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both halves of the following stage receive an equal AC signal component at high frequency.

TR310 and TR307 are the ‘pre-driver’ transistors, which act to buffer the outputs from the preceding stage and drive the Darlington output power transistors. TR309 and R321 act as a current limit, to ensure that the emitter current of TR310 does not exceed 30mA in a fault condition. TR306 and R323 provide the same function for TR307.

R338 and R339 are to loosely couple the outputs of the pre-driver stage to the inputs of the Darlington power output devices. This is so that the inbuilt temperature sensing diodes of the output transistors can accurately control the quiescent current of the output stage as the junction temperature of the power devices varies. C312 and C318 ensure that both halves of the output stage receive an equal AC signal component.

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, fast-responding and reasonably accurate control of quiescent current (one of the major headaches of class B amplifier design!)

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.

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 non-inverting.

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 non-inverting servo built around IC100. The amount of 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. However the high pass 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.

A85 specifications

The Arcam P85 and A85 amplifiers are a testament to Arcam's commitment to high-quality audio performance. These components are well-known in the audiophile community for their unique blend of state-of-the-art technology and superior sound quality, catering to discerning listeners who value fidelity and craftsmanship.

The Arcam A85 is a high-performance integrated amplifier that features a robust design and advanced circuitry. It offers a powerful 85 watts per channel output, which is more than sufficient to drive a wide range of speakers with clarity and precision. The A85 incorporates a dual mono design with separate power supplies for the left and right channels, ensuring low crosstalk and high channel separation. This design choice also enhances the amplifier’s dynamic response, allowing it to handle complex musical passages with ease.

One of the standout features of the A85 is its use of high-quality components, including low-noise transistors and capacitors that are specifically chosen to minimize distortion. The amplifier exhibits an impressive signal-to-noise ratio, contributing to a clean audio signal that preserves the nuances of the music.

On the other hand, the Arcam P85 is a powerful, dedicated power amplifier that pairs perfectly with the A85. It utilizes a Class G amplification technology, which provides significant power efficiency. This means that the P85 can deliver peak power output without generating excessive heat, making it an ideal choice for long listening sessions.

The P85 offers 85 watts per channel into 8 ohms but can also deliver 140 watts into 4 ohms, allowing it to drive even the most demanding loudspeakers with authority. Its balanced XLR inputs, alongside standard RCA inputs, ensure compatibility with various audio sources, while the sturdy construction ensures durability and performance.

Both the A85 and P85 feature a simple yet elegant design, characterized by a sleek chassis that minimizes resonances and maximizes aesthetic appeal. They also come equipped with a variety of connection options, including multiple line-level inputs and outputs, making integration into an existing audio system straightforward.

In summary, the Arcam P85 and A85 amplifiers represent a harmonious fusion of innovative technology, impeccable sound quality, and user-friendly design. Their powerful performance, combined with high efficiency and low distortion, makes them a prized possession for any audiophile seeking to enhance their listening experience. Whether used in a stereo setup or as part of a more extensive home theater system, the A85 and P85 deliver exceptional audio performance that stands the test of time.