HeadRoom BlockHead headphone amplifier Page 2
More technical mumbo-jumbo
The BlockHead, HeadRoom will have you know, is the world's first and only commercially available, fully balanced, "double dual-mono, monoblock headphone amplifier." As I've said, the BlockHead's machined aluminum front and rear panels hold together two enclosures that contain the separate left and right audio electronics. Each noninverted and inverted audio channel has completely separate electronics and power supplies, all the way back to separate Avel-Lindberg toroidal transformers. Four transformers, four power supplies, four audio channels—two of them inverted, of course.
"The amp comes standard with all sorts of sweet stuff," Mr. Hertsens informs readers of his website: such as high-performance Burr-Brown 627 audio op-amps (footnote 1), low-temperature-coefficient 0.1% metal-film resistors and matched polyphenoline-sulphide film capacitors in the signal path, Nobel or Swiss-made stepped Elma potentiometers, Neutrik connectors, three-step gain control, three-step filter controls, and the HeadRoom Crossfield Audio Image Psychoacoustic Processor Circuit (whew, that's a mouthful).
"Yeah," enthused Hertsens, "the parts are very carefully matched, and surface-mount techniques have really come a long way in the last few years. We use NASA-spec Vishay resistors, and man, are they expensive. Usually you pay half a cent for a resistor, but these are 70 cents each in quantities of 1000! The stacked capacitors are surface-mount parts with materials that give them a very low equivalent series resistance (ESR). There's so little resistance, in fact, that they act almost perfectly—high speed with no resistance because of their low dielectric absorption. The only time they're in the signal path is when the processor is engaged. With the Crossfield Processor off, the BlockHead is a pure-DC design.
"You know," he continued, "it's the same electronics package we use in the Max, which runs about half the price. The only real difference is that, with the Max, you do have that common return [explained below] that can put crosstalk into the headphones and lose some of the astonishing clarity and magic of the BlockHead when its special cable is used.
"Another cool part we make ourselves is the stepped attenuator. We get the switch detents and wipers from Elma, a Swiss switch manufacturer. We make our own circuit boards for the attenuators with a lot of gold on 'em—three or four times the amount typically found on other stepped switches. You know the circuit boards the wiper contacts as it moves? Well, that's where the gold is thickest. And we use 2%-silver solder paste.
"Surface mount now is just as good as the old 'through-hole' soldering technique, but this way there are no wires! It's very cool. You squirt the solder paste, set the parts down, and put it in this device containing an inert gas vapor which is heated to 600 degrees. You're essentially submerging the printed-circuit board into this inert gas so the solder melts in an atmosphere with no oxygen.
"And that's not all!" Here I inserted a Ginsu knife joke, but the peripatetic Hertsens barely slowed. "Within the solder is plastic, so when you reach the melting point, the plastic emerges, covers, and seals the joint! No oxygen, and sealed in plastic before it's pulled out! That's NASA quality, buddy-boy. And we use that technique with all our amps," he chortled.
Perhaps sensing that that was enough, Hertsens hurriedly said, "Hey, don't hang up yet, J-10. I wanna ground-plane ya." Uh-oh. "Yeah, you asked about the grounding earlier. Well, the internal audio reference ground plane inside the unit is shared—you need a very good ground reference. Danny Bartlett, our chief designer, does better than star-wired grounding in the BlockHead. He used to be an RF engineer. Anyway, the circuit board is one solid piece, but here's the trick: various cuts are made in the ground plane so that return current paths always flow in the region where any particular circuit is implemented, and that's the technique used over the entire board."
Another feature of the BlockHead is that it can double as a fully balanced line-level preamp. You've got to make sure the Processor and Filter switches are off when listening through speakers to "avoid goofing up your imaging and frequency response," as Happy Hertsens puts it.
Double your pleasure, double your fun...
Now to that special connection for the headphones. Normally, 'phones share a common connection on the non-driven side of the driver elements. Take a look at a standard ¼" headphone plug: the left-channel connection is at the tip, the right-channel connection is the ring, and the common connection is the remainder of the plug shaft, called the sleeve. The problem for dual-mono operation is that the summed left- and right-channel return current develops a signal across the series resistance of the common return path, which, according to Tyll Hertsens, muddies headphones' stereo presentation with crosstalk. Therefore, he says, it's that pesky connector that's responsible for making it impossible to drive the return side of the headphone coils with the separate left and right inverted signals of a fully balanced amplifier. (I swear he banged his desk with his fist when he explained it!) Thus the need for a special cable with two entirely separate signal paths. The cables are shielded, of course, but connected only at the source, not at the 'phones.
Footnote 1: See the June 2001 Stereophile, Vol.24 No.6, for Ben Duncan's thorough examination of high-performance audio op-amps. Hertsens, it seems, is a big fan of the 627 op-amp.