ImPasse Preamp now online

[SY]

The ImPasse preamp (made for driving low gain bridged power amps or low gain power amps with balanced inputs) article is now available online, with permission from AudioXpress (Audio Amateur Incorporated). By grounding the output coming off the phase splitter plate, this can also be used as a conventional single ended output line amp, albeit one with a lot of gain (near 26dB).

The ImPasse Preamp

[Tony]

thanks SY, downloaded the file....i have a bunch of 4KN8's that is am using in place of ecc88...and russian 6H8c's instead of 6SN7.....

[rongon]

Hi SY, just read the article. That's a really good one, extremely informative. I'd like to build something like that, which I have a question about...

I already have an aluminum plate punched and ready to go with two octal sockets (scavenged from a previous project). I was thinking of making a diff-pair 6SN7 per channel, but worried about the high output resistance. The ImPasse solution is just what the doctor ordered for that. But, having two octal sockets already punched, what do you think of subbing in a 6SN7 split-load inverter? Would that be a giant step down from the 6DJ8? Or would it mainly result in higher output resistance, so take care of cable lengths, etc?

How would the lower gm of the 6SN7 impact performance, other than higher output resistance? Worse distortion?

Thanks!

added -- I forgot to mention, the gain of 20 won't be a big deal, although I really only need a gain of about 9 or 10. Since a 6SN7 diff-pair would yield a gain of a bit less than 10, do you think the 6SN7 diff-pair makes more sense? Input impedance of the power amp is about 200k (diff-pair 6N6P w/ balanced input).

[Tony]

not SY, but the last stage has a unity gain being a concertina.......the desiderata for the small tubes is its tranconductance, the lowish output impedance you can get for a single cathode......paralleling sections of the 6SN7 might help, but that is doing it hard already...

[rongon]

Thanks for the reply, Tony.

I was just reading a post by Allen Wright about how a high gm triode is the best for a cathode follower, which implies that a FET source follower would be better yet. Since a concertina splitter has many of the same characteristics of a CF, I suppose that discussion was applicable to this situation.

I was just looking at that plate I have, and there is a hole for a 9-pin socket too. I suppose that means I could use a 6DJ8 after all. And I should do that, as I have a whole bunch of them and I keep trying to find a way to use one.

Or I could use a 6CG7 as the voltage amp and parallel each 6SN7 for each channel's concertina.

What I need is a line stage with unbalanced inputs and balanced outputs, to go with a 5-input switch box and inductive attenuator/vol ctrl (both unbalanced). The power amp I'm using needs 4V peak at its input for full power. Since you figure a CD that's mastered at a low level might only output 1V peak, I only need a gain of 4X. Maybe the differential pair would be a better choice in this situation? But then a tube with lower rp than a 6SN7 would be a better choice. Oh well, I keep trying to find a way to use a 6SN7 or two.

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[Tony]

What I need is a line stage with unbalanced inputs and balanced outputs,

doing that with the Impasse is easy, just get rid of the input traffo....but i'am sure SY and SteveEddy will disagree....

TBH i was stunned at first when i saw a big tube driving a small tube, but hey, if it does the job, then i am for it....

there is another thread here: ImPasse Preamplifier

[buzzforb]

This was on my REAL SHORT list. Tube preamp built by a guy who believes in numbers over opinions and knows live music. Wonderful gift, Sy.

[SY]

The 6SN7 will work, but yes, not as well as a 6DJ8 or the equivalent. There may be a nice high gm dual triode with an octal base, but I haven't found it yet. If your other hole doesn't work, it's not hard to fit a sheet metal or perf plate cut for a 9 pin miniature into the largr octal hole. You could even use fiber-epoxy (e.g., FR-4) perforated circuit board for that.

[rongon]

If your other hole doesn't work, it's not hard to fit a sheet metal or perf plate cut for a 9 pin miniature into the largr octal hole. You could even use fiber-epoxy (e.g., FR-4) perforated circuit board for that.

I finally dropped the coin for a couple of gen-yoo-wine Greenlee punches, 3/4" and 1-1/8". Those will make it possible to fabricate some kind of adapter plate to put over the 1-1/8" hole for the octal socket. Nothing like having the right tools, eh?

Was the input transformer necessary for connecting balanced sources to the 6SN7 voltage amp? Or was there a deeper purpose?

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[SY]

Deep and mysterious reasons, known only to the High End Cognoscenti.

1. To allow balanced or single-ended sources
2. To remove common-mode noise
3. To provide galvanic isolation
4. To give some bandwidth limiting (f3 at 90-100kHz)