"Along with helping me with the design of the custom SPZR regulators for heating the DHT cathodes with one SPZR per tube, Brian Lowe of Belleson also worked with me on the super regulators for the anodes (again one per tube), with the balanced input/output stages as well as the regulation for the optional L2 phono and Dac modules. Beyond his super regulation, Brian also helped me with the design and testing of the analog gain stages of the phono stage and the DC offset nulling servo on its output. I had my layout guru mentioned above map the L2 phono stage on a 4-layer PCB for similar reasons. The switching between DHT/Jfet stages is accomplished via one internal toggle per channel, not via jumpers as we use for setting the gain of the L2 phono stage. Those toggles are located on the motherboard near each tube socket."
On to good DHT housekeeping, "it's very important to insure a good connection. Some DHT have variations in their pin diameters even from batch to batch. If you use a tube with a thicker pin, the pin sleeves of the Yamamoto UX4 sockets will stretch to accommodate. But if you go back to a tube with thinner pins, the connection may not be as good. With the power off, simply use a jeweler's or eyeglass screw driver to gently close the diameter of the contact sleeves. The tube pins themselves too must be clean or you might get an intermittent connection to have the occasional pop or crackle that wasn't there before."
By late February, my loaner shipment was imminent. "A few 'stupid' parts were back-ordered like the glass spheres and spring I use for the knobs to give them their unique feel and sound but I'm finally getting them and will wrap this up for you. The good news is that I'll now have parts to make a batch of 25 units without delay other than standard lead time. The only 'problem' with this design is that it'll be very hard for me to surpass it one day with a new and better one. And, I'm just going to have to include my TAK TA-300B for you to try along with the stock tubes – my favorite so far!" Pain and suffering lay ahead. Actually, I had a most unusual experiment lined up.
My favourite SR1a driver from all available in-house amplifiers was this Pass Labs XA-30.8 class A behemoth with 20 transistors per side.
The Raal Requisite SR1a from Serbia is the world's first true-ribbon dipole headphone. It floats freely before your ears like AKG's bygone K-1000. Due to 0.02Ω impedance plus low sensitivity to keep weight down—higher sensitivity would have meant bigger heavier magnets whilst built-in transformers too would have driven up weight beyond wearable—this very comfortable earspeaker ships with a resistive impedance-matching box. That connects to an amplifier of at least 50wpc/8Ω to source the necessary current. The amp coasts on a steady ~6Ω load to sound its very best. The adapter box burns up most of its power as series resistance to feed the SR1a at just a safe fraction. The upshot is bandwidth of 30Hz to beyond audibility with zero ear-cup issues like resonance, reflections, trapped-air compression and sweaty ears. The whole ear-muff syndrome is banished. It's open-baffle big-speaker sound at extreme proximity. It's of the highest resolution and speed beyond the best dynamic or planarmagnetic headphones. It's unusually perceptive of even small amplifier changes which on standard speakers might not register. One hears everything. Such a high-power sonic microscope on hand eliminates the room as the biggest problem with conventional loudspeakers.
That made me curious. What might replacing our COS Engineering D1 solid-state DAC/preamp accomplish when Vinnie's L2 drove the Pass amp driving the ribbon earfield monitors? I also was curious how Vinnie's ElectroHarmonix and Takatsuki 300B would compare on headphones not just loudspeakers.
Once my loaner was airborne, Vinnie emailed these instructions. "Box 1 is 61 lbs so be careful lifting it. Inside the box are the following:
♦ L2 travel case (everything is inside the travel case for protection)
♦ L2 Signature Preamplifier
♦ L2 Isolation Base. You will need to install this via the included 4 screws and Allen wrench. Simply align the 4 holes in the base with the 4 holes on the bottom of the L2 and tighten the screws.
♦ L2 Remote
♦ Stock EH300B Gold Grid tubes
♦ The two mesh tube covers. Insert the side labeled 'down' into the L2 as that side has the unpainted edge that is conductive to the tube socket bases. It is normal for the covers to be snug on the tubes when you install.
♦ Stock power cord
"Box 2 is only 4 lbs and contains the Takatsuki TA-300B. You can tell just how much I love them by how well I packaged them. Before turning the machine on, make sure the filament switch on the rear panel is in the middle 5V position for the 300B. Once turned on, warm-up (the display will say 'L2' during this process) is 25 seconds. The remote is full function and the same type as LIO including left/right balance control in 12 x 1dB steps. If you want to open up the chassis, do the following:
"If you want to bypass the tubes, there are two toggle switches, one next to each tube socket inside. The motherboard is labeled to tell you the switch positions. Left is DHT enabled, right is DHT bypass. With DHT enabled, voltage gain is 8.6dB. I tested it and these are matched tube sets. DHT bypassed sets gain at unity i.e. 0dB.
"SE or BAL? If you have the option to use either SE and BAL i/o and your cables are under 4 meters, I personally prefer going single-ended. Now the i/o don't go through the active BAL-to-SE and SE-to-BAL conversions. Each channel inside is single-ended so that use keeps it all as clean as possible. On paper, the BAL stages have incredibly low distortion to add negligible THD+N but why bother unless you are running very long cables and are in a noisy environment?
"I purposely did not want this design to be internally balanced which would have required extremely well-matched tube pairs per side to get the noise-reduction benefits of balanced in the first place and I really love our spud approach. Bandwidth is ultra-high and the 2nd harmonic exactly where I want it. With two tubes per channel, it all gets rather more complicated.
"Transformers can be nice for creating true balanced i/o but they wouldn't have had anywhere near the bandwidth to keep up with the direct-coupled DHT circuit. The L2 Signature Preamplifier is the product I am by far the most proud of designing and implementing in my 15 'non-stop' years."
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