Doepfer

--- Dieter Doepfer <hardware@doepfer.de> wrote:
> From: "Dieter Doepfer" <hardware@doepfer.de>
> To: "Bakis Sirros" <synth_freak_2000@yahoo.com>
> Subject: AW: [Doepfer_a100] Re: Modified my A-117
> Digital Noise Generator
> Date: Thu, 5 Jun 2003 18:58:58 +0200
> 
> > Re: Modified my A-117 Digital Noise Generator
> >
> > > The modification requires cutting 4 tracks and
> adding 4 straps (and
> > > is moderately fiddly). If anyone else has been
> experiencing this
> > > frustration, and wants to know the details, let
> me know and I shall
> > > write some instructions and post them (and if I
> can persuade my mate
> > > with the digital camera to visit and take a few
> snaps of it, post a
> > > few pictures too).
> >
> > For those that expressed an interest in this, I
> have created a folder
> > in the 'Files' section containing a text file of
> instructions, and
> > three photos of my modified unit. (Thanks go to
> Mike for the photos.)
> >
> > Any questions or comments, then get in touch!
> 
> Roland Mayer informed me about a simple solution for
> the described A-117
> problem.
> 
> He found the reason for the problem described: the
> A-117 clock input is DC
> coupled (simply a resistor that is connected to the
> base of a standard npn
> transistor). This leads to problems if AC coupled
> signals are used as the
> capacitors in the AC coupling circuit cause charging
> the transistor base.
> The combination A-111/A-117 is such an example as
> the waveform outputs of
> the A-111 are AC coupled. One solution is to bypass
> the capacitors in the
> A-111 output section (i.e. short circuit the
> capacitors). But the more
> general an more simple solution is to add a 100k
> resistor that connects the
> A-117 clock input to GND (e.g. simply across the
> jack socket). We will
> modify all A-117 in the future and add this resistor
> to all A-117 clock
> inputs.
> 
> We will add this item to the FAQ and DIY section in
> the near future.
> 
> Best regards
> Dieter Doepfer
> Doepfer Musikelektronik GmbH
> Email address: hardware@doepfer.de
> Web Site: www.doepfer.com
> 


=====
synthfreak(parallel worlds)
athens-greece
[Doepfer_a100] group owner

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> > Roland Mayer informed me about a simple solution for
> > the described A-117
> > problem.
> > 
> > He found the reason for the problem described: the
> > A-117 clock input is DC
> > coupled (simply a resistor that is connected to the
> > base of a standard npn
> > transistor). This leads to problems if AC coupled
> > signals are used as the
> > capacitors in the AC coupling circuit cause charging
> > the transistor base.
> > The combination A-111/A-117 is such an example as
> > the waveform outputs of
> > the A-111 are AC coupled. One solution is to bypass
> > the capacitors in the
> > A-111 output section (i.e. short circuit the
> > capacitors). But the more
> > general an more simple solution is to add a 100k
> > resistor that connects the
> > A-117 clock input to GND (e.g. simply across the
> > jack socket). We will
> > modify all A-117 in the future and add this resistor
> > to all A-117 clock
> > inputs.
> > 
> > We will add this item to the FAQ and DIY section in
> > the near future.

Dieter,
Thanks for this - yes, a much simpler solution: indeed I tried it 
out, and for my A-111 it does seem to solve the problem (using the 
square output only). However with my A-110's it is still marginal (at 
many frequencies you can hear the 'noise' breaking into a clearly 
periodic tone). Anyhow I have added a comment to my instructions 
pointing this out to people, so that they know a solution may be 
possible with an easier fix.

(I shall have to read up on the mechanism as to why this helps: with 
the resistor, SPICE simulation shows a significant improvement on the 
rise times at the collector, but little difference on the fall times, 
which ties in with my current understanding that base-collector 
capacitance has a greater effect as the transistor comes out of 
saturation. Since the 4006 is clocked on the high to low transition, 
I thus don't see why it should help at all!)

Regards,
Tim

hi list

yes it seems easy to modify the A177.
I really want to do this.
Is there any possibility to have a pictutre or scheme about the 
modification.
I am french and not so specialist of electronics so I am scared to 
not totally get the manipulation.

Also yes, why not start a pole for the manual trigger mixer.

Alexandre