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Pre-amplifier circuit description
The A85 preamplifier is a high-performance, DC coupled design
with microprocessor control of input select, two independent tape
loops, electronic volume control, tone bypass and electronic tone
control.
It features a discrete power supply and low-noise linear circuitry to
obtain very good distortion and noise performance, suitable for
high quality source material such as CD or DVD-A.
Input switching
Each of the inputs has a pair of diodes to the ±15V rails to prevent
static spikes from causing damage to the CMOS multiplexers. In
addition, there is a simple resistor-capacitor filter with a corner
frequency of approximately 340kHz to remove any unwanted high
frequency interference from the signal. This uses high-quality
polypropylene capacitors for best performance.
Z104 and Z105 are the main input select multiplexers, which are
configured in a ‘virtual earth’ unity gain arrangement with Z115
and Z116. This arrangement is slightly lower distortion than the
‘normal’ one, at the cost of a slightly higher noise floor. It is an
inverting configuration, which is restored to correct polarity by the
inverting electronic volume control which follows.
Z115B and Z116B are integrating servos, which take out any DC
from the input signal before the following stages. The servos are 2-
pole, with a passive 2
nd
pole being formed by R180 and C147 (for
the left channel) to remove broadband noise from the output of the
servo and improve speed of response.
Z100 thru Z103 are the input selectors for the 2 tape loops. These
are normal non-inverting selectors which are buffered before being
passed on to the phono sockets.
Z109A output is decoupled by R108 which is included in the
feedback path. Local high frequency feedback occurs around C108
to allow the tape loop output to be very low impedance, whilst
being stable into a capacitive load such as may be presented by a
screened interconnect cable. This is the same for all tape outputs.
Z106 is configured as a double pole changeover switch, used to
select the tone controls. The tone controls are bypassed when not
required so that the noise and distortion can be minimised.
Tone control circuit
The tone control circuit is a non-inverting one, using a gyrated
‘bell’ filter for the bass and a simple shelving filter for the treble.
Left channel description
The input is attenuated by 6dB and biased to a voltage of +2.5V
DC by C111, R113, R112, R110, R111 and C110. This is so the
signals fall within the 0 - 5VDC required by the digital
potentiometer Z108.
Z111B and its associated components form an active equivalent of
a series resonant LCR circuit. This has an impedance minimum of
5.4k at around 80Hz with Q=0.7 The reason the bass is done as a
band-boost filter rather than a shelving filter is so that you can
boost the ‘real’ bass without causing lots of sub-audio loudspeaker
cone excursion which wastes power and may damage the drive
units.
The digital pots Z108D and Z108A control the bass and treble
respectively. This is done by moving the wiper connected to the
frequency-sensitive impedance between the non-inverting and
inverting terminals of Z112A, effectively changing the ratio of
feedback boost and feed-forward attenuation of the circuit at the
desired frequencies, thus providing a EQ gain control that is
symmetrical on a logarithmic scale, with the use of a linear pot.
Z112B provides the 6dB of gain necessary to bring the nominal
signal level back to unity. C116 and C117 remove the 2.5VDC
offset from the output, to prevent clunks when the tone controls
are activated.
Z108 is controlled by a simple 3-wire serial interface from the
microprocessor. Each of the digital lines has its own ground return
to minimise electromagnetic interference. They are connected
together only at the GND pin of the IC.
Volume control
Z107 is a VSDVC electronic volume control IC. It works, in
conjunction with an external op-amp, by varying the feed-forward
and feedback resistors in an inverting gain configuration. In this
way, it can allow output signal swings of up to 22Vpp whilst
operating from a single +5VDC power supply. Also, it allows the
user the choice of external circuitry to fine-tune the performance.
The gain is controlled from the microprocessor via a 3-wire serial
interface. The analogue supply rail is derived from the local +5V
via R185 and C156 // C157.
Z117 is the output op-amp. Its outputs are decoupled via R186,
R187, C158 and C159 so that it has a low output impedance but
can drive cable capacitance without oscillation. R186 and R187
are included in the audio frequency feedback loop to reduce output
impedance when driving ‘difficult’ cables.
RLY100 is a mute relay which shunts the preamp output to
ground. This is to prevent thumps and squeals when the units is
powered up or down.
Power supply
The transformer winding is connected to SK300. The voltage is
rectified and smoothed by D300, D301, D306, D307 and C300,
C310. The unregulated voltage should be around ±27VDC. F300
and F301 are secondary fuses, as the low power preamp winding
would not blow the primary fuses if short circuited.
The voltage regulators are discrete compound emitter followers. I
will describe the +15V supply as the negative is essentially an
exact mirror image.
Q300 and R300 act as a constant current source, supplying around
7mA into D310. C302 and C314 reduce ripple and broadband
noise on the zener diode. Q305 and Q306 form a complementary
Darlington NPN transistor which is configured as an emitter
follower, producing the +15VDC at its output. C303 is to provide
bulk charge storage and to reduce the AC output impedance of the
power supply. D302 prevents reverse bias of the supply during
power down.
Z301 is a conventional LM317 type circuit to drop the +15V rail
down to +5V for the tone and volume control circuits.
Star point SP300 explicitly connects the differently named ground
nets together at one point, to minimise hum.