HP Aikido Stereo 9-Pin PCB - Manual (Page 9)

GlassWare Audio Design 8
High transconductance output tubes are best for driving headphones, for example, the
6DJ8, 6H30, 12BH7, and ECC99. A coupling capacitor of at least 33µF is required
when driving 300-ohm headphones; 330µF for 32-ohm headphones. Use a high-quality,
small-valued bypass capacitor in C2’s position. Capacitor C3 can be bypassed by placing
a small film capacitor across the leads of resistor R11.
Configuring the PCB as a Headphone Amplifier
The standard Aikido is a thoroughly single-ended affair, nothing pulls while something
else pushes. Unfortunately, wonderful as single-ended mode is sonically, it cannot
provide the larger voltage and current swings that a push-pull output stage can. Single-
ended stages can only deliver up to the idle current into a load, whereas class-A push-
pull stages can deliver up to twice the idle current; and class-AB output stages can
deliver many times the idle current. For a line stage, such big voltage and current swings
are seldom required; headphones, on the other hand, do demand a lot more power;
really, a 32-ohm load is brutally low impedance for any tube to drive. Unfortunately, a
heavy idle current is needed to ensure large voltage swings into low-impedance loads.
R4
R2
in
R8
R11
R3
R9
R1 C11 R5
R6
R7
C2
C12
B+
C1 out
R15
C6
C5
R10
R16 R12 R13
J8 (& J9)
C3
Rt C1
Rt C2
Lt C1
Lt C2
Right HP
Left HP
Right Line Output
Left Line Output
output
Headphones
Mute
Line
Switch Front
R14