Doepfer

Hello list, I've an A101-2 LPG and think a mixed bag. Basically I like
it's sound *but* totally dislike it's self oscilation. Even though the
filter itself is rather smooth sounding the self oscilation comes all
of a sudden and hardly sounds musically to my ears.

Is there any way to help it? Using the trimpots? Changing the
vactrols? (I've to shop for VTL5C3 vactrols anyway.)

Hi Ingo,

>  Even though the
> filter itself is rather smooth sounding the self oscilation comes all
> of a sudden 

I'd be quite happy to cast an eye over the circuit to see if I can come 
up with a suggestion as to how to change this, but unfortunately I 
don't actually have this module and so do not have access to the 
schematics, so unless someone can send me a copy ... Dieter? Anyone?

Tim

Hello Tim, any hint is highly appreciated but unfortunately can't
supply you with the schematics.

BTW I always enjoy your contributions to this list! Even though I
often don't grasp them completly since my knowledge on
electro-engineering is poor.

--- In Doepfer_a100@yahoogroups.com, "Tim Stinchcombe" <timothy@...>
wrote:

>
> Hi Ingo,
> 
> >  Even though the
> > filter itself is rather smooth sounding the self oscilation comes all
> > of a sudden 
> 
> I'd be quite happy to cast an eye over the circuit to see if I can come 
> up with a suggestion as to how to change this, but unfortunately I 
> don't actually have this module and so do not have access to the 
> schematics, so unless someone can send me a copy ... Dieter? Anyone?
> 
> Tim
>

> Hi Ingo,
> 
> > Even though the
> > filter itself is rather smooth sounding the self oscilation comes all
> > of a sudden 
> 
> I'd be quite happy to cast an eye over the circuit to see if I can come 
> up with a suggestion as to how to change this, but unfortunately I 
> don't actually have this module and so do not have access to the 
> schematics, so unless someone can send me a copy ... Dieter? Anyone?
> 
> Tim

Tim,

I'll send you the schematics.

Dieter

Hi Ingo, Dieter, list,

After looking at the schematic Dieter kindly sent I believe the 
reason for the 'suddenness' of the resonance/self-oscillation is the 
type of filter itself: it's a standard Sallen-Key low pass topology, 
and I've read in several books that for normal use these are not 
regarded as very practical filters, due to the sensitivity of the 
resonance to the component values used, i.e. it is hard to guarantee 
any given value of resonance because small changes in component 
values give rise to large changes in the resonance. I came across 
this before in the work I've done to date on the Korg35-based 
MS10/MS20 filters (there's a little algebra on it in my report), as 
these are also Sallen-Key types, and as then, I'm finding it hard to 
come up with a good way of sensibly representing this problem, and 
how it might be affected and/or alleviated by different circuit 
topologies (and for example different pot laws). However I have 
breadboarded a very simple fixed-frequency Sallen-Key filter, and 
comparing experimental results with some simulations, I now have a 
greater trust in what the simulations are saying (which can be hard 
to interpret), and the upshot is I believe it should be possible to 
make the resonance come in in a more controlled fashion. This should 
be achievable by replacing the resonance pot P5 with a 'reverse log' 
type, law 'C' (keeping it at 50k); and another thing which would seem 
to be a further step in the right direction would be to connect the 
pot wiper to the end of the pot connected to R17 (=47k).

Unfortunately since I don't have one of these modules I cannot try 
it, and so cannot be sure for certain. However Dieter has said he 
will look back at the notes made during the module development to see 
if anything similar was tried then. The simulations show that one 
possible problem with doing this is that the peak in the resonance 
might occur for a lower knob setting: trying it out for real is going 
to be the only way to find this out, but even if it does, the 
solution should be quite simple - tweaking the value of R17 (although 
since we know the topology is very sensitive to small changes in 
component values, the required value might mean the use of a preset). 
Also, since I have only simulated the *filter* configuration of the A-
101-2, it is possible that the change has an undesirable effect on 
the VCA side of things (!?).

