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Busted? Trying to boost two in-house SETs with the Burson amp—not a feature you get to experiment with every day—at first rewarded with noise so excessive, you could have heard it down a transatlantic phone line. No scheme of floating grounds would kill it. Reporting this to Burson, my first correspondent misunderstood the severity: "The booster function of our power amp is a little different than the Nelson Pass F4 design. We do have a small amount of voltage gain. We find that it allows better matching to a wider range of speaker systems. However, this also means our booster is more sensitive to preceding filament and AC hum. If the filament is regulated, the noise you have would not be an issue. What you are hearing at the moment is the tube amps' noise floor. Our PP-160 has 29dB of gain of which 6dB are applied to the booster circuit. The tube amp sees the Burson as an 8-ohm load."


A few frustrated emails later, a different Burson correspondent entered the picture. He finally acknowledged that a valve amp of sufficiently low output should not be noisier with the Burson than on its own (certainly not to the extent he now understood I was describing). I was asked to return the amp to Oz for a bench test.

"We evaluated the returned PP-160 and found one transistor semi detached from the PCB. That caused the excessive noise leakage. The damage could have occurred in transit. We have also re-evaluated the PP-160 on various tube amps and found that on 93dB+ speakers you will notice a slight increase in noise. This is not the fault of the PP-160. It only amplifies everything upstream. On less sensitive speakers, this noise will hardly be noticeable. And it was the less sensitive speakers our design focuses on primarily. The booster function allows SE tube amp lovers to expand their speaker options. It allows a 300B or 2A3 enthusiast to select speakers other than the most sensitive kind while retaining the intonation of their tube amp."

Boosted. Boosting my 3wpc MiniWatt N3 (APPJ version) and 8wpc Yamamoto with the repaired Burson proved no noisier than the amps were on their own - dead quiet with the Chinese, very quiet with the Japanese. Burson's solution did not cause ground noise between series-connected amps. AC-heated SETs without feedback were prospective candidates as advertised after all. Given that my Tango R speakers don't really require a boosted Yamamoto, I would use them to satisfy personal curiosity about which SET qualities boosting would retain, which it would alter. Grafting low output impedance and high current drive to a SET should affect the composite sonics after all. High output impedance with low damping factor and limited current drive are two acknowledged SET weaknesses. High THD interactions with loudspeakers are another. Would improving those aspects improve the sound; or did SET sound depend on them?


A fortuitous turn of events allowed me to test this question also on Inès Adler's Voxativ Ampeggio. That's a 98dB sensitive single-driver speaker with proprietary drive unit and an enclosure built by Schimmel Pianos of Braunschweig. Despite my unconventional open-backed layout, this back-loaded horn design primed for triode drive still had solid 40Hz to 20.000Hz response. Described in more detail in my review of Yamamoto's new passive volume control, the combination of Yamamoto DAC, volume controller and direct-heated triode amp into these exotic transducers was a fantastically pure window onto legendary 300B tone and soundstaging (so good in fact that a writeup of Tianjin's new Full Music 300B SE, new 6SN7 and matching Synergy Hifi valves was partially conducted in this very context).

Boosted = busted? Functionally, boosting worked perfectly. Sonically there were gains, shifts in gestalt and specific losses or changes depending on perspective. The buoyant freely gushing tension-free feel I'd described in the AT-03-1A review as being nearly limpid yet uniquely luminous became something tauter and more tensioned. As such it was less fluid - more material, less spiritual if you like.


Bass amplitude particularly with the Ampeggio actually reduced a tad but not with my usual 91dB 6-ohm ASI Tango R. On the Voxativ the additional damping was perhaps higher than ideal to actually restrict/overdamp the AC-3X drive unit (2.3 Gauss field strength, 5.5g of moving mass, goat-leather suspension, 33Hz free-air resonance). Bass articulation and definition did improve and firm up of course. The valve amp solo was looser, freer and more energetic. Contrary to expectations, the sharpening of transients into clearly transistorized turf was quite mild. I'd unnecessarily feared that semiconductor behavior would overlay typical 300B traits beyond recognition. To that degree it did not.


Color temperatures too translated reasonably intact. While lower than with the valve amp pure, they were certainly higher than with the raw Burson. Even so, I considered the gains in bass articulation with the Voxativ overshadowed by the parallel reduction of bass amplitude and extension and even more so, overall fluidity and luminosity. 300Bs—and particularly the vintage-flavored new Full Music which improve upon the Western Electric reissues—excel in elasticity and freely decaying tones. The Burson's greater grip altered that aspect in particular. Applied damping firmed up transients and damped (i.e. shortened) fades. The semi-eerie holographic sculpting of the soundstage and peculiar intimacy with solo vocals also diminished for a more ordinary presentation in those regards.


If you own multiple systems, it's sensible to have them represent opposing/alternate views for greatest variety and perhaps even to optimize specific playback material. If you own just one, long-term satisfaction might instead want to stay 'in the middle' to not exhaust fascination with particulars and have a system that does all music equal justice. If you belong in the first category, boosting could be somewhat counter-productive. It does move tube sonics towards a more neutral middle ground. If you're a stout hifi monogamist on the other hand who enjoys a lot of modern music with big dynamic swings, serious bass and structural complexity; and if rather than deep triode you're after a lighter distilled version to gladly trade some elasticity and holography for tauter control and more accurately damped bass... then boosting could become very attractive even on speakers which play plenty loud and without obvious shortcomings straight off a low-powered valve amp.

Boosted = blessing: With my Tangos, boosting's loss/gain balance shifted in Burson's favor. The most overt improvement was in bass control. Considering the Tango's prodigious talent there, this was significant. In a nutshell, it's the difference between playing unreasonably loud without obvious distortion (SET solo) and playing with proper control (SET + Burson). I also suspect that regular dynamic drivers in general respond better to higher amplifier damping than the ultra-light cones that front the extreme powerful motors of so-called widebanders (AER, Feastrex, Fostex, Lowther, PHY, Voxativ et al). Standard drivers appreciate added amplifier control whilst still passing pure tube—rather than unpredictable amp/speaker interaction—harmonics to a good degree.


Regardless, the final sound is a composite or hybrid. The harmonic complexity or tone color richness will echo if not outright duplicate tubes. Overall transients will be slightly softer than pure transistors, slightly sharper than with pure tubes. Bass will be better than valves in all regards save for a degree of 'loose swing'. Without owning speakers that couldn't be driven to unpleasant levels by the hi-gain Yamamoto solo, I sadly cannot report on 86dB variants with steeper phase angles and more challenging impedance swings. With them boosting would become a requirement rather than option.


Concluding with this feature, combining valve voltage gain with transistor current gain—in Burson's case the semiconductors actually contribute 6dB of voltage gain—in many ways duplicates what hybrid circuits like Moscode, Unison Research and Aria Audio accomplish in one box. The 'recombinant' sonics show traits of both devices. On speakers of standard efficiency, the actual balance is surprisingly centered. Neither tubes nor transistors dominate. With true high-efficiency designs, jacked-up damping becomes counterproductive to actual frequency response in the bass. Plus, their more or less uncompromised performance with pure valve amps shifts core parameters of their signature sound too much on alien turf to remain completely compelling. In culinary terms, why make a mushroom taste like chicken - just because you can?