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.
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:
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. |
@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. |
@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. |