Remember, tubes are not yardsticks that never change, being more like car tires—
wear out. Just as a tire’s weight and diameter decrease over time, so too the tube’s
conductance. So the fresh 6DJ8 is not the same as that same 6DJ8 after 2,000 hours of
use. But as long as the two triodes age in the same way—
which they are inclined to do,
as they do the same amount of work and share the same materials and environment
the Aikido amplifier will always bias up correctly, splitting the B+ voltage between the
triodes. Moreover, the Aikido amplifier does not make huge popping swings at start
up, as the output does not start at the B+ and then swing down a hundred or so volts
when the tube heats up, as it does in a ground
-cathode amplifier.
This circuit eliminates power-
supply noise from the output, by injecting the same
amount of PS noise at the top and bottom of the two-
tube cathode follower circuit.
The way it works is that the input stage (the first two triodes) define a voltage divider
of 50%, so that 50% of
the PS noise is presented to the CF's grid; at the same time the
100k resistors also define a voltage divider of 50%, so the bottom triode's grid also
sees 50% of the PS noise. Since both of these signals are equal in amplitude and phase,
they cancel each
other out, as each triodes sees an identical increase in plate current
(imagine two equally strong men in a tug of war contest).
If the output connection is taken from the output cathode follower's cathode, then the
balance will be broken. The same holds
true if the cathode follower's cathode resistor
is removed. (Besides, this resistor actually makes for a better sounding cathode
follower, as it linearizes the cathode follower at the expense of a higher output
impedance. Unfortunately, it should be removed and the bypass capacitor C3
be used when driving low-impedance headphones, 32-
ohms for example. When used
as a line stage amplifier, n
o cathode resistor bypass capacitors should be used, as these
capacitors are very much in the signal path and ver
y few do not damage the sound,
unless high quality capacitors are used.)
Rt C1
Rt C2
Lt C1
Lt C2
Right Output
Left Output
C1 & C2
C2
C1
How do I wire up a rotary switch for switching between the two coupling
capacitors? We need a four- pole, three-
position switch and some hookup wire. All
four coupling capacitors attach to the input contacts and the two channels of output
can receive either
coupling capacitors C1’s or C2’s or both capacitors’ outputs. The
drawing below shows the knob on the faceplate and the rotary switch from behind.
(The switch is shown on the "C1 + C2" position.)
Switch Rear
Switch Front
GlassWare Audio Design
3
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