Passive Attenuator anyone?

Steven Stone got me thinking about preamps. Again. The reason is pretty straightforward — it’s really hard to do right something that’s not supposed to do anything at all. Let me untangle this a bit, tell you where I’m coming from and what my concern is.

First, what does a preamp do, exactly? In short, it’s a volume control and a source selector. That’s it. That’s all we want. It’s not supposed to add anything to the audio stream and it’s also — and perhaps more importantly — not supposed to take anything away, either.

Apparently this is much easier to say than it is to do.

I’m not at all privy as to why this is, but the short of it is that the parts have to be quite good to keep them from “veiling” the source signal. And most preamps don’t really do a good job of this — that is, most actually do veil, obscure, cloud, and otherwise rob the source signal of life, detail, and, well, everything. That’s the story, anyway.

I was chatting with Bill Baker about all this at Capital Audiofest, in between drooling on and attempting to steal one of his impressive Purity Audio Design preamps. Yeah, I want one of those bad boys. The reason I’m so interested in his work is, to all reports, the Reference and Statement preamps are some of the most neutral you can buy — and don’t actually rob the signal blind when passing it along. Music to my ears! Sadly, this level of performance comes at a rather dear price point. Hmm.

Given that you have a single system, and that system plays at a high level, perhaps a Purity Audio preamp is your ticket to audio heaven. If you’re building several systems, at several price points, perhaps another solution might be on offer?

Steven Stone recommends passives. I asked him about the frequency attenuation issue — he cites impedance mismatches as the culprit there. Which is one of the things a powered/active pre should sort out for you — but in a passive, well, there’s not a lot in there to separate your source from your amp. Like, nothing. So, finding the right fit is important. But do that, and, well, Bob’s your Uncle — right?

We’ll see — I want to bring in a few passive preamps and see what’s what.

I found Luminous Audio at Capital Audiofest, and Tim Stinson was telling me about his marvelous $400 “Walker Audio” mod for his Axiom II passive. I was very curious, naturally, and in a move of sheer awesomeness, he’s sending me one to play with. Score!

First Watt also has a passive, the $1,000 B-1. Given how enthralling the J-2 I have here on loan is, I’m betting that the match is pretty much perfect.

Another I’ve heard about is the Lightspeed Attenuator. This little doojabber is a bit different from the rest, it uses diodes or something … clearly, I need to do some more research. But I’m told this little guy is amazing. $525 Aussie Bucks and it’d be on the way.

And then there’s the AVC from Intact Audio — which is neat, because the autoformer mechanism is designed by Dave Slagle, and that AVC was used in the Bent Audio Tap-X and is currently being used by Bill Baker at Purity Audio. Not sure if Dave is actually still making these for sale, but even if he is, the last price he had up there for a “fully assembled unit” is something north of $1,200. But look at that aesthetic. It’s awesome! Like the Terminator, after it got fucked up a little. Yeah!

Anyway, that’s where I’m at. What am I missing? Any thoughts on what I need to keep in mind? Any others I need to try?

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12 Responses to Passive Attenuator anyone?

  1. John Doyle says:

    Doesn’t the Hippocratic Oath allude to something like, “First, do no harm.”? Maybe we could learn from that approach in audio… For whatever reason, we (ok, I…) tend to think the bigger, the more complex, the better. Maybe not.

    If you need lots of inputs or you’re doing the vinyl thing, I get why you would want/need a more traditional preamp. But if you’re listening primarily through a disc player or digital, why wouldn’t more people go the approach you’re suggesting?

    The Luminous preamp, as an example, is such a great deal it’s crazy… biggest problem is that you have to suspend your belief that it costs as little as it does and isn’t that big to sound that good.

    I replaced a Musical Fidelity A3 2CR preamp with the previous model Axiom I on a whim, figuring I’d only be out $200, could flip it if I wasn’t happy… and was blown away. I was listening at the time only to CD’s (MF A3.2 CD) and had Bryston amps (lots of gain) and was slack-jawed at how much better, how much more information was getting thru. Humbling.

    And no I’m not related to Tim or have any interests in his company. ;-)

    Look forward to your impressions!

    • Part-Time Audiophile says:

      I’m certainly interested. I’ll certainly be watching the mail to see if one of these little buggers shows up! I may have to try a few out to see what’s what.

