beginner questions

[Magnus Danielson]

From: michael <michrice at earthlink.net>
Subject: beginner questions
Date: Fri, 28 Jan 2000 19:30:46 -0500

> i have a few questions relating to designs on Tom's Synth Cookbook site (
> www.geocities.com/efm_gc/cbook/cookbook.html ), and since the email link
> there doesn't work i'm hoping that someone here can help me......

Nice place yes...

> first, what are the trimmers on many of the designs for?

Trimming the properties to stable and sound defaults. Please find specific
comments below.

> ie the balanced ring-modulator 
> www.geocities.com/efm_gc/cbook/pics/brm1a_pc.gif

Check the schematic:
http://www.geocities.com/efm_gc/cbook/pics/brm1a_sd.gif

Where you have one trimmer for compensate for carrier and modulator DC offset
errors. Those errors would result in leakage of the other waveform and this is
an non-ideal behaviour.

> and the ms-20 filter
> www.geocities.com/efm_gc/cbook/pics/vcf8b_pc.gif

Yeat again, check the schematic:
http://www.geocities.com/efm_gc/cbook/pics/vcf8b_sd.gif

Here the trimmers set the scale offset correctly for the filter sections.

So, for the ring-modulator I would say that the trimmers should really be in
there, or you would not feel too happy. For the MS-20 filter you could skip
the trimmers, but I would advice against that. A trimmer is a cheap way of
getting a better known property, if operated properly.

Some people feel that trimmers are a waste, I can not agree with this opinion.

Some people feel that trimmers are not as good as designing you curcuit
correctly. This has a truthness to it. However, the better preformance you
would like to have, and the better repeatability and lifetime eqaulness you are
looking for, some fashion of calibration of properties to adapt to the current
state of the device is a good thing. In production trimmers cost most money in
the time spent for actually trimming them, but for some stuff you can't get
away with less than a trimmer. More advanced instruments trim themselfs up as
part of their warmup sequence, this is done through A/Ds and D/As.

A common misstake with trimmers is to set them up so that they span a too large
trimming range. Occasionally one also see them given a too small range. This
will make it hard (or even impossible) to trim something to specs. Ensuring
the propper feasable range for a trimmer will make the actual trimming process
easier and less of an hair-pulling experience.

So, my advice is to use trimmers wisely and to actually use them for trimming
up the property they should compensate.

> second, would there be any easy way to run the designs here from 15V?

You are refering to runing them on only +15V as compared to dual supply with
-15V and +15V. You can do it, but I would not recommend it.

> third, my past experience etching circuit boards didn't turn out so well,
> but i've seen "professional" systems with heaters for the etchant. is this
> worth the money? any advice about this?

This is certainly a good step upwards.

> i know this is alot to answer, but if someone could take the time i'd
> appreciate it!

This was no sweat!

Cheers,
Magnus