Why do so few low power amplifiers using 300B have balanced inputs?

Most 300b amplifiers are single-ended.

Most designers think that there is no point to adding a balanced input to such an amplifier.

But the fact is there is a way to do it (without an input transformer too) that allows the amplifier to process both phases of the balanced signal.

Any single-ended amplifier can be easily modified to do this without changing the character of the amplifier. FWIW, most single-ended designers don't know this is the case.

Most designers think that there is no point to adding a balanced input to such an amplifier.

What would be the point?

What would be the point?

Depending on the design of the specific preamp that is being used, perhaps less sensitivity to cable effects, less susceptibility to ground loop issues, and in some cases perhaps better sound from the preamp’s balanced outputs than from its unbalanced outputs.

There seems to be a belief among some audiophiles that balanced interfaces can only be beneficial if the internal signal paths of each of the components are balanced. I see no basis for the belief. Depending on the specific designs a balanced interface may provide better results or worse results than an unbalanced interface, even if some or all of the components are not "fully balanced."

Regards,
-- Al

Thanks Al and Ralph
 
I have a 5 meter run of Audience SX balanced interconnect between my VAC Sig IIa preamp and my Clayton M300s and I'm thinking about a Mastersound 300B amplifier to use with some monitors. I do not have a quality 5 meter run of unbalanced interconnect. May I use a female XLR to male RCA adapter at the unbalanced amplifier input?  If not I'll just purchase a 5 meter RCA run or let Ralph custom modify the input to balanced.😊

Audiobrian, your question gets into the fact that most XLR-to-RCA adapters connect the inverted signal on pin 3 to ground (pin 1). That can be appropriate for many preamps that have transformer coupled outputs (as yours does), but would not be suitable in many cases where both the inverted and non-inverted outputs are directly driven from active devices. (Although for those situations I believe Cardas can supply adapters on special order that leave pin 3 unconnected).

I note the following statement in the manual for your preamp:

Main outputs are available as unbalanced via RCA jacks or fully balanced via XLR jacks. You may use either set, but not both simultaneous/y. The GROUND switch must be set correctly for the output configuration being used. Incorrect setting will result in either excessive buzz, or cause gross distortion of frequency response and output.

For power amplifiers with RCA input jacks, you will normally use the "RCA GND" setting. You may also choose "XLR/RCA UNGND" if it is necessary to break a ground loop.

For power amplifiers with XLR input jacks, you must determine if the input is truly balanced, or if either pin 2 or 3 is actually tied to ground (unbalanced XLR).

For true balanced inputs you may choose between "XLR GND" and "XLR/RCA UNGND" for lowest hum level.

For power amplifiers with unbalanced XLR inputs, select "XLR/RCA UNGND". If the amplifier ties pin 3 to ground, you may also use "RCA GND". Do NOT use "XLR GND".

This leaves me somewhat confused, because in looking at photos of the front and rear panels of the preamp, including the photos in the manual, I don’t see a "Ground" switch anywhere. But if there is such a switch, based on these statements and on the fact that the outputs are transformer coupled I suspect it would be suitable for you to use typical XLR-to-RCA adapters that ground pin 3, but I suspect you would want to set the Ground switch per the last paragraph, as if you were connecting the preamp to an amp having unbalanced XLR inputs. It would probably be best, though, to give Kevin Hayes a call at VAC, and present the question to him.

Good luck. Regards,
-- Al