I've placed a small file in the 'Files' section showing some plots of 
the effect of changing the pot on my simple experiment. The schematic 
shows the Sallen-Key set-up: the key thing it shares with the A-101-2 
is the topology around the final op amp providing the gain; other 
component values were chosen for convenience (and note since I have 
equal value capacitors, the max gain for self-oscillation needs to be 
three, as opposed to 2 in the A-101-2, hence the different values 
there). The blue trace shows how the filter gain shoots up in about 
the last 5% of the pot travel for a linear pot, which effectively is 
the problem. The pink trace shows how the gain rises much more 
steadily throughout the pot rotation, using a law 'C' pot with the 
wiper connected to the one end as stated above. I should point out 
that the traces were, of necessity, obtained via slightly different 
means for the different pot types: using switches I rigged it so that 
as well as measuring the filter gain, I could simply switch the 
filter caps and resistors out and measure the gain of the non-
inverting op amp set-up. For the linear pot, having measured both 
gains, I inverted the theoretical relationship between the pot 
position and the op amp gain (simple, since the pot is linear), so 
this enabled me to plot the filter gain versus pot position - this 
gave a satisfyingly very close trace to that output from the 
simulation (in which it is easy to relate pot position to everything 
else), which is what convinced me that I could trust what the 
simulations are telling me. For the 'C' pot (robbed from my A-102 as 
I don't have a spare one lying around!), I have no way of telling 
exactly what the relationship between pot position and its resistance 
is (the log pots I use in my simulations are 'guesstimates' from 
datasheets), so I have nothing that I can invert, so the figures are 
literally taken from the physical settings I used (and hence why the 
graph is nowhere near as smooth).

I'm hoping that Dieter or someone can try this suggestion easily 
enough and let us know whether it is as useful as I'm hoping it is!

Tim

--- In Doepfer_a100@yahoogroups.com, <hardware@...> wrote:
>
> > Hi Ingo,
> > 
> > > Even though the
> > > filter itself is rather smooth sounding the self oscilation 
comes all
> > > of a sudden 
> > 
> > I'd be quite happy to cast an eye over the circuit to see if I 
can come 
> > up with a suggestion as to how to change this, but unfortunately 
I 
> > don't actually have this module and so do not have access to the 
> > schematics, so unless someone can send me a copy ... Dieter? 
Anyone?

> > 
> > Tim
> 
> Tim,
> 
> I'll send you the schematics.
> 
> Dieter
>

Hello Tim, thank you very much for your efforts. Highly appreciated.
Thanks to your suggestion and my solder sucker I've swapped the B50K
pot for a C50K in a fly. The result is much more to my liking.
However it effects the signal/self osc ratio which is no prob. since
one can turn down the input level to adjust the ratio. Now, setting
the res. pot fully cw the filter resembles the MS10 one. At least IIRC
since I sold my one a long time ago.
Still I think the overall smooth character of the vactrol filter and
the rather rude self osc. seems to me a bit contradictory even though
others will actually like this.
I still like to give your 2nd mod a try but I'm not sure wether I get
it. Even worse my A101-2 is a prototype thus there are no part numbers
on the board. Pls verify if I've understood how to proceed:
I'm going to link the res. pots wiper (= 2nd pin i.e. the one in the
middle) with one resistor on the board. Since I've no part numbers:
You're talking about one of the 2 resistors located btn the brackets
of the res. pot: the 47k one? One of it's leads is already connected
to the wiper! The remaining lead should go to the wiper as well?
Sorry, just want to make sure.
Many thanks for you efforts! You did me a big favor! Regards, Ingo



--- In Doepfer_a100@yahoogroups.com, "Tim Stinchcombe" <timothy@...>
wrote:

