help calibrating my ASM-1

[Magnus Danielson]

From: Hess Hodge <hess at surrealistic.net>
Subject: help calibrating my ASM-1
Date: Tue, 15 Aug 2000 13:13:34 -0700 (PDT)

> Hey, remember me?

Oh yes, you are the sticky guy! ;D

>  I just finished building my ASM-1...  Now I need help
> in calibrating it...  I thought for sure that I'd be able to find
> references to this procedure in the archives, but to no avail...  So, does
> anyone have any helpful hints they could give me?  The things I am having
> trouble with are:
> 
> VCO : v/oct
> VCF : v/oct and offset trimmers

In both Gene's material as well as in the ASM-1 homepage at
http://home.swipnet.se/cfmd/synths/friends/stopp/

You find a description on how to do this. However, it is a bit sparse I see and
the comment:
"Precise instructions for this are beyond the scope of this document."

does not really help.

> Now, I understand the concept behind v/oct calibration.  The specific
> procedure is what I am looking for.  I have been trying to calibrate the
> VCOs by using the frequency counter on my Fluke, but I have not had much
> luck.  Should I be listening for a beat frequency between the two
> oscillators to do the calibration?  Oh, and I'm pretty much clueless on
> how to calibrate the VCF...   ;)

Fluke's are nice, but the frequency counter is not very usefull for this I'd
say, you are probably better of tuning it by ear to a synth playing a sine or
something. The you should listen to the beat frequency to that reference
rather than between the oscillators. You will tweak away the oscillators
against each other anyway, but you want the scale to be right.

The trouble with the ASM-1 VCO tuning is that the non-modulated frequency of
this oscillator is about 909 Hz, so you will have to start of tweaking it to
880 Hz, but as you are starting to tune the scale the offset trimming will
break. I have a proposed mod for this. Anyway, here is how you should trim your
ASM-1:

0. You have an init trimpot and a separate pot for which you are able to tweak
   the offset voltage.
1. Have your separate pot disconnected for now.
2. Set init freq so that the V/oct sum op-amp output reaches 0 V. You should
   now have a 900 Hz sawtooth available.
3. Set init freq so that the oscillator hits A5 (880 Hz).
4. Set your separate pot to +1 V.
5. Connect the separate pot. Tweak the Scale trimpot until you have no beating
   at A6 (1760 Hz).
6. Disconnect the separate pot. Tweak the Init Freq against A5 for no beating.
7. Goto 5 until both step 5 and 6 is passed without trimming.
8. Set Init Freq so that the oscillator has no beating against whatever tone
   you selects to be 0V. This last step is an option if you want a specific
   pitch offset.

Now, if you stick in a trim-pot in series with the 150k resistor (R15 on the
web schematics), a 5 or 10 k pot should be fine, the procedure would be much
simpler:

0. You have an init trimpot and a separate pot for which you are able to tweak
   the offset voltage.
1. Have your separate pot disconnected for now.
2. Set Init Freq so that the V/oct sum op-amp output reaches 0 B. You should
   now have a 900 Hz sawtooth available.
3. Set the ref-pot (the added pot from above) so that the oscillator hits A5
   (880 Hz) with no beating.
4. Set your separate pot to +1 V.
5. Connect the separate pot. Tweak the Scale against A6 for no beating at A6
   (1760 Hz).
6. Disconnect your separate pot and set Init Freq so that the oscillator has no
   beating against whatever tone you selects to be 0V. This last step is an
   option if you want a specific pitch offset.

Both the description and the practice makes the modded version much simpler and
easier to use.

Your Fluke will come to good use in measuring the voltages with accuracy.

For the VCF you should run a common voltage to both the VCO and VCF and feed
the VCO to the VCF. Then, listen to the notch output and set the scale so that
for both A5 and A6 (using the 1 V pot from above) you get the maximum reduction
of the fundamental.

Unfortunatly there is no Init Freq pot for the filter.

Cheers,
Magnus