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The amp is a c-j ET250S. Its rated input sensitivity is...

...1.85V for 250W into 8. If I did my arithmetic correctly, the Voltage-gain ratio is 1:24.2 or about 28dB! It has a 0.1uF cathode-bypass capacitor bypassing a stack of 4 zener diodes. It has no overall negative feedback. Speakers are Audio Physic Avanti IIIs rated at 88dB.

Input impedance is rated at 110K and measured at 120K. I've added a high-pass 0.1uF cap at the RCA inputs and a 56K resistor in series to achieve a high-pass filter of about 90Hz; the MR drivers have no HP caps in their x-overs. The gain decreased about 3dB at 1K from adding the 56K resistor. I'm running the preamp in a range of about -50dB to -30dB (ref. full gain). I'd like to be rid of 10- to 15dB of gain in the poweramp.

The tube section's schematic is here...
http://s89.photobucket.com/user/jeffreybehr/media/C-j%20equipment/conrad-johnson%20ET250S%20poweramp/ET250_frontends_zpsswodrqfl.jpg.html?sort=3&o=0

...and the driver and output stages here...
http://s89.photobucket.com/user/jeffreybehr/media/C-j%20equipment/conrad-johnson%20ET250S%20poweramp/ET250_output%20stages_zpsmwnwfwrr.jpg.html?sort=3&o=1

I've thought about increasing the values of one of two quads of coupling resistors--Rs 33, 34, 50, and 51 or Rs 37, 35, 54, and 53--but have no idea what bad might happen.

Of course, I could increase the value of the added input-series resistor to something like 300K, but of course with higher resistance, capacitance must decrease for the same FP, and that requires finding 5-star-quality input-filter caps in the 0.01 to less-than-0.1uF values, which is dificult.


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