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STEREO POWER AMPLIFIER

P550

Ola-pavallel push-pull output stage delivevs 270 W/B

ohms -2.Cunent leedback ampliﬁer design eliminates

phase shvftsOBndged operation as monophonic power

amulmer possibleOEalanced inpulsoml signal paths

gold-platedOOversize speaker terminals

If an amplifier is to accurately provide a large

current to its load, that is to say a loudspeaker,

there are two essential prerequisites: the power

supply must be capable of delivering ample

energy, and the high-current output stage must

be designed with low impedance so that its

operation is not adversely affected by the load.

The latter requirement is especially important.

because the impedance of a loudspeaker fluc-

tuates considerably, depending on the fre-

quency of the signal. Only an output stage with

very low impedance will be able to assure

accurate music reproduction under these

demanding conditions.

To fulfill these demands, the P~550 employs

multi-emitter type power transistors specially

developed for high-power audio applications

These advanced devices have wide and flat

frequency response, and their linearity of

forward-current transfer ratio as well as their

switching characteristics are excellent. With

these transistors arranged in a 10-parallel

push-pull configuration. the P-550 achieves an

impressive power output rating of 270 watts

into 8 ohms per Channel. By using bridged

mode. the P-550 can be turned into a mono-

phonic power amplifier with an output of 840

watts into 8 ohms.

Another design highlight of the P-550 is the

current negative feedback" principle. As

opposed to conventional voltage NFB designs,

there is virtually no phase shift in the upper

frequency range, and frequency response does

not change when gain is altered, This new type

of circuit therefore provides ideal amplification

characteristics, combining operation stability

with excellent frequency response. Phase com-

pensation can be kept at a minimum, since

only moderate amounts of NFB are required

This assures outstanding transient response,

and the advantages are clearly audible as

superb sonic realism and extraordinary detail

resolution.

The basic source of energy for an amplifier is

of course the power transformer and the filter-

ing capacitors. Consequently, the sonic end

Powerful and subtle-this amplifier makes music

come alive. Current feedback design puts an end to

phase shifts. Multi-emitter transistors in 10-parallel

push-pull arrangement provide quality power: 270

watts per channel into 8 ohms, or 840 watts into 8

ohms monophonic mode.

result depends to a considerable degree on

how much of a performance margin is built into

the power supply, The P-550 makes absolutely

no compromises in this regard, featuring a

large toroidal power transformer and enormous

filtering capacity. And to maintain absolute

sonic purity, all vital parts where the audio

signal passes, such as the traces on the

printed circuit boards and input and output

terminals are gold-plated.

In the original Accuphase tradition, the P-550

features a champagne gold colored front panel

made from heavy, 15-mm thick extruded alumi-

num with a hand-brushed finish its simple and

uncluttered design gives the amplifier an ele-

gant and sophisticated visual appeal.

Powerful output stage with 10-parallel push-

pull configuration delivers 550 watts into 2

ohms, 420 watts into 4 ohms, or 270 watts

Into 8 ohms per channel

The output stage uses multi-emitter type power

transistors specially developed for audio appli-

cations and rated for a collector power dissipa-

tion of 130 watts and collector current of 15

ampere. These devices offer extraordinarily

wide and flat frequency response, and their

linearity of forward-current transfer ratio as well

as their switching characteristics are outstand-

ing. By arranging these transistors in a 10-

parallel push-pull configuration, the P-550

achieves impressive power output capabilities,

providing a full 550 watts into 2 ohms, 420

watts into 4 ohms, or 270 watts into 8 ohms per

channel.

Figure 1 shows the output circuit configuration

of the P-550. The driver stage employs power

MOS-FETs with negative temperature coeffi-

FIgJ Clrcuil Diagram of the Ampliﬁer Unit (One Channel)

cient. These devices cancel out the positive

temperature coefficient of the bipolar power

transistors, which guarantees perfectly stable

operation under all conditions.

Current feedback circuit topology prevents

phase shifts

When the gain of an amplifying circuit

increases, frequency response, i.e. the ban-

dwidth that can be handled by the amplifier,

becomes more

narrow. To

counter this

effect, a com-

monly em-

ployed tech-

nique called

negative feed- Frequencyﬂ (in)

back (NFB) mzfrmncymmwlmvolwludbuk

routes part of (Responsu changes when inlnls vanedi

the output signal back to the input. If phase

shift is disregarded, applying a high amount of

NFB results in a circuit with high gain and wide

frequency response, as shown in Figure 2,

Conventional amplifiers employ voltage NFB,

whereby a fraction of the output voltage is

used for the feedback loop. For the lP-550,

Accuphase developed a new type of feedback

circuit topology which uses the signal current

(as) 7

urea...

Trans-impedance pews,

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converter Output

g.

t Input

- Currant NFB m

network

Flgﬂ Princlplo cl currlnt feedback ampliﬁer

rather than voltage Figure 3 shows the operat-

ing principle of this circuit. At the input point of

the feedback loop, the impedance is kept low

and current detection is performed, A trans-

impedance amplifier then converts the current

into a voltage to a, ,

be supplied to

t h e o u t p u t .

S i n c e t h e

impedance at

th e c u r r e nt

feedback point

(current adder

in Fig. 3) is very an Emmy moonwwllhwrrmtlndwck

'0Y there is (Knows-does rm chewewtengsln hurled)

almost no phase shift. Phase compensation

therefore can be kept at a minimum, resulting

in excellent transient response and superb

sonic transparency.

With this circuit, there is virtually no change in

frequency response when gain is altered. Fig-

ure 4 shows frequency response for different

gain settings of the current feedback amplifier,

The graphs demonstrate that response remains

uniform over a wide range.

utagﬁ

Frequency -. mu