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From: manfredi@rockwell.com (Bert Manfredi, 747-6735)
Subject: Re: Bridging - what is it & how?
Date: Tue, 28 Jun 1994 14:11:00 GMT

In article <2ui442$13b7@introl.introl.com>, cam@ukelele.GCR.COM (Christian MacDonald) writes...

>What is meant when one says that they are 'bridging their amp'?  My
>understanding is that this increases the power of the amp, but my
>understanding of electronics is so bad that I don't know what is being
>bridged or how this benefits a system.

Normally, an amp's output consists of one hot and one ground. Picture the
hot lead, usually a red connector, as being the one which carries the
music signal, and the ground is merely a reference point (voltage is a
potential difference between two points).

Now, what if we had that second lead, the one which would normally be ground,
also carry the signal, but exactly 180 degrees out of phase with the
original hot lead? The two stereo channels of an amp would be needed to
drive one speaker, but you would have at least twice the power for a
given load impedance.

>I have a Rotel RA-820BX integrated amp.  Can this amp be bridged?  Why or
>why not.

Any amp can be bridged, but most would require the addition of a phase
inverter on one of the inputs. If the Rotel is set up at the factory for
bridging, you'd see a switch on the back panel for this purpose.

To bridge an amp which was not set up for this, a simple way is to use
an opamp (many people cringe, I know) set up as a 1X gain phase inverter
on one channel, say the left channel, and send the audio signal from
the right channel's RCA input jack to this phase inverter as well as
to its usual right channel. The left channel's input jack would be unused.

The output of the amp is now available from the left and right channels'
red connectors. Only one channel of output, of course. You would wire the
speaker's + terminal to the right red output connector and the speaker's
- lead to the left red output connector. The black output connectors
would be unused.

Bert