WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

Page: 12 / 196

12
WhitakerAudio
Transformer T1/T2 couples the audio amplifier output to the speaker. Taps are provided on
the primary windings for connection to the screen grid of each output tube. This mode of
operation reduces distortion of the amplifier, albeit with a small power output penalty. The taps
on the secondary of the transformer match the plate-to-plate impedance of the output stage to the
speaker voice-coil impedance. Although the transformer specified provides output impedances of
4
Ω
, 8
Ω
, and 16
Ω
, only the 8
Ω
tap is provided at the back panel for connection to the
loudspeaker. Negative feedback of approximately 20 dB is coupled from the secondary of the
output transformer to the cathode of the input stage to reduce distortion and to improve circuit
stability. Use of taps other than 8
Ω
can compromise the distortion performance of the amplifier
unless appropriate adjustments are made to the feedback circuit. For users requiring an output
impedance of something other than 8
Ω
, a custom implementation can be provided.
For lowest distortion in a push-pull amplifier stage, it is important that the operating
parameters of the two output tubes are well matched. The first step in achieving high
performance, therefore, is the use of matched output tubes. These are readily available from a
number of vendors. The second step is to make certain that the components supporting the output
tubes are matched as well. To this end, resistors R6, R7, R9, R10, R11, and R12 for the right
channel and resistors R23, R24, R26, R27, R28, and R29 for the left channel are all 1% precision
devices. In addition, coupling capacitors C4 and C5 for the right channel and capacitors C10 and
C11 for the left channel are all 5% tolerance devices.
Two front-panel jacks are provided for connection of one or two headphones to the amplifier
for private listening. One jack is a conventional 1/4-inch stereo phone; the other is a miniature
(3.5 mm) stereo jack. For headphone listening, the speaker outputs would normally be turned off.
While this function could be accomplished with a simple switch, it would require bringing the
speaker output leads from the back of the chassis to the front panel and then back again to the rear
panel. To avoid the potential for feedback or noise that such an arrangement could set up, the
Speaker On/Off switch is fed from a +5 V dc source and used to control relays that are located
adjacent to the speaker terminals. For the condition where the speakers are switched off, an 8
Ω
,
10 W load is placed across the output transformer secondary.
It should be noted that the front panel headphone jacks are wired across the output transformer
secondary windings. The headphones used must be wired for stereo operation. A mono plug will
short-circuit one of the output channels. Series resistors in the headphone circuit are provided to