both halves of the following stage receive an equal AC signal component at high frequency.
TR310 and TR307 are the
R338 and R339 are to loosely couple the outputs of the
The output transistors are TR318 and TR319. These are Sanken SAP15N and SAP15P devices respectively. They are specially designed for audio power amplifier use. In addition to high current gain (Darlington with a typical hFE of 20,000) they provide an inbuilt emitter resistor (thick film power resistor of 0W22) and temperature sensing diodes which closely and rapidly track the VBE versus temperature characteristic of the power transistors, allowing for easy,
RV300 is for fine trimming of the quiescent current. PL300 provides a convenient measuring point for this, which is short- circuit protected in the event of a slip with the multimeter probe!
All of the remaining circuitry to the right of TR318 and TR319 is essentially for output stage protection...
Transistors TR312 and TR304, along with the network of resistors and capacitors to which they are connected, provide instantaneous overload protection of the output stage. This is a conventional single slope VI protection scheme, which allows much greater current to be delivered into a rated load than into a short circuit. The values allow for 18A peak delivery (at clip) into a purely resistive load, 7A peak (at clip) into a purely capacitive load and around 4A peak into a short circuit. R345, C303, R346 and C304 allow these values to be doubled for short transient bursts (approximately 2.7 milliseconds) so that impulsive musical transients can be delivered cleanly with minimal risk of damaging the output transistors.
TR313, TR302 and their associated components send a signal to the microprocessor when the instantaneous protection circuits are having to work ‘hard’ to prevent amplifier overload. This instructs the micro that the user is severely abusing the amplifier and will switch off the loudspeaker relays to prevent possible permanent damage. In reality, if you short circuit the outputs at any appreciable volume level, this circuit will trigger and the microprocessor will turn off the loudspeaker relays and send a signal to the user.
R308, R314 and C320 form a low pass filter from which the DC detection circuits can sense excessive DC at the loudspeaker outputs. If there is any positive DC present, then TR316 will turn on, which turns on TR305 and thus activates the DC protection line to the micro, turning off the loudspeaker relays.
If there is any negative DC present, then TR308 will turn on, which turns on TR317 which then turns on TR305 in turn, causing the same effect.
R350 and C319 are the Zobel network which is provided to ensure the amplifier ‘sees’ a constant and resistive load at very high frequencies, to aid stability, although the amplifier will be stable without the Zobel fitted.
C313 locally couples the ‘high frequency’ and loudspeaker ground returns together at the output to overcome the effects of track inductance back to the star point. C309 couples the ‘high frequency’ and signal grounds together at the input for the same reason.
D303 and D304 are ‘flyback’ diodes to protect the output transistors from reverse bias when the amplifier is heavily clipped into an inductive load (such as a loudspeaker voice coil!)
Sheet 4 is an identical copy of sheet 3 so I will not describe it separately.
L870 Phono Circuit Description
The Phono board is a simple single stage RIAA amplifier. It consists of two channels of high gain amplification, and switching 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 an actively loaded 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. The active load 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 the feedback network: C101,110,111,112,119,120,&R115,112. C115 is used to correct between MM & MC gains as the amplifier is
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 in parallel 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
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.
