Over the last year or so, some merchants – on eBay and elsewhere – have been selling some very compact – and very heavily hyped – headphone amplifiers designed around the 6DJ8 vacuum tube. Despite their low costs – usually around $50 USD – they’re touted as being high-quality audiophile devices for the discerning listener.
Are they? Yes and no.
The two most visible merchants of this amp are eBay merchants who sell them as “Bravo Audio” and “Indeed Audio”. They didn’t design the amps, despite what they’d like you to believe; they just resell ’em for some Chinese factory somewhere. That factory didn’t design the amp, either; it’s a reasonably standard hybrid design popularized about six or seven years ago by a Korean DIY audio enthusiast called Sijosae.
This isn’t the first commercial product based on that design, which uses a single 6DJ8 or compatible dual-triode tube, buffered by a single field-effect transistor per channel. Previous incarnations include the (much larger) Xiang Rong Audio XR010, which is known in some audio circles as the Super Simple. My bad on the name, by the way.
Having built several amps based on this design, and having bought a “Bravo Audio” version on eBay last year – seen above – I probably know it as well as anyone does, meaning I’m at least marginally qualified to opine on its shortcomings and weaknesses.
Weaknesses? Yes, the original design has a few. Most notably is that it can experience noticeable bass roll-off, owing to under-sized coupling capacitors. The folks who built the Bravo/Indeed 6DJ8 amplifier apparently decided to correct for this by… omitting these capacitors altogether. I assume this was done to save money, but it might, as counter-intuitive as it sounds, actually make the sound worse.
Another major issue is the inadequate size of the power-supply reservoir/filter capacitor. The original design calls for 1000uf here, which is… way, way too low. I consider 8600uf to 12000uf to be ideal, and several others agree with me. The Bravo and Indeed versions of this amp use 6800uf capacitors here – rated at 25VDC, and manufactured by Rubycon. This is bad because A, Rubycon makes some of the lowest-quality capacitors in the world, and B, the nominal voltage of the power supply is 24VDC. Even a 5% variation – fairly normal in real-world circumstances – in power-supply voltage can exceed the capacitor’s rating. Not cool. Not cool at all.
It’s not too hard to replace the Rubycon capacitor with something from a better manufacturer, rated at 35VDC, but you really shouldn’t have to.
The amplifier dissipates 10-12 watts of energy as heat, most of this from the output FETs and the voltage regulators which supply the tube heater voltage. The heatsinks on these amps are laughably under-sized. (This is, in part, why mine lives on its side, in the hopes of slightly improved convection cooling.) I get that they had to make some compromises to make it small and cheap, but I can’t help think these amps are all doomed to die an early death from heat-related issues.
Aside from the crap capacitors, the Bravo and Indeed amplifiers use el-cheapo, literally generic resistors; there’s nothing at all wrong with that, but it’s a bit laughable in something marketed as an “audiophile” product, and belies the claims about ‘high quality components’. On mine, at least, the output devices are IRF630s. I assume these were chosen because they were available cheaply; the compatible IRF510 is a better part in this design, owing in part to its lower internal capacitance. Output capacitors are 1000uf, 25V Panasonics meant for power-supply, rather than audio, use. There’s nothing wrong with them, but, again, it’s slightly laughable in an “audiophile” product. The surplus Chinese 6N1 tube supplied with mine is serviceable, but highly microphonic; I’m using an inexpensive vintage General Electric 6DJ8 from 1964 in mine. I have doubts about the long-term survival of the volume potentiometer, and the input and output jacks, none of which inspire a great deal of confidence.
Audibly, it’s not bad, and I can see how people who don’t know what the design is really capable of would be impressed. It has a cheerful, reasonably neutral sound to it; noise is pretty much nonexistent, assuming your tube isn’t microphonic, but THD, as with most simpler tube amp designs, probably isn’t anything to write home about. Bass response is decent but unspectacular, and the craptastic power supply capacitor appears to negatively impact transient response in a pretty noticeable fashion; treble seems to be fairly rolled-off as well. This is not obviously objectionable per se, and probably somewhat favors classical or jazz music, those eternal favorites of die-hard audiophiles everywhere, but the design, properly implemented, is capable of so very much more…
Overall, the Bravo Audio 6DJ8 amp isn’t bad, but neither is it great. The design, while basic, can sound phenomenal when built using high-quality components of the right values; regrettably this product has been designed to be small and cheap, with quality pretty much an afterthought. For the price, it’s a decent enough basic headphone amp, serviceable but unspectacular. Mine is attached to my Debian desktop at home, where I primarily use it for watching movies, streaming moderate-bitrate music, and playing ancient 8-bit SNES games via an emulator, tasks it performs perfectly adequately. It’s not really “audiophile” quality, by any stretch of the imagination. If you need an inexpensive headphone amplifier – or preamplifier, I suppose – you could do worse, but as an introductory tube amp, such as it is, it leaves a bit to be desired.
(Update 25 June 2010: See an update on this amp and the clear evidence of the cost-cutting measures taken in its assembly.)