This review page is supported in part by the sponsors whose ad banners are displayed below

Passive aggression: While an obvious teaser at comparing passive preamps, the aggressive bit does deserve closer scrutiny. Being conceptually neutrality incarnate when properly designed and used, a passive will not smooth over, flesh out, prettify or otherwise enhance/condition the source signal in ways we expect from active preamps (and in the absence of which we might as well go passive and simplify). Where desired and appropriate, the list of passive assets are directness, immediacy, crispness, speed, incision power, micro resolution and energy. Where additive measures are required to balance out a system's overall performance, potential negatives are brashness, leanness, thinness, angularity, stridency & Co.

If radio-ready Pop factors high on your music menu but you really favor a warm mellow saturated sound, your amp/speaker combo alone might not sufficiently massage the source signal. Should your recordings major on lovingly mastered jobs where your amp/speaker combo does all you want already, the invisible volume control at the heart of the passive concept could be tailor made. It all depends. The ultimate passive is the variable-gain source where attenuation occurs in the analog—not digital—domain. This eliminates a preamp of any persuasion and with it an entire box and the additional cabling involved. Most such sources also eliminate source switching to narrow you down to a single-source system but there are exceptions (the Ancient Audio and Ayon players for example have analog inputs and USB to become CD player, PC DAC and mini preamp all in one).

I had such a flexible source in Burson Audio's HA160D DAC/preamp/headphone amp with precision stepped pot and three analog inputs. I also had three different passives with Yamamoto's resistor volume control AT-03-1A; Bent Audio's Tap X remote-controlled autoformer unit (this has replaced my previous far more expensive Music First Passive Magnetic while adding vastly superior functionality of finer volume adjustments and comprehensive remote control); and StereoKnight's transformer variant. Running the Burson direct would act as bypass to determine whether these passives were audible in specific ways. My Esoteric C-03 set to zero gain would contribute a curve ball by way of an activated passive (no voltage gain but high supply rails). ModWright's LS100 would stand in for a wide bandwidth tube-rectified valve-output unit (1 x 5AR4, 2 x 6SN7). The scene for passive aggression was all set.

With the Burson set to max output (ca. 10V) and the one long 5-meter interconnect my setup mandates hanging off it rather than the passives which drove the FirstWatt F5 as a known ultra-resolved reference amp, the verdict was that these passives did not sound exactly alike; that the differences were primarily a matter of surface textures; and that source-direct was warmest and fullest and as such clearly superior in this context.

Yamamoto's resistive passive didn't seem to offer quite the same extended bandwidth top to bottom as the two conceptually similar TVC/AVC units but in turn had a particular silken texture which they did not. If the physically bigger units were water pure, the Yamamoto injected a small atomised dose of oil. This subtle softening or beautifying treatment showed up on timbres like ultra suede vs. smooth hard leather. Just enough to tell, the transformer/autoformer machines were starker and more matter of fact. No matter, connecting the 5-meter leash between Burson and FirstWatt whilst controlling volume from the Aussie box instantly demonstrated higher color saturation from more powerful fleshier bass, greater overall solidity/robustness and grippier dynamics. Put differently, I expect most listeners would insist on a warmer amp with any of the three passives. Using the Burson as variable source instead—i.e. no preamp whatsoever—they'd be happier or fully contented, period.


Granted, I'd not peg the very lucid, direct and 'unplugged' F5 as necessarily ideal to showcase the advantages of passives preamps. For that you'd want something like my tubed Trafomatic Audio Kaivalya monos or Yamamoto's A-09S SET. Yet the purpose of this exercise wasn't an idealized mating but a simple show 'n' tell.

Using the same cables in reverse—short LiveLine between Burson and Esoteric preamp, the 5-meter length between C-03 and F5—retained source-direct dynamics and tone-color fullness but in trade inserted a layer of softness and distance. Audiophiles call this opacity no matter how mild. This was the proverbial clean and thin pane of glass which reviewers love to write about as being magically removed but which here was mundanely added. There was no getting around a plain fact. The simplest (and cheapest) connection of source direct was sonically best. The passives were in a sort of Jenny Craig business where weight loss does not mean just greater fitness but also involves some gauntness and paleness. Again, this was in the context of an amp/speaker combo which wanted additional body and tone/image density. The passives simply did what they were designed to do - nothing. The debate about whether they actually subtract while active gain stages are neutral; or whether passives are neutral whereas active gain stages are really additive... that debate is academic. The practical upshot is the same. What I personally learnt was that to my ears transformer/autoformer volume controls seemed slightly superior to even ultra-purist resistive variants in bandwidth whilst a source with sufficient gain/drive and analog volume was superior again to one more box and interconnect. I also concluded that if there were any sonic differences between the copper-wired StereoKnight and Bent Audio units, my hearing wasn't keen enough to reliably tell them apart. The Tap X was simply superior in functionality not merely with its remote—StereoKnight's costlier version adds that—but with its far finer 1dB steps across the entire range.
Enlarge!