Pre-amplifier circuit description
The A85 preamplifier is a
It features a discrete power supply and
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
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 2nd 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
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
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
The digital pots Z108D and Z108A control the bass and treble respectively. This is done by moving the wiper connected to the
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
Volume control
Z107 is a VSDVC electronic volume control IC. It works, in conjunction with an external
Z117 is the output
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.