A visit with Dick Olsher’s DIY Slagle Autoformer Volume Control, or, The Brotherhood of the Traveling Preamp

In 2011, Enjoy The Music posted an article by Dick Olsher on a DIY project, the Slagle Autoformer Volume Control (AVC for short). I remember reading it, getting excited about it, and flailing fruitlessly around, wondering where I could buy one — that is, one already made. Yeah, I’m lame. But since I don’t really “cover” DIY on Part-Time Audiophile, it’s either pre-made or out-of-scope. Oh well, I figured. Too bad, so sad ….

But guess what? Heh heh! Ahem.

In case you’ve missed it, a transformer-based preamp has some potential and inherent benefits. Srajan Ebaen wrestled with this on 6moons a few years back:

The rationale for TVCs [transformer-based volume control] is that traditional attenuation in active preamps is a matter of loss. The preamp generates gain that’s completely redundant in the context of high-output digital sources, hi-gain amps and reasonably efficient speakers. For normal listening, most that gain needs to be strangulated. The way that’s done is to throw it away as heat via resistive action. After all, that’s what volume pots are – resistor-based devices. The pro-TVC thinking considers this counterproductive and champions current/voltage conversion. Why pay for gain only to throw it away? And what else do you throw away while you’re throwing out gain? Resolution perhaps?

It’s this sort of talk, which I started scouring the ‘net for after reading the Enjoy the Music article, that gets an audiophile all hot and bothered. And not just me, apparently — certain preamp designers seem swayed too, including the now-discontinued Bent Audio TAP and the new line from Purity Audio Design.

So, when Dave Slagle handed me a rather unassuming box at this year’s CAF, I was psyched — Dave was hooking me up! But then … I was a little puzzled. WTF was this little gray box? Dave grinned — yes, this was in fact that very same box that I’d read about in Olsher’s review! Dave said, “try it out, see if you like it; when you’re done, we’ll talk about who it goes to next.”

Goes to … next? Did I just I join the Brotherhood of the Traveling Preamp?

Sweet!

Inserting Olsher’s AVC into my reference rig, currently a pair of DeVore Fidelity Orangutan O/96 loudspeakers driven by an S10 stereo amp from BorderPatrol, was … eye-opening. I mean that literally. I could actually feel my eyebrows attempting to flee up and over the top of my head, taking most of my face with it. Yeah. In a good way.

Why? In a word — dynamics. In another — transparency. In a third — sha-ZAM.

My reference amp is a bad ass single-ended job based on 300b tubes and sports a pair of external, tube-rectified choke-filtered PSUs. This amp is a monster. It also has a 100,000 ohm input impedance, meaning that there won’t be issues driving it from a passive. But the best thing? Bandwidth! I’m sure you’ve heard that 300b tubes are “all midrange, with a flabby ass and no air”. Okay, well I certainly heard that. Tube rolling helps, I was told, but not much, and at the end of the story, what I should expect to be left with is either a “vintage” or “romantic” sort of sound — which is what that 300b tube is “all about”. Well, on that metric, the BorderPatrol amps just … don’t measure up. Heh heh. See, I get oodles of “midrange magic” — but … (cue the drumroll) … I also get air … and … a tight, fast bass. Heh heh, heh heh. The why and the wherefore of this apparently magical result have to do with some very straightforward and entirely clever engineering choices, but however it happens, it seems to be unique amongst 300b amps especially compared with certain Far Eastern cousins. Anyway, with all that said, I can honestly say that I haven’t heard this system sound like this. Just really … open.

If I was picking nits, I’d admit that the presentation is a tad leaner than I’m used to. My reference preamp, the Control Unit EXT1 (also from BorderPatrol), brings a huge sound stage that is dense with color and tone. By contrast, the AVC was less ripe but felt faster, and kept the sound stage. That was a surprise — most preamps (especially my solid-state preamps) tended to constrict the ‘stage. Happy days! Detail and texture showed some remarkable staying power as the volume dropped down to night-time levels. On the other end of the dial, the lack of grain made for more than a couple of “oh shit!” dashes over to the AVC.

Some curiosities — there isn’t a volume knob on the box. There are four. Each channel is controlled separately, which makes for an impromptu balance control. Note that there are two dials per channel. On Olsher’s AVC, the dial on the left is for “big changes”, that is, steps of 3.75dB each. The dial on the right is for more granular dialing in. It has 3 positions: nothing (middle), +1.25dB (up) and -1.25dB (down). Weird arrangement? Well, yes, but it works fine. In practice, having to fiddle with four dials was less of an issue than a lack of remote control, but then again, my BorderPatrol pre doesn’t have one either. Max attenuation on the AVC is -41.25dB before the you hit the Wall of Silence. I tended to listen to the unit rather loud (a couple positions off of unity), and the lack of steps simply meant I fiddled with it less. You learn, you adapt, you get used to being startled during swells — it’s fun!

The AVC is really in it’s sweet spot when the volume is turned down. Think digital sources — I used this preamp quite a bit to test out another analog attenuator, the Seta Buffer. In that instance, I was testing the digital volume control on several DACs and comparing that to the analog attenuation. In nearly all cases, I vastly preferred the AVC to using the digital volume controls built into the DACs themselves, and used it about four stops from wide-open (-15dB — any more, and I started hearing a bit of hum come through the AVC). It wasn’t until I inserted the Seta Buffer inline, with it’s fixed-value attenuation, that the digital volume controls were able to catch up to the AVC.

For those of you that are curious, Dave still sells the AVC units for $200/pair over on his Intact Audio page if you get the uncontrollable urge to break out the soldering iron. He also offers some options, including AVCs with different gain structures and even an option (gasp!) for a remote-controlled version. If you’re like me and believe you have a Damn Good Reason to not solder, I’d encourage you to check out Emia for a suite of finished, ready-to-play components (including AVCs!) from Dave and partner Jeffrey Jackson. If you’re ready to damage your PayPal account, you can find the Emia portfolio available over at Stereodesk.

As for me, I’m not sure how to sum this up. I really liked this little $400 DIY experiment. Clearly, gain matching your system to your preferences is going to be high on the list of things to keep in mind with this, or any, passive attenuator. Will your system have enough gain? How much attenuation do you really want or need? Like I said, it’s something you’ll need to consider, and generally, audiophiles aren’t terribly good at this.

Overall, my biggest complaint? Olsher’s ugly ass box. If he’d have dropped this into an aluminum chassis, or maybe a vintage Western Electric wooden box, or best of all, something with a Faraday Cage (thanks to Steve Rochlin for that suggestion), I’d have stolen that bitch and tried to play all innocent with Dave. Mine! As it is, well, yes, I’m a vain little girl. Sue me.

Having the AVC in my system was a ton of fun. My recommendation? If you have an amp that can handle the variable load and don’t mind a little exercise with your stereo experience (standing up to make changes — you vinyl guys know what I’m talking about), then make sure you check out this hilariously good attenuator design. If you don’t love it, you’ve clearly done something horrible to your audio system and probably deserve to be beaten about the head and face.

Associated Equipment

• DeVore Fidelity Orangutan O/96 loudspeakers
• BorderPatrol S10 EXD amplifier
• BorderPatrol Control Unit EXT1 preamplifier
• Thöress Phono Preamplifier
• TW Acustic Raven AC-3, with Raven 1.5 tonearm and Ortofon Windfeld cartridge
• Planus AG speaker cables and interconnects from MG Audio Design
• Power cords from Triode Wire Labs