The Fi Primer: Preamps

Full Function Pre-Amp

3mv Phono/250mv Aux

This circuit represents a great sounding preamp which nonetheless does not require much messing around in order to get it going. The general features include a two stage phono preamplifier and a one stage line amp with a volume control input. Provisions for switching and recording can be easily adapted to anyone's needs. The phono stage provides enough gain for most moving magnet cartridges and incorporates passive RIAA equalization.

A brief circuit description follows: a single gain stage of 6DJ8 drives a passive (RC) network which accomplishes the necessary equalization for records, the output is fed to a second 6DJB gain stage for a total gain of 32 db. 1% tolerance resisitors and capacitors should be chosen for the filter components. The passive balance and volume controls involve a loss of -6db. The line amp, a paralleled 6DJ8, provides gain of roughly 24 db at an output impedance of 1.3K ohms.

In addition, the power supply is tube regulated and the filament voltage is also regulated. These regulators play a large part in the high performance of the whole unit. The only thing to keep in mind is that good shielding and mechanical isolation exists between the power supply and preamp circuitry. This can be done a variety of ways, including a separate enclosure for the power supply. However, the entire circuit can be point to point wired in one chassis if care is taken while laying it out. This means short neat wiring and some sort of shielding around the tubes and small signal area.

Do not underestimate the importance of good mechanical construction and some form of vibration control. Bolting everything to a Bud Box will not make for a quiet preamp. Look instead at well known chassis design for hints and ideas. The Marantz 7C, MacIntosh C-22, and early Audio Research preamps are good places to cop construction practices from. These preamps are not layed out exceptionally well but are well-wired and mechanically sound designs. The places where your own improvements can make a difference are in keeping the signal path as short as it can be done. The long runs of cable between pots and switches that exist in the aforementioned products left them noisy and vulnerable to airborne interference (I'm not much of a Marantz/Mac fan, but you may be!). For example, the Marantz style of horizontal tube mounting with the input/output jacks, volume pot, and switching arranged as close to the tube sockets as possible, might be one possible arrangement to consider. Or, consider a shock mounted (foam washers) sheet of copper, onto which the entire preamp circuit is hardwired to, is then installed inside an appropriate chassis. Short jumpers connect the preamp board with the selector, volume pots, and jacks. The power supply is built into a separate chassis and joined to the preamp with an "umbilical cord". These are just two examples of what is possible.

"7C" Style Full Function Preamp

2mv Phono/250mv Aux

Almost universally loved, the Marantz 7C set new standards for preamp performance and musicality. In many ways it represented the first really "high-end" mass market preamp. Today, most of the well known American and Japanese makers of tube audio equipment have imitated it and improved upon its design at one point or another (I won't name names but those Californian dudes know who they are...). This design can still provide an eye-opening audio experience if a couple of important details are dealt with first.

First, the tone controls should be eliminated and the switching simplified. And second, the circuit should be wired as short as possible. The feedback networks need a little attention and a general streamlining of the circuit wouldn't hurt. What you end up with is circuit number 2. The general construction comments for number 1 are equally pertinent here. While technically no longer a "7C", you can see the direct descent without much difficulty. The point of including this circuit is to provide an excellently performing example of the feedback type of preamp, which is the dominant topology used in american tube design. A topology, I might add, that is still much appreciated all around the world.