>
> Hi Ingo, Dieter, list,
> 
> After looking at the schematic Dieter kindly sent I believe the 
> reason for the 'suddenness' of the resonance/self-oscillation is the 
> type of filter itself: it's a standard Sallen-Key low pass topology, 
> and I've read in several books that for normal use these are not 
> regarded as very practical filters, due to the sensitivity of the 
> resonance to the component values used, i.e. it is hard to guarantee 
> any given value of resonance because small changes in component 
> values give rise to large changes in the resonance. I came across 
> this before in the work I've done to date on the Korg35-based 
> MS10/MS20 filters (there's a little algebra on it in my report), as 
> these are also Sallen-Key types, and as then, I'm finding it hard to 
> come up with a good way of sensibly representing this problem, and 
> how it might be affected and/or alleviated by different circuit 
> topologies (and for example different pot laws). However I have 
> breadboarded a very simple fixed-frequency Sallen-Key filter, and 
> comparing experimental results with some simulations, I now have a 
> greater trust in what the simulations are saying (which can be hard 
> to interpret), and the upshot is I believe it should be possible to 
> make the resonance come in in a more controlled fashion. This should 
> be achievable by replacing the resonance pot P5 with a 'reverse log' 
> type, law 'C' (keeping it at 50k); and another thing which would seem 
> to be a further step in the right direction would be to connect the 
> pot wiper to the end of the pot connected to R17 (=47k).
> 
> Unfortunately since I don't have one of these modules I cannot try 
> it, and so cannot be sure for certain. However Dieter has said he 
> will look back at the notes made during the module development to see 
> if anything similar was tried then. The simulations show that one 
> possible problem with doing this is that the peak in the resonance 
> might occur for a lower knob setting: trying it out for real is going 
> to be the only way to find this out, but even if it does, the 
> solution should be quite simple - tweaking the value of R17 (although 
> since we know the topology is very sensitive to small changes in 
> component values, the required value might mean the use of a preset). 
> Also, since I have only simulated the *filter* configuration of the A-
> 101-2, it is possible that the change has an undesirable effect on 
> the VCA side of things (!?).
> 
> I've placed a small file in the 'Files' section showing some plots of 
> the effect of changing the pot on my simple experiment. The schematic 
> shows the Sallen-Key set-up: the key thing it shares with the A-101-2 
> is the topology around the final op amp providing the gain; other 
> component values were chosen for convenience (and note since I have 
> equal value capacitors, the max gain for self-oscillation needs to be 
> three, as opposed to 2 in the A-101-2, hence the different values 
> there). The blue trace shows how the filter gain shoots up in about 
> the last 5% of the pot travel for a linear pot, which effectively is 
> the problem. The pink trace shows how the gain rises much more 
> steadily throughout the pot rotation, using a law 'C' pot with the 
> wiper connected to the one end as stated above. I should point out 
> that the traces were, of necessity, obtained via slightly different 
> means for the different pot types: using switches I rigged it so that 
> as well as measuring the filter gain, I could simply switch the 
> filter caps and resistors out and measure the gain of the non-
> inverting op amp set-up. For the linear pot, having measured both 
> gains, I inverted the theoretical relationship between the pot 
> position and the op amp gain (simple, since the pot is linear), so 
> this enabled me to plot the filter gain versus pot position - this 
> gave a satisfyingly very close trace to that output from the 
> simulation (in which it is easy to relate pot position to everything 
> else), which is what convinced me that I could trust what the 
> simulations are telling me. For the 'C' pot (robbed from my A-102 as 
> I don't have a spare one lying around!), I have no way of telling 
> exactly what the relationship between pot position and its resistance 
> is (the log pots I use in my simulations are 'guesstimates' from 
> datasheets), so I have nothing that I can invert, so the figures are 
> literally taken from the physical settings I used (and hence why the 
> graph is nowhere near as smooth).
> 
> I'm hoping that Dieter or someone can try this suggestion easily 
> enough and let us know whether it is as useful as I'm hoping it is!
> 
> Tim
> 
> --- In Doepfer_a100@yahoogroups.com, <hardware@> wrote:
> >
> > > Hi Ingo,
> > > 
> > > > Even though the
> > > > filter itself is rather smooth sounding the self oscilation 
> comes all
> > > > of a sudden 
> > > 
> > > I'd be quite happy to cast an eye over the circuit to see if I 
> can come 
> > > up with a suggestion as to how to change this, but unfortunately 
> I 
> > > don't actually have this module and so do not have access to the 
> > > schematics, so unless someone can send me a copy ... Dieter? 
> Anyone?
> > > 
> > > Tim
> > 
> > Tim,
> > 
> > I'll send you the schematics.
> > 
> > Dieter
> >
>

