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The ACOUSTAT SERVO-CHARGE AMPLIFIER
Service and Owner's Manual
Con fen rs:
Sysrem Sef-Up lnstrucﬁons
Shielding and Grounding of Audio Systems
Descriprion of the Servo-Charge Amplifier
Theory of Operation
Constant Charge Electrostufic Panels
lnpu? and Equalization Amplifier
lnpuf Area Assembly
Primed Circuit Assembly
SET-UP INSTRUCTIONS FOR
The set-up procedure, exclusive of assembly, is the same for all Acoustat
electrostatic speaker systems. We have been able to make the Monitor series
speaker systems more tolerant than the Acoustat X systems for small aberotions
in the set-up and room tuning; but we still advise that the preferred procedure
should be followed fully to obtain the best possible performance from the
Some of the principal considerations which will determine the performance
potential of the speaker system as installed are:
The Wall Behind the Speakers:
This wall should be hard, reflective, and sufficiently rigid that
it will not rattle-~sliding glass doors are a No-No".
The Orientation of the Speaker System Within the Room:
Unless there is an overriding consideration to the contrary, the
speakers should proiect sound along the longer axis of the room.
This can be considered a universal rule of stereo speaker systems
and is in no way peculiar to Acoustat products.
The Grounding of the System:
It is essential that the power supply ground be connected to prevent
any buildup of charges in the Servo-Charge amplifier power supplies.
THE THIRD WIRE GROUND MUST BE CONNECTED.
While the system may function reasonably well with only the
power supply ground, this leaves the main signal ground floating.
THERE MUST BE ONLY ONE SIGNAL GROUND IN THE SYSTEM
and this should normally be provided at the inr v of the preamplifier.
Electrostatic loudspeakers produce a strong electric field which can
influence the operation of any low level electronic circuitry in
close proximity and can result in feedback at frequencies well above
the audio spectrum--this has the effect of high power dissapation and
decreased headroom in the systen The tone arm of the turntable
and the input of the preamplifier should be kept at least six to eigh
feet from the nearest speaker to ensure that no feedback can occur.
Acoustat speaker systems are designed to preserve correct absolute
phase throughout the system including the final coupling of the
signa' to the air. This ensures that vocalists and instruments will
sound as natural as possible. Therefore, make sure that the
connections from the speakers to the amplifiers are correctly oriented:
the wires should enter the three prong Pomono plug from above.
THEORY OF OPERATION
in order to understand the operation of the Servo-Charge amplifier, it is very
usefull to have at least a rudimentary understanding of the load which it was
designed to drive,
Acoustat uses what are referred to as constant charge electrostatic transducer
panels in which the electrostatic "plates" are driven in a push-pull format and
the driven diaphram is biased in such a manner that its electrostatic charge
remains constant despite the application of high voltage audio signals to the
electrostatic plates .
l C =q/V
I 1 J =CV2/2
e E is _
s l l1 l w ' F =_Eq
l I Where: C capacitance in fared:
* . + 41 ' V voltage in volt:
l + l ' J energy in ioules
. * V F Force in newton;
4 l ' l _ E electric field in
I | newton: per coulomb '
I * " q charge in coulomb:
measure at e lectvic
newtons per coulomb
and volts per meter
[ are alternate units at
While we admit that the physics is not "obvious at a glance" we will, for the
purposes of this amplifier service manual, simply state the electrical requirements
to drive the electrostatic loudspeaker panels that we use at Acoustat Corporation
and not dwell on the physics of electrostatic loudspeaker design.
Unit shuts down
to standby, but
comes back on
Unit will not
shut down to
Unit will not
turn on from
or Arcing tube
Detective LC. ,
Q5 Transistor or
A detective integrated
circuit can cause this problem.
The OP-ll device was found
to be particularly prone to this
rare problem. An arcing tube
can also affect proper operation
of the turn on circuitry.
The attending tube will
usually arc at the shutdown
to standby point and bring
back the amplifier to the
full 0n condition. This tube
can sometimes be spotted in a
darkened room. The simplest
standby. bad Relay solution is to replace all
White noise in Leaky 2N3440 Be sure the noise isnt
background below (Ql,Q3) originating Upline in the
level of music
preamp or elsewhere by removing
the input to the servo charge
amplifier. It noise persists
replace QI or Q3.
Amp distorts ,
by a low level hum.
Ql or Q3 transistor
in volume, about
by low level hum.
Replace Q2 and Q4
Unit turns on from standby
due to AC line surges-
sometimes induced by an
Earlier production amplifiers
are sometimes found to do this.
Eliminate the problem by
installing a 680 ohm resistor
in series with the cathode
(banded end) of CRI4. This
resistor is factory installed
_-eginning with s/n I700.