Doepfer

Hi list,

I have been taking some detailed measurements from my A-127 (for comparison against a heavily-modified Plan B M12) and have noticed that one of the three sections is adjusted quite differently from the other two, which agree very closely. Thus I'm not sure which sections are adjusted as they would have been when I originally bought it (it's likely I have 'fiddled with it' over the years!), so if anyone has the service manual and can detail or send me the calibration procedure I would very much appreciate it!

Many thanks,
Tim

> Hi list,
>
> I have been taking some detailed measurements from my A-127 (for
> comparison against a heavily-modified Plan B M12) and have
> noticed that one of the three sections is adjusted quite
> differently from the other two, which agree very closely. Thus
> I'm not sure which sections are adjusted as they would have been
> when I originally bought it (it's likely I have 'fiddled with it'
> over the years!), so if anyone has the service manual and can
> detail or send me the calibration procedure I would very much
> appreciate it!
>
> Many thanks,
> Tim

There are four trimming potentiometers on each A-127 board:

P6/100k: frequency offset (defines the absolute filter frequency when the
VCF Frq. control is fully CCW)
P7/2k5 : frequency scale (defines the sensitivity of the CV input, can be
adjusted e.g. to approximately 1V/oct)
P8/100k: OTA offset 1 (adjusted for minimal CV feedtrough)
P9/100k: OTA offset 2 (adjusted for minimal CV feedtrough)

This is the order of the trimming potentiometers from left to right on the
pcb (front view): P7, P6, P8, P9

Hope this helps

Best wishes
Dieter Doepfer

Hi Dieter,
Thanks for the info.

> P6/100k: frequency offset (defines the absolute filter frequency when the
> VCF Frq. control is fully CCW)

It was exactly how this one gets set I was particularly interested in, i.e. just where the overall frequency range of the filter ends up. Do you apply a signal of known amplitude and frequency, and then adjust so that the output has such-and-such an amplitude (hence setting the cut-off at the desired point), or what?!

Thanks, Tim

> Hi Dieter,
> Thanks for the info.
>
> > P6/100k: frequency offset (defines the absolute filter
> frequency when the
> > VCF Frq. control is fully CCW)
>
> It was exactly how this one gets set I was particularly
> interested in, i.e. just where the overall frequency range of the
> filter ends up. Do you apply a signal of known amplitude and
> frequency, and then adjust so that the output has such-and-such
> an amplitude (hence setting the cut-off at the desired point), or what?!
>
> Thanks, Tim

We use white noise as audio input signal and max. resonance setting at the
A-127. Then the filters are adjusted by ear.

Best wishes
Dieter Doepfer

> We use white noise as audio input signal and max. resonance setting at the
> A-127. Then the filters are adjusted by ear.

Ah, interesting. I'm still having difficulty understanding how this means you can reference the frequency range to an absolute set of values, but it has given me a couple of things to try!

Thanks,
Tim

> > We use white noise as audio input signal and max. resonance
> setting at the
> > A-127. Then the filters are adjusted by ear.
>
> Ah, interesting. I'm still having difficulty understanding how
> this means you can reference the frequency range to an absolute
> set of values, but it has given me a couple of things to try!
>
> Thanks,
> Tim

The exact measuring procedure is a bit more complex for filters that do not
have available self-oscillation. You need a measurement equipment that gives
you the frequency response of the filter and how it changes with the control
voltage (combination of a frequency modulated sine wave over the audio range
as audio signal and an envelope follower at the filter output which gives
you the filter response curve). But if you use a noise signal and set the
filter's resonance to it's maximum (and use the bandpass output - if
available) you will hear kind of a "pitched noise". If you want to adjust
the filter CV input to 1V/octave a difference of one Volt of the CV has to
correspond to a pitch change of one octave. This parameter is adjusted with
the trimming potentiometer for the scale. After that the absolute frequency
of the filter is adjusted to the desired value (i.e. the frequency if all CV
inputs are zero Volt and the manual control is fully CCW). For this the
trimming potentiometer for the frequency offset is used.

Hope this helps

Dieter Doepfer

> The exact measuring procedure is a bit more complex for filters that do not
> have available self-oscillation. You need a measurement equipment that gives
> you the frequency response of the filter and how it changes with the control
> voltage (combination of a frequency modulated sine wave over the audio range
> as audio signal and an envelope follower at the filter output which gives
> you the filter response curve).

Yes, I have been doing this manually, because of that lack of self-oscillation!

> After that the absolute frequency
> of the filter is adjusted to the desired value (i.e. the frequency if all CV
> inputs are zero Volt and the manual control is fully CCW).

It is what these values are for the factory setup-up of the A-127 that I was really wondering about - e.g. is the cut-off range set to 8 -> 4096Hz, or 16 -> 8192, or 32 -> 16384 or ... what?

Thanks again,
Tim

> It is what these values are for the factory setup-up of the A-127
> that I was really wondering about - e.g. is the cut-off range set
> to 8 -> 4096Hz, or 16 -> 8192, or 32 -> 16384 or ... what?
>
> Thanks again,
> Tim

The factory adjustment is ~ 40Hz (+/- 10 Hz) with VCF Frq. and CV Ampl. both
fully CCW.

Best wishes
Dieter Doepfer

> The factory adjustment is ~ 40Hz (+/- 10 Hz) with VCF Frq. and CV Ampl. both
> fully CCW.

Wow, amazing! Last week I took three measurements from my 127 (at 1, 3 & 5V in), noting the frequency giving a signal 26dB down from the input; I then extrapolated the 26dB back up again, to find the nominal 'cut-off' frequency; I took logs and plotted the points, which gave a pretty good straight line; I fitted the points by regression, and then took anti-logs and plotted the resulting exponential curve - the frequency this gave corresponding to 0V CV in? 40.3Hz! :-) I'm pretty happy that I'm on the right path now! Many thanks for the info Dieter.

Tim