Roger and I decided to do a general purpose =
output PC board=20
using the current-boosted THAT1646 (or DRV134) I've been writing about =
since=20
2007.
The web page to order is here:
http://www.ka-electronics.com/KAelectronics//Dual_Class-A/Dual=
_Class-A.htmProduction DRV134=20
Build
Dual Class-A Output using a =
Current-boosted DRV134.Brief=20
DescriptionThe Dual Class-A Output uses the =
THAT1646 or DRV134 with discrete BD139/BD140 outputs and a THAT1240 or =
THAT1246=20
for balanced input.
A second BD139 serves as a Vbe multiplier.
If =
desired,=20
the Dual Class-A can be used fully direct-coupled with low output offset =
typ=20
<5mV.
Gain is unity or -6dB depending upon the application and =
choice of=20
line receiver.
A low impedance open-loop output (<2 Ohms) permits =
driving=20
capacitive loads either with or without build-out resistance.
Inputs =
can be=20
paralleled or cross-coupled via jumper selection.
Phoenix connectors =
are=20
provided for input, output, power and optional level controls for =
headphone=20
use.
The Dual Class-A board can be used=20
as1) A stereo headphone amp.
2) Dual=20
single-ended class-A line output.
3) Fully-balanced line output with=20
cross-coupled differential input.
4) A high-current line =
"re-balancer."
5)=20
Dual single-ended or differential transformer driver.
6) Dual output=20
distribution amp.
7) Parallel operation (without external ballast =
resistors)=20
for double output current.
8) Passive EQ network driver. =
(Pultec)
9)=20
Control Room monitor driver.
10) A few more things we haven't thought =
of=20
yet.
I'm going to be using a set of these to buffer the sound =
card output=20
to drive stuff on the bench and another set as a precision headphone=20
amp.
Schematic of the DRV134=20
build
Dual Class-A Output =
schematic using a=20
Current-boosted DRV134Large file suitable for =
printing:
http://www.ka-electronics.com/images/jpg/DCAO_173_DRV134_Sch=
ematic_Large.JPGDual Class-A Output Circuit=20
DescriptionInput for the dual Class-A is =
received=20
by a "0dB" gain THAT1240 (INA134) or "-6dB" gain THAT1246 (INA137). The =
choice=20
of line receiver gain is up to the user but generally-speaking 0 dB =
parts are=20
used for headphone amplifier applications or when the board is used as a =
dual=20
single-ended output. -6dB gain is the best choice for fully-balanced =
outputs=20
when the inputs are cross-coupled because it provides overall unity gain =
input=20
to output.
Input links allow (1) dual operation, (2) parallel =
input in=20
which both outputs are driven in the same polarity, or (3) cross-coupled =
which=20
drives the outputs in anti-polarity to provide a fully-balanced =
differential=20
output.
A Phoenix terminal block permits level trim or headphone =
volume=20
control to be inserted prior to the output stage.
The dual =
Class-A output=20
does not provide gain. For applications as a headphone amplifier with=20
professional line levels or with consumer level ("-10") equipment =
driving=20
moderately sensitive headphones, gain is not usually required. In the =
rare=20
application in which gain is required, an external stage providing both =
gain and=20
volume control can be inserted after the input stage.
The dual =
Class-A is=20
fully DC-coupled with the exception of C13. C13 is optional for =
headphone=20
applications where there might be concern about having DC in the wiper =
of the=20
potentiometer. The bias current of the THAT1646 or DRV134 is satisfied=20
internally but can return through the wiper creating excess noise. In =
practice=20
several boards have been built without C13 and have been quiet. R17 (0R) =
exists=20
only to provide a PC board jumper.
The THAT1646 or DRV134 are =
used in the=20
dual Class-A unconventionally to provide "common mode drive" and a =
differential=20
Vbe bias voltage for the output stage consisting of Q1 and=20
Q3.
Conventional applications of the THAT1646 or DRV134 as a =
balanced=20
line driver have pin 4 driven by input and pins 2 and 7 used to provide =
common=20
mode feedback and ground sensing.
In the dual Class-A circuit, =
the DRV134=20
pin 4, the "differential" input, is used to provide a Vbe reference for =
the=20
output devices that is "voltage mirrored" around the input signal =
appearing on=20
pins 2 and 7. If the input is held at ground, the Vbe reference voltage=20
appearing on pin 4 appears as +Vbe on pin 8 and -Vbe on pin 1. If +1 =
volt is=20
applied to pins 2 and 7, then the output on pin 8 is +1V + Vbe and on =
pin 1 +1V=20
- Vbe. (Note: The THAT1646 has a differential gain of 2, the DRV134 in =
this=20
configuration a gain of 4. The DRV134 requires R1 and R2 to provide a =
Vbe/2=20
reference.) The "voltage mirror" is actually a Vbe or bias =
mirror.
Q2, a=20
BD139 and an identical device to NPN output Q3, provides a =
temperature-dependent=20
Vbe reference to set output idle current and provide thermal feedback =
for=20
temperature regulation. Q2, which has identical geometry to the output =
devices=20
it measures and a mounting hole, is located between output devices Q1 =
and Q3 on=20
the heat sink. R3 provides a current source for Q2. R5, R6 and trim pot =
R35 set=20
the output stage idle current and create Vbe multiplication. C13 =
provides noise=20
filtering and turn-on delay. When the THAT1646 is used, R1 is not =
installed and=20
R2 is 0R. The DRV134, when configured for common mode drive, has a =
differential=20
gain of 4. R1 and R2 scale the collector voltage of Q2 by 1/2 when a =
DRV134 is=20
used. The voltage at the collector of Q2 appears on pin 8 and its =
inverted "Vbe=20
mirror" image on pin 1.
Q1 and Q3 form an open loop output stage. =
D1 and=20
D2 provide reverse Vbe protection for Q1 and Q3. R4 and R7 provide =
emitter=20
degeneration and provide ballasting when both channels' outputs are=20
paralleled.
R35 sets the output stage current which is measured=20
indirectly as a voltage at TP1 and TP2. Currents from 50 mA to 85 mA are =
typically used to maintain heavy class-A operation. 50 mA produces 165 =
mV; 85=20
mA, 280 mV.
R8 is an optional build-out resistor and is not =
essential to=20
maintain stability with highly-capacitive loads. The selection of R8's =
value=20
depends on a number of factors including short-circuit protection =
capability and=20
optimum matching to the load. Generally, for headphone applications, R8 =
can=20
range from 0 to 33 Ohms or more. For balanced line driving applications, =
which=20
are usually terminated in 100-120 Ohm shielded twisted pair, 45-50 Ohms =
per leg=20
provides maximally-flat transient response, reduced DIM distortion and =
reduced=20
peak current requirements.
The output connector for each channel =
has two=20
ground connections. The "GP" (ground plane) connections may be used to =
connect=20
cable shields. The "PG" (power ground) connection has a dedicated return =
trace=20
to the power connector ground and should be used for headphone driving =
where=20
there is significant current in the return.
D9 and D10 provide =
reverse=20
polarity protection. R31 and R32, which are "defined interruption" =
(fusible)=20
resistors, serve to limit current in the event of reverse polarity. In=20
applications where R8 is made low, e.g. 0 Ohms, the builder might want =
to=20
consider making R31 and R32 larger to provide short circuit protection. =
C17 and=20
C18 provide bulk bypass capacitance. R19, R22 and C1-C6 (100 nF) provide =
local=20
bypass.
Distortion =
performance30=20