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Thread

A-131 Increased Gain modification

2006-08-21 by Sean

I'm trying to modify my old style A-131 Exp. VCA module to get a 
gain >1 as the output often seems weedy and I would like the option 
of a little (or a lot of) overdrive.

I'm guessing that I can do this in 2 ways:
1. change the resistors connected to the audio In pots to a lower value.
2. change the resistor connected to the output voltage of the CV circuit.

So the question is... is there any information in english that will 
help me identify the correct resistors to change?

Re: [Doepfer_a100] A-131 Increased Gain modification

2006-08-21 by Florian Anwander

hi Sean

> I'm trying to modify my old style A-131 Exp. VCA module to get a 
> gain >1 as the output often seems weedy and I would like the option 
> of a little (or a lot of) overdrive.
> 
> I'm guessing that I can do this in 2 ways:
> 1. change the resistors connected to the audio In pots to a lower value.
> 2. change the resistor connected to the output voltage of the CV circuit.
I think the VCAs OTA is not the place where you should change the level. 
Usually a change of amplification should be done by decreasing the 
feedback resistor of an OpAmp. As far as I remember, the old A131 with 
the Curtis CEM3382 does not use an separate buffer stage at the output 
(thats where you might do this). So I suggest to modify a Mixer A-138, 
where you could change either the feedback of the summing amp (to 
amplify all four inputs), or change the input resistor of one channel.

Florian

AW: [Doepfer_a100] A-131 Increased Gain modification

2006-08-21 by hardware@doepfer.de

To increase the output level I'd recommend to increase R17 (is now 15k). To
increase the input level causing clipping/overdrive at higher input levels
I'd recommend to increase R12 (is now 47k) or to lower R13 (is now 10k).

Best wishes
Dieter Doepfer

P.S. These statements are valid only for the new version of the A-131 (not
for the CEM based old version).

Show quoted textHide quoted text

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com
> [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von Sean
> Gesendet: Montag, 21. August 2006 13:29
> An: doepfer_a100@yahoogroups.com
> Betreff: [Doepfer_a100] A-131 Increased Gain modification
>
>
> I'm trying to modify my old style A-131 Exp. VCA module to get a
> gain >1 as the output often seems weedy and I would like the option
> of a little (or a lot of) overdrive.
>
> I'm guessing that I can do this in 2 ways:
> 1. change the resistors connected to the audio In pots to a lower value.
> 2. change the resistor connected to the output voltage of the CV circuit.
>
> So the question is... is there any information in english that will
> help me identify the correct resistors to change?
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>

AW: [Doepfer_a100] A-131 Increased Gain modification

2006-08-21 by hardware@doepfer.de

Sorry, I overlooked that Sean was asking for the old version of the A-131:

Increasing the input level: increase R10 (now 47k)
Increasing the output level: increase R12 (now 47k)

Best wishes
Dieter Doepfer

Show quoted textHide quoted text

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com
> [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von Florian Anwander
> Gesendet: Montag, 21. August 2006 14:14
> An: Doepfer_a100@yahoogroups.com
> Betreff: Re: [Doepfer_a100] A-131 Increased Gain modification
>
>
> hi Sean
>
> > I'm trying to modify my old style A-131 Exp. VCA module to get a
> > gain >1 as the output often seems weedy and I would like the option
> > of a little (or a lot of) overdrive.
> >
> > I'm guessing that I can do this in 2 ways:
> > 1. change the resistors connected to the audio In pots to a lower value.
> > 2. change the resistor connected to the output voltage of the
> CV circuit.
> I think the VCAs OTA is not the place where you should change the level.
> Usually a change of amplification should be done by decreasing the
> feedback resistor of an OpAmp. As far as I remember, the old A131 with
> the Curtis CEM3382 does not use an separate buffer stage at the output
> (thats where you might do this). So I suggest to modify a Mixer A-138,
> where you could change either the feedback of the summing amp (to
> amplify all four inputs), or change the input resistor of one channel.
>
> Florian
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>

