2. amps-preamps
  3. 176243-139155

Autoformer/Passive Preamp vs. 6SN7 vs. 12AT7 with Class A, Class D, and Tubes

I’m having some interesting listening experiments to relay.

Here are the things being mixed and matched:

Preamps:

An autoformer passive preamp based on the Bent Tap-X preamp.
A 6SN7 Preamp modeled (with upgrades) after the deHavilland Ultraverve III preamp
A Quicksilver Line Stage (with NOS 12AT7s) in the mix.

Amps:

A hypex based DIY amp which can deliver 550wpc in to 8 ohms
A Pass XA 25 amp
Quicksilver Mono 60s amps with a variety of output tubes.

Speakers:

Salk SS6M
Ascend Sierra Towers with RAAL tweeter

Obviously, the combinations here add up very quickly, but I’ll focus on one thing — the passive preamp.

At it’s best, the autoformer preamp has the ability to lift a veil from the sound — everything becomes clearer, sharper, more fully present in the room. It’s quite revealing and startling to hear. You’d think that this would mean "game over" for the other preamps — but not so fast. It depends on the amps involved.

When the autoformer is in combination with the hypex amp, there is virtually no depth to the soundstage; things are clear but they can also be a bit raw and even "thin." In the case of the Salk speakers, the gain on the preamp needs to be put too high, above unity gain, and this can have the effect of not presenting the full range of frequencies adequately. The tonalities become unbalanced.

The autoformer in combination with the tube amps seems like a winning combination, but again — not quite. There is still a thinness to the sound, a lack of depth in the soundstage overall, and lack of excitement which both the 6SN7 and the Quicksilver bring to the fore.

The autoformer is most engaging, full, and more layered in the soundstage presentation with the Pass Labs amp. I don’t know why. I could almost imagine going with only the autoformer preamp with the Pass — it’s that good. Especially with the Ascends.

Anyway, I wanted to relay this experiment because there is something really helpful about having a passive preamp to play with — it’s almost like an MRI for the rest of what my gear is doing to the sound. And yet, when it’s clear that a bit more power is needed, the weakness of having a passive preamp is immediately clear.

Here are the guts of the autoformer:

 

Here is the 6SN7 deHavilland type unit:

 

Here’s some of the other gear:

 

128x128hilde45

Showing 4 responses by hilde45

hilde45's avatar
hilde45 OP
4,876 posts
 

@bhvf I'm glad the post is of interest. I'm still learning, and the combinations are complicated! I also tried the MiniGan5 with my Salks and they had a lot of biff.

@daledeee1 You are so far advanced from where I am, I'm grateful that you've taken the time to weigh in. I need a much better sense of how the various electrical factors match -- or don't match -- up. I did change the pot on my Line Stage to a TKD pot, and improved the caps. But I hear you about the voltage output on the source components. The wizard who built both my autoformer preamp used what is reportedly quite excellent -- Slagle's stuff. His suggestion: put the streamer's output on maximum.

hilde45's avatar
hilde45 OP
4,876 posts
 

@antigrunge2 

All goes to confirm that with Dacs outputting 2V, all you need is a pot

Incorrect. If you read the OP, you'll see that this does NOT confirm that, overall. It's a mixed picture. Re-read.

hilde45's avatar
hilde45 OP
4,876 posts
 

@clio09 As I was typing my reply to include a link to Intact Audio, you posted about the same thing! Thanks for your input.

hilde45's avatar
hilde45 OP
4,876 posts
 

@brownsfan Thanks for relating that. Because I was comparing the passive to such a good 6SN7 (with excellent tubes) I really thought it could not be beat. But so much matters regarding the other things in the chain!

All that said, what I think I've realized is that until I believe I've hit an end-game preamp, I won't be thinking that passives are the "answer." It all depends.

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