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cgs60 question

cgs60 question

2005-10-04 by Gerald Stevens

Question about the cgs60 stomp box adapter and using it before a filter.
I built a modular C64 SID synth that is a lovely beast, with CV ins and
outs, and I bought a cgs60 to bring its output up to PAIA/Blacet levels.
This works fine - a 3 Vp-p is brought up to about 5Vp-p, nice and clean, and
you can hear the difference if you route it through a VCA. When I run this
through an EFM VCF-2F moog filter, the filter frequency works, but the
resonance control no longer works. This filter uses a LM13700 OTA, and it
has been noted that its output isn't buffered properly... maybe that is
contributing to the problem. Running the amplifed SID signal through a PAIA
9730 multimode filter doesn't work very well either ... but is harder to
characterize. It just seems that the filter's range isn't quite as wide, and
the cuttoff not so severe. Is it possible that these filters require more
current than the op-amp in the cgs60 can give? My PAIA oscillators work
perfectly through both filters. It is a little odd that not all 5V signals
were created equal. Maybe I'm having impedance matching problems?
-gerald


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

Re: cgs60 question

2005-10-04 by sasami@hotkey.net.au

Something odd is going on. The CGS60 gives a 20x gain, and is very high
impedance - around 150k. If you are only getting a fraction of a gain, there
is something seriously wrong with what's driving it. Add a CA3140 or maybe a
TL071 as a non-inverting voltage follower/buffer to lower your output impedance.

Ken
Show quoted textHide quoted text
>Question about the cgs60 stomp box adapter and using it before a filter.
> I built a modular C64 SID synth that is a lovely beast, with CV ins and
>outs, and I bought a cgs60 to bring its output up to PAIA/Blacet levels.
>This works fine - a 3 Vp-p is brought up to about 5Vp-p, nice and clean, and
>you can hear the difference if you route it through a VCA. When I run this
>through an EFM VCF-2F moog filter, the filter frequency works, but the
>resonance control no longer works. This filter uses a LM13700 OTA, and it
>has been noted that its output isn't buffered properly... maybe that is
>contributing to the problem. Running the amplifed SID signal through a PAIA
>9730 multimode filter doesn't work very well either ... but is harder to
>characterize. It just seems that the filter's range isn't quite as wide, and
>the cuttoff not so severe. Is it possible that these filters require more
>current than the op-amp in the cgs60 can give? My PAIA oscillators work
>perfectly through both filters. It is a little odd that not all 5V signals
>were created equal. Maybe I'm having impedance matching problems?
> -gerald
>
>
>[Non-text portions of this message have been removed]
>
>
>
>
>The CGS Modular Synth home page: http://www.cgs.synth.net/
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
_______________________________________________________________________
Ken Stone sasami@... or sasami@...
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>

Re: cgs60 question

2005-10-04 by Andrew Sanchez

Hi Gerald,
I wonder if the EFM filter input is being overdriven? I have an EFM VCF-1e,
and if I feed an Oakley or MOTM osc to it, I get the same symtom as you
describe. If I attenuate the oscillator outputs before feeding them into the
EFM filter, the resonance works as it should.

Because of the above, I've assumed that the EFM filters I own were designed
to be used with standard line level signals, but I could be wrong.

Andrew Sanchez
Show quoted textHide quoted text
On 10/3/05, Gerald Stevens <logo64@...> wrote:
>
> Question about the cgs60 stomp box adapter and using it before a filter.
> I built a modular C64 SID synth that is a lovely beast, with CV ins and
> outs, and I bought a cgs60 to bring its output up to PAIA/Blacet levels.
> This works fine - a 3 Vp-p is brought up to about 5Vp-p, nice and clean,
> and
> you can hear the difference if you route it through a VCA. When I run this
> through an EFM VCF-2F moog filter, the filter frequency works, but the
> resonance control no longer works. This filter uses a LM13700 OTA, and it
> has been noted that its output isn't buffered properly... maybe that is
> contributing to the problem. Running the amplifed SID signal through a
> PAIA
> 9730 multimode filter doesn't work very well either ... but is harder to
> characterize. It just seems that the filter's range isn't quite as wide,
> and
> the cuttoff not so severe. Is it possible that these filters require more
> current than the op-amp in the cgs60 can give? My PAIA oscillators work
> perfectly through both filters. It is a little odd that not all 5V signals
> were created equal. Maybe I'm having impedance matching problems?
> -gerald
>
>


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

Re: cgs60 question

2005-10-04 by Gerald Stevens

Ken,
Sorry... I forgot to mention that I set it up for 2x gain... not much, but
just enough to bring the SID up to modular specs.
The reason that I built it in the first place is that I ran into the same
problem with the VCF-2F not having any resonance
with the SID output fed in directly. I thought the VCF just needed a
slightly hotter input signal, but that doesn't seem to
be the case. Is it possible that the VCF wants a lot more current than the
TL071 can give?
-gerald
Show quoted textHide quoted text
On 10/3/05, sasami@... <sasami@...> wrote:
>
> Something odd is going on. The CGS60 gives a 20x gain, and is very high
> impedance - around 150k. If you are only getting a fraction of a gain,
> there
> is something seriously wrong with what's driving it. Add a CA3140 or maybe
> a
> TL071 as a non-inverting voltage follower/buffer to lower your output
> impedance.
>
> Ken
>
>


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

Re: cgs60 question

2005-10-05 by sasami@hotkey.net.au

>Is it possible that the VCF wants a lot more current than the
>TL071 can give?

I'd hope not. Input impedances should be 100k or greater if its designed right.

Ken



>Ken,
> Sorry... I forgot to mention that I set it up for 2x gain... not much, but
>just enough to bring the SID up to modular specs.
> The reason that I built it in the first place is that I ran into the same
>problem with the VCF-2F not having any resonance
>with the SID output fed in directly. I thought the VCF just needed a
>slightly hotter input signal, but that doesn't seem to
>be the case. Is it possible that the VCF wants a lot more current than the
>TL071 can give?
> -gerald
>
> On 10/3/05, sasami@... <sasami@...> wrote:
>>
>> Something odd is going on. The CGS60 gives a 20x gain, and is very high
>> impedance - around 150k. If you are only getting a fraction of a gain,
>> there
>> is something seriously wrong with what's driving it. Add a CA3140 or maybe
>> a
>> TL071 as a non-inverting voltage follower/buffer to lower your output
>> impedance.
>>
>> Ken
>>
>>
>
>
>[Non-text portions of this message have been removed]
>
>
>
>
>The CGS Modular Synth home page: http://www.cgs.synth.net/
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
_______________________________________________________________________
Ken Stone sasami@... or sasami@...
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>

Re: cgs60 question

2005-10-05 by Dave Kendall

Hi.

FWIW, I got exactly the same symptoms with an EFM VCF6C. When feeding a
K2000 signal into it, it worked fine, but an EFM VCO3D higher (pk to pk)
produced the same effect. The VCF6C, like the 1E and 2F, also has a
LM13600/13700. Both buffers are used in the 6C, and in the output stages of
both filters in the 1E circuit. Input impedance maybe?

Dave.


on 5/10/05 1:25, Andrew Sanchez at synthasaurus@... wrote:

Hi Gerald,
I wonder if the EFM filter input is being overdriven? I have an EFM VCF-1e,
and if I feed an Oakley or MOTM osc to it, I get the same symtom as you
describe. If I attenuate the oscillator outputs before feeding them into the
EFM filter, the resonance works as it should.

Because of the above, I've assumed that the EFM filters I own were designed
to be used with standard line level signals, but I could be wrong.

Andrew Sanchez

Andrew Sanchez
Show quoted textHide quoted text
On 10/3/05, Gerald Stevens <logo64@...> wrote:
>
> Question about the cgs60 stomp box adapter and using it before a filter.
> I built a modular C64 SID synth that is a lovely beast, with CV ins and
> outs, and I bought a cgs60 to bring its output up to PAIA/Blacet levels.
> This works fine - a 3 Vp-p is brought up to about 5Vp-p, nice and clean,
> and
> you can hear the difference if you route it through a VCA. When I run this
> through an EFM VCF-2F moog filter, the filter frequency works, but the
> resonance control no longer works. This filter uses a LM13700 OTA, and it
> has been noted that its output isn't buffered properly... maybe that is
> contributing to the problem. Running the amplifed SID signal through a
> PAIA
> 9730 multimode filter doesn't work very well either ... but is harder to
> characterize. It just seems that the filter's range isn't quite as wide,
> and
> the cuttoff not so severe. Is it possible that these filters require more
> current than the op-amp in the cgs60 can give? My PAIA oscillators work
> perfectly through both filters. It is a little odd that not all 5V signals
> were created equal. Maybe I'm having impedance matching problems?
> -gerald




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

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