Doepfer

Is it possible to mod the A143-9 to have an external sync input?  I love this thing, but the one thing it's missing for me is a sync (reset).

> Is it possible to mod the A143-9 to have an external sync input?
> I love this thing, but the one thing it's missing for me is a
> sync (reset).

I'm sorry but that's not possible. We thought about this feature during the
design but the circuit is kind of a filter in self resonance. If one would
add a sync circuit similar to other VCOs (electronic switch that discharges
the timing capacitor) the oscillation would not start with full amplitude
but increase little by little until the full level is reached. That's
probably not what you would expect from a sync input.

Best wishes
Dieter Doepfer

> > Is it possible to mod the A143-9 to have an external sync input?
> > I love this thing, but the one thing it's missing for me is a
> > sync (reset).
> 
> I'm sorry but that's not possible. We thought about this feature during the
> design but the circuit is kind of a filter in self resonance. If one would
> add a sync circuit similar to other VCOs (electronic switch that discharges
> the timing capacitor) the oscillation would not start with full amplitude
> but increase little by little until the full level is reached.

I've been thinking about this, and reckon it might actually be possible, though how 'accurate' it would be I can't be sure. I think the following ought to work: on a reset pulse, discharge the capacitor of the sine integrator to zero volts, but _charge_ the cosine cap to its maximum value (just over 3V on mine); then when the reset pulse completes and the oscillator runs, the sine output should climb away from zero, and the cosine should decrease from its maximum, exactly as it should under normal operation.

How useful this would be I'm not sure, as there will naturally be some small delay due to charging/discharging the caps, and if the charged cap doesn't match the normal max amplitude of the output, that will introduce some error too (and the amplitude of mine varies by several hundred millivolts over the range of the LFO - the amplitude decreases as the frequency increases).

Does that make sense?

Tim

> I've been thinking about this, and reckon it might actually be
> possible, though how 'accurate' it would be I can't be sure. I
> think the following ought to work: on a reset pulse, discharge
> the capacitor of the sine integrator to zero volts, but _charge_
> the cosine cap to its maximum value (just over 3V on mine); then
> when the reset pulse completes and the oscillator runs, the sine
> output should climb away from zero, and the cosine should
> decrease from its maximum, exactly as it should under normal operation.
>
> How useful this would be I'm not sure, as there will naturally be
> some small delay due to charging/discharging the caps, and if the
> charged cap doesn't match the normal max amplitude of the output,
> that will introduce some error too (and the amplitude of mine
> varies by several hundred millivolts over the range of the LFO -
> the amplitude decreases as the frequency increases).
>
> Does that make sense?
>
> Tim

That's an excellent idea. I'll have to try it out. The magnitude of the
amplitude variation depends upon the matching of the two OTAs in the
LM13700.

Best wishes
Dieter Doepfer