RIAA Phono Preamplifier


Since modern sound
systems usually lack inputs for record players, separate MD preamplifiers
are becoming increasingly popular. They are needed not only by people
who still regularly like to listen to vinyl records, but also by those
who want to finally transcribe their LP collections to CD using a CD
recorder. The author has built innumerable phono preamplifiers for
friends and acquaintances. In many cases, for the sake of simplicity
these were based on an old circuit design with two µA741s, which was
originally described by B. Wolfenden in a 1976 issue of Wireless World.

In that simple design, the first 741 simply amplified the full range
of the frequency spectrum, while the second one was fitted with RIAA
frequency compensation — a fairly common configuration at that time.
However, a variant on this classic design was recently born after a bit
of experimenting. It also uses two opamps, with the difference that the RIAA
frequency compensation is distributed over both opamps. The
accompanying figure shows the schematic diagram of this preamplifier. The
first opamp attenuates the signal at 6 dB/octave starting at 2.2 kHz,
while the second opamp looks after the other corner frequency.

The objective of the new design was to keep the feedback factor as
high as possible in both stages. To the considerable surprise of the
developer, this modification turned out to have an unexpected side
effect: when records were played, certain scratches were no longer
audible! The difference between the new and old preamplifiers could be
clearly heard; it was certainly not just imagination. What could be the
cause of this? A quick calculation showed that a 0.05-mm scratch in a
record groove moving past a needle at a speed of 0.5 m/s produces a
square-wave pulse with a frequency of 10 kHz. Evidently, there is a lot
to be gained by attenuating such pulses with a low-pass filter as early
as possible, which means in the first stage, in order to prevent them
from over-driving the rest of the circuit.


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