[sdiy] 1v/oct trimming: how does it work?

harry harrybissell at prodigy.net
Sun Aug 11 18:54:42 CEST 2002


Hi Jeffrey...

Adjusting the trimpot is adjusting a resistance... it "is" the circuit
parameter
that is being tweaked.  In most cases, resistance is the easiest and
cheapest
component to trim, as variable resistors can have a wide change of values...

To get 1V/octave... we are usually calibrating an exponential converter.
There are
usually two trims, the offset (to make 1V = an arbitrary pitch, like 'C')
and the
span (to make a change of 1 volt = exactly 2x the frequency).

They interact so you usually have to go back and forth until the adjustments
converge
(don't get any better).

Circuit tolerances can be critical... but usually its the stability of the
components that is more important.

Transistor based expo converters also have a temperature dependance that
must be
compensated... either by adjusting for the change in temperature (tempco
resistor or other active circuit) or by holding temperature constant
(heater).

H^) harry

Jeffrey Pontius wrote:

> I've assembled several kits where adjusting a trimpot is used to set a
> 1v/oct 'scale', which I've done by using a keyboard, Korg tuner, and
> adjusting the trimpot until (usually, but not always) the octave pitches
> on the tuner stabilize (say, at A). I'm sure most of you have done this.
>
> I would appreciate a basic explanation of how adjusting the trimpot and
> sending 1 volt jumps (from the keyboard) enables the circuit to
> 'stablize' at 1v/oct.  To me, a trimpot is a small potentiometer, so I
> assume that there is some component in a sub-circuit that is being
> 'permanently' changed.
>
> Are the tolerances of components used critical to achieving a 'stable'
> 1v/oct?
>
> Thanks [novice electronics module assembler], Jeff

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