Re: A-131 Increased Gain modification

2006-08-21 by stupidpaddy

--- In Doepfer_a100@yahoogroups.com, <hardware@...> wrote:
>
> Sorry, I overlooked that Sean was asking for the old version of the A-131:
> 
> Increasing the input level: increase R10 (now 47k)
> Increasing the output level: increase R12 (now 47k)
> 
> Best wishes
> Dieter Doepfer
> 
> 
> > -----Ursprüngliche Nachricht-----
> > Von: Doepfer_a100@yahoogroups.com
> > [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von Florian Anwander
> > Gesendet: Montag, 21. August 2006 14:14
> > An: Doepfer_a100@yahoogroups.com
> > Betreff: Re: [Doepfer_a100] A-131 Increased Gain modification
> >
> >
> > hi Sean
> >
> > > I'm trying to modify my old style A-131 Exp. VCA module to get a
> > > gain >1 as the output often seems weedy and I would like the option
> > > of a little (or a lot of) overdrive.
> > >
> > > I'm guessing that I can do this in 2 ways:
> > > 1. change the resistors connected to the audio In pots to a lower value.
> > > 2. change the resistor connected to the output voltage of the
> > CV circuit.
> > I think the VCAs OTA is not the place where you should change the level.
> > Usually a change of amplification should be done by decreasing the
> > feedback resistor of an OpAmp. As far as I remember, the old A131 with
> > the Curtis CEM3382 does not use an separate buffer stage at the output
> > (thats where you might do this). So I suggest to modify a Mixer A-138,
> > where you could change either the feedback of the summing amp (to
> > amplify all four inputs), or change the input resistor of one channel.
> >
> > Florian
> >
> >
> >
> > Yahoo! Groups Links
> >


hi Dieter

Is there a PDF available for the service manual for the A-131 to help me identify the 
correct resistors?

vielen dank
sean

Re: A-131 Increased Gain modification

2006-08-22 by ilanode

> The numbers of the resistors are printed on the pcb.
The part numbers are often concealed by the part itself.

Re: [Doepfer_a100] Re: A-131 Increased Gain modification

2006-08-22 by Florian Anwander

Hi Sean

>>Increasing the input level: increase R10 (now 47k)
>>Increasing the output level: increase R12 (now 47k)
> Is there a PDF available for the service manual for the A-131 to help me identify the 
> correct resistors?
The numbers of the resistors are printed on the pcb.

Florian

AW: [Doepfer_a100] Re: A-131 Increased Gain modification

2006-08-22 by hardware@doepfer.de

Sean,

I'll send you the pcb silk screen to your email address.

Best wishes
Dieter Doepfer

Show quoted textHide quoted text

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com
> [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von stupidpaddy
> Gesendet: Montag, 21. August 2006 19:51
> An: Doepfer_a100@yahoogroups.com
> Betreff: [Doepfer_a100] Re: A-131 Increased Gain modification
>
>
>
> --- In Doepfer_a100@yahoogroups.com, <hardware@...> wrote:
> >
> > Sorry, I overlooked that Sean was asking for the old version of
> the A-131:
> >
> > Increasing the input level: increase R10 (now 47k)
> > Increasing the output level: increase R12 (now 47k)
> >
> > Best wishes
> > Dieter Doepfer
> >
> >
> > > -----Ursprüngliche Nachricht-----
> > > Von: Doepfer_a100@yahoogroups.com
> > > [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von Florian Anwander
> > > Gesendet: Montag, 21. August 2006 14:14
> > > An: Doepfer_a100@yahoogroups.com
> > > Betreff: Re: [Doepfer_a100] A-131 Increased Gain modification
> > >
> > >
> > > hi Sean
> > >
> > > > I'm trying to modify my old style A-131 Exp. VCA module to get a
> > > > gain >1 as the output often seems weedy and I would like the option
> > > > of a little (or a lot of) overdrive.
> > > >
> > > > I'm guessing that I can do this in 2 ways:
> > > > 1. change the resistors connected to the audio In pots to a
> lower value.
> > > > 2. change the resistor connected to the output voltage of the
> > > CV circuit.
> > > I think the VCAs OTA is not the place where you should change
> the level.
> > > Usually a change of amplification should be done by decreasing the
> > > feedback resistor of an OpAmp. As far as I remember, the old A131 with
> > > the Curtis CEM3382 does not use an separate buffer stage at the output
> > > (thats where you might do this). So I suggest to modify a Mixer A-138,
> > > where you could change either the feedback of the summing amp (to
> > > amplify all four inputs), or change the input resistor of one channel.
> > >
> > > Florian
> > >
> > >
> > >
> > > Yahoo! Groups Links
> > >
>
>
> hi Dieter
>
> Is there a PDF available for the service manual for the A-131 to
> help me identify the
> correct resistors?
>
> vielen dank
> sean
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>