by Fred Nachbaur, Dogstar Music ©1998, 2000


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 context of the full integrated amplifier, there is no reason why you couldn't build standalone preamps for specific applications using this design.

The phono/mic preamps and the tone control/line preamps use a universal preamp module with 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, and the tone/line preamp section is detailed in Figure 3. Note that these are identical except for the feedback elements. The phono preamp equalisation is accomplished using a small "daughter" card directly on the main board; the mic/line equalisation is accomplished using point-to-point wiring to the gain trim controls; and the tone control equalisation is done on a separate card which include the tone control pots.

Phono/mic/line preamps
Phono and Mic/Line Preamplifier

Tone control preamp
Tone Control Line Amplifier

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.)

Preamp Gain vs. Feedback
Preamplifier Gain vs. Feedback Ratio

As detailed here, the phono/mic preamp is selectable (using a jumper or switch) between magnetic phono (RIAA equalization, high gain), ceramic phono (flat response, low gain). An external switch or switching mic input jack selects between the phono and mic/line (flat response, variable gain) modes. See Figure 7, which graphs open-loop response and the two closed-loop phono curves.

Preamp Frequency Response Curves
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 Preamp Applications
Other Possible Preamplifier Applications

Phono mode

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.

Mic/Line mode

The mic/line mode is similar to the ceramic phono mode, in that it is set for essentially flat response over the audible frequency spectrum. The exception is that a separate "Gain Trim" pot is provided for each channel. Minimum gain is approximately 1.5 (about 4 dB), handy for line-level devices such as some older CD players that may not quite have enough drive for the standard line-level inputs. Maximum gain is over 200 (46 dB) which is plenty for virtually any microphone or direct instrument input. It should be noted that you should use only as much gain as is necessary, since frequency response and distortion figures degrade at the highest gain setting.

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