Using a Mac Stereo Amp for Mono Operation?

[70sMac]

A question came up on another forum board that got me to thinking. I've definitely heard of Mac stereo amps that [literally] allow switching between stereo and mono operation, but what about those that don't come with that kind of a switch?

I also seem to remember that "bridging" changes the output impedance of the transformer secondaries to half their design value. As everything we presently have for a speaker is 8 ohms, I'm thinking that -- regardless of what you do -- you end up with the original rated output. That's to say that the bridging of two amps theoretically renders twice the power, but, in the end, the impedance mismatch between the amplifier output and said speaker reduces the bridged power increase by a factor of 0.5. Does that sound about right?

I'm certainly no audio expert, but one of my USAF specialties, when I wasn't mobilized on one of Reagan's many alerts, was to repair, align and test aircraft test equipment...so parallel resistance and complex impedance aren't entirely alien to me. I'm just looking for some thoughts from the audio circuitry experts here on the AAF ... and, in case you're wondering exactly why, I'm thinking about looking for a stereo amp that can be used to drive a single center speaker.

Thank you for your thoughts.

[W9TR]

Bridging doubles the voltage across the speaker terminals. Power goes as the square the voltage, (P=E^2/R) so theoretically you get 4 times the power by bridging.

But current also doubles when you bridge and that usually limits the amount of power you can get out of a bridged configuration.

It is common the have bridged amps only double the output power.

Also with McIntosh amps that have impedance taps, since the outputs are essentially in series you have to use taps that are each half of the speaker impedance. So for example if you have an 8 Ohm speaker you would run it off both 4 ohm taps. A 4 Ohm speaker would run off the 2 Ohm taps.

I think your MC 300 can be run bridged with a switch on the back panel. Cool.

[70sMac]

Originally Posted by W9TR

Bridging doubles the voltage across the speaker terminals. Power goes as the square the voltage, (P=E^2/R) so theoretically you get 4 times the power by bridging.

But current also doubles when you bridge and that usually limits the amount of power you can get out of a bridged configuration.

It is common the have bridged amps only double the output power.

Also with McIntosh amps that have impedance taps, since the outputs are essentially in series you have to use taps that are each half of the speaker impedance. So for example if you have an 8 Ohm speaker you would run it off both 4 ohm taps. A 4 Ohm speaker would run off the 2 Ohm taps.

I think your MC 300 can be run bridged with a switch on the back panel. Cool.

Tom..... Do you know whether or not it would be possible to use an amp in a Mac receiver in this way? We use our MAC4200 as an independent amp, for our two 805Ns (rears) ... so I was wondering if it could somehow be made to operate in mono? If not, how about a MAC4300V?

[W9TR]

I don’t think the receivers have the grunt to pull this off. You also need a circuit to invert the phase of one of the inputs, and the receivers don’t have that circuit.

Your MC 300 can be run bridged, so you could use it for the center and buy another McIntosh amp for the mains.

[cleeds]

I've always considered bridging to be not much more than a parlor trick - the result is never a better-performing amplifier, imo. While it may be useful in a pinch, I'd never suggest someone buy an amplifier with the intent to bridge it. I think it's much, much better to buy an appropriately-sized amplifier from the get-go.

[Msegal]

Originally Posted by cleeds

I've always considered bridging to be not much more than a parlor trick - the result is never a better-performing amplifier, imo. While it may be useful in a pinch, I'd never suggest someone buy an amplifier with the intent to bridge it. I think it's much, much better to buy an appropriately-sized amplifier from the get-go.

I would disagree. I am running MC275LE monos.

Mike S.

[damacman]

Doubling the Voltage really isn't a parlor trick.

[JemHadar]

Originally Posted by cleeds

I've always considered bridging to be not much more than a parlor trick - the result is never a better-performing amplifier, imo. While it may be useful in a pinch, I'd never suggest someone buy an amplifier with the intent to bridge it. I think it's much, much better to buy an appropriately-sized amplifier from the get-go.

Most high power fully balanced mono amplifiers are internally bridged designs. I would hardly consider it a parlor trick...just sound engineering.

That being said. If it is an afterthought, and the stereo amp is not designed with bridging to mono in mind it is better not to do it.

[cleeds]

Hmmm, looks like my opinion about not liking bridged amplifiers has ruffled some feathers.

Originally Posted by Msegal

I would disagree. I am running MC275LE monos ...

The 275 is designed for both mono and stereo operation. While I've never owned one, my understanding that it runs in parallel operation - and not bridged - when it is configured for mono. So it avoids some of the limitations common to use of a bridged amplifier, such as the potential for increased distortion.

Originally Posted by Jem666

Most high power fully balanced mono amplifiers are internally bridged designs. I would hardly consider it a parlor trick...just sound engineering ...

I consider a "fully balanced mono amplifier" as a differential amplifier, such as is common in Audio Research designs. That's something different than bridging a stereo amplifier for mono operation.

In any event, my negative opinion of bridged amplifiers wasn't intended to offend anyone.

[W9TR]

When done incorrectly bridging makes everything worse, especially if the amps in question were bridged as an afterthought.

When done correctly it cancels out any distortion common to both amps, which is a big benefit.

Since this is the McIntosh forum:

It is the core of McIntosh’s quad balanced amplifier topology where two bridged amps are used per channel.

The MC 275 runs mono parallel, not bridged.

The MC 2102 runs either mono parallel or bridged. When run bridged, clarity and soundstage improve dramatically over the stereo or mono parallel modes. I ran two MC 2102’s bridged before I switched to MC 2301’s.

So no offense taken. As with a lot of things audio there is some nuance to the story.