Having put this amp to rest for a while I suddenly got awaken by a suggestion from Pete Good via e-mail. He uses a positive powersupply for the output stage without a negative rail. Since the output is balanced and not referred to ground, this is possible. This obvious (one of those ‘of course, this is so obvious’ thing once somebody comes up with it) solution enables the input to be at ground level and eliminates the noise problem of the original as well as eliminating the need for capacitors in the signal path.
The only setback is the limited voltage available for the VAS and currentsinks in the output stage. But if a few volts can be sacrified, meaning design with a few extra volts than needed for full output swing, no real problems should arise other than a little wasted power dissipation.
Damn it, I was just about to start another tubed project…
Below is a suggestion, derived from Pete's and my design, for a 30watt/8ohm with a few volts to spare. The idle current of both halves need to be 2.8 amperes, which will produce a voltageswing of 22volts over 8ohms. Power dissipation will be 30*2.8*2=168watts/channel.
This improvement came about because of an e-mail with a few suggestions as well as a few questions, exactly the way I like it, fun. Thanks Pete.