But first more on basic challenges of classic output transformer design. I asked Sasa Cokic of Trafomatic Audio who is an expert in this dark art of compromise: "Generally speaking, if we deal with a single-ended 300B output transformer, as it gets bigger, its inter-layer capacitance increases to roll off the high frequencies. Meanwhile to transfer low frequencies to the loudspeakers, the transformer must get bigger. I aim for 10Hz at -3dB and in my Experience 2 MkII, our in-house wound output transformer is 15cm² which grows to 24cm² for my PSE 300B Glenn monos. If we unspooled the E2's primary winding, we'd arrive at around 250 metres of single wire which represents ~65Ω." From that brief paragraph we appreciate that designing steel-core output transformers is a fine balance between mutually conflicting requirements and inserts high resistance from copious coiled wire between amp and speaker. Classic output-transformer-less aka OTL tube amps by Julius Futterman and Harvey Rosenberg used lower-Ω triodes in series-connected banks of mass-paralleled tubes then added copious negative feedback. Atma-Spere's smallest Circlotron OTL needs five 6AS7G tubes per channel to manage output impedance of 7Ω. Their 60wpc model needs eight to make ~4.1Ω. To get that value to 1.75Ω, Ralph Karsten's approach needs 20 triodes per channel and draws 800 watts. Hello AC-gobbling space heater.

Tubes. Using them in high-performance audio isn't exactly a walk in the park. That said, applying them to headphones which typically present from 30-600Ω not the generic 4-8Ω of loudspeakers automatically reduces the impedance step-down a valve amp needs to create the recommended minimum 1:10 ratio. Again, Velo's exotic RF impedance converters manage 1.1Ω using one twin triode per channel with a sectional plate resistance of ~7'000Ω. How glassy does Velo tube pricing get? Sticking with current-production Slovakian JJ, at time of publication one 6SN7 wanted €25.80, one 12AT7 €17'70. Call it ~€100 for the set; delivered. That's not the €1'500 a matched pair of current WE 300B got just then. Going with Chinese Psvane conveniently available on Amazon, I saw matched pairs of 12AU7/12AT7 for €62, 6SN7 for €152 and 6SN7 SE for €203/pr. A pair of Linlai's best E-6SN7 showed $265 on their webshop. More current options not of Russian origins come from China's TJ Full Music brand; and Sophia Electric. NOS glass from Mullard, GE, RCA, Philco, Hytron, Sylvania even Shuguang's WE-6SN7 could get pricier but making sound with Velo won't have to break the bank. And to state the obvious implication, disregarding hot-pluggable opamps on the transistor side, tube rolling requires tubes to begin with. Sock(et) 'n' roll?¹

Velo and Aero for size comparison.

¹ Tube fanciers already understood that 12AX7 are not compatible with Velo so stay away from E.A.T.'s ECC803S and others of its ilk. But with adapters like those shown at left, single-triode versions of the 6SN7 can be paralleled to make one dual triode. Options include E-1148, 7193, CV6, DET20, 6J5, CN1932 and more. True cognoscenti can really get quite Caetano Veloso on voicing this amp.

Velo's published specs show a voltage gain of 13.5dB into 100Ω, 12.4dB into 8Ω. With a DAC like iFi's Pro iDSD Signature offering ~6V max on its single-ended outputs so far beyond the standard 2V, it's potentially feasible to season up voltage gain at the source. This depends on the input sensitivity. "Our input stage is preceded by an attenuator so the true limit will be when the source voltage gets high enough to limit the usable range of steps. Velo's input sensitivity should be close to the original Microzotl spec of 0.6Vrms before clipping. But again, that's 0.6Vrms coming out of the attenuator not going into the amp. With a 6V source you should have around 100 steps before the amp clips. The unknown thing is how loud it will be at step 1 and how many steps are in your preferred listening range with your particular headphones."

The power ratings span from 1.9W into 2.5Ω to 35mW into 300Ω. 14Ω nets 0.68W, 30Ω does 0.34W, 100Ω 100mW. Given those figures, my Susvara and Immanis loads looked to be rather unlikely candidates. For in-house comparators, my most applicable compact preamp was Lifesaver Audio's icOn 4 from the UK, a sterling passive magnetic with relay-switched autoformer secondaries providing non-resistive attenuation in mostly 1dB steps. In headfi mode, my most sensibly priced contrast agent was Kinki's solid-state THR-1 when Cen.Grand's Silver Fox, Enleum's AMP-23R and Vinnie Rossi's L2 Signature aim at rather stuffier wallets. For them even LTA have more loaded options like the MicroZOTL full-size preamplifier with 6.3mm port which Mark Schneider describes in the next video. The brand's crowning headfi glory is their $6'950 Z10E, a 12-watt electrostatic headphone amp with 580V bias spur which will also drive standard planars and dynamics with 3wpc into 32Ω. Velo's brief simply was to go down not upmarket. So I thought that's where I'd play.

In that sensible vein, FiiO's 32Ω FT3 and Meze's 109 Pro headphones were my sane options, Final's D-8000 and Audeze's LCD-2/XC the semi bonkers, HifiMan's Susvara and Raal's Immanis the totally nuts and probably well off the reservation. Prospective buyers would simply want to know. So that box needed ticking. To hail an über ride in roughly the Velo+LPS 'hood, you'd want the recently reviewed Soundaware P1x. That does a hulking 12 watts class A into 32Ω but with transistors. That's a different concept with no built-in voicing options nor what in preamp mode is mandatory remote and from-the-seat visibility of the volume setting. Whilst the P1x is fully balanced unlike the purely single-ended Velo, for LTA to bolt on true not convenience XLR would require either opamps or transformers. Owners of fully balanced sources who insist they sound superior over XLR can always detour into an outboard converter box like this ~$600 transformer unit. I simply have no clue whether that'd be at all sensible on the sound-for-pound meter; or vis-à-vis allocating the same funds to the linear power supply instead. Mentioning it just ticked an otherwise empty XLR box. Check.