Differential input, SET output

[tomchr]

I'm mulling over the design of a 300B amp. I want to run the 300B as a single-ended triode. The rest of my audio chain uses differential signaling so I would like this 300B amp to have differential input.

One could, of course, just ground the (-) pin of the XLR connector and take the signal single-ended off the (+) pin, but that defeats the purpose of the differential signaling. Another thought would be to start with a resistively loaded differential stage and use one of the outputs from it to drive the 300B (via appropriate cathode/source follower, etc). And finally, one could use an op-amp diff amp to deliver a single-ended signal to a normal single-ended 300B amp. Oh, and transformer input is also an option, though, in my humble opinion, not a viable one.

Of these options, I like the differential pair the best. But does it make sense to go about it this way?

~Tom

[leadbelly]

Look at kenpeters' work:

converting BAL to UNBAL using tubes and without transformers

[Miles Prower]

Any differential input stage can do this, NBD.

[tomchr]

Sure, any differential stage CAN do this, but is that the best approach?

The diff pair is my preferred approach - mostly because I'm very familiar with it - and it looks like everybody else has arrived at the same conclusion.

thanks,

~Tom

[SY]

Oh, and transformer input is also an option, though, in my humble opinion, not a viable one.

Why not? Something like a Jensen JT11-P1 or a Cinemag 15/15 would give you superb CMR, galvanic isolation, and have bandwidth and distortion well better than any SET is likely to have.

[Gordy]

tomchr, I'm in the process of building a small SET with a balanced input served simply by an XLR feeding into a transformer, and then into a volume control attenuator, which then feeds into a driver tube before the output tube.

Nowt wrong with an input transformer!

[jlsem]

Why not keep it differential right up the the 300B and use an interstage transformer to couple the driver to the output tube and convert to single-ended? This would be the best approach IMO.

John

[tomchr]

I was discarding transformers on the grounds of my preconceived notions of high distortion and high cost. Looking at the Jensen and Cinemag products, I definitely take back the "high distortion" part. Those trannies are IMPRESSIVE!! and offer CMRR that rival many modern op-amps. THD < 0.02 % even at 20 Hz! Nice. The Jensen is $80-ish. Cinemag don't publish prices. I'm a little afraid to ask. If you have to ask, you can't afford it.

Has anyone of you ordered from Cinemag and have an idea of the pricing?

I know interstage transformers have been used to drive a 300B in the past. I also understand that the 300B wants a low-impedance drive. I wonder what the output impedance of those interstage transformers are if they're driven by a source/cathode follower. I think that would be the bottleneck on performance of the system.

Thanks,

~Tom

[Eli Duttman]

Originally Posted by tomchr

I was discarding transformers on the grounds of my preconceived notions of high distortion and high cost. Looking at the Jensen and Cinemag products, I definitely take back the "high distortion" part. Those trannies are IMPRESSIVE!! and offer CMRR that rival many modern op-amps. THD < 0.02 % even at 20 Hz! Nice. The Jensen is $80-ish. Cinemag don't publish prices. I'm a little afraid to ask. If you have to ask, you can't afford it.

Has anyone of you ordered from Cinemag and have an idea of the pricing?

I know interstage transformers have been used to drive a 300B in the past. I also understand that the 300B wants a low-impedance drive. I wonder what the output impedance of those interstage transformers are if they're driven by a source/cathode follower. I think that would be the bottleneck on performance of the system.

Thanks,

~Tom

Contact Steve Eddy. He switched from Jensen to CineMag because of favorable pricing.

A 6SL7/5751/7F7 SRPP should provide the voltage gain needed. An IRFBC20 in Tubelab's Power Drive will neatly take care of dragging a 300B's grid slightly positive.