2. amps-preamps
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Balanced outputs vs balanced design

Friends, I am looking for a balanced tube pre to mate with Halcro MC20. I have since learned that there are single ended designs with balanced outputs and fully balanced designs, like the BAT's. In a response to a thread below the author suggests that the "magic" happens in the amp and that the pre does not have to be fully balanced to benefit from the noise reduction qualitites. If so, is thre any inherent advantage to a fully balanced preanp? And more particularly for my purposes which would not run connects more than 10' from the amp. If there is no great advantage to a fully balanced pre, I can widen my search. As always, tell me about your favorite tube pres to run with this SS amp. Many thanks.
deliberate1
jmaldonado
57 posts
 
ELDARTFORD: As for balanced circuitry in the power amp the advantage over single ended is largely theoretical for well designed equipment. It is often said that balanced is quieter because the signal is twice as strong, but there is also twice the circuit noise. Common mode noise, as for hum from the power supply, will be canceled, but there shouldn't be any such noise in a good unit.
Circuit noise is not twice as strong (+6 dB) in balanced designs. Rather, it's +3dB stronger, due to stochastic signals adding up logarithmically. That gives balanced units a +3 dB advantage in noise over single-ended ones. You are also neglecting the benefits caused by reduced distortions caused by cancelling, as well as the improved stability at the power supplies. The advantage is largely theoretical for BADLY designed equipment. In well designed units, the advantage is subtle but real, and I think detectable by ear if you care enough.

Cheers,
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atmasphere's avatar
atmasphere
11,588 posts
 
The advantage of a fully differential preamp or amplifier is that distortion is canceled at every stage in the device. As a result fully differential products can be lower distortion for a given amount of gain. Noise can be lower too- and the effects of noise and distortion always compound from stage to stage. The more differential circuitry is used, theoretically the more transparent the preamp or amp will be as well.
eldartford
4,262 posts
 
Jmaldonado...Yes. As you say random circuit noise in each leg will appear at 1.414 amplitude at the differential output, wheras the signal will appear at twice amplitude. So there is a bit of noise reduction. S/N of my power amps is spec'd at <100 dB, and the source equipment is better. I'm not sure what another 3dB would buy me. As for distortion, there really isn't any to cancel.

The more even loading of the two rails of the power supply is a real benefit IF the power supply design is marginal.
Some amps are spec'd for power, both channels driven, with the two signals out of phase (like a balanced design amp).
atmasphere's avatar
atmasphere
11,588 posts
 
Eldartford, its likely that your amplifier has a lot of differential circuits already- many transistor amplifiers do.

There is more than one stage of gain in most transistor amplifiers, so you get more than 3 db on the noise figure. Doubtless that is part of why your amp is as quiet as it is.

The area where the technology is still under-utilized is preamps (tube and transistor) and tube amplifiers.
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