A HI-FI VACUUM TUBE AMPLIFIER
by Fred Nachbaur, Dogstar Music ©1998, 2001
3-A) HIGH-PRECISION PHONO PREAMPLIFIER: OVERVIEW
Considerable effort was taken to design a preamp that is as clean as possible to satisfy discerning
audiophiles, while maintaining a relatively low parts count to satisfy limited budgets. It should be
noted at this point that, while the discussion is in the spcific context of an RIAA phono pre-amplifier,
this design lends itself well to a wide range of other uses.
This "universal preamp module" sports a high gain (about 60 dB) and differential input stages
that allow for differential input (as for balanced mic preamps) as well as either inverting or
non-inverting single-ended applications. Frequency response shaping and gain setting are accomplished
using the appropriate feedback networks.
The schematic diagram of the phono/mic preamp is shown in Figure 2. If built on a PCB, the
phono preamp equalisation is accomplished using a small "daughter" card directly on the main
board; if you use the point-to-point wiring approach, these networks can be wired on separate
terminal strips for easy modification if necessary.
Phono (Magnetic and Ceramic) Preamplifier
The main circuit board was designed to be virtually universal in applicability. Small outrigger
"feedback cards" are used to customize the response to almost anything you might need. Figure 6
plots gain vs. feedback ratio for the non-inverting configuration as used in the phono/mic preamp.
There is an obvious linear relationship, except at high gain. (The "porch" at the low end is due to
the "+1" when using the non-inverting mode; see below for further details. In the inverting mode,
as used in the tone preamp section, the relationship remains linear at low gain settings.)
Preamplifier Gain vs. Feedback Ratio
As detailed here, the phono preamp is selectable (using a switch) between magnetic
(RIAA equalization, high gain), and ceramic (flat response, low gain). See Figure 7,
which graphs open-loop response and the two closed-loop phono curves. If you only need one or the other
mode options, the switch and the appropriate associated components can be omitted.
Open-loop, Magnetic Phono and Ceramic Phono Response Curves
There is no reason why you couldn't adapt this universal preamplifier module for any other
gain/equalization combinations, simply by modifying the feedback networks. (Figure 8 gives a few
ideas, covered in more detail in
Other Preamp Applications.)
Other Possible Preamplifier Applications
Phono equalisation
In magnetic phono mode, the RIAA curve is achieved by using negative feedback to decrease the
gain at higher frequencies, according to the RIAA specification. The result is that the amplifier
runs almost open-loop (about 60 dB gain) at 40 Hz, and rolls off smoothly to a gain of less than
20 dB at 20 kHz. This means that the "tube sound" caused by even-order distortion will be most
pronounced in the bass region, giving it that warmth that is so highly prized by vacuum tube
aficionados. At higher frequencies, this distortion is increasingly cancelled out by the negative
feedback that sets the gain, keeping the mids from sounding "brassy," and the highs from
sounding "splashy" (two common complaints about open-loop tube amplifiers).
In the optional ceramic cartridge mode, response is flat within 0.5 dB over the audio range, and
voltage gain is set at 5 (about 14 dB). At this relatively low gain, the circuit is very clean and
distortion-free. However, discerning ears may hear a subtle quality of warmth not present in
solid-state gear.
The tube types and their operating points were carefully chosen to minimize power supply
requirements, and to maximize tube life. Both channels together (four preamps) draw only about
15 mA from a 430 volt supply.
