Dynaco Aikido Features
Overview
The circuit board is the same size as the original PC5 board, 5.6×3.6 inches, and mounts identical to the original board.
The AIKIDO circuit delivers the sonic goods. It offers low distortion, low output impedance, a great PSRR figure, and feedback-free amplification. The secret to its superb performance which lies in its internal symmetry, which balances imperfections with imperfections. As a result, the circuit works at least a magnitude better than the equivalent grounded-cathode amplifier, and much better than the 50 year old Dynaco design.
Low Distortion
The AIKIDO circuit produces far less distortion than comparable circuits by using complementary, balancing load design. By loading a triode with the same triode, under the same cathode-to-plate voltage and idle current and with the same cathode resistor works well to flatten the transfer curve out of that triode.
PSRR
The circuit self-corrects power supply noise by incorporating the noise into its normal operation. The improved PSRR advantage greatly unburdens the power-supply, and results in a better than -30dB PSRR figure.
A conventional grounded-cathode amplifier with the same tubes and current draw yields only a -6dB PSRR). In a conventional circuit, critical voltages will vary as the power line voltage falls and climbs with normal variations in utility power, but with this design the amplifier will tracks and correct these voltage changes symmetrically.
This circuit eliminates power-supply noise from its output, by injecting the same amount of PS noise at the inputs of the top and bottom tubes in the two-tube cathode-follower circuit.
Since both of these signals are equal in amplitude and phase, they cancel each other out, as each triode sees an identical increase in plate current.
Age Tolerant
Tube characteristics change with age and wear out. In other words, a fresh 12AX7 is not the same as that same 12AX7 after 2,000 hours of use. But as long as the two Triode’s within the 12AX7 age in the same way—which they are inclined to do—the amplifier will always bias up correctly, splitting the B+ voltage between the Triode’s.
NOTE: tubes require 24VDC for the filaments, NOT 6.3V or 12.6VDC!
Low Output Impedance
This circuit uses a modified cathode follower circuit as the output stage. Cathode follower s are famous for providing low distortion and low output impedance’s, but no voltage gain. This modified cathode follower scrubs away the power-supply noise from its output and provides a complementary non-linear load for the top Triode’s cathode. The top Triode’s capacitor resistor is in series with the output, so its resistance must be added to the cathode follower output impedance. Had the output connection been taken from the top Triode’s
cathode, then the output impedance would be slightly lower, but the symmetry would be broken and the PSRR enhancement would be lost.
Gain
The gain from this circuit is around 12dB with the 12AX7 type tube.
There may be some instances where this gain of 12dB may be too high for your setup, with other words, your amp seems to max out in volume with less that 50% turn on the volume control.
The gain can be reduced to 6dB, by changing the values of the following components.
Change C1 from 0.22uF to 0.47uF. Change R15 from 1M to 470K. Make sure you make these changes for BOTH channels!