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:

 

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As I built my system, kind of on my own, volume control was not as simple as just putting a preamp in the system and walking away. Volume control, attenuators have many different configurations and levels of quality playback.

System specs:

DAC output: 2.0V single ended

Pass XA25: 47Kohms input impedance

Speakers: about 100 dB efficient

Streamer: Digital volume control

This has been a two year saga. My original active preamp failed. I bought a passive preamp, not knowing what it really was, except a volume control. It had an Alps volume control($15 part). It sounded OK at high volume. Why? The streamer volume "bit stripped". Twenty years of inactivity did not prepare me for this. I thought the problem was my amp. I bought a Schitt Aegir but my kids could kick out it’s touchy protection circuit. Not the amp’s fault. It was asked to play above it's comfort zone. . So I bought a Pass XA25. Problem solved. Couldn’t kick it out. But back to the preamp. So I was turning up the passive all the way and adjusting the volume on the streamer. Partially due to laziness(no remote) and partially because I didn’t understand the problem. At the same time, I had some listener fatigue from some sharpness in the trebel.  So let’s try a tube buffer. A passive with a tube and it had a remote. Now we’re talking. So I had good sound, much better sound. Wait a minute, the volume isn’t linear and low level listening just isn’t right.

Enough of that, now that I understand all this input, output, impedance stuff, let’s solve this problem once and for all. I was hoping to be able to swap my current preamp for a good volume attenuator without spending a lot more than I can sell the current preamp for. Enter Khozmo. Talked to Arek Kallas. Figured out the correct impedence of the attenuator, beautiful color options, upgraded to Vishay and Takman resistors and it is remote. There is only one resistor in the signal path.  There are three type of Khozmo: ladder, series and shunt.  I am getting the shunt style. Cost roughly $600. Per Arek, the ratio of passive preamp resistance to the amp input impedance should be 1:4 or thereabouts. This unit will be arriving in a week or so. Khozmo and Hattor is the same company in Poland. Their preamps get great reviews. The Khozmo is a basic model. The Hattor can have op amp based gain switchable to 3,6, or 9 gain and has nicer enclosures but the volume attenuator used is the same.. The passive I ordered should be very transparent.

There are other designs but this one is known well and reasonable. There are TDR and another kind of implementation that are supposed to be good, but the cost is higher.

@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.

I've got several passive preamps and have recently built an autoformer version from Intact Audio that uses a 47 position Elma attenuator. In comparison to my Berning ZOTL Pre-One the autoformer preamp gives up nothing. Source is a low output impedance Audio Note Kits DAC 4.1x and I use high input impedance amplifiers, Music Reference OTL-1 and Atma-Sphere Class D. I've also built these using the Kozmo attenuators which are quite nice, although I prefer using Seiden attenuators or NOS Noble potentiometers.

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

I`m using a battery powered Tortuga Audio LDR passive pre and it sounds pretty good I must say.

Clarity and definition with a pitch black background.  Also has a remote that is very handy.