Hi Ingo,

> Hello Tim, thank you very much for your efforts. Highly appreciated.

My pleasure!

> Thanks to your suggestion and my solder sucker I've swapped the B50K
> pot for a C50K in a fly. The result is much more to my liking.

Great, sounds like we're moving in the right direction!

> I still like to give your 2nd mod a try but I'm not sure wether I 
get
> it.

OK, but...

> Even worse my A101-2 is a prototype thus there are no part numbers
> on the board.

...thus your module *might* be slightly different!

> Pls verify if I've understood how to proceed:
> I'm going to link the res. pots wiper (= 2nd pin i.e. the one in the
> middle)
<snip>

As you look at the pot from the front, simply short the middle pin to 
the right-hand end - if your module *is* as the schematic I have, 
this end should go to the 47k resistor, and pin 2 on the amplifier 
chip (LM324), and a 100p cap. If the wiper is already connected to 
this point, then it looks like it may have got changed for the 
production version (it is a fairly subtle change after all!) - the 
other end of the 47k should go to ground, and you *don't* want to 
connect the wiper there! If you're still not sure, get back to me.

I'm just about finished writing up some slightly more detailed notes, 
mostly for my own benefit, but I will send a copy to Dieter so that 
he can see what I have done/am suggesting.

As I mentioned before it is possibly that this change will move the 
whole resonance peak down to a lower pot setting - if this is 
happening you should detect that the resonance *decreases* again as 
the pot is rotated further clockwise from some given point. If you 
think this is happening let me know, and I can probably make a 
suggestion on how to cure it (basically replacing that 47k with a 
slightly larger value).

Good luck,

Tim

Hello Tim, the wiper of my prototype is connected to pin 2 of the
LM324 and an 100pF cap! Actually I thought the prototype would be
technically the same as the A101-2 in production. (If Dieter "sneaks",
it reads "Pr. 2" on the back of the frontpanel).

To summerize the results: Now the self osc. can be dialed in and is
absolutly not dominating the overall sound since the original signal
is always in the foreground. Thus you can no longer overdrive the
input signal and have a dominating self osc. since you have to dial
down the input signal to alter the signal/self osc. ratio in favor of
the later. In my opinion absolutely no problem for I think the module
now much more versatile than before. It will only self. osc. in the LP
mode.

BTW usually, even though not always, a TL074 is prefered over an
LM324, do you see any reason why the LM324 has been choosen for the LPG?

Again, your contribtions are highly appreciated! Best, Ingo


--- In Doepfer_a100@yahoogroups.com, "Tim Stinchcombe" <timothy@...>
wrote:

