John, I am very interested to jump on board with the vfet amps, is there still a possibility (30 pairs does not go very far)? Could you send me the details?
Sounds like you are running this in parallel with the MDAC project, can you manage it all?!
TonyB,
Basically the last couple of units – it looks like I'll be building around 10 pairs of these amps (excluding my own units).
Yes - they will be designed in parallel with the MDAC2 project as I’ll need a decent pair of "reference" grade amps to evaluate the MDAC2. It will also be nice to have an Amp / DAC combination that I have designed myself to work together as a system - at least I can have confidence in the DAC / Amp combination.
WRT to the MDAC2 design I find that I can only work effectively (concentrate) on the design for the first 6 to 8 hours each day, after that I tend to slow down so being able to work on something different will "boost" my spirits.
Managing myself is not my strongest point - for sure I'll be pushing my limits come Christmas, however I tend to work best under a little pressure. Actually, the Amps are a much easier project to the MDAC2 – also much easier for me as I can hand build them on my lab bench without needing to work with an external SMD subcontractor – I can also source the components here in Europe (for the MDAC2 I need to arrange certain critical components from Asia due to cost and availability)…
Sure, I’ll be talking to myself in harsh tones while I’m hand building the PCB’s – but I’ll be very proud of them once they are completed, just as with all my designs
The final design is subject to refinement (just as the MDAC2 'features' have developed as I've progressed with the design), here is the general overview of the design:-
These amplifiers are intended to serve as my own “Reference units” and designed to drive my Martin Logan’s CLSIIz’s with their notorious 0.5 Ohm impedance dip at HF. Due to the limited availability of the VFET’s, Tubes & Vertical MOSFET’s used in the current Dumper section they are sadly limited in numbers to around 10 to 20 pairs.
The amplifier topology will be based upon a JAN6112 high gain, Military grade miniature Triode used in the input Diff Stage – this “gain block” is based upon the unique circuit I’ve developed for the TDAC – it’s a fully balanced, direct coupled circuit (with no interstage coupling capacitors) – the circuit operates in high impedence‘Current mode’ to achieve the highest linearity from the Tube.
The Tube Gain stage drives the VFETs operating in ClassA (about 5W) this is coupled to the speaker load via a Bridge “sensing” circuit (AKA Quad Current Dumping). The ClassA stage via the bridge circuit controls the operation of the MOSFET Current Dumpers – Basically the current dumpers’ ‘Bootstrap’ the VFET ClassA stage, so that the VFETS ‘believe’ they are driving a fixed 5W load at all times.
While operating within normal signal limits, the ClassA stage delivers 5W +/- a small error signal that compensates for any non linearity of the current Dumper section which will be typically less then 0.01%.
The VFET ClassA stage delivers the first 5W to the speakers with the “Dumpers” providing the balance. As the Current Dumpers are controlled via the ClassA stage (the ClassA stage is always delivering the first 5W, even when “idling” with no audio signal), the Amplifiers sound quality is determined by the VFETs
This allows high power from a single pair of VFETs’ – also, the “precious” VFETS are protected from external adverse load conditions.
Target output power is 200W to 250W – with balanced XLR inputs (which can be used with single ended sources).
The Current dumpers Mosfets are NOS vertical Hitachi's, sadly limited stock (I still have a small stock from a project my father and I had planned for Pink Triangle).
It’s a low feedback design (the Feedforward Bridge helps to maintain the output stage linearity), yet despite low feedback will meet a target THD of 0.005% at 1W, 1KHz.
These Amplifiers are very much a ‘background’ project (while the MDAC2 takes shape) – this also allows me time to perfect the design; hopefully the first units will be built around Christmas (this year)
As these VFET Amps and MDAC2 are being developed together at the same time, they will be designed sonically to complement each other.
The VFET based amps are pretty costly as they are handbuilt "custom designs", I'm building them to serve as my reference amps to aid in the development of MDAC2.... however so long as I can purchase the VFETs then I'm more then happy to also build them for others
The price is GBP3500 for a pair of Monoblocks (in Silver or Black), the cost is split into two payments, the first for £2300 is partially used to secure the new donor 8200MB's, VFET's & Tubes etc – and the balance of £1200 would be payable once the final units are being constructed towards the end of the year – in time for Christmas.
These amps will be Reference grade units designed to achieve the very best with the MDAC2 / TDAC...
PM me if your interested in reserving a pair.
