The MRP-10 Preamplifier.
THE MRP-10 PREAMPLIFIER.
25 OCT 98 MRP10.TXT
This, as its name implies, is the tenth preamplifier worthy of
the name that I designed, and the third to be published, in
the Oct 1983 issue of Wireless World. (now Electronics World)
This article may not be easy to find, so I am looking into
putting it on this site.
You may be wondering what happened to the MRP5, MRP6, etc up
to MRP9. All these designs existed, though not all were
properly put in a case and preserved for posterity. Maybe one
day the truth will be told about these "lost" designs...
This my first IC opamp preamplifier design. It was conceived
and constructed when the wonderful 5534 opamp was relatively
commonplace in high-quality audio circuitry, but still
sufficently expensive that you thought twice before designing
it in. (Note that the 5532 is a dual 5534; it is the dominant
version. I tend to use the numbers interchangeably)
I was prompted that the time of the 5532 had come by an
article in the Audio Amateur. ["A Low Distortion IC
Preamp/Control Unit" Feb 1979, Issue #1.]
AIMS.
There were several aims when I designed the preamp:
1) To find out how good preamp performance could be, given
5532 opamps throughout. This meant back to dual supply rails,
with IC regulators.
2) To use an active volume control based on the Baxandall
concept, to hopefully eliminate channel imbalance problems.
SHORTCOMINGS.
1) Turn-off noises. For several years the MRP10 was use with a
power amp with no turn-off muting, and it let out a sort of
faint dying squeal when the power went off. This was far too
low in level to damage speakers etc, but it was annoying.
Therefore a FET shunt muting circuit was added to the preamp
output; this is so far unpublished.
2) Over the last ten years, the volume control has got rather
crunchy, sounding as though it has DC on it. The circuit
diagram shows this should not happen, and it is possibly just
down to elderly electrolytics. I will investigate when I get
time...
3) Different sources at different levels; change from tape to
disc etc and the volume control needs adjustment. The only
answer to this appears to be separate gain controls on each
input, and they would need to preserve the interchannel
balance.
MOVING-COIL PHONO INPUT.
The MRP-10 was born into a world where the moving-magnet phono
cartridge was completely dominant. Moving-coil (MC) cartridges
had existed for many years, but were desperately expensive and
required exotic step-up transformers or head amplifiers.
Over the next five years this changed, and it became necessary
for a properly-equipped preamp to include an MC input. The MC
input preamp for the MRP-10 was published in Wireless World
in Dec 1987.
Douglas Self.
to The Institute
