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

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?

May I use a female XLR to male RCA adapter at the unbalanced amplifier input?

No. That is the same as single-ended.

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.

With the input I described earlier, this is essentially not true. While no SET has a balanced input, they **all** have differential inputs, else the amplifier would be unable to amplify. Try shorting the input grid to the cathode of the input tube and you will see what I mean. The tube amplifies because of what is different between the grid and cathode.

No. That is the same as single-ended.

Ralph, yes, it would amount to a single-ended connection. But he wants to use his existing 5 meter balanced cable to connect to the single-ended input of the amp he is considering. It seems to me, given the info I quoted from the manual for his preamp (and aside from the uncertainty I stated about the existence of the ground switch the manual refers to), that he should be able to use that cable with an adapter.

Best regards,
-- Al
 

^^ He can, but 5 meters is a long ways to go single-ended, even if the cable is balanced. The adapter, if properly wired, will tie pin 3 to ground. At that point the cable capacitance will increase.

But with a simple mod (which is done by adding a coupling capacitor into the input tube's cathode circuit, which is tied to pin 3 of the XLR, pin 2 being tied to the RCA signal connection) then the balanced preamp can drive the amp directly with no need for an adapter.

The amp at that point will be operating with a differential input. It won't be balanced, but it will work better than with single-ended.