>
> Hi Ingo,
> 
> > Hello Tim, thank you very much for your efforts. Highly appreciated.
> 
> My pleasure!
> 
> > Thanks to your suggestion and my solder sucker I've swapped the B50K
> > pot for a C50K in a fly. The result is much more to my liking.
> 
> Great, sounds like we're moving in the right direction!
> 
> > I still like to give your 2nd mod a try but I'm not sure wether I 
> get
> > it.
> 
> OK, but...
> 
> > Even worse my A101-2 is a prototype thus there are no part numbers
> > on the board.
> 
> ...thus your module *might* be slightly different!
> 
> > Pls verify if I've understood how to proceed:
> > I'm going to link the res. pots wiper (= 2nd pin i.e. the one in the
> > middle)
> <snip>
> 
> As you look at the pot from the front, simply short the middle pin to 
> the right-hand end - if your module *is* as the schematic I have, 
> this end should go to the 47k resistor, and pin 2 on the amplifier 
> chip (LM324), and a 100p cap. If the wiper is already connected to 
> this point, then it looks like it may have got changed for the 
> production version (it is a fairly subtle change after all!) - the 
> other end of the 47k should go to ground, and you *don't* want to 
> connect the wiper there! If you're still not sure, get back to me.
> 
> I'm just about finished writing up some slightly more detailed notes, 
> mostly for my own benefit, but I will send a copy to Dieter so that 
> he can see what I have done/am suggesting.
> 
> As I mentioned before it is possibly that this change will move the 
> whole resonance peak down to a lower pot setting - if this is 
> happening you should detect that the resonance *decreases* again as 
> the pot is rotated further clockwise from some given point. If you 
> think this is happening let me know, and I can probably make a 
> suggestion on how to cure it (basically replacing that 47k with a 
> slightly larger value).
> 
> Good luck,
> 
> Tim
>

[Apologies to anyone who saw the first version of this - I spotted 
yet *another* mistake, but then found I could delete the post...
For those that know the movie "The Long Kiss Goodnight", I'd like to 
use "Oh darn it, I burnt the muffins!" euphemistically.]

Hi Ingo,

First I must correct two things I said in my previous post (I do so
hate putting incorrect information out there...):

> > As I mentioned before it is possibly that this change will move
the
> > whole resonance peak down to a lower pot setting - if this is
> > happening you should detect that the resonance *decreases* again
as
> > the pot is rotated further clockwise from some given point.

The last part of this, about the resonance decreasing again at higher
pot settings, appears to be complete and utter tosh! That's the
trouble with relying on simulation traces too much, as I have a
tendency to do: the simulation traces climb to a peak, and *do* then
decrease, but since I'm using a SPICE *AC* analysis it is
rather 'analytical', and requires the correct interpretation, which I
have clearly not found yet (I think it is probably to do with the
poles in the right-half plane or something...). In reality it is
clear from my simple breadboard S-K filter, that once it is
oscillating, even as you continue turning the knob past the 'optimum'
point to start oscillations, it *stays* oscillating. In terms of the
gain this is obvious - once you're past the minimum point for
oscillation, you do not go back below it, despite what my simulation
traces might look like! (Doesn't look like too many other people are
paying attention though, so I seem to have got away with it! - "Pay
attention, I'll be asking questions later!") Lesson learned!

My second correction has caused this confusion:

> Hello Tim, the wiper of my prototype is connected to pin 2 of the
> LM324 and an 100pF cap! Actually I thought the prototype would be
> technically the same as the A101-2 in production.

As long as the above means you were mistaken about the 47k going to 
the wiper as you said before (post 13172), I think they are the same, 
as this *was wrong*:

> the right-hand end - if your module *is* as the schematic I have, 
> this end should go to the 47k resistor, and pin 2 on the amplifier 
> chip (LM324), and a 100p cap.

Actually the schematic shows the right-hand pin going to the 47k 
resistor *and that is all*; the wiper goes to the 100p cap and LM324 
pin 2. The second suggestion was to connect these two points (I must 
have gotten ahead of myself), but it sounds like you may not need to 
bother with it anyway (easy enough to do/undo though, with a small 
bit of bare wire).

> To summerize the results: Now the self osc. can be dialed in and is
> absolutly not dominating the overall sound since the original signal
> is always in the foreground. Thus you can no longer overdrive the
> input signal and have a dominating self osc. since you have to dial
> down the input signal to alter the signal/self osc. ratio in favor
of
> the later. In my opinion absolutely no problem for I think the
module
> now much more versatile than before.

