7
X
s
Owner’s Manual
Initially our prototypes used the conventional X series front end,
but as we tweaked the output stage we slowly worked up newer
circuitry for the voltage gain system. We continued to use cascoded
NOS (
New Old Stock
) Toshiba Jfets that still set the standard for
input devices, but for the heavy voltage lifting we switched to better
Toshiba Mosfets (NOS, also discontinued). Much more heat sinking
was allowed for these, and we were able to increased the bias current
of the front end by a factor of about three, giving much better
linearity and much higher drive current capacity.
A new topological wrinkle was introduced for the first time in this
front end, CLF (Cascode Local Feedback), an alternative method
of controlling linearity around a single stage without the usual
trade-offs. This innovation allows arbitrary shaping of the relative
distortion harmonics and was tweaked to precisely complement the
character of the output stage.
This new front end has a 100 Khz open loop bandwidth and a gain
figure which allows about 10 dB of feedback around the output
stage. The output stage could easily be operated without feedback,
but it sounds better with this little bit. As an aside, all this was
accomplished without frequency compensation – there are no little
capacitors in the circuit to trim or stabilize the response. For that
matter, the circuit is also DC coupled. The only non-power-supply
capacitors are across the DC bias voltage generators.
Also, some of you will be delighted that the new front end has a
200 Kohm input impedance (balanced) with very small capacitance.
Anything will drive it.
Some things remain the same. The amplifiers still run hot, and the
meter on the front panel still reads the current draw of the output
stage. Normally it sits there contentedly in the center position, but
when the output current of the amplifier exceeds the Class A bias
setting, you will see the needle bounce upwards.
I think though, that the Xs amplifiers will set a new standard for
musical reproduction.
Nelson Pass
3/19/2012