and a trimmer/resistor between the summing point and ground with the pot
>wiper feeding into the expo.

This way has a caveat. If you are going to connect and disconnect
inputs to the summing resistors (like in a patchable modular...)
when you make or break the connection, there will be a slight voltage
shift at the expo tranny base... so the pitch will change.

This method can be used if the source imprdance never changes... as
in prepatched syntheizers.

There are some benefits for each method. No one way is ideal. I'd use
whatever method the designer recommended.


>
>Question is: what's the difference (if any) in the methods?
>
>Answers in words of one syllable please, I'm still learning...

This is the hardest question I've ever been asked in words of one
syllable. Its hard to understand even if you know why things are being
done.

Design is a series of trade-offs between conflicting problems. A good
designer gets the most bang for the buck by clever use of circuits.

H^) hary

>
>Regards,
>
>
>Merv Thomas
>UNIX CAE SysAdmin - Portsmouth
>*Email:	mervyn.thomas at astrium-space.com
>*Tel.:	 +(0)23 9270 5394
>  * Fax.:	 +(0)23 9270 8663
>
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