The break has also given me time to think about the design. I like to get components as close to each other as possible. PCBs, generally speaking, have poor quality copper track on them. So I was building the circuit using no printed circuit board and with components very close together. Because of this the first three production runs have blown up, the last causing one of my speaker drivers to melt (NAIM SBLs!). So I will now eat humble pie and agree to use PCBs and I can now see why everyone uses them.
I also came to the conclusion that I wanted to get rid of a lot of the drive circuitry to the power MOSFETs (I will be using 2SK1058 and 2SJ162 in push-pull configuration). Components do useful things but they must bring both distortion and noise so if it is possible to remove them, I do. It is also the area that keeps shorting out. So I started to do some research and came across a component from TI (it used to be NSC but TI took them over), the LM4702. They use a minimal amount of external components and can drive these transistors directly with no driver circuitry. I can’t believe my luck really. How amazing is that – you need something and someone has already made it (it only came out in 2005). I know some believe that op-amps should not be in a high end HiFi system but there are some compelling reasons for using them; you get a lot of close matching when all components are on the same substrate and components are close up to each other so control is good. Single-ended, Class A, no NF are good solutions and I may go there myself one day but this design needs something sonically way above everything else in its price bracket and I want that bracket to be within most people’s grasp – not just wealthy people.
There are two amps in each chip but I want totally separate power supplies for each channel so I will use the second amp to drive an output for using with a sub-woofer or rear speakers or speakers in another room.
The PCB I will get made in China and will get them silver plated (I do like silver when it comes to connections).
I am quite excited by the new design direction and feel this will be a good solution to my build problems.