Ecleree PAM2600, Service Manual

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MODULE CIRCUIT 11.0504B OPERATION - DESCRIPTION
The control element is the operational NE5534. This is a very low noise operational, especially designed
for very high quality applications in professional audio equipment, control equipment and telephony channel
amplifiers.
The operational is internally compensated for a gain equal to or higher than three. Frequency response
can be optimized with an external compensation capacity, for several applications (unity gain amplifier, capacitive
load, slew-rate, low overshoot, etc...).
Characteristics:
Small-signal bandwidth: 10Mhz
Output drive capability: 600 Ω 10V(rms) at Vs=
"
18V
Input noise voltage: 4nV/
%
Hz
DC voltage gain: 100000
AC voltage gain: 6000 at 10KHz
Power bandwidth: 200KHz
Slew-rate: 13V/µs
Supply voltage range: " 3 to " 20V
POWER SUPPLY
The BF871 and BF872 transistors are mounted in a common
base configuration, in a current source structure. The current sources
have a double function: polarizing the gate-source links in the MOSFETs
to the limit of the conduction and moving the voltage variations at the
operational output which are refered to ground to voltage variations
refered to high voltage power supply. The polarization point is calculated
so the voltage dropout in Rc(R112+R111) is the limit voltage of
conduction of the MOSFETs (
.
2 to 3V), enough to carry the bias
current. If we modulate in AC the base-emitter voltage, the Ic and VRc
will vary proportionally. In our configuration, as the reference voltage Vref
is constant (it is a part of the operational power supply), we add the
operational output voltage to the transistors emitter through Re
(R107-R108).
The Rc value fixes the source output impedance. We do not
recommend to raise it higher than 1K Ω because of frequency response
and slew rate reasons. This voltage circuit's gain is, as usual in a
common base configuration with Rc/Re emitter resistor, 0.45.