M2tech – M2TECH Joplin User Manual

Page 31

JOPLIN
384

/32bit ANALOG-TO-DIGITAL CONVERTER

REVISION P

B – SEPTEMBER 2012

M2Tech

www.m2tech.biz

The curve has three parameters:

1. the turnover frequency, that is the frequency below which the low frequencies start

to be attenuated during the recording and amplified during the playback (500Hz for
RIAA);

2. the roll-off, that is the value of the high frequency amplification at 10kHz during

recording and attenuation during playback (16dB for RIAA);

3. the shelving frequency, that is the frequency below which the low frequency

attenuation during recording and high frequency amplification during playback are
limited to a fixed value (50Hz for RIAA).

Before the introduction of the RIAA standard curve, each recording company used her own
“secret” curve (FFRR from Decca/London, HMV, Capitol, Columbia…), with big troubles
for music lovers, because no amplifier could be provided with all equalization circuits to
accommodate all the different curves. For this reason, all amplifiers were provided with
tone controls: not to compensate speakers response nor room acoustics, but to adapt the
one and only phono curve provided with the amp to the different curves of the various
LP’s. Not only LP’s are equalized: equalization was used for 78rpm, too, as well with great
variety of choices.

A good phono preamplifier, or an ADC suitable for direct phono acquiring as is the Joplin,
should allow the vinyl collector to choose the right curve for each LP pressed before 1954
(but it’s known that many recording companies adopted the RIAA curve years after the
official introduction date. It seems that some eastern Europe labels only adopted RIAA
around 1975!).

Most collectors use the RIAA curve to listen to all their records, often resulting in a sound
which is not the one really recorded on master tape. To show the reason of that, Figure 23
on next page shows some of the most famous playback curves superimposed on the
same graph.

Differences are not subtle!

Note the different low frequency amplification due to different turnover frequencies and the
different high frequency attenuation due to different roll-offs. Also, note that no shelving is
applied to HMV and Capitol curves. In fact, shelving is a relatively recent choice to avoid
phono preamps excessively amplifying the turntable rumble, thus saturating. Older curves
were thought for playback systems in which the low frequency limit was relatively high (50-
80Hz) and for recordings with poor low frequency content, setups in which the turntable
rumble was not a problem.