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Phono stage
The Phono board is a simple single stage
RIAA amplifier. It consists of two channels of
high
gain
amplification,
between moving magnet (MM) and moving
coil (MC) settings.
PSU:
The unit derives its ±15V regulated rails from
the unit it is fitted into with only local
decoupling capacitors on board.
Interface:
The unit connects to the host unit via a 8 way
connector:
Amplifier:
The left channel has designators beginning
with 100, and the right with 200. For the
purposes of this description the left channel
will be described, as the right channel the
same in all respects.
The amplifier is a small signal class A voltage
feedback amplifier with switchable gain. The
input
consists
of
differential pair of very low noise PNP
transistors (TR106, 107). These transistors
are very specific and should only be replaced
with identical parts with the E grade high
gain. TR100 & TR101 form a current source
for the pair, which sets the quiescent current
for the entire amplifier.
consists of TR110 & TR111, which forms part
of a differential current mirror with TR112,
113 & 114. This differential stage also has an
active load (TR102 & TR103) to keep gain to
a maximum.
Both of these differential stages are designed
to have as much gain as possible to enable
the single stage design. The RIAA response
is
achieved
in
C101,110,111,112,119,120,&R115,112.
C115 is used to correct between MM & MC
gains as the amplifier is non-inverting.
and
switching
an
actively
loaded
The active load
the
feedback
network:
SW100 switches between MM & MC. Two
poles of the switch change between the
different loading required for each type of
cartridge: R108 & C109 for MM, and added
inparallel for MC R104 & C108. The other
two poles change the feedback resistor value
to alter the gain. MM: R105 and in parallel
for MC R123.
The DC offset is controlled by a non-inverting
servo built around IC100.
servo current is different for each gain setting
via R111 (MM) & R124 (MC) so that the low
frequency high pass point remains the same
for both settings.
point for the circuit is set by C113. This gives
a warp filter, stops DC startup thumps from
upsetting DC coupled circuitry and an
approximation of the RIAA/IEC curve (-2dB
@ 20Hz).
The output is class A buffered by a dual
mirror follower (TR104, 105, 108, 109). The
quiescent current is set up by D100 and
R118, 119.
Closed loop stability is achieved with C116,
117 giving symmetrical slewing capability.
The amount of
However the high pass
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