The Supratek push pull tube power amps have been continually evolving since I started to prefer them to the Single-Ended amps I built for such a long time in the 90's and early 00's.
SE can sound exceptional with the right speaker, but a high efficiency speaker that is reasonably accurate is so damn rare, a push pull amp is much more practical and suitable for the majority of purposes.
But becoming used to the purity and clarity, and especially the vividness of SE amps, most of the push pull amps I'd heard and built after the SE's left me cold. Sure they had better power and control, but they all sounded veiled, constricted and lifeless .
So all my design efforts from then on concentrated on building a push pull amp with control and power AND with the immediacy and vividness of a good SE amp.
Fortunately I had Kevin Covi and his amazing computer simulations to do all the hard nitty gritty tube operating point work while I concentrated on fine-tuning the sound to find that elusive magic I was chasing.
I believe the biggest mistake manufacturers make is the overuse of electrolytic capacitors-these are poison to sound quality, and the least you can use the better. The alternative is plastic caps, but when they are 20 or 30 times the size of a comparable electro and many times more expensive, most manufacturers baulk at the idea of using them.
Thet Supratek Monduese amp uses a couple of electros after a set of high speed hexfred rectifiers- the HT voltage is then pushed through a parallel pair of 5AR4 tube rectifiers, used as tube dampers in order to get a nice soft start on turn on, and for some "tube" sound. On each side of the 5AR4 is a polyprop cap with a choke to make a nice CLC(capacitor-inductor-capacitor) filter.With this approach we get the solidity and speed of a solid state power supply combined with the slow turn on features and sound of a tubed CLC . Again, too bulky and expensive for most manufacturers.
The approx 500V HT voltage is sent to the output tubes, and to a tube regulator using a 6H30 tube to shunt regulate the voltage for the input and driver stages.
Surprising to some, we use a single 6H8C/6SN7 for the input and phase splitter/driver. This results in a low gain/sensitivity amplifier circuit, but this is exactly what we are trying to achieve.
The purpose of a power amplifier is to turn the signal voltage from the preamp into current to drive a loudspeaker-it should be nothing more, but these days most power amplifiers are in effect integrated amps, with quite high gain. This seems to have evolved from a desire to use the typical 2V output from a CD player directly into a power amp.
However there are all sorts of problems with this approach- firstly the impedance matching between the source and the power amp may not be optimum and a flat ,boring, un-involving sound is often the result.
A high quality preamp before the power amp will provide much better matching and will give real tangible benefits to the sound. Let the preamp do the signal gain work and the power amp provide the power is our approach.
The second problem with high gain power amps is that of noise- if they are used with a high gain source or a preamp with voltage gain, the power amp will amplify the noise floor of the preamp and the noise will become audible.
A properly designed preamp and power amp, designed to work in conjunction with each other, will be very quiet, even with very high efficiency horn speakers with outputs of 100+dB.
10 years ago I sold quite a few SE Merlolt amps to Avantegard horn owners as they were one of the very few SE amplifiers quiet with horn drivers.
Back to the Mondeuse- the single 6H8C/6SN7 input tube circuit is based on the classic Williamson circuit, with the input tube direct coupled to the phase splitter and then cap-coupled to the output tubes via a pair of polyprop caps . Very simple, very effective circuit, the only concession to modernism is a Constant Current Source(CCS) loading of the input tubes plate.
There is a total of 13 capacitors in the Mondeuse amplifier- compare that to a typical amplifier with maybe 50 ,usually all of them electrolytic- capacitors are bad-mmmk?
At the grids of the output tube , instead of using grid resistors we use grid chokes, which do the opposite of "choking" and provide a real improvement in sound quality. The difference between grid resistor loading and choke loading is quite profound, the soundstage is bigger, imaging takes on a more 3D presentation.
There is a bit of fine tuning to be done as the inductance of the chokes does give some subsonic accentuation, but this gives us a great opportunity to tune for flat deep bass. This is done with a compensation filter that is non-obstrusive. Again, too bulky, expensive for mainstream manufacturers.
Biasing of output tubes is usually done by either fixed bias or cathode bias- both have advantages and disadvantage, but we do something different (again)
Kevin Covi came up with a couple of different ideas to improve the traditional biasing methods and which have the extra bonus of eliminating crossover distortion which is a problem with traditional Class AB biasing methods. After much evolution, a CCS is used under each of the output tubes and is adjusted to give constant, unwavering current through each tube that mimics pure Class A operation, with close to the power of Class AB. Another advantage is the CCS devices are bolted to the copper chassis which acts as a heatsink and keeps the heat out of the chassis interior.
As each tube is adjusted for constant current with a small pot on each tube, different tubes can be set up with the amp- KT88 are standard on the Mondeuse, but 6550, KT120,KT90, EL34 can also be used, although some technical knowledge is required to do the adjustment.
