A HI-FI VACUUM TUBE AMPLIFIER
by Fred Nachbaur, Dogstar Music ©1998, 2000

3: HOW IT WORKS

A block diagram of the overall amplifier is shown in Figure 1. (The "Figures" are shown here in reduced size. Click on the image to enlarge it, preferably by right-clicking and opening the image in a new window.) Full-size, printable versions are also available in the Resources section. It is highly recommended that you print out these files and have the diagrams at hand as you study this document.



The mic/line input and phono input(s) share a common high-gain, precision preamplifier utilising a differential input stage.

The Tape, CD, and AUX inputs are of sufficiently high level to be applied directly to the volume control, and thence to the line / tone control amplifier module. This module is essentially identical to the phono/mic preamp, except that its feedback network is the tone control assembly. It is therefore a true active filter, featuring three bands of equalisation (bass, midrange and treble) with approximately +/- 12 dB of adjustment.

The output of the tone control preamp is coupled, via the balance control and stereo/mono switch, to a direct-coupled driver module. This is another differential amplifier, except that in this case we also use the balanced outputs to drive the final push-pull power amplifier. Again, direct coupling is used to improve performance; and again, a feedback loop is included (mainly, in this case, to reduce the effective output resistance of the final amplifier). The loop can be bypassed (with gain compensation) so you can do direct A-B comparisons between the "classic" tube sound and the new "clean" sound as implemented in this design.

Not shown in the block diagram is the rather sophisticated power supply, which provides us with the required regulated and unregulated DC voltages.

Figures 2 through 5 in the chapters that follow are a series of schematic diagrams of the Hi-Fi Tube Stereo Amplifier. Since both channels are identical, only one is shown. Please note that part numbers refer to the various sections as follows: (We'll be assuming the left channel in this discussion.)

1-99	Common parts (power supply, etc.) Also tube designations.
100-199	Left phono/mic preamp
200-299	Right phono/mic preamp
300-399	Left tone/line preamp
400-499	Right tone/line preamp
500-599	Left driver/PA
600-699	Right driver/PA
700-899	Power meters

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