Balanced amplifiers. Beamed down from the heavens as the answer to everyone's amplification nirvana. The touted features of these wonderful machines are, according to ATI:
1) Lower Noise
2) Double the slew rate
3) Apparent gain in volume
4) Immunity to stray fields
5) Lower Distortion
6) Reduction in all types of amplitude distortion
7) Immunity to hum
8) Better transparent performance
Here, I intend to rip these "features" a new asshole.
First, some important background. Whatever you might hear from manufacturers, and whatever your mommy told you, "balanced" amplifiers (really, the correct term is differential, and I will use that from now on)
were created mainly to reduce the heat generated by multichannel amplifiers which have as many as 7 channels stuffed into a single chassis. Without differential design, the multichannel amplifiers we have today would not be possible because they would generate far too much heat. As a consequence, they would need so much expensive heatsinking and/or fans as to render them either far too expensive for consumers, or too noisy from the fans.
Differential amplifiers pull off this heat reducing trick by reducing the + and - rail voltages by as much as a half. Where a traditional 200 watt class A/B amplifier might have rail voltages of +/- 80VDC, a differential class A/B amplifier would typically have rail voltages of +/- 40-50 VDC. By having lower rail voltages, there is less voltage potential across the output transistors, and as a consequence, less power dissipated by the output transistors in the form of heat (remember - class A/B amplifiers are at best 60% efficient).
A 200 watt differential amplifier is really nothing more than two 50 watt amplifiers in bridge mode. Yes, that's right...nothing more than that. If you have any old stereo 50 watt amplifier which could be put into bridge mode, then you would have exactly the same thing as a "balanced" amplifier.
Let's do some math. A 50 watt amplifier puts out 20 volts into an 8 ohm load. If you take two of those 50 watt amplifiers, invert the polarity of one of them and connect the 8 ohm load across the "hot" terminals of the two amplifiers, then you would see a total of 40 volts across the 8 ohm load. Using Ohm's law, E (voltage) squared divided by 8 (ohms), equals....viola....200 watts!
That is how a differential amplifier works. The only other requirement is that each channel, and the power supply, must be robust enough to support that 200 watts output. So in practice each channel of a differential amplifier has more output transistors than would be required for a "normal" 50 watt amplifier, and the power supply is capable of more amperage.
Differential amplifiers usually have "balanced" (that's really differential) XLR inputs. When you are inputting a signal which is already differential (one phase normal, and one inverted), those two polarities are sent directly to the two halves of the amplifier. When you input a single ended signal into a differential amplifier, the signal goes directly to the "+" side of the amplifier, and also, via an inversion stage, to the "-" half.
Now for the alleged "benefits" (which are really bullshit marketing to make something which doesn't sound all that sexy (heat control) into whiz-bang "features"):
1) Lower Noise: A differential amplifier behaves basically in the same way as any differential circuit - it has common mode rejection. If the cables going to your power amplifier are picking up hum and noise, then that will be largely canceled. Unless the cables to the power amplifier are really long, are routed near large motors, or are in an area where EMI/RFI are prevalent, then there is no reason to buy a differential amplifier for this feature.
2) Double the slew rate: This one is true. However it should be noted that every competent "regular" power amplifier has more than enough slew rate to reproduce the highest audio frequencies. If an amplifier has a specification that it can output 200 watts all the way out to 20kHz, then it has enough slew rate to do the job. Having "more" slew rate is not buying you anything.
3) Apparent gain in volume: BULLSHIT, BULLSHIT, B U L L S H I T ! ! ! ! ! Basically all home theater amplifiers have a standardized gain of 28dB, which is the same gain specified in THX specs. The differential (XLR) and single ended (RCA) inputs have buffering which reduces the voltage of the differential input by 6dB, which makes the net gain of the amplifier the same, whether the differential or single ended inputs are used.
4) Immunity to stray fields: Nope, bullshit. This is because differential circuits rely on the fact that the "+" and "-" polarities of the signal are
intimately placed next to each other - i.e. in a cable, the two wires are twisted tightly together. This assures that any interference which impinges on the cable
equally effects both wires (both polarities). The input of a differential circuit sums the signal on the two wires together. That which is different between the two wires is passed (i.e. the audio), and anything which is common and equal on both wires is canceled (i.e. the noise). The relatively widely spaced physical layout of the circuitry inside a power amplifier makes it impossible for any real cancellation at any frequencies which would be useful.
5) Lower Distortion: Yep, that's true...but be careful what you wish for. When talking about distortion, the percentage of distortion is not nearly as important as the
spectra of the distortion. Even order harmonic distortion is both easily accepted by the ear (it's simple octaves of the original frequency), and its presence tends to mask the presence of the odd order harmonic distortion components. Odd order harmonic distortion is of a much more complex relationship to the fundamental frequency, and as a consequence is more grating to the ear. Differential amplifiers get their "lower" distortion spec
by eliminating the even order harmonic distortion components and leaving the odd order ones laid bare.
6) Reduction in all types of amplitude distortion: To be honest, I don't know what the fuck they're talking about here. Amplitude "distortion" is non-flat frequency response, and differential amplifiers are no better than regular ones in this respect. This is an example of what happens when marketing people take too many drugs...
7) Immunity to hum: Bullshit for the same reason as #4 above.
8) Better transparent performance: Bullshit. Transient response is directly dependent on bandwidth, and differential amplifiers don't have bandwidths any better than conventional ones. A "transient" wavefront is nothing more than a fundamental frequency with many harmonics riding on top of it - just like a square wave, the most "transient" signal of them all is nothing more than a fundamental with every harmonic component present at equal amplitude, theoretically up to infinity. If the bandwidth of an amplifier is sufficient to reproduce all the frequencies up to and slightly beyond the audio spectrum, then it has good enough "transient performance" to reproduce the steepest waveform the human ear can hear.
Wow...I feel much better now....