However, once done no other tube adjustment is required.
Output transformers are oversized and optimally chosen for the circuit operation- the device we use is a high bandwidth transformer that does not require feedback to give this high bandwidth response.
Feedback- another contentious issue. The Supratek push pull amps use both negative and positive feedback.
There is nothing wrong with a sensible amount of negative feedback, as long as it used properly and not as a cure-all for design faults or poor quality output transformers. Some speakers really benefit from a touch of negative feedback which lowers the amplifiers output impedance and can drive the speaker easier.
Go too far with excessive feedback though and the sound degradation is very obvious- veiling and a loss of 3D presentation is usually the result.
The Supratek amplifiers have adjustable feedback via a 6 position switch- the first position is no negative feedback and the next 5 positions give an increasing amount to a final sensible,but not excessive amount of feedback. Using my Tannoy Golds with an EL34 Mondeuse I might use the first or second position, but driving ribbons or stats requires the higher positions.
Positive feedback. Kevin uses Wolcott amps on his Soundlab stat speakers. The Cotters are maybe the most technically proficient tube amps available and maybe the only tube amp capable of driving difficult speakers like the Soundlabs accurately. The Wolcotts use positive feedback to lower the output impedance of the amplifier quite drastically, to achieve the same damping factor as the biggest, meanest solid state amps.
However, imo the amps are very complex and whilst I wanted to investigate positive feedback I wanted something a little less complicated.
Positive feedback is nothing new, it was used on quite a few amps back in the "golden age" of tube design, but was less appreciated then as speakers were relatively easy to drive.
However my stat speakers are definitely not easy to drive and Kevin and I spent a lot of time getting a positive feedback system to work with the Mondeuse and Malbec amplifiers.
The result is quite pleasing- in theory the positive feedback can be adjusted to give extremely high damping factors, and while it still cant drive the most difficult speakers it does make the 100 watt tube amplifier a formidable weapon to drive all but the most difficult speaker.
It's greatest potential though is the sheer adaptability of the amplifier- with both adjustable and switchable negative and positive feedback the combinations of different sound perspectives is greatly enhanced. From romantic lush to master tape accuracy the choice is yours.
The Mondeuse and Malbec amps have a negative feedback switch and adjustment on back panel and positive feedback switch and adjustment knob on front of amp.
Setting both positive and negative for best speaker performance can be tricky at first, but with a bit of experience it becomes relatively easy. With my Tannoy Golds, which are a easy load for most amps, I use very lttle, if any, positive feedback and maybe none or 1 click of the negative feedback .
With my hybrid stats/ribbons it is a different approach. These speakers are a difficult load and the best way to do this is to start with the 2nd position of negative feedback and slowly raise the positive feedback while listening. Then try the 3rd position of neg feedback and raise the positive feedback. You'll soon find the spot where the amps have combined with your speakers and are close to infinite damping and thus very accurate sound. You can choose to use this or back off for a less damped sound- the choice is yours.
Sometimes the gain of these low gain/sensitivity amplifiers is too low, for example with a preamp with low or no gain (I cant see the point, but there are quite a few available)
BTW sensitivity has nothing to do with power- sensitivity is the amount of volume required to reach full power- a 5 watt amp can be built to have much more gain than a 100 watt amp, but it will not be able to drive most speakers. The 5 watt amp will be distorting at 9 o'clock while the 100 watt amp will be clean and have power in reserve at 11 or even 3 o'clock. (depending on preamp gain)
Another example of when the gain of the Supratek amp could be too low is when an additional device is inserted between preamp and power amp - I have a DEQX digital crossover in one of my systems that soaks up gain and requires higher sensitivity power amps.
In this situation I use an optional switchable input step up transformer. The advantage of this method is that we get the required gain, but dont have the disadvantage of extra input noise, hum etc that would result if we used an extra tube for more gain.
Again it is switchable so it is there when needed and out of circuit if it is of no advantage.
The Supratek push pull amps are basically simple in concept- the basic design of all tube amps has remained the same for over half a century, but there are some very clever design variations in the different stages- the low gain input/driver stage to reduce noise and component count, and minimise phase issues. Minimal use of capacitors - less is more with capacitors. CCS devices used for constant reliable operation of output tube bias/current. Voltage shunt regulation of driver/phase splitter stages using tube regulator. Grid chokes.
Switchable and adjustable negative and positive feedback. Optional higher gain if needed.
High quality power supply using hexfred diodes and tube rectifiers.
All of these improvements make a very sophisticated amplifier that is very reliable and sturdy with a choice of adjustments that make it possible to fine tune the sound to get the best out of your speaker.
