jnorman wrote:

1. what is the simplest preamp design? ie, what exactly is required
in the circuit and why?

I think the simplest possible preamp would be an input transformer
followed by a single unity-gain active stage.

The transformer would have to have a high turns ratio in order to
provide all the gain necessary.

The active device would have to have a nearly infinite input impedance
and an output impedance near zero.


2. what are the basic problems with the simplest possible design?
(noise, fidelity, whatever)

The transformer will have a limit to its operating level and bandwidth,
will cause distortion, and will be bulky and expensive. The high ratio
required to obtain the necessary gain will compound every one of these
limitations.

The active device will require peripheral components including a power
source, and will have limits to its linearity and bandwidth, in
addition to adding noise.

There is no control over the gain, and the output signal is unbalanced.
There are no utility features such as input pad to prevent overload,
polarity switching, phantom power, etc.

The preamp will be susceptible to interference from environmental
noise, be it sound pressure, electrical, or magnetic.

3. what are the approaches to turning the basic design into a
reasonably clean and useable mic preamp? (better parts, additions to
the basic circuit, etc.)

Adding more active stages to allow for a lower-ratio transformer.
Adding a gain control or an input attenuator. Adding still more gain
stages to compensate for losses in the attenuator or gain control.
Adding a differential amplifier or transformer for balanced output.
Adding contol over polarity, an input pad, and phantom power. Housing
the whole thing in an enclosure to protect it from damage and noise.
Adding a control surface to allow the user to easily and effectively
manipulate all the features you just added. Some connectors to allow
quick and easy interfacing with different microphones and destinations.

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3 when they are
both designed to be clean and transparent, or why tubes vs
transformers for more "colored" preamp dsigns?

Dunno.

ulysses