To get the voltage gain needed for a normal installation, the DoZ preamp can be used. Everyone who has built this circuit has commented on the exceptional sound quality, and it is ideally suited to this application.
Figure shows the modified version of the preamp, the output of which would be connected directly to R1 in the circuit. The quiescent output voltage is now set by VR1 in the preamp, and the voltage at the source of T1 should be set to 19.8V as shown in Figure 1 by means of VR1 - the voltage at the gate (preamp output) should be 4V higher, i.e. 23.8V. The DoZ preamp board is stereo, and can drive a pair of the power followers with ease. Q2 and Q3 should be fitted with small "flag" heat sinks to allow them to dissipate the increased power caused by the higher operating voltage.
As shown, the gain is 3.2, so it will require nearly 4V RMS input for full power. To change the gain, I suggest that R4 be changed to 3k3 to obtain a gain of 7.7 (17.7dB), which will give an input sensitivity of about 1.5V for maximum output. C3 will also need to be changed, and a value of 100uF will be more than adequate. I do not recommend that R4 be reduced to less than 2k7, which will give a gain of 9.15 (19.2dB). To maintain good low frequency response, C3 will need to be about 100uF, although even with 25uF, the low frequency response is maintained to 2Hz. Ideally, the input network should define the low frequency limit, so the higher value is recommended if R4 is reduced.
Unless a preamp is used in front of the amp, a pot will be needed at the input for gain control. 10k is fine here, and will not cause excessive loading on the source.
