Doepfer

Hi

Can someone tell me if my A102 filter is acting correctly - The resonance seems to be set really high.  The filter will self oscillate with the resonance set to just 4 (when the cut off is set between 3 and 6).

Is this normal?

As a result of this the range of usable resonance is really low and there is a sudden change in sound from zero resonance to a setting of 1.

Is there any way to adjust the resonance to a more usable range?

Or is the filter faulty?

Thanks

Mick

I have two of them. They both act like you describe.
One is a bit milder.
I would be interested to know how to adjust too.
Only to match them better.

grtz

Ernst





On Fri, Jan 22, 2010 at 12:40 AM, mickgrit <carterguitarist@yahoo.co.uk>wrote:

>
>
> Hi
>
> Can someone tell me if my A102 filter is acting correctly - The resonance
> seems to be set really high. The filter will self oscillate with the
> resonance set to just 4 (when the cut off is set between 3 and 6).
>
> Is this normal?
>
> As a result of this the range of usable resonance is really low and there
> is a sudden change in sound from zero resonance to a setting of 1.
>
> Is there any way to adjust the resonance to a more usable range?
>
> Or is the filter faulty?
>
> Thanks
>
> Mick
>
>  
>


[Non-text portions of this message have been removed]

Hi Mick,

> Can someone tell me if my A102 filter is acting correctly - The resonance seems to be set really high.  The filter will self oscillate with the resonance set to just 4 (when the cut off is set between 3 and 6).
> 
> Is this normal?

Yes, both mine behave similarly - basically there is way too much gain around the feedback loop.

> Is there any way to adjust the resonance to a more usable range?

I tried a couple of things very quickly (the first one was spectacularly ineffective!). However the following seemed OKish: between C10 and Q7 there are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold the faceplate nearest you); swap this for a 5k6 say, which will cut the gain to a third, so it won't resonate until the pot is further round. A 4k7 may not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the 2k2) may restrict the range of frequencies at which it will oscillate. Unfortunately it will also reduce the output amplitude somewhat.

To do the job 'properly' would mean re-vamping the entire 'gain regime' around the loop, which would require considerably more thought, and replacing maybe half-a-dozen or so resistors. My initial idea was to *double* R5 (the 10k in the middle of the only group of 5 resistors, central to the board), but this seemed to have little impact, mainly I think because the resonance pot is a 'law C' ('rev log'), and so naturally 'all the action' is in the first part of its rotation.

Let us know if you have any success with this!

Tim

@Tim: WRONG! No, relax, I'm joking ... ;)

@all: 
May I ask you by the way how you like the sound of the diode-ladder filter?
I mean do you find it more or less musical than the transistor-cascade e.g. for bass sounds or leads or so? 
Last question: Does the Doepfer transistor ladder filter really sounds like the Moog filter?

I've read your paper about the comparison between the diode- and transistor filter, Tim. Good work!

Ollie


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

>
> Hi Mick,
> 
> > Can someone tell me if my A102 filter is acting correctly - The resonance seems to be set really high.  The filter will self oscillate with the resonance set to just 4 (when the cut off is set between 3 and 6).
> > 
> > Is this normal?
> 
> Yes, both mine behave similarly - basically there is way too much gain around the feedback loop.
> 
> > Is there any way to adjust the resonance to a more usable range?
> 
> I tried a couple of things very quickly (the first one was spectacularly ineffective!). However the following seemed OKish: between C10 and Q7 there are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold the faceplate nearest you); swap this for a 5k6 say, which will cut the gain to a third, so it won't resonate until the pot is further round. A 4k7 may not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the 2k2) may restrict the range of frequencies at which it will oscillate. Unfortunately it will also reduce the output amplitude somewhat.
> 
> To do the job 'properly' would mean re-vamping the entire 'gain regime' around the loop, which would require considerably more thought, and replacing maybe half-a-dozen or so resistors. My initial idea was to *double* R5 (the 10k in the middle of the only group of 5 resistors, central to the board), but this seemed to have little impact, mainly I think because the resonance pot is a 'law C' ('rev log'), and so naturally 'all the action' is in the first part of its rotation.
> 
> Let us know if you have any success with this!
> 
> Tim
>

@ Ollie: it is my favourite filter. (Next to the wasp filter, that's
something different).
I use the A102 with very moderate resonance settings. I just love diode
filters.
But the A108 is also very good. But the character is completely different.
It's like comparing apples and pears I'd say.

