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“Fighting For Power”
By Dave Rat
Here is a way to increase the actual power delivered to your speakers by 5% to 15%, for free, by doing a simple wiring change.
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A bit about power amps.
You can look at a power amp as a device that has two reservoirs of energy, one positive, one negative. The signal you send to it acts as a control to release the energy from the reservoirs to the output. The problem is that when an amp is driven hard, the energy from the reservoirs is depleted. The AC power going to your amp, from the wall or distro, refills these reservoirs at a rate of 60 times a second in the US and at 50 times a second in many other countries.
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Yet the audio your amp is putting out drains the power at much higher rates. The problem is further compounded by the fact that most amps are stereo and both channels often carry very similar signals, so both channels are fighting for the limited supply of available energy when the amp is driven at its maximum. This “fighting for power” issue is the focus of this article.
A bit of technical, kinda.
Imagine a kick drum beat or simpler yet, a positive pulse. Both the "A” and “B” channels pull energy from the “positive reservoir” at the same time while the negative reservoir does nothing. Then a negative pulse comes along and both channels fight for the available energy in the negative reservoir. Meanwhile, the wall AC is refilling these reservoirs at a relatively slow rate. Even when the AC is refilling, in larger systems or with long AC cable runs, the wall voltage may not even be enough to refill them completely.
What if…
What if there was a way that when the “A” channel was pulling power from the positive reservoir, the “B” channel could pull from "negative” reservoir. Instead of both channels fighting for the same thing, they would take turns and use the available sources more efficiently. The system power draw would be more consistent rather than a series of positive and negative peaks.
The cool part…
All that you would need to do, to use the amp in this most efficient way is to reverse phase on the signal going to one of the channels. How about the “B” channel? Sounds good, so now the “B” channel is doing the exact opposite of everything the “A” channel is doing. When “A” pulls positive energy, the “B” pulls negative and vice versa.
But what about phase?
The output to the speakers of the “B” channel is now out of phase, that’s bad (actually it’s reverse polarity, but you know what I mean). That won’t sound good at all, and you now have all kinds of problems UNLESS you reverse the speaker wires on the output of the “B” channel. Perfect, everything is in proper polarity, the amp uses power more effectively and your system just got a little bit louder.
Adaptors suck.
Adding a polarity reverse adaptor to every “B” channel in the system is kinda an irritating thing to do but there is an easy way when both amp channels are seeing the same signal. Nearly all pro amps nowadays have a “Bridge Mono” switch on them somewhere. All that the Bridge Mono switch does is reverse polarity to the signal going to the “B” channel. So that’s it, just put the amp in bridge mono, wire it like a stereo amp on the outputs and don’t forget to reverse the speaker leads on the “B” channel output and you are done.
Ahhh, so you want proof that it’s worth the effort!
I measured a Crest 4801 on the bench today. I did not do distortion measurements, just two 4 ohm loads, a scope, an AC meter and a signal generator. The output power in normal stereo mode was 535 watts on channel “A” and 542 watts on channel “B”before I could visually see it clip on the scope. I used a 30 hz tone and the power was calculated as RMS output voltage squared, divided by load resistance (4 Ohms).
Simply flipping the switch on the back of the amp from stereo mode (inputs were “Y”ed) to bridge mono mode brought the output power up to 645 watts on channel “A” and 658 watts on channel “B”. Poof, free power and like14% more at that!
Hmmmm, but will I hear the difference?
Well, I don’t know, but the theory is solid and the testing proves it’s
measurable, not to mention that it can’t hurt. Amps that are run in bridge mono mode naturally benefit from the balanced power draw. All of our main systems use amps in bridged mode whenever possible. The advantages are more pronounced in amps driving low frequencies and this is one of the many things that we at Rat Sound do to keep our systems optimized.
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