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

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

Cost. I know some professional amplifiers that have balanced inputs, but internally short pin 3 to ground. It's kind of a waste, but it's a necessary convenience feature there.

In the residential world, you could add an XLR input, and do the same thing, but you won't get any benefit from it unless you actually have a balanced input stage, and even then, whether it's a benefit or not would be debatable. It would involve more circuit complexity.

By leaving the XLR connector out they can save money, keep the price a little lower, and leave it up to you if you must have XLR. I know of very few preamps that don't offer RCA outputs, so most of the time this makes sense.

If you need XLR to minimize noise due a longer run,  you'll probably want to use a transformer at the amp end, but only very long runs prone to noise, or near powerful sources of noise such as a radio tower would benefit.

Best,

Erik

To audiobrian's original question ...

I'd hazard a guess that in the population of SET owners, less than 1% have balanced preamps.  I don't have industry numbers on this.  Anecdotally, I know of only one SET owner who's running a balanced preamp.

There comes a point when making design decisions that you don't design for such small numbers.

Cheers,
Thom @ Galibier Design

Ralph, 
Are you using diodes to separate signal from the single output tube per channel?