On Fri, Jan 22, 2010 at 6:16 PM, okmog <olli@lalug.de> wrote:

>
>
> @Tim: WRONG! No, relax, I'm joking ... ;)
>
> @all:
> May I ask you by the way how you like the sound of the diode-ladder filter?
> I mean do you find it more or less musical than the transistor-cascade e.g.
> for bass sounds or leads or so?
> Last question: Does the Doepfer transistor ladder filter really sounds like
> the Moog filter?
>
> I've read your paper about the comparison between the diode- and transistor
> filter, Tim. Good work!
>
> Ollie
>
>
> --- In Doepfer_a100@yahoogroups.com <Doepfer_a100%40yahoogroups.com>,
> "Tim" <timothy@...> wrote:
> >
> > Hi Mick,
> >
> > > Can someone tell me if my A102 filter is acting correctly - The
> resonance seems to be set really high. The filter will self oscillate with
> the resonance set to just 4 (when the cut off is set between 3 and 6).
> > >
> > > Is this normal?
> >
> > Yes, both mine behave similarly - basically there is way too much gain
> around the feedback loop.
> >
> > > Is there any way to adjust the resonance to a more usable range?
> >
> > I tried a couple of things very quickly (the first one was spectacularly
> ineffective!). However the following seemed OKish: between C10 and Q7 there
> are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold
> the faceplate nearest you); swap this for a 5k6 say, which will cut the gain
> to a third, so it won't resonate until the pot is further round. A 4k7 may
> not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the
> 2k2) may restrict the range of frequencies at which it will oscillate.
> Unfortunately it will also reduce the output amplitude somewhat.
> >
> > To do the job 'properly' would mean re-vamping the entire 'gain regime'
> around the loop, which would require considerably more thought, and
> replacing maybe half-a-dozen or so resistors. My initial idea was to
> *double* R5 (the 10k in the middle of the only group of 5 resistors, central
> to the board), but this seemed to have little impact, mainly I think because
> the resonance pot is a 'law C' ('rev log'), and so naturally 'all the
> action' is in the first part of its rotation.
> >
> > Let us know if you have any success with this!
> >
> > Tim
> >
>
>  
>


[Non-text portions of this message have been removed]

> @ Ollie: it is my favourite filter. (Next to the wasp filter, that's
> something different).
> I use the A102 with very moderate resonance settings. I just love diode
> filters.

Wow, that's impressive. I really have to try them!
Thank you.
I think I should try them all! ;)

> But the A108 is also very good. But the character is completely different.
> It's like comparing apples and pears I'd say.
> 
> On Fri, Jan 22, 2010 at 6:16 PM, okmog <olli@...> wrote:
> 
> >
> >
> > @Tim: WRONG! No, relax, I'm joking ... ;)
> >
> > @all:
> > May I ask you by the way how you like the sound of the diode-ladder filter?
> > I mean do you find it more or less musical than the transistor-cascade e.g.
> > for bass sounds or leads or so?
> > Last question: Does the Doepfer transistor ladder filter really sounds like
> > the Moog filter?
> >
> > I've read your paper about the comparison between the diode- and transistor
> > filter, Tim. Good work!
> >
> > Ollie
> >
> >
> > --- In Doepfer_a100@yahoogroups.com <Doepfer_a100%40yahoogroups.com>,
> > "Tim" <timothy@> wrote:
> > >
> > > Hi Mick,
> > >
> > > > Can someone tell me if my A102 filter is acting correctly - The
> > resonance seems to be set really high. The filter will self oscillate with
> > the resonance set to just 4 (when the cut off is set between 3 and 6).
> > > >
> > > > Is this normal?
> > >
> > > Yes, both mine behave similarly - basically there is way too much gain
> > around the feedback loop.
> > >
> > > > Is there any way to adjust the resonance to a more usable range?
> > >
> > > I tried a couple of things very quickly (the first one was spectacularly
> > ineffective!). However the following seemed OKish: between C10 and Q7 there
> > are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold
> > the faceplate nearest you); swap this for a 5k6 say, which will cut the gain
> > to a third, so it won't resonate until the pot is further round. A 4k7 may
> > not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the
> > 2k2) may restrict the range of frequencies at which it will oscillate.
> > Unfortunately it will also reduce the output amplitude somewhat.
> > >
> > > To do the job 'properly' would mean re-vamping the entire 'gain regime'
> > around the loop, which would require considerably more thought, and
> > replacing maybe half-a-dozen or so resistors. My initial idea was to
> > *double* R5 (the 10k in the middle of the only group of 5 resistors, central
> > to the board), but this seemed to have little impact, mainly I think because
> > the resonance pot is a 'law C' ('rev log'), and so naturally 'all the
> > action' is in the first part of its rotation.
> > >
> > > Let us know if you have any success with this!
> > >
> > > Tim
> > >
> >
> >  
> >
> 
> 
> [Non-text portions of this message have been removed]
>

Thanks folks, its good to know at least the filter is not faulty.  Another thing on the list of modifications I will get round to doing one day!

