"I guess I don't need to tell you about the advantages of DHT tubes in preamps. They have an ease and immediacy that is better expressed than IDHT like 6SN7 etc. even though I have built IDHT that come very close. DHTs are hard to engineer with though. It's very hard to get them completely hum and noise free. Because the electron flow is coming from the filaments rather than the cathode being heated by a separate heater as in IDHT tubes, it takes extra engineering to get a DHT working optimally.

"As far as I know, Supratek was the first company to do a commercial DHT preamp. That was the 300B, many years before Manley brought out theirs. I also did PX4 and 45 preamps and one that could do 300B, PX4 or 45 interchangeably. Then I went to the 101D and used it for quite a few years despite reservations about its bass. I partly solved this by building the Cabernet Dual [positively reviewed by Srajan back in 2006 - EK] which used the 101D and a 6H30 in two separate but integrated line stages. This was ideal for speakers which could be bi-amped to use the 101D on the mid/ treble and the 6H30 circuit on the bass.

"I started using Russian DHT about ten years ago. The engineering of Russian tubes is mostly excellent and some tubes are exemplary. Most of the early 1930s’ American DHT are hard to match perfectly but it's not uncommon to get a batch of 4P1L that all measure exactly the same. I really like the 71A and 801A/10/10Y of the early 30s US options in preamps but getting matched pairs is quite difficult now. Of all the Russian DHT I tried, I like the 4P1L most. It has a very good bass, sometimes the Achilles heel of DHT tubes, and has a very extended but sweet top end. The midrange is maybe not as euphonic as some but that after all is a coloration and usually becomes unconvincing over time.

"Supratek preamps are always high gain. I'm convinced that high gain equals higher dynamics and better resolution of microdynamics. This is usually achieved by putting a 6SN7 in front of the DHT as an input gain tube. In the Reference preamp, I elected to use a step-up transformer instead and make it switchable so as to enable user choice of utilizing extra gain or not.

"As per usual with Supratek, I also fitted an output transformer. This drops the gain if switched in and also drives down the already lowish output impedance of the 4P1L to a very low value suitable for driving amps or multiple amps with low input impedance. By using combinations of input and output irons with their respective gain contributions, it's possible to find a gain setting to suit any amp or taste. The Supratek tube shunt regulator remains as always. I remain convinced that super-tight voltage regulation is essential for fast responsive tube sound. Some like the sound of unregulated warm and romantic tubes but for me, no thanks. Tube rectification isn't quite as important an issue. It's possible to use solid-state rectification that sounds as good as the old tube type rectification but I like the latter’s slow-start feature. It's worth noting that there is quite a bit of solid-state engineering in the circuit. The 4P1L plate is loaded with a combination of depletion-mode Mosfets and Jfets. This gives high gain and superb performance. I've been using this combination for quite a few years now and it is very robust and reliable. Reliability is something I value highly and the Reference is built to last the lifetime of the owner and beyond.

"The phono stage is a bit different than the normal Supratek phono stages. Just about all of them used the 6922/ECC88/E88CC in combination with a E180F/6688-Jfet cascode input. The 6922 tube variants are still produced and very suitable for the gain structures needed in phono stages. They can sound very good but need to be selected carefully. It's a bit hit and miss and they do tend to have a sonic signature on the hot side that needs to be engineered out. For the Reference I wanted to do something a bit different. I wanted to use a tube that sounds good and is consistent from sample to sample and easily available. That sounds like the good ol’ 6SN7 or Russian NOS 6H8C. Gain is down a bit but still suitable for the majority of low-output moving coil cartridges. No moving magnet option, sorry I don't like them. The Reference phono stage is a hybrid of LCR and CR passive RIAA equalization. LCR phonos are a bit fashionable at the moment. I’ve built quite a few over the years but was never totally convinced that they were tonally accurate. They sound a bit "phasey" and "rich" to me. Of course many people like this sound but eventually it gets exposed as untruthful. After many years of playing around with all sorts of concepts, I found that using CR EQ for the treble and LCR for the bass gave the perfect combination of accuracy and tone."