I have continued looking at various aspects throughout the day,
mostly improving my SPICE models of 'log' and 'reverse log' taper
pots. Even those these are now not so 'optimistic' as the ones I was
previously using, they do still point to the use of a 'C' law pot as
going in the right direction to remove the rather 'binary'
resonance/no resonance characteristic using the linear pot. I'm
pleased that having made this change it has made the module work
better for you!

> BTW usually, even though not always, a TL074 is prefered over an
> LM324, do you see any reason why the LM324 has been choosen for the
LPG?

I cannot see any glaringly obvious circuit reason for using a higher-
spec op amp, and I guess Doepfer themselves are aware that A-100
stuff probably doesn't appeal to those people who absolutely insist
on low-noise *everything*, so my best guess would be that it simply
comes down to economics - I suspect the LM324 is just cheaper than a
TL074. I might be wrong though, and so I am willing to be corrected
by Dieter!

Regards,
Tim (who really hopes he's got it right this time!)

> Hello Tim, the wiper of my prototype is connected to pin 2 of the
> LM324 and an 100pF cap! Actually I thought the prototype would be
> technically the same as the A101-2 in production. (If Dieter "sneaks",
> it reads "Pr. 2" on the back of the frontpanel).

Sorry - we do not keep the technical documents of prototypes but only of the
regular modules. The prototypes are sold "as they are". They are preliminary
versions of the final products and may differ in some details.

Best wishes
Dieter Doepfer

--- In Doepfer_a100@yahoogroups.com, <hardware@...> wrote:
Never mind. Anyway, I used to think that those prototypes which are
depicted as "von den Funktionen her identisch zu der endgültigen
Version"* would be the same as the modules in production apart from
their "DIY-appearance". Regards, Ingo

* = "functions identically to the final version"


> Sorry - we do not keep the technical documents of prototypes but
only of the
> regular modules. The prototypes are sold "as they are". They are
preliminary

> versions of the final products and may differ in some details.
> 
> Best wishes
> Dieter Doepfer
>

> I cannot see any glaringly obvious circuit reason for using a higher-
> spec op amp, and I guess Doepfer themselves are aware that A-100
> stuff probably doesn't appeal to those people who absolutely insist
> on low-noise *everything*, so my best guess would be that it simply
> comes down to economics - I suspect the LM324 is just cheaper than a
> TL074. I might be wrong though, and so I am willing to be corrected
> by Dieter!

The economic aspect is totally negligible (LM324 and TL074 are in the same
price range below Euro .20). The fine adjustments of the vactrol modules
(A-101-1/2/3) were made at that time by Roland Mayer and I found a note in
his records that he had problem with HF oscillations if the TL074 was used.

Best wishes
Dieter Doepfer

Hi Ingo

 > * = "functions identically to the final version"
the "functions"(!) are the same, but the technology how the function is 
achieved (vulgo the schematic) nay differ.

Florian

> --- In Doepfer_a100@yahoogroups.com, <hardware@...> wrote:
> Never mind. Anyway, I used to think that those prototypes which are
> depicted as "von den Funktionen her identisch zu der endgültigen
> Version"* would be the same as the modules in production apart from
> their "DIY-appearance". Regards, Ingo
>
> * = "functions identically to the final version"

Even if the "functions are identically to the final version" there may be
some on-board differences compared to the final version. For example
electronic parts may be located on other positions on the pc board or any
corrections or improvements have been made. If for example a missing part
has been added (e.g. an additional resistor or capacitor) the prototype will
have the same behaviour/function as the final version of the module - but it
will look internally different.

Best wishes
Dieter Doepfer

Interesting bit on the LM324, thanx for the responses.