Mick

--- In Doepfer_a100@yahoogroups.com, "okmog" <olli@...> wrote:

>
> > @ Ollie: it is my favourite filter. (Next to the wasp filter, that's
> > something different).
> > I use the A102 with very moderate resonance settings. I just love diode
> > filters.
> 
> Wow, that's impressive. I really have to try them!
> Thank you.
> I think I should try them all! ;)
> 
> 
> > But the A108 is also very good. But the character is completely different.
> > It's like comparing apples and pears I'd say.
> > 
> > On Fri, Jan 22, 2010 at 6:16 PM, okmog <olli@> wrote:
> > 
> > >
> > >
> > > @Tim: WRONG! No, relax, I'm joking ... ;)
> > >
> > > @all:
> > > May I ask you by the way how you like the sound of the diode-ladder filter?
> > > I mean do you find it more or less musical than the transistor-cascade e.g.
> > > for bass sounds or leads or so?
> > > Last question: Does the Doepfer transistor ladder filter really sounds like
> > > the Moog filter?
> > >
> > > I've read your paper about the comparison between the diode- and transistor
> > > filter, Tim. Good work!
> > >
> > > Ollie
> > >
> > >
> > > --- In Doepfer_a100@yahoogroups.com <Doepfer_a100%40yahoogroups.com>,
> > > "Tim" <timothy@> wrote:
> > > >
> > > > Hi Mick,
> > > >
> > > > > Can someone tell me if my A102 filter is acting correctly - The
> > > resonance seems to be set really high. The filter will self oscillate with
> > > the resonance set to just 4 (when the cut off is set between 3 and 6).
> > > > >
> > > > > Is this normal?
> > > >
> > > > Yes, both mine behave similarly - basically there is way too much gain
> > > around the feedback loop.
> > > >
> > > > > Is there any way to adjust the resonance to a more usable range?
> > > >
> > > > I tried a couple of things very quickly (the first one was spectacularly
> > > ineffective!). However the following seemed OKish: between C10 and Q7 there
> > > are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold
> > > the faceplate nearest you); swap this for a 5k6 say, which will cut the gain
> > > to a third, so it won't resonate until the pot is further round. A 4k7 may
> > > not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the
> > > 2k2) may restrict the range of frequencies at which it will oscillate.
> > > Unfortunately it will also reduce the output amplitude somewhat.
> > > >
> > > > To do the job 'properly' would mean re-vamping the entire 'gain regime'
> > > around the loop, which would require considerably more thought, and
> > > replacing maybe half-a-dozen or so resistors. My initial idea was to
> > > *double* R5 (the 10k in the middle of the only group of 5 resistors, central
> > > to the board), but this seemed to have little impact, mainly I think because
> > > the resonance pot is a 'law C' ('rev log'), and so naturally 'all the
> > > action' is in the first part of its rotation.
> > > >
> > > > Let us know if you have any success with this!
> > > >
> > > > Tim
> > > >
> > >
> > >  
> > >
> > 
> > 
> > [Non-text portions of this message have been removed]
> >
>

> I tried a couple of things very quickly (the first one was
> spectacularly ineffective!). However the following seemed OKish:
> between C10 and Q7 there are 4 resistors - the 2k2 amongst these
> is R24 (the 'bottom one' if you hold the faceplate nearest you);
> swap this for a 5k6 say, which will cut the gain to a third, so
> it won't resonate until the pot is further round. A 4k7 may not
> be enough of a difference, and a 6k8 (I temporarily added a 4k7
> to the 2k2) may restrict the range of frequencies at which it
> will oscillate. Unfortunately it will also reduce the output
> amplitude somewhat.
>
> To do the job 'properly' would mean re-vamping the entire 'gain
> regime' around the loop, which would require considerably more
> thought, and replacing maybe half-a-dozen or so resistors. My
> initial idea was to *double* R5 (the 10k in the middle of the
> only group of 5 resistors, central to the board), but this seemed
> to have little impact, mainly I think because the resonance pot
> is a 'law C' ('rev log'), and so naturally 'all the action' is in
> the first part of its rotation.
>
> Let us know if you have any success with this!
>
> Tim

R5 is the right resistor. It defines the max. resonance. If it's value is
increased the max. resonance (and consequently the position where the self
oscillation starts) should be lowered.

Best wishes
Dieter Doepfer