Odp: [sdiy] 1/8th inch questions

[Gene Stopp]

It's a good question - the answer's not obvious and it does bring up some
side issues. Many times a signal is used internally to the module as well as
being brought out as an output. For example, in a VCO, the core could be
triangle-based and then the triangle wave is used as the source of the
triangle-to-sawtooth converter, the sine converter, and the pulse wave
comparator. If the triangle wave is brought directly out without a series
resistor, and then you insert a tip/ground cable into that output, and then
start to patch it into another jack, when the tip of the jack touches the
sleeve of the (common grounded of course) other jack, the whole oscillator
will "die". If you are listening to it while you are patching it, you'll
hear it glitch when you plug in that last cable. Series output resistors
will prevent this.

Also, no matter what the data sheets say, I just get nervous if I know that
I'm temporarily shorting mutli-volt signal sources to ground repeatedly. Not
nervous because of the energies involved, not nervous because I fear
embarrasment in front of a huge crowd under hot stage lights, but rather
nervous because I don't want to have to pull the *&%&%$#^% thing apart to
change a chip every couple months. Homebuilt stuff is sometimes just not
easy to take apart.

Also with the main pitch CV, the direct (non-resistor) op-amp output is best
to avoid scale errors due to loading, but this is subject to the same
"patchcord events". What I do with those is provide two or three output
jacks, and have a single op-amp buffer on each alone.

- Gene


-----Original Message-----
From: Seb Francis [mailto:seb at is-uk.com]
Sent: Tuesday, October 22, 2002 4:46 PM
To: John Blacet
Cc: synth diy
Subject: Re: Odp: [sdiy] 1/8th inch questions


So why use a resistor on the output at all if the buffer opamp can withstand
indefinite short circuits?  Is it something to do with not glitching the
power bus?  I guess the normal short circuit situation is simply plugging
and unplugging jacks.

Seb



John Blacet wrote:

> I believe that this refers to the practice of including the output
> resistor (1K) in the feedback loop of the op amp. For example, if you
> have a 100K feedback R and a 1K output R, the actual output is taken
> from the junction of the two, not the output pin of the amp. Of course,
> this changes the actual gain, so you have to figure the Rs for that. I'm
> not sure if this is something you can do with a buffer configuration.
>
> In practice, we have opted to use a 100 ohm resistor for the output. The
> 074X series for example, is rated for indefinite short circuits to the
> supplies and ground. The 100 ohm is just extra current limiting, but
> gives us 10X the "fanout" of the 1K typically used.
>
> --
> Regards,
> --/////--
> John Blacet
> Blacet Research
> http://www.blacet.com