  2. David Gatti says:

    Timely discussion. I’m in the process of building my own passive preamp after coming to the same conclusions as yourself. Mine will be 6-channel, fed from my Oppo BDP-95 and then feeding 3 separate stereo amplifiers to give me my 5.1 setup. The Oppo BDP-95 can easily drive a passive preamp (it can drive as little as 600 ohms). I’ve already sold my complicated, noisy, compromised, costly receiver! The heart of any passive preamp is the potentiometer or switched attenuator. Good ones aren’t cheap. I sourced mine from Goldpoint level controls.

    • Part-Time Audiophile says:

      I’m really impressed with what I’ve learned about Slagle’s AVC. I really want to get one in to play with!

  3. marty keane says:

    Forget passive vs active and go straight to ‘no preamp’. I have an old Adcom GFP-750 that lets me switch from active to passive — and I love the passive side of it. Just a completely different sound and a lot quieter with my McCormack amp. I’m thinking about going further, though, and getting an NAD M51 DAC that has digital volume and just run that to the amp. No extra preamp. I’d love to see that in the mix of what you’re looking at.

    • Part-Time Audiophile says:

      I’ve been running my Berkeley Alpha DAC straight into my amp for a while — and it works great!

      Here’s a problem. Any DAC that uses a digital volume control “throws bits” away to attenuate. This isn’t a problem if you’re fairly close to unity gain (0dB), but as you crank it down, because it’s late or you’re too old to rock it that hard or you have some semblance of sanity or simply a small desire to preserve what’s left of your hearing, you’re eating into the signal and losing resolution, detail, and, well, music. You need some kind of “attenuation system” to bring the levels down to the point where you can use your digital control in it’s sweet spot. A passive is perfect for this, but there are other ways to do it, I’m sure.

      The other thing? I’d like play LPs occasionally. And I’m not interested in encoding to digital and decoding back to analog if I can avoid it. I know, I know — “Luddite”. It’s a problem and my therapist says I’m making progress.

      • marty keane says:

        Makes perfect sense and appreciate the feedback. So for those late night listening sessions you would run the DAC full out and then use the passive to attenuate down — thus requiring a preamp. I get it now. Is a DAC really thowing that many ‘bits’ away to get to that lower volume, that it would degrade sound quality audibly? Will be curious what you find when you get a passive to play around with vs the Berkeley direct. Thanks. Love the blog and the show reports.

  4. Robert says:

    There are different ways to do digital attenuation that aren’t as basic as throwing bits away. The Weiss implementation of digital volume control is a bit more sophisticated: See http://www.weiss-highend.ch/computerplayback/Digital_Level_Control.pdf for an explanation about their method.

    I firmly believe in the DAC direct to power amp minimalist setup being the superior method when multiple sources aren’t needed and budget constraints don’t allow kilobuck preamps to be taken into account.

    My most recent experience was of a Wyred4Sound DAC-2 connected straight to a Plinius SA-103: for that price I found that combination to sound superb.

    Kind regards

    PS: I had to register to get to the Weiss Audio downloads section but it’s free and easy.

  5. Kory says:

    I use the Placette RVC before a custom gain stage. It works like a champ and has tons of clarity.

  6. Phil says:

    So when do you publish your impressions? Very curious.
    You could add the Khozmo Akustyk (http://www.akustyk.com/) starts at $299. Why not include the Bent Tap-X? I’m very curious how the Placette compares to this group as well. And since you have it and are already a fan, I’m guessing you’ll include the WFS STP-SE.

    I’ve been recently comparing borrowed Khozmo, a DIY Lightspeed, and a Modwright SWL 9.0SE. with my Quicksilver monoblocks. I really like the purity of the passives. It just sounds so uncolored.

    Love the blog. Keep writing!

    • Part-Time Audiophile says:

      Thanks!

      I have a Wyred4Sound STP SE and an Luminous Audio Axiom II here, but haven’t had a chance to do any head-to-head comparisons. It’s in the queue ….

      I tried to find a Bent Audio Tap-X, but they’re no longer being made, which is a bummer. The volume control, Slagle’s AVC, is pretty interesting, but I haven’t had a chance to hear it.

    • Mike Casellas says:

      Hello Phil,

      Did you like the Khozmo better than the DIY Lightspeed?

      Regards

      Mike

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