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Introduction

The DC-50 is a high quality,
DC-coupled Class-A/AB power amplifier, nominally
rated at over 50W into 8 Ohm per channel. With 3
pairs of output devices and proper power supply
and heat sinking it can deliver over 100W into a
4 Ohm load.

The EB-693/129 driver circuit
uses the same topology as the Class-A lineamp,
described in the 3/90 issue of The Audio
Amateur. Cascode JFETs have been added to the
monolithic input FETs to improve linearity and
to reduce the input capacitance. It is also
using special, high frequency second stage
transistors with very low collector-base
capacitance. This results in very good linearity
across the audio range.

The EB-693/114
output stage consists of two or three pairs of
TO-3P power MOSFETS, driven by TO-220 MOSFET
drivers. It is recommended to use three pairs of
output devices when driving 4 Ohm loads. The
output devices are connected directly to the PC
board, which simplifies
assembly.

Circuit Description

Transistors Q1 through Q10 form
the double cascode input stage. Q1 and Q2 are
monolithic JFETs of the type 2SK389 and 2SJ109.
These are low-noise, medium gm devices. They have
relatively high internal capacitances, which
result in high input capacitance. This, in turn,
might cause stability problems, especially with
high impedance feedback network. Q1 and Q2 are
first cascoded with JFETs which are referenced to
the sources of the input FETs. Then, the cascode
JFETs themselves are cascoded with bipolar
transistors, which are also referenced to the
sources of the input FETS. This results in very
good linearity and very low input
capacitance.

The input JFETs are operating
at 2mA each. The input stage current it set up by
trimpot P2. Trimpot P3 is used to adjust the
offset of the amplifier. Constant current diodes
D1 and D4 are supplying a 1mA current to the 5
Volt reference diodes D2 and D3. Resistors R7/R18
and capacitors C4/C9 are pre-filtering the supply
voltage to the current diodes and are reducing the
voltage across them at the same time. If the total
supply voltage of the amplifier is less than �45
Volt, there is no need for the series
resistors.

The second stage, which is a
folded cascode circuit, consists of Q11 through
Q14. Q11 and Q13 are TO-92 devices, they are
operating at approx. 2mA, and they provide the
bias for the Q12/Q14 TO-126 pre-drivers. Q12 and
Q14 are operated at approx. 10mA to drive the
input capacitance of the output stage. Q12 and Q14
need to be heatsinked. A suitable heatsink can be
made of a 120mmx25mmx2mm Aluminum. The devices
need to be insulated with mica and should be
mounted with a generous amount of silicone grease.
Alternatively you can use a silicon rubber
insulator, which do not require silicon grease.
The quiescent current is adjusted with trimpot P4.
Diodes D5 through D8 provide a current limit with
very low impedance loads or short
circuit.

The DC-50 Driver
Stage

The DC-50 Output Stage

The EB-693/114 output stage is
a source follower circuit, consisting of TO-220
drivers and TO-3P plastic output devices. The
advantage of the TO-3P package is that it is
bolted with single-hole mounting, directly to
the heatsink. The three legs are connected
directly to the PCB. Naturally, TO-3P packages
need proper heatsinking, just like metal cans.
The heatsink has to have a 0.5-0.6 K/W thermal
resistance per channel. The output devices have
to be insulated from the heatsink with mica or
silicon rubber. It is recommended to install a
thermal breaker with a cutout temperature of
approx. 70 degree C on the heatsink.

A
word of caution concerning shorting the output
of the amplifier. Although it can survive a
short-term short, it can't take a long term one,
because of thermal overload. If you are likely
to run into frequent and long shorts we
recommend that you install a more sophisticated
protection for your
amplifier.

The DC-50 Output
Stage

Power Supply

It is recommended to use a
200VA transformer per channel, or 400VA for two
channels for this amplifier. It is also
recommended to have separate windings for the
driver stage: 2x36VRMS/100mA, giving approx.
+50V DC. The output stage needs 2x30-32V RMS at
2.5/3A per channel. The filter capacitors should
be minimum 10000 uF/50V for the output stage
(22000uF is preferred) and 1000 uF/63V for the
driver stage. Larger transformer(s) and
capacitor(s) improve bass significantly. Both
supplies have to be grounded at the same point,
for example at the PGND of the driver board, or
a star ground point elsewhere. Connect a
0.luF/100V film capacitor between the two supply
pins on the boards (plus-to-plus and
minus-to-minus), to establish a common high
frequency reference between the two. You can
also use a single supply for the driver and the
output stage. You have several possibilities in
running such a system. You can for example run
both from an unregulated supply of �40-45V, or a
common, high current regulator of the same
output
voltage.

Proposed wiring diagram for the
DC-50 Home Theater amplifier (one channel
shown)

�2001-2007 Borbely Audio

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