General Overview

The M124 is a completely discrete design from Ward Beck. The card was designed as a line amplifier and has a transformer input. The transformer can be set for 600, 150, or 75 ohm input impedance. The signal is amplified with a few stages before going balanced. The output stage is completely balanced with a set of transistors for the inverted and non-inverted outputs.

M124 as a Microphone Preamplifier

Since the M124 was designed as a line card, the selection of input impedances was limited. Several experiments were performed listening to the results before the 600 ohm tap was chosen for the input transformer.

Gain is set with a common mode feedback resistor between the balanced output amplifiers. The total gain swing is only 16dB. To allow for a decent amount of gain setting, a 16dB balanced input pad was designed and placed in-line before the transformer. This allows for 32dB of gain adjustment, which is low, but with a 48V supply line, headroom is not an issue. This will limit the usefulness of this circuit as a microphone preamplifier, but experiments have shown applicability for vocals and drum kits which covers a wide range of gain requirements.

Noise tests after assembly of the circuits show that the best performance is obtained by using only one side of the output amp in an unbalanced configuration. Despite replacing the capacitors on these modules, the tolerances of the devices and resistors lead to less noise performance when operating into a common-mode input.

Power Supply

The Power Supply required for the card operation is +48V. The supply was designed using a torroidal power transformer and over specified switching application diodes for the bridge rectifier. The DC passes through four parallel one ohm resistors for filtering and is then filtered to DC. The regulator is a standard LM317 regulator which provides a lower noise regulation than a fixed value regulator.

Documentation

A full set of documentation for the M124 line amplifier was included with purchase of these cards. Since this information is quite difficult to find, a scanned version is located here (4.5MB).