Passive RIAA Preamplifier


There are two types of
preamplifiers for magnetic phono cartridges. An example of the most
common type is the one described in the March 2002 issue of SILICON CHIP. It has the RIAA
equalisation network in the feedback loop. The second type was
previously used in valve circuits which typically had no feedback loop
and used passive RC networks to provide the phono equalisation. This
experimental preamp was put together using inexpensive FETs to compare the performance of these two types of preamp. The first stage, consisting of Q1 and Q2, is a simple FET audio amplifier, where the FETs are connected in parallel to reduce noise. This is followed by a passive RIAA network consisting of 240kO and 15kO resistors and the associated 0.1OF .022OF and .0047OF capacitors.

Circuit diagram:

Passive RIAA Preamplifier Circuit

Passive RIAA Preamplifier Circuit Diagram

Some of the gain loss in the passive network is then made up by FET
Q3. It also has a 51kO drain resistor and is buffered by bipolar
transistor Q4 which is connected as an emitter-follower stage. All
resistors are 1% tolerance metal film type while the capacitors for
equalisation are MKT polyester types. Ideally, the Idss of all FETs
should be matched for both channels. Resistors R3 and R8 should be
adjusted so that the drain voltage in each stage is between 13V and 14V,
to give symmetrical signal clipping. The power supply can be three 9V
batteries connected in series. Current consumption is only 3mA for the
stereo circuit.

Author: Sam Yoshioka – Copyright: Silicon Chip Electronics


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