Accuphase M 100 Brochure

Page 2

Anyone engaged in audio engineering will agree that an ideal power amplilier
would have to possess the following prerequisues.
l1) Full capability to handle large static and dynamic input signals without distortion.
(2) Wide bandwidth characteristics. (3) Low output impedance under open loop con
ditions (without NFB application), (4) Surplus power supply capability for highest
peak current requirements. (5) Full complement output stage. (6) Durable. high
stability construction that ensures complete absence of electrical and mechanical
resonances.

Prerequisites l3). (4) and (5) are particularly important to achieve ideal speaker
driving conditions. For example, when the speaker is driven by the energy from the
amplifier its cone vibrations create a counterelectromotive force that flows back-
ward toward the arnpiitier. This force must be eliminated instantly by the amps
output circuit.

The ideal as measured from the speaker side would be practically a short circuit
(very low impedance) condition oi the output Circuit. Thus power amplifier must not
only completely overcome this counterelectromotive lorce, but it must simultane-
ously deliver new energy to the speakers.

This calls for the ideal power amptilier to possess a low output impedance under
open loop condition. a very heavy-duty output stage and also a power supply with
surplus current supplying capability. ,

The Accuphase Monophonic Power Amplifier M100 is a product of a design
approach which pursued an ideal heavyduty. high quality sound producing power
amplifier. its array of eight power transistors in a parallel-push-pull (octo»parallel
push-pull) output stage and its large capacity Series Regulator type power supply
both attest to the seriousness with which we tackled the challenge. The very low
impedance characteristics under open loop conditions that was achieved together
with the capacity to deliver continuously stable 500 watts of electrical energy at 8
ohms marks another successful fonivard step in audio technology.

The large power output of the M100 does not merely mean big sound" capa-
bility. Its also means that a higher grade. quality sound is now available which is the
direct result of Accuphase's positive design approach in introducing this outstanding
monophonic power amplifier.

The Accuphase M-too also features. for the first time in the world. 3 Peak Power
Direct Readout System With Digital Display. lt permits direct reading of power
output peak values which are numerically displayed in walls. It also permits peak
value Hold Time" selection up to 30 minutes tor studies of peak power recorded
during a choice of three Hold" periods.

The surplus power and high quality sound of the M400 will enable everyone to
enjoy the unlimited beauty and depth of music to their hearts' content.

HEAVY-DUTY OCTO-PARALLEL PUSH-PULL
OUTPUT STAGE

The rated output at the M100 is 800 watts at 4-ohms and 500 watts at 8-ohms.
Accuphase also guarantees its operation at 2-ohms when multiple speakers are
connected in parallel to form a 20th load. in this case the rated output is 1.000
watts.

In order to guarantee such a high power output. a total of sixteen bipolar power
transistors. each of which has a superior wideband characteristic and a maximum
power dissipation (Pc) of 200W rating, form an cote-parallel push-pull output stage.
The 16 transistors have a total electrical power capacity oi 3.2KW which shows the
very large extra margin that supports our output ratings.

Figure 1 shows the amplifying circuit of the M100. The array of transistors from
021 to 036 are the output transistors. Eight each are attached to two separate giant
heat sinks located at each end of the M100 to dissipate heat eiiectively. 021 to 028
are attached to No. 1 Heat Sink. while 029-036 are located on the No. 2 Heat Sink.

Since the two heat sinks are not thermally coupled, a "Bias Balance Circuit
was newly developed to balance the bias current flow to the output transistors and
control their respective heat generation. This effectively prevents the possibility of
any thermal imbalance between the heat sinks and serves to give lull protection to
the output transistors.

The Bias-Balance Circuit is centered around the four transistors 017020 and
connects to the common base of the output transistors. Two transistors each are
attached to each cther's heat sinks to detect their mutual thermal conditions. They
function to keep both heat sinks at approximately the same temperature and ensure
stable operation by controlling bias current flow to the power transistors.

MOS FET DRIVER STAGE IMPROVES HIGH
FREQUENCY PERFORMANCE AND ELIMINATES
SWITCHING DISTORTION

If a high power amplifier is used in the home at normal listening levels it would
be required to supply a mere several watts or so. This means that sound quality at
low reproduction levels also becomes a very important factor in designing an ideal
power amplifier.

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Fig. 2 Diagram of Series Regulator type Power Supply Circuitry

We adopted a sure way to achieve this in the M100 with the use oi power ampli-
iication devices MOS FETs (Metal Oxide Semiconductor Field Eiiect Tran s)
in the driver stage. which is the Accuphases original MOS FET applicati is
stage requires low output impedance and large current capacity to drive the final
output stage. which means a periect role for MOS FETs which not only can fully
meet such requirements. but also improves high frequency performance.

A further advantage of MOS FETs is that their thermal characteristics helps
to stabilize bipolar transistor operation which also stabilizes the bias current of the
output stage and permits the use of low value emitter stabilizing resistors. More-
over. the total equivalent resistance value of the output stage is further reduced to
1/8 because of the octo~parallel push-pull circuit. This low value of emitter resist-
ance has practically eliminated switching distortion that is normally created when
the bias current ol the output transistor is out off.

011-012 constitute a Cascode push-pull pre-driver stage which features the
coexistence of excellent high frequency performance characteristics. together with
large amplitude handling capability. The high frequencies are free of Miller eilects
and the pre-driver stage feeds high quality signals to the MOS FET driver stage.

The M400 is further designed so that the 16 output transistors are supplied with
comparatively large bias current so that they can be worked in pure Class-A
operation up to 10 watts output. This further ensures highest quality sound at low
reproduction levels.

CASCODE BOOTSTRAP, PUSH-PULL
DIFFERENTIAL AMPLIFIER INPUT SECTION

The input section employs the traditional Accuphase complementary-symmetry
push~pull amplifier in every stage in which all desirable characteristics have been
improved to the utmost limits under open loop conditions before NFB application.
The input signal is led to dual type FETs 01a and 02a. 03 and 04 form a cascade.
push~pull amplilier configuration with bootstrap circuit.advantages which include
high gain. improved high frequency performance and no distortion when input
impedance is increased. as when the input level control is varied.