Q: Is it possible to bridge a Phase Linear amplifier for mono operation?

A: It's possible but I don't recommend bridging Phase Linear consumer amplifiers. In the bridge mode, each channel sees half the load impedance; i.e., 4 ohms when driving an 8 ohm speaker. The amp will run warmer as a result. Early 400's had a single heatsink per channel which is insufficient for 4 ohm loads at high power. Series One 200's and 200B's don't drive 4 ohm loads well either. I advise buying an amp already designed for bridging. All Phase Linear Professional Series amplifiers have a separate mono bridging input. Many other professional and commercial amplifiers have mono bridging inputs as well. However, if you must bridge your amplifier, read on.

1. Bridging a stereo amplifier requires inverting the signal to one channel and taking the output from the two positive (+) terminals. The grounds are connected internally.

2. Inverting the signal can be done with an active phase inverter circuit or with a centertapped transformer (see the original mono service bulletin). The transformer method is simpler.

3. Bridging will quadruple the single channel power output. For example: a 200 watt/channel amplifer will theoretically produce 800 watts in mono. Actual maximum power is limited by the power supply and typically will produce about three times the single channel power output (600 watts in this example).

4. The above service bulletin refers to a Triad G-336 transformer which has been out of production for many years. Any high quality shielded audio transformer will work. The input and output impedance should be approximately the same (the centertap cuts the output impedance to each channel in half). Check with your local industrial electronics shops. Radio Shack doesn't stock such a transformer.

5. For 4 ohm operation use 8 A supply fuses for either a 400 or 700 amplifier.

6. If all else fails, see answer above. You're on your own!

NOTE:
NEVER parallel the outputs of a stereo amplifier (tie both channel + leads together) for mono operation. Doing so may cause the amplifier to fail. This applies to all direct coupled amplifiers.