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"The problem with DHTs in low-level applications is hum from the AC heater. If DC is used the hum vanishes but now the electrons migrate to one side of the heater. This is sonically not too bad if the grid is minus 60 volts or more. In a headphone or line-level amp however there is no good reason to operate the tube at max plate voltage and current. In fact the sweet spot is at about 1/3rd the max ratings. This means that the grid voltage is much lower. Now the tube sounds odd because the electron cloud is less likely to cover the grid smoothly. Some have tried to use a heater current source rather than DC voltage source. They claim it makes a difference. In my opinion that's because the higher source resistance of the current source adds a level of isolation from the solid-state sound that's induced into the cathode circuit.

"I tried AC heating the BA and was immediately impressed by how much deeper and more natural the soundstage became. Of course hum was intolerable now. The solution became high-frequency AC on the heaters. My heater circuit in the external PSU runs off a dedicated 40kHz 18-watt power amp. This feeds two ferrite-core isolation transformers which are quad-filler wound so the electrical center on the secondary is absolute. Now we have a perfect electron cloud evenly dispersed like a water sprinkler—back and forth 40.000 times per second—and the signal comes off dead center. The transformers also provide isolation between channels and drive circuit. The heater amp has current limiting to start up very gently. The output tubes are operated much below their rated power to greatly extend tube life. The best-sounding output transformer is the Electra-Print psss. That stands for partial silver-stranded secondary. The transformer's magnetic field changes to electrical current differently into silver than copper."

The Electra-Print website adds: "For many years silver-wired audio transformers have been considered an improvement in audio reproduction. Due to silver’s higher sensitivity to flux change (200%) it can reproduce lower level information produced by the output stage. This results in an audible increase of the low-level higher harmonics that do exist with various musical instruments. Recently the price of silver increased to a point where not many builders can afford it over standard copper OPTs.

"We have tested lower-priced partial silver or silver-plated multi-strand wire and found that the results were equal to solid silver wire. The amount of silver:copper was measured and found to be 20:80. The reason this worked as well as solid silver is that it is plated around the circumference of each copper wire. The flux generated by the primary would excite these surfaces first. The many strands then result in a larger silver surface area than one singular silver-plated wire.

"We have developed a winding technique that uses psss wire effectively with a standard copper primary. Primary windings do not have to be silver and they cannot be wound with silver-plated stranded wire due to the final size being too large in diameter. Finally a silver primary would not offer a performance improvement as it provides the flux for the secondary."

On DHT types: "I prefer the sound of cathode bias on headphones and as a preamp. The BA can be set up for 2A3/45 or 300B/PX4. There is a two-position bias switch on the rear. In the case of the 2A3/45, the heater transformer secondaries are wired 2:1 so 5V equals 2.5V at twice the current. With the 300B/PX4 option the heater voltage is adjusted to read 4V for the PX4 and 5V for the 300B. To have a universal amp that accepts all four tube types would be challenging. I have found the 45 to be great sounding on high-efficiency horn systems. In this scenario bass extension is either not important or handled by a subwoofer system.

"With headphones the higher plate resistance of the 45 equals less bass extension. To compensate for this we need more primary inductance. That rolls off the highs. The Czech KR Audio PX4* has been the best compromise albeit at a cost. It's expensive. It has 300B electrical characteristics with a slightly richer 45 presentation plus bass extension. Most opt for the 300B/PX4 option because there are so many good-sounding new production 300B tubes and more come out each year. I would recommend delivering the loaner outfitted with KR PX4s."

* TJ/Full Music also has a current production PX4 in solid-plate and mesh versions. See above.

Craig's plan was perfect. I had 300Bs by Western Electric, Synergy, Emission Labs (300B XLS and 320B XLS), Euro Design Team, Shuguang and Gold Aero but no single PX4. While I used to own Art Audio's 6wpc PX25 SET, I had never yet listened to a PX4. Live a bit longer, learn a lot more. "With the KR PX4, gain is 12.5dB, power into 50Ω is 1.25 watts. Into 300Ω it becomes 270mW, into 600Ω 150mW. In preamp mode, the headphones are disconnected. Max output voltage then is 9.5V, Zout is 5Ω. The 45, 2A3 and 300B have the same gain so unlike with speaker drive, power into the easier loads of headphones will be the same.

"Naturally the different plate resistances affect bandwidth. That of the 2A3 and 300B is the same but the 45's is almost twice as high. With the same output transformer this will affect bass response to translate for a 2A3/300B into something like minus 0.5dB at 15Hz and for a 45 as minus 0.5dB at 30Hz. Kron's PX4 has the plate resistance of a 300B but more gain. The Sophia/TJ PX4 meanwhile has the plate resistance and gain of a 45. However the Sophia/TJ PX25 might be an interesting consideration. It would play with more power because of its far higher gain."

Some much-needed perspective. Reader Joe Eagleeye: "I've got the Audez'e LCD-2s (recabled with Stefan Art Audio's balanced Endorphin wire) with an Eddie Current Balancing Act headphone amp. The sound is spectacular

"My source is the Metric Halo LIO-8 pro DAC which is quite the rage on That replaced a Weiss Minerva and a tweaked Technics SL1200 turntable. I lost my listening room which prompted the move to headphones.

"I haven't regretted it for a moment. I used to have Avantgarde Duo Omegas and Shindo electronics in a 1,500sf open loft."


Precursor to the Balancing Act with 101D driver

Let that sink it. Not only are them fighting words, they mirror what I've been saying without counting on most audiophiles believing any of it. The Audez'e with a superior headphone amp preceded by a quality source can make you forget some very expensive speaker systems. Harebrained pricing and silly rigs of monstrous complexity would stop selling like bad fakes and worse jokes if this caught on. As is, many listeners won't accept the different presentation headphones deliver. So trophy hifi is safe to proliferate. Just don't buy into the necessity or justification of it all. True, $4.000 headphone amps like the Balancing Act and Model 5 or €3.000 Emillé Labs Ara strike the average headfier as trophy kit already. It's when you factor what they can go up against and best—easily by a decimal point—that things come to a point.