[sdiy] ASM-1 VCO

[Gene Stopp]

I know what it could be - the PWM summing amp has a 100K feedback resistor
and my original schematics should have shown a 100K input resistor for the
PWM Imput (-5 to +5 gives 0% to 100%) and 300K for the PW Pot (-15 to +15
gives 0% to 100%). I looked at my schematic just now and both inputs are
100K (sorry). If you hook the PW pot across +/- 15v to a 100K input then the
PW Pot will be way too sensitive. So - change the PW Pot input resistor to
300K and the pot sweep range should be all better. In fact you may want to
use 270K instead so that there are dead spots at both ends, but only for a
small part of the rotation. I personally like to be able to dial in 0.001%
width pulse waves sometimes, so the dead zone being there lets me do this. I
may not use that very much, but at least I *can* use it.

Actually a 300K input resistor may allow this anyway. The 5-volt core
sawtooth gets boosted to 10v and level-shifted to be bipolar, and the
resistors I spec'ed for that were just picked from standard values to be
close enough. So the final saw is also the one that drives the PWM stuff,
and it may not be perfectly centered around ground. It will probably be
exactly 10v p-p (assuming perfect resistors).

Some questions:

Is your LFO putting out -5v to +5v?
Did you use the resistor values in the VCO schematic all around?
Is your power supply +/- 15v, or 12v?
Is your core sawtooth running from zero to 5v?

that's all i can think of at the moment...

- Gene

-----Original Message-----
From: Oren Leavitt [mailto:oleavitt at ix.netcom.com]
Sent: Monday, November 17, 2003 4:44 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] ASM-1 VCO


Hello,

Are you saying that the pulse width sweeps over its entire 100% range within
a small 15% range movement of the pulse width control?
Or is it just the difficulty of getting it on the 50% perfect square wave
mark that's troubling you?
Would adding a separate comparator with a dedicated 50/50 square wave output
help?

If it is over responsive, it could be that the signal going to the pulse
comparator is very small.


Oren

-----Original Message-----
From: "Brett, Bill" <BJB at dolby.com>
Sent: Nov 17, 2003 2:14 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] ASM-1 VCO

Greetings,

I have built up several ASM-1 PCB's and the VCO pulse width modulation
control does not work as I would like. A control signal from an LFO will
sweep the PWM control until the square wave output goes to the power rails.
There is only a narrow range in the middle of the control signal swing that
results in a square wave output. I also used a 100k pot to set the PWM
manually This control also has a very narrow range (about 15%) that results
in a square wave output before the output goes to either of the power rails.


Does anyone have advise about modify this part of the VCO so that either
control does not shut down the square wave output. 

Thanks

-Bill