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

@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.

@hilde45 In some respects, my experience with a very good passive is similar to yours. I still own a Prometheus (sic) Signature passive TVC. It has been out of my main rig for 10 years now. I still have it because it is worth more to me as a back up than it would bring if I sold it. It was regarded by some as the best passive TVC ever made. It is of course dead silent and is uncolored in its sound.

When I bought my Coincident CSL line stage, I never looked back. Dynamics and imaging markedly improved, without loosing anything in terms of the blackness of background (running balances ICs) or purity of tone.

Well, the Coincident cost 2x the price of the Prometheus not counting the expensive 101D tubes, so it should be better.

I think this is how I would view this question regarding passives. If you are able to spend the bucks, you can get things in an active where even a good passive is going to struggle. If you are on a modest budget, there is a lot to be said for going with one of the better passives. Some of these are an excellent value proposition.

@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.

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

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. 

@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'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.

@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.

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.

Thanks for posting this. I’m interested in reading more about your experiments. I’ve got a Quicksilver linestage and amps and have had Salks in the recent past. I also run my Teac NT-505 direct at times and it’s interesting how it works well with some amps but not others. One thing I’ve found is using the the balanced outs of the Teac direct has yet to work well with any amp but the same amp using RCA’s might work great. I’m not sure whether it’s the balanced out of the Teac that’s not great or if it’s the flavor of the VH Audio RCA cables I use that I really like. This hobby can drive an experimenter like myself crazy. I also have wanted to try that Pass amp with my Devore O/96’s. I’ll probably pick one up someday. As far as class D amps, I tried the AGD amps with my Salks and it was a really nice combo.  I'd like to try more class D.