EM/IA Remote Autoformer Volume Control
Dick Olsher (October 2023)
For the past decade I’ve used and enjoyed a version of this autoformer volume control (AVC), aka the Passive Aggressive, built for me by Jeffrey W. Jackson of Experience Music. For some reason, I hadn’t had the opportunity to say much about it in print, so this is my chance to correct matters. It should be recognized that these designs have always been a collaborative effort between Jeffrey W. Jackson of Experience Music (EM) and Dave Slagle of Intact Audio (IA) who provides the autoformer modules. Details can be found at: https://www.myemia.com/Remote.html.
Of course, John Chapman of Bent Audio (www.bentaudio.com) is also a major factor in this design by providing the remote control capability. According to Chapman: “I was on the lookout for a new device some years back when I heard and fell in love with the sound of the Autoformer made by Dave Slagle of www.intactaudio.com. It's this unit that together we packaged up to create the Slagleformer used in the TAP System and in the AVC-1 Pre-amp Parts Package.” The remote provides full volume control, right/left balance, and mute function. Most noteworthy is the 1 dB volume step size over the range from -54 dB to +7 dB. That makes it a breeze to nail down a desired playback volume level. And the balance control is an important bonus as I find it occasionally useful to tweak balance by 1 to 2 dB.
An AVC is simplicity itself: a single coil of wire wound on a high permeability core such as HyMu 80 alloy, an unoriented, 80% nickel-iron-molybdenum alloy. The entire signal path consists of magnet wire and high quality switching elements. The input inductance is about 150 H which generates a sufficiently high input impedance even in the bass range. The AVC’s output impedance is a function of the source impedance driving it and the attenuation setting. It varies between the source impedance of the source component at 0 dB attenuation and essentially zero output impedance at maximum attenuation. For example, my Denafrips Terminator DAC is specified at an output impedance of 625 Ohm. When hooked up to the AVC, the output impedance will vary between 625 Ohm and zero Ohm, perfectly fine even for power amps with 10 kOhm input impedance, and suitable for driving even long interconnect cables. It’s worth noting that an AVC’s output impedance is much lower than that of a resistor-based passive volume control.
Switching from an active line preamp, even one with a quality resistive volume control, results in quite a sonic revelation. Active stages, whether tube or solid-state, possess a sonic character due to intrinsic non-linearities and the flavor of resistors and capacitors in the signal path. The AVC dispenses with all that. Its sonic hallmarks are pristine textures, a consequence of its harmonic purity, and exceptional soundstage transparency. If you want to know just how colored your line stage really is, take the EM/IA AVC for a test drive and judge for yourself.