John Mentioned....
Just for your records,
Cary mades a such differential power amp with tube CCS & without zero
NFB:
http://www.caryaudio.com/products/classic/CAD211FE.html
Hp
As usual they are keeping the cct a secret! But 6CA7 seems far too much
of a tail for the front end difference 6SL7 amp, so perhaps the 300Bs?
To run AB on the 845s surely no tail there.
Cheers, John
It seems Cary Audio are copying the Chinese made Ming-Da monoblocs which use 2 x 845, 2 x 300B drivers, and 2 x 6SN7 input, but with no CCS tail anywhere.
I couldn't find a schematic of the Cary amp so I cannot endorse their methods. I always assume all high end makers include gross shortcomings in their designs because every time a sample lands on my bench for singing lessons, de-noising, and to stop them smoking, I have always had to do serious mods to get them to work properly.
I totally rebuilt Ming-Da a few years back and owner was very happy. Using 300B as drivers with very low anode current of about 5mA each may seem stupid but it works OK.
The owner found the sound of reformed Ming-Da excellent even though there was a total of 4 amp stages - AND 12dB of global NFB. I was able to get a good 85W in class AB, and with 845 biased with only 35mA and with the B+ at 1,200Vdc.
Using 2 x EL34 in triode as drivers would be better, and with a CT choke and choke to anode resistances to allow 20mA Ia and high Z RLdc, and hence wide V swing at low THD, as shown at my website. All too hard for Chinese to figure out, and too hard for Cary either.
I cannot respect a marketing ploy where the tube line up is given and the use of constant current source ( EL34 ) is touted without saying where or how. BS is BS as usual.
CCS work best in an input stage pair where you have an input signal and FB signal and you just want to amplify the difference. The result is always good sounding, low noise, low THD and perfect balance. Such a stage can also be used to work as a floating balanced input amp as any common mode grid signals can't produce much anode signal because of the high Z tail.
Other uses for CCS are for anode loads, and cathode loads. I find 2SK1924 works very well as an anode load for EL34, and to voltage proof them a bit you can run two in series - special trick of mine.
Patrick Turner.