TR


>From: <hardware@doepfer.de>
>Reply-To: Doepfer_a100@yahoogroups.com
>To: <Doepfer_a100@yahoogroups.com>
>Subject: AW: [Doepfer_a100] Re: A-101-2 Low Pass Gate and self oscilation
>Date: Mon, 10 Sep 2007 12:21:57 +0200
>
> > I cannot see any glaringly obvious circuit reason for using a higher-
> > spec op amp, and I guess Doepfer themselves are aware that A-100
> > stuff probably doesn't appeal to those people who absolutely insist
> > on low-noise *everything*, so my best guess would be that it simply
> > comes down to economics - I suspect the LM324 is just cheaper than a
> > TL074. I might be wrong though, and so I am willing to be corrected
> > by Dieter!
>
>The economic aspect is totally negligible (LM324 and TL074 are in the same
>price range below Euro .20). The fine adjustments of the vactrol modules
>(A-101-1/2/3) were made at that time by Roland Mayer and I found a note in
>his records that he had problem with HF oscillations if the TL074 was used.
>
>Best wishes
>Dieter Doepfer
>
>
>
>
>
>
>Yahoo! Groups Links
>
>
>

_________________________________________________________________
More photos; more messages; more whatever. Windows Live Hotmail - NOW with 
5GB storage. 
http://imagine-windowslive.com/hotmail/?locale=en-us&ocid=TXT_TAGHM_migration_HM_mini_5G_0907

I'm late, it's an old thread. However, I've done a small demo after
the pot change. The demo has no musical approach whatsoever but to
demonstrate the tamed self oscilation of my modified module. Still
you'll note significant differences in volume when cranking up the
resonace pot however to me it's much more usable. Keep in mind my
A101-2 is a prototype even though I *guess* it's soundwise very
similiar to the produced modules, I could be wrong on that. Tim,
thanks again for your assistence.
The link:
http://www.twango.com/media/Ilanode.public/Ilanode.10004 
Best rgds, Ingo

--- In Doepfer_a100@yahoogroups.com, "Tim Stinchcombe" <timothy@...>
wrote:

>
> Hi Ingo,
> 
> > Hello Tim, thank you very much for your efforts. Highly appreciated.
> 
> My pleasure!
> 
> > Thanks to your suggestion and my solder sucker I've swapped the B50K
> > pot for a C50K in a fly. The result is much more to my liking.
> 
> Great, sounds like we're moving in the right direction!
> 
> > I still like to give your 2nd mod a try but I'm not sure wether I 
> get
> > it.
> 
> OK, but...
> 
> > Even worse my A101-2 is a prototype thus there are no part numbers
> > on the board.
> 
> ...thus your module *might* be slightly different!
> 
> > Pls verify if I've understood how to proceed:
> > I'm going to link the res. pots wiper (= 2nd pin i.e. the one in the
> > middle)
> <snip>
> 
> As you look at the pot from the front, simply short the middle pin to 
> the right-hand end - if your module *is* as the schematic I have, 
> this end should go to the 47k resistor, and pin 2 on the amplifier 
> chip (LM324), and a 100p cap. If the wiper is already connected to 
> this point, then it looks like it may have got changed for the 
> production version (it is a fairly subtle change after all!) - the 
> other end of the 47k should go to ground, and you *don't* want to 
> connect the wiper there! If you're still not sure, get back to me.
> 
> I'm just about finished writing up some slightly more detailed notes, 
> mostly for my own benefit, but I will send a copy to Dieter so that 
> he can see what I have done/am suggesting.
> 
> As I mentioned before it is possibly that this change will move the 
> whole resonance peak down to a lower pot setting - if this is 
> happening you should detect that the resonance *decreases* again as 
> the pot is rotated further clockwise from some given point. If you 
> think this is happening let me know, and I can probably make a 
> suggestion on how to cure it (basically replacing that 47k with a 
> slightly larger value).
> 
> Good luck,
> 
> Tim
>