Doepfer

> Hi,
>
> I seem to be having a problem with my A-105  SSM LPF.
>
> If I modulate either of the cut off  (FCV1/FCV2) inputs with any
> shape from an LFO, I get a loud audible click coming from the
> filter  as the LFO goes through it's cycle - with audio going in
> to the filter or not... also with resonance set to zero or otherwise.
>
> I have tested other Doepfer filters with the same lead/LFO/ etc
> and the problem does not materialise.
>
> Any suggestions, please.
>
> Best wishes

I suppose that the module is faulty and has to be repaired. Maybe the SSM
circuit is defective but it may be also another circuit. Please return the
module for repair to the dealer where you purchased it.

Best wishes
Dieter Doepfer

Can you post a detailled description of a test patch which can be used 
to reproduce the problem. Create the patch only with Doepfer modules and 
make the patch as simple as possible. Describe each parameter setting 
for each module. And finally you may provide an audio example of the 
problem based on this patch on soundcloud or similar.

Maybe then it is possible to reproduce (and explain) the behaviour.

Florian

On 11.06.14 21:23 , isoldeisobelle@gmail.com [Doepfer_a100] wrote:
>
>
> Hi again.
>
> I just received the replacement A-105 and on testing the module it still
> generates the same short click on modulation of the CV inputs with any
> shape wave.
> It seems quite unfeasible that two of the same modules would be faulty
> in the same way, so maybe it is me at fault!!!
> Should this be happening? It is quite a loud sound that it produces as
> the LFO modulates... like I stated before: with no other inputs and the
> level at zero.
>
> Any suggestions would be most welcome.
>
>
> Kind regards
>
>

> Hi again.
>
> I just received the replacement A-105 and on testing the module
> it still generates the same short click on modulation of the CV
> inputs with any shape wave.
> It seems quite unfeasible that two of the same modules would be
> faulty in the same way, so maybe it is me at fault!!!
> Should this be happening? It is quite a loud sound that it
> produces as the LFO modulates... like I stated before: with no
> other inputs and the level at zero.
>
> Any suggestions would be most welcome.
>
>
> Kind regards

I'll take an A-105 module from stock and try it out. The A-105 nothing but
the standard application of the SSM2044 filter circuit. How loud is the
sound compared to the normal audio signal (i.e. with full audio level just
before clipping). Maybe it's the control voltage feedthrough which is
specified with -36dB in the SSM2044 data sheet. So if the click is
about -35dB compared to the full signal it's probably the control voltage
feedthrough.

Best wishes
Dieter Doepfer

Hi Dieter

Am 12.06.2014 10:48, schrieb yahoo@doepfer.de [Doepfer_a100]:
> So if the click is
> about -35dB compared to the full signal it's probably the control voltage
> feedthrough.
Listen to the soundcloud example, there is no audiosignal at all.

But the soundexample is indeed quite "funny", because the modulation 
sounds like an asymmetric pulse wave - while he states, that he is using 
a triangle from the A-145.

@isoldeisobelle:

Can you turn up the resonance so we can hear whether it is in fact a 
sine, which is doing the modulation.

Maybe it is not the 105 which is broken, but the A-145 (I don't have the 
schematics here, so I don't know whether some broken OpAmp in the sine 
out could act as an comparator).

Florian

> Hi Dieter and thank you for the reply.
>
> I can not be to certain to the accuracy of my monitoring of the
> sound - but, with the settings I mentioned above it is almost
> touching the -4dB level on my hardware mixer (with no extra gain)
> and roughly the same in my DAW.... so pretty loud!
>
> I should have recorded a sample with audio input for comparison,
> but with lower frequency input the sound is fairly noticeable as
> the LFO cycles... even more so with higher frequencies as the
> sound is fairly low.... it makes a useful kick drum btw! ;)
>
> I only had the filter for a short period before I sent for the
> replacement, but I am almost certain that it never behaved this
> way at first... it seems striking that the replacement would be the same.
>
> Having said that; I have eight filters in my system - five of
> them being Doepfer and this is definitely one of my favorites - a
> wonderful smooth sound and I personally prefer its more
> ''darker'' tone compared to its nearest rival.
>
>
> Best wishes

I took an A-105 module from stock and measured indeed a control voltage
feedthrough of about 30mVpp (i.e. if the CV changes the output voltage
changes up to 30mV without audio input). The full level of the module before
clipping is about 15Vpp. So the measured control voltage feedthrough is
25mV/15V = 0,017 which is -35dB. That's very close to the specified -36dB
control voltage feedthrough in the SSM2044 data sheet. So I suppose that
it's just the control voltage feedthrough which you hear. Maybe we should
add this information to the A-105 specs (which are in the end nothing but
the spec of the SSM2044). The output voltage follows not exactly the CV
input signal but is assymetrical (or distorted), i.e. modulating the filter
frequency e.g. with a sine wave does not generate a sine wave at the output
but a very distorted sine (like a waveshaper). If the CV input is in the
audio range you will hear the frequency of the modulation LFO at the A-105
output even without any input signal (about -35dB compared to the full 15V
signal with audio input).

Best wishes
Dieter Doepfer

> Hello .
>
> Any more suggestions for my problem, please, or should I return
> the module to my distributor again.
>
> Kind regards

Please refer to my previous email. If the control voltage feedtrough is not
acceptable for you please return the module for refund (replacing does not
make sense as all A-105 show this behaviour).

Best wishes
Dieter Doepfer

> Hi Dieter.
>
> Apologies, I did not see your previous detailed explanation.
> Thank you very much for your reply and taking the time to test my query.
> I never noticed this behavior when I first used the filter but
> -36dB is, for me, a usable level to deal with - I suspect that I
> may be patching things now a little differently, possibly with
> regards to VCA gain, which has made it noticeable than it did to
> begin with.
>
> Either way, it is great sounding filter and so long as I know
> that this behavior is inherent in the design... it is something
> that I am prepared to live with.
>
>
> Kind regards and best wishes
>
> Tolly Hamilton

As a rumor Solid State Music (SSM) was aware of this drawback and developed
a new SSM2044-compatible circuit named "SSM2046" which should have a 10dB
better control voltage feedthrough. But it's quite impossible to find more
information about this circuit and I have never seen an exemplar of this
circuit.

Best wishes
Dieter Doepfer