Custom Cortese
I've been building preamps for nearly 40 years and to hear an old recording, that I thought I knew every last detail and nuance of, to sound like I'd never heard it before, with a freshness and amazing retrieval of detail and vividness , makes it almost impossible to believe I'm hearing the same recording!
I thought I knew the old Beatles records very well, but I had no idea that so much musical info was there that I'd never heard that way before- a presentation that is like listening to a completely different recording.
What this preamp does so well is the 3D thing, it lets you hear way into the music, the depth and width of the soundstage seems to be much magnified , it sounds less like a recording and simply more lifelike, digital is less digital, and vinyl is more vinyl!
Supratek preamps have always had a reputation for that, but now there is a new standard.
To get such a big step up in sonic gains with a new design is unusual- preamplifier design usually results in little steps over a long timeframe to achieve a remarkable design, which in essence is what I've done when one considers that I've been playing around with the 6SN7 tube for so many years.
What happened over the last couple of years was a very rewarding coming together of a lot of synergy, lots of knowledge and experience that resulted in a mix of design elements being put together , that went KABOOM and resulted in something special.
Not that there was , or is , anything deficient with any of the previous Supratek designs, they are all very highly thought of world class preamps.
And I think there may be some who still prefer the warmer, delicious sound of a Supratek DHT preamp, with its slight emphasis of tone and perhaps a touch of veiling.
But I'm finding the new 6SN7 preamp to be perfect in just about every way, its a magical looking-glass into the heart of the music , its exciting, AND soothing and so damn musical , yet is also so very accurate and transparent- theres no tube shortfall or compromises, its an exciting state of the art design that I'm going to find a real challenge to improve.
I'd like to go into some detail of the design and component elements of this preamp- I don't usually get too technical , but I would like to shed some light on the numerous elements that go together to make this such an extraordinary and incredible value preamplifier.
Everything starts with the power transformer and a big 165VA transformer that is the size normally used in a power amplifier provides plenty of electrical energy comfortably and reliably.
A tube rectifier provides DC voltage into a CLCR filter , designed specifically for use with the tube rectifier (I've lost count of the number of tube preamps I see with inappropriate filter design. Tube rectifiers cannot use the same values as a solid state design, but the wrong component values that suit solid state are all too common)
Electrolytic capacitors are not used in the High Tension voltage supply, electros have limited life, and a thinner , less satisfying sound. They are used extensively in commercial products because they are cheap, and polyprop/oil caps are too bulky to fit in a one chassis design.
The DC voltage is then fed into a tube shunt regulator- one for each channel. I've been using tube shunts for over 20 years and they are part of the reason for the Supratek sound- they keep the voltage of the preamp tubes constant no matter what the current draw is , and help with the definition and "blackness" around all dimensions of the music.
The power supply also contains a solid state rectifier for the tubes heaters- very over-engineered and mounted on a large heatsink for reliabilty.
This is attached internally to the chassis of the power supply, made of 1.2mm copper plate which is an excellent heat transfer .
copper chasis before polishing and plating
Copper plate is also used for the preamp chassis, it has a different sound than the steel chassis's used in virtually all other preamps, its not a sound instantly recognisable, but is a softer , less harsh sound over long term listening.
On to the preamp circuitry.
The first tube preamp I built 40 years ago was a phono stage that used 12AX7 and 12AU7 tubes, which are still very common these days and the first choice today for commercial tube manufacturers - they are readily available and cheap.
They were also designed to be used in televisions.
The 6SL7 and 6SN7 tubes were the lower distortion predecesors of the 12**7 tubes and came from audio applications, but back in the 1970's they were largely unused , and for a decade or so until they came back into vogue , one could buy really nice NOS 6SN7's for peanuts.
The first 6SN7 preamp I built I fell in love with the sound - lush , inviting and so much better than the newer , modern tubes which sound strained and distorted in comparism.
Simple circuits - plate resistor loading , capacitor outputs , highish output impedance that demanded an easy load, preferably a tube amp.
Over the next 40 years I tried just about every possible combination of tube/circuitry/components till I have finally arrived at todays 6SN7 circuit.
Both triode sections of the 6SN7 (It's a double triode with two seperate triode gain sections with a common heater) are cascaded, ie the line inputs are connected to the first section , which is then connected to the second gain stage section, which becomes the output stage. It's not quite as simple as that as there are some design tricks I'm not going to divulge , but that's a basic description.
This results in a high gain circuit, which to my ears always sounds more dynamic and flowing than a low gain circuit, and there are ways to make the gain useable and useful.
6SN7 double triode
Both sections are plate loaded with solid state constant current sources, (CCS) which have been refined over many years to be as transperent and musically coherent as passive devices without the limitations.
The cathodes of both sections also use constant current sources, but in this case its to avoid the use of capacitor/resistor bias devices.
CR filters must introduce phase anomalies , and this affects the imaging and sound-staging of the preamp. By eliminating the CR filters common to almost all other preamps , the Supratek preamp gains a tremendous advantage in realism that preamps with CR filters always degrade.
The CCS's used in the cathodes are discrete solid state devices, mounted on a small PCB with the plate CCS devices.
Traditionalists could argue that passive loading use transformers or steel/nickel/wire chokes sound more "tube like" . I did use metal/wire chokes and transformers for these purposes for quite a few years, but I was always aware of the severe limitations the resonances and frequency anomalies of the devices brings to the sound when using them for plate or cathode loading.
I like a preamp to sound beguiling and interesting, but there's no reason it cant be accurate and technically proficient as well.
However, there is a place for transformers and chokes in a well designed preamp , as long as they are very well made and used accordingly.
I use Lundahl and Magnequest input and output transformers and grid chokes - these are expensive , high quality products that are the result of many years experience with transformer winding techniques.
The use of transformers for the output stage has two distinct advantages. Its a step down transformer which transforms the high gain of the circuit into a lower gain , and also lowers the output impedance
of the circuit. This 6SN7 circuit has an output impedance of around 1500 ohms, which will drive any solid state or tube amp.
It also has a centre tapped winding which allows the use of balanced or unbalanced output switching.
This is by far the best way to achieve a balanced output. We can use a Single Ended Class A circuit throughout the preamp and then get a perfectly symmetrical fully balanced or unbalanced output signal.
(I'm sorry but if you think an all balanced preamp, with double the components working in push pull sounds better, then you need to get new speakers. 😊 (but not as critical with balanced power amps which have their uses)
People like to talk about the use of super expensive and exotic capacitors. I do use very high quality caps in my preamps, but the overall determination to the sound is the correct value of a high quality cap, rather than some exotic all-talk device designed to make money.
Using high quality film capacitors rather than electrolytic caps has far more effect than replacing a capable film cap for an exotic one.
My line stages have no electrolytic caps , and the phono stages have only a couple of Elna Silmics electro caps in them.
The result of this circuit design, high quality components and dedicated handmade craftsmanship is a preamp that is sophisticated and elegant.
It is technologically state of the art design , but is also an instrument of music - its purpose is to convey the music in as pure a form as it can.
It will startle you and calm you, it will enchant and beguile you.
It is the result of my dedication to the art of the tube preamplifier over a very long period and a lot of hard work, combined with a lot of joy taken from the amazing and heart lifting world of music.
Custom Cortese. Power supply on left with tube shunt regulators and tube rectifier.
