The operation of the front panel input selector is straight forward. The selected input is
indicated by a blue LED above the knob, and the selection can be moved by rotating the
knob. The knob is attached to an optical encoder indicating the direction of rotation to the
microcontroller which drives the input relay switches.
The Left and Right input level controls and the Master level control are designed not only to
provide level adjustment and balance adjustment, but also to optimize the performance of the
circuit against various input levels.
The input of the gain stage is directly coupled to the input selected, without an intervening
volume or balance control as seen in other preamplifiers. This provides the highest
performance from the Mosfet gain devices.
Because there is a wide variety of possible source levels, the intrinsic gain of the circuit
should be adjusted to provide optimal performance. This is something you will not see on
other preamplifiers. The intrinsic gain of the circuit itself is adjusted by the Left and Right
level controls. We set the gain lower for sources which have high levels, and higher for
sources with low levels, minimizing the distortion and noise for each.
We have set the maximum input so that the circuit will remain linear with balanced input
peaks up to about 18 volts. If you are concerned that your source will approach or exceed
this level, you may use the internal attenuator switch to reduce the input by 12 dB.
The switch default position is 1,2,7,8 = ON and 3,4,5,6 = OFF. For a 12 dB input
attenuation, 1,2,7,8 = OFF and 3,4,5,6 = ON.
This switch can also be used to effectively lower the gain of the preamp by 12 dB. It does
not otherwise affect the performance of the preamp.
At the output of the active gain circuits is the master level control. This is a low impedance
discrete level control which sits between the preamp active circuitry and the input to your
power amplifier. It attenuates the output of the preamp in the same manner as you would
experience with a “passive preamp” except that it has a precision 256 position 4 pole
attenuator for both balanced channels matched to .1%. It also has a low impedance, which
assures that the typical unbalanced output impedance is between 100 and 1000 ohms.
There are good reasons for having such an arrangement:
The gain stage operates at a constant level regardless of the setting of the master level
control, and so the sound of the this circuit will not alter at various level settings.
The preamplifier will drive a low impedance load without alteration in the signal, in fact it can
be used as a current source driving a 0 impedance mixer junction without any distortion.
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike
circuits where the volume control is before the input.
