The MRP-4 Preamplifier.
THE MRP4 PREAMPLIFIER.
25 OCT 98 MRP4.TXT
This, as its name implies, is the fourth preamplifier worthy
of the name that I designed, and the second to be published.
It was published in the Feb 1979 issue of Wireless World. (now
Electronics World)
This article is not going to be easy to find, so I am looking
into putting it on this site. Please be patient.
This is one of my last discrete-component preamplifier
designs. It was conceived and constructed before the wonderful
5534 opamp appeared; in those days the world was young, and if
you wanted a decent amplifier stage you designed it yourself.
AIMS.
There were several aims when I designed this preamp:
1) To demonstrate that good performance could be achieved
without the complexity of the MRP1.
2) To use a single supply rail with no regulation, just
passive RC smoothing, and still obtain totally hum-free
operation. It worked well- no hum- but there is one minor snag
to this sort of scheme. It may appear that the lowest
frequency the smoothing network would have to deal with would
be 100 Hz, but there are also short-term variations in mains
voltage that feed right through as VLF disturbances. They are
at far too low a level to cause operational problems, but it
does mean that noise measurements over the full 22 - 22 kHz
bandwidth are difficult, and a 100 Hz high-pass filter may be
required to get meaningful results.
SHORTCOMINGS.
1) Volume control interchannel balance. A normal log pot
volume control is made of two linear slopes whose matching is
not perfect. However, since the control is being used as a
potentiometer, the actual track resistance cancels out. In
this design the absolute value of the track does matter, and
there are two sources of channel imbalance instead of one.
This realisation led directly to the MRP10 active volume
control, which uses a linear control with no dual-slope
errors, and also cancels out the absolute value of the track
resistance.
Douglas Self.
to The Institute
