ABOUT PHONO PREAMPLIFIERS
When playing phonograph records, you might normally use the phono input provided on your amplifier,
often called a RIAA (Recording Industries Association in America) phono input.
The RIAA equalization curve specifies two turnover points (a ‘turnover’ point is where the level changes
±3dB): one at 500Hz, and the other at 2125Hz. The RIAA playback curve has the following theoretical
response:
1. +17dB from 20Hz to 50Hz, then falling about 6dB/octave to
2. 0dB from 500Hz to 2125Hz, and then falling again about 6dB/octave to
3. -13.7dB at 10kHz.
You use the RIAA playback curve to compensate for the RIAA equalization that was originally used to make
the record.
Normally this works well on single, EP and LP records, issued from 1955 and later. But with earlier LPs
(some record companies didn’t use RIAA before 1960), this curve is not correct.
The cutting equalizations on all 78 RPMs and LPs before 1955 have been very different. 78rpm records
in particular used curves very different from the RIAA curve. For this reason, you should not use the RIAA
phono input on your amplifier if you want good quality reproduction of 78rpm records.
I want to thank Roger Wilmut for basic information about reproduction of 78rpm records. For a better un-
derstanding of the problems of reproduction of records, read the following articles (with a few edits from my
hand) taken with kind permission from Roger Wilmuts home page: ”Reproduction of 78rpm records” - www.
rfwilmut.clara.net/repro78/repro.html.
EQUALIZATION OF PHONOGRAPH RECORDS
The simplest form of disk cutter consists of an amplifier, similar to that used to drive a loudspeaker, con-
nected to a cutting head having a stylus connected to a coil, which is placed in the field from a strong
magnet (or, more usually in later designs, a magnet within a coil). When the signal is applied to the coil, the
stylus moves and engraves a groove in the blank disk. (There is of course a lot more to it than that, but we
are considering only the basics here.)
However, because the cutter head’s movements translate the amplitude swings of the original signal into
velocity - the rate at which the stylus moves during its swings - low-frequency signals would be recorded
with a much larger swing than high-frequency signals of the same original amplitude. In order to keep the
movements of the groove much the same at all frequencies (given equal level signals) it is necessary to
use a circuit to introduce - in the theoretical situation - a 6 dB/octave cut as the frequency decreases - i.e.,
halve the frequency and you halve the voltage.
In the reverse situation, that of a reproduction head, the principal is that of a wire moved in a magnetic field
- it is the rate of cutting ‘lines of force’ that matters. The cutter head works exactly in reverse, like a simple
motor, where increased voltage means increased speed. Therefore, the constant amplitude groove theo-
retically achieved produces a signal where the bass is low and the treble high: so a 6 dB/octave cut with
increasing frequency would be called for.
In the real world, losses in the head with increased frequency complicate the issue. Early cutter heads
were highly inefficient, and so, while the bass cut described above was used, the treble trailed away, result-
ing in equal groove modulations (movements) up to mid frequencies, but decreasing above that.
To compensate for this, the playback characteristic boosted the bass below 200 Hz but left it flat above that
- effectively providing a 6 dB/octave boost to the higher frequencies (and the surface noise). With the later
improvements in cutters, it was possible to pack more treble onto the records, and so new equalizations
provided for a 6 dB/octave cut above a turnover frequency which varied between 3.4 and about 6 kHz,
depending on the system.
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