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CGS33 Matrix Mixer Basic Circuit Questions

CGS33 Matrix Mixer Basic Circuit Questions

2007-03-12 by peng3002

I'm building a Matrix Mixer for a portable modular instrument. I have
a couple questions about the circuit.

http://www.cgs.synth.net/modules/cgs33_matrix_mixer.html


The first schematic is the buffer stage. I've built many a buffer
stage like this except without the 1K resistor. What purpose does this
1K resistor serve?


The second schematic (column mixer) op amp output stage. I'd like to
know more about what's going on here. I've seen an Oakley design that
uses a 75R (instead of 220R) and a 22K (instead of 47K)in the feedback
loop. In the Oakley user guide it explains that this enables it to
drive larger capacitive loads. So, I think I get that part of the
circuit.
My question is: What are the parallel diodes and 4.7K and 390R that
are in the lower feedback loop doing? This is very interesting.

Thanks,
peng

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-12 by bbluthang

--- In cgs_synth@yahoogroups.com, "peng3002" <peng3002@...> wrote:
Show quoted textHide quoted text
> The first schematic is the buffer stage. I've built many a buffer
> stage like this except without the 1K resistor. What purpose does this
> 1K resistor serve?
>
I assume the 1k/100k were there to provide a hi-Z input, as Ken wrote
- " Any signal applied to an input is first buffered by a voltage
follower based on a TL071 op-amp so as to not load down the output of
whatever module is driving it. "


> My question is: What are the parallel diodes and 4.7K and 390R that
> are in the lower feedback loop doing? This is very interesting.
>
this was asked a few weeks ago -

">What do the diodes do in the CGS33 matrix mixer? Do they prevent
>signal overload (limiting)?

Yes, they are a little clipper on the output amp. They and their
associated
resistor divider can be omitted if desired."

hope this helps a bit
andrew

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-12 by peng3002

--- In cgs_synth@yahoogroups.com, "bbluthang" <jumpswine@...> wrote:
Show quoted textHide quoted text
> >
> I assume the 1k/100k were there to provide a hi-Z input, as Ken wrote
> - " Any signal applied to an input is first buffered by a voltage
> follower based on a TL071 op-amp so as to not load down the output of
> whatever module is driving it. "

>
> hope this helps a bit
> andrew
>


Thanks for answering the second question.

As far as the buffer stage goes, even without the 1K resistor it has a
high impedance (100K). What does the 1K resistor do? I'm guessing that
I'd be fine without it, but I would like to employ it in other designs
if it lends some added benefit.

Thanks,
peng

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-13 by Richard Brewster

You sometimes see a series resistor, 1K or maybe 10K, on the input pin
of an op amp connected to a panel jack. The resistor does nothing to
change the behavior of the circuit, which is a unity-gain buffer. It
may provide a measure of protection for the op amp from static electric
charges that could enter via the jack by limiting the current that such
very high voltages might generate.

Richard Brewster
http://www.pugix.com
Show quoted textHide quoted text
>
> Thanks for answering the second question.
>
> As far as the buffer stage goes, even without the 1K resistor it has a
> high impedance (100K). What does the 1K resistor do? I'm guessing that
> I'd be fine without it, but I would like to employ it in other designs
> if it lends some added benefit.
>
> Thanks,
> peng
>
>
>

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-13 by Andre Majorel

On 2007-03-13 00:40 -0000, peng3002 wrote:
Show quoted textHide quoted text
> As far as the buffer stage goes, even without the 1K resistor it has a
> high impedance (100K). What does the 1K resistor do? I'm guessing that
> I'd be fine without it, but I would like to employ it in other designs
> if it lends some added benefit.

Current limiting in case someone plugs ±15 V into the module
while it is not powered ? Though I would expect the resistor to
be followed by a pair of reverse-biased diodes for good measure.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
Do not use this account for regular correspondence.
See the URL above for contact information.

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-13 by peng3002

Thanks AM and RB for the explanation. Sounds like good design
practice. I'll use it from now on. Thanks, Ken, for exposing me to yet
another nice subcircuit.


peng
Show quoted textHide quoted text
--- In cgs_synth@yahoogroups.com, Andre Majorel <aym-htnys@...> wrote:
>
> On 2007-03-13 00:40 -0000, peng3002 wrote:
>
> > As far as the buffer stage goes, even without the 1K resistor it has a
> > high impedance (100K). What does the 1K resistor do? I'm guessing that
> > I'd be fine without it, but I would like to employ it in other designs
> > if it lends some added benefit.
>
> Current limiting in case someone plugs ±15 V into the module
> while it is not powered ? Though I would expect the resistor to
> be followed by a pair of reverse-biased diodes for good measure.
>
> --
> André Majorel <URL:http://www.teaser.fr/~amajorel/>
> Do not use this account for regular correspondence.
> See the URL above for contact information.
>

Re: CGS33 Matrix Mixer Basic Circuit Questions

2007-03-13 by sasami@hotkey.net.au

>The first schematic is the buffer stage. I've built many a buffer
>stage like this except without the 1K resistor. What purpose does this
>1K resistor serve?

Protection for the chip. It allows the chip to deal with anything odd that
gets plugged into it by restricting current flow. As we are using this as a
voltage follower, limiting the current flow has virtually zero effect on
normal operation.

>The second schematic (column mixer) op amp output stage. I'd like to
>know more about what's going on here. I've seen an Oakley design that
>uses a 75R (instead of 220R) and a 22K (instead of 47K)in the feedback
>loop. In the Oakley user guide it explains that this enables it to
>drive larger capacitive loads. So, I think I get that part of the
>circuit.

220R restricts the maximum drive output from the op-amp, again to protect
the op-amp from whatever it gets plugged into - such as accidently plugging
it into another output. the 47k is arbitary. You could replace them all with
100k.

>My question is: What are the parallel diodes and 4.7K and 390R that
>are in the lower feedback loop doing? This is very interesting.

Output clipper. It merely limits the maximum swing of the output. It can be
left off with no ill effects. I don't know why I bothered to include it in
the first place.

Ken
_______________________________________________________________________
Ken Stone sasami@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>

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