A couple of sections of the 4PDT TB switch in my Soundcraft Delta mute the
L/R inputs to the control room monitor line amps by shorting the inputs to
ground.

Like many of the more modern consoles, I'd like to have variable dim
so that a full-duplex conversation could take place.

Curiously, the current L/R paths to ground in the TB switching circuit go
through 0 (zero) ohm resistors. I'd never seen such a thing but yup, there
they are on the schematic and on the PC board (color code is a single
black stripe, and they do indeed measure 0 ohms).

So I'm wondering now if those resistors are in there as "placeholders" for
some value that would yield a dim function, possibly even a place to
attach the wiper and one leg of a pot to provide variable dim. (Though why
they wouldn't just use wire links in place of the resistors is an
interesting question.)

I'm thinking a 10K pot would do it, maybe a 20K or even a 50K if I still
had too much muting. The amp input resistor is 4.7K; the amp feedback is
20K (this is a TL072) so there is a bit of gain.

Does that dim pot value sound reasonable? (Obviously, one wants to beware
of feedback due to insufficient dimming.) Is there something special about
a zero ohm resistor?

Any Delta owners make a similar mod?

Thanks in advance,

Frank Stearns
Mobile Audio
--

So I'm wondering now if those resistors are in there as
"placeholders" for
some value that would yield a dim function, possibly even a place to
attach the wiper and one leg of a pot to provide variable dim.
(Though why
they wouldn't just use wire links in place of the resistors is an
interesting question.)



Yes 0 Ohm jumpers are often used as placeholders just as you say.
With automatic machine placement of SMT parts, they are essentially
free.

Mark

In article writes:

A couple of sections of the 4PDT TB switch in my Soundcraft Delta mute the
L/R inputs to the control room monitor line amps by shorting the inputs to
ground.

Curiously, the current L/R paths to ground in the TB switching circuit go
through 0 (zero) ohm resistors. I'd never seen such a thing but yup, there
they are on the schematic and on the PC board (color code is a single
black stripe, and they do indeed measure 0 ohms).

So I'm wondering now if those resistors are in there as "placeholders" for
some value that would yield a dim function

Could be. Put a real resistor in their place. Start with 5K and see
what it does for you.

possibly even a place to
attach the wiper and one leg of a pot to provide variable dim. (Though why
they wouldn't just use wire links in place of the resistors is an
interesting question.)

I suspect that they used the zero ohm resistors rather than wire
jumpers because they could be used with automatic component insertion
equipment rather than using a hand operation in PC board assembly. You
probably don't really need to make it variable, just find a level
where you can hear and the talkback mic doesn't feed back.



--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

4.7 k will dim -6 db. 470 ohms will dim -20 db.

Have Fun!

Jim Williams
Audio Upgrades

4.7 k will dim -6 db. 470 ohms will dim -20 db.

Have Fun!

Jim Williams
Audio Upgrades