single 12AT7 in a tubelab simple se

[martyhektik]

This is my first post and this will be my first build. I search the forum and www for some time and am not finding the answer to this; so here it goes.....

On the tubelab simple se design it uses a single 12AT7 in the preamp to handle both channels. I realize that this is a "dual triode"- but can you really get true stereo separation running both channels through the same tube? Would there be any advantage to separating the pre-amp into two individual circuits (sounds like I am trying to build simple se monoblocs now!?!) I am hoping that I am just naive about this. Your first amp has such a fast learning curve but I am loving it!

[Sheldon]

Originally Posted by martyhektik

This is my first post and this will be my first build. I search the forum and www for some time and am not finding the answer to this; so here it goes.....

On the tubelab simple se design it uses a single 12AT7 in the preamp to handle both channels. I realize that this is a "dual triode"- but can you really get true stereo separation running both channels through the same tube? Would there be any advantage to separating the pre-amp into two individual circuits (sounds like I am trying to build simple se monoblocs now!?!) I am hoping that I am just naive about this. Your first amp has such a fast learning curve but I am loving it!

Stereo separation is, in my opinion, less of an issue than it's often made out to be. Many people think LP's sound better than CD's. But you are not going to get better than 30dB separation (if you even get much better than 20 dB in most cases) from the cartridge. One reason headphones sound artificial, is the unnatural separation (for which there are deliberate strategies to mitigate).

I used to use separate supplies for left and right channels, because I could hear a small amount of crosstalk if I had input on one channel only (BTW, I could hear no crosstalk after separating the supplies, meaning I could hear none from the dual triode input tube). I found that I could not hear any difference in actual stereo playback, so I no longer worry about it. In fact, you'll find some folks who think that significantly less separation sounds better. I think, for typical designs, it's more important to consider isolation between amplifier stages, than it is between channels.

The small amount of crosstalk you'd get between typical dual triodes is, IMO, completely harmless to good stereo reproduction.

Sheldon

[martyhektik]

Thanks, all I needed to hear.

[Miles Prower]

Originally Posted by martyhektik

I realize that this is a "dual triode"- but can you really get true stereo separation running both channels through the same tube?

Look at the spec sheet for the 12AT7. Here, the capacitance between grids is given as: 0.005pF, and 0.4pF between the plates. At audio frequencies, this is insignificant. The 'AT7 was designed to be an RF tube for mixer/oscillator service. The section-to-section separation is more than good enough into VHF.

Cross talk from stray wiring capacitance will be much worse. Cross talk within the 12AT7 will be the least of your concerns.

[martyhektik]

Thanks for your reply. Answered my question and made me realize the answer was right there amongst the specs. Learning, learning. I had a hard time putting my head around the fact that the plate and grid share such close proximity without any cross-talk.