8
instrumentation amplifier topology
able from our balanced circuitry by including a balanced XLR input connector version of the phono preamplifier modules.
No32 XLR phono module recommended wiring
NC
1 2
LEFT +
3
LEFT -
TONE ARM GROUND
NC
1 2
RIGHT +
3
RIGHT -
Combined with a shielded, twisted pair cable from cartridge to preamp, the XLR input phono preamp offers the ultimate in noise rejection and low level signal resolultion. (An
Madrigal uses a fully balanced implementation in its phono preampli- fier. The gain stage produces a perfectly symmetrical pair of opposite- polarity signals, based solely on the difference voltage appearing at the amplifier input. Each half of the balanced signal sees identical signal path and circuitry from input to output. The RIAA equalization stage and even the DC servo is duplicated for each signal half. (A single resis- tor change on the input amplifier would let each channel operate as a stereo pair of phono preamplifiers.)
The balanced output signal from the phono preamp is received on the Nº32 audio input selector board with the same treatment as a line level balanced input.
The classic method of resolving tiny electrical signals from potentially noisy environments is called an “instrumentation amplifier.” The fully balanced, input to output instrumentation
The industry’s lowest noise amplifiers implement the input side of the phono circuit. Gain is distributed over two stages with ample band-
