Crown D75 Modification

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What is the modification procedure for configuring the Crown D75 for
bridge-mono?
 
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Never mind. I found it. Looks like a fairly simple task to bring the
jumper functionality out to a switch between the two input jacks.

Greg Taylor wrote:

> What is the modification procedure for configuring the Crown D75 for
> bridge-mono?
 
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Greg Taylor <gtaylor@umd.umich.edu> wrote:
>What is the modification procedure for configuring the Crown D75 for
>bridge-mono?

Make a Y cable with pins 2 and 3 swapped on one output, so that the
two outputs have different polarity. Apply your signal to the inputs
through that. Your signal comes off between the two driven leads of
the output. I think the grounds are already tied together internally
but adding a jumper won't hurt.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
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Greg Taylor <gtaylor@umd.umich.edu> wrote:
>
>Which leads me to a more general question. If the normal phase
>difference between the two channels is 180 degrees does it make sense to
>swap pins 2 and 3 on one of the inputs, or rather change the polarity of
>the speaker connection on one channel, in 'stereo' mode to avoid LF
>cancellation effects from the two speakers?

I'm not sure what you're talking about here? Are you talking about bridged
mono?

The whole idea of bridged mono is that you have both outputs referenced
to ground. You put a signal of opposite polarity into each input, so
now you have the two outputs swinging in opposite directions. This is
like putting two amplifiers in series with one another... you get twice
the output voltage but you don't get any more current. You can't swap the
polarity on the output side because the grounds are already tied together.
If you floated one output with a transformer you could, but that gets silly.

If you are talking about the easiest way to swap polarity on one channel
of a stereo system, it really doesn't make any difference whether you swap
on the input or the output. If you're using banana plugs for speakers, it
is easier to swap the speaker side. If you're using Speakon connectors,
it's probably easier to swap the input side.
--scott


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"Greg Taylor" <gtaylor@umd.umich.edu> wrote in message
news:KaaHd.338$UN1.255@news.itd.umich.edu...
> My question was: If the normal stereo (not bridged mono) operation of
> this amplifier internally reverses the polarity of one output with
> respect to the other doesn't that create a 180 degree phase shift
> between the two channels? If so, wouldn't you encounter phase
> cancellation issues with mono material, especially low frequency signals?

Yes; that's why amps like this had the output terminals hooked up backwards
in the reversed channel, so the red connector (or equivalent) was hooked to
ground, the black to signal output. The manuals had stern warnings not to
use speaker switchers that connected the two black terminals.

The discussion is academic, though; I haven't seen an amp like that in
*decades*. They date from the very early solid-state era; having the two
channels in opposite polarities, on a single-supply amp, meant that
(mostly-mono) bass signals theoretically put less demand on the cheap power
supply, since one would be swinging upward while the other swung down, and
the current demands would cancel. Of course, record warps would then add...

Peace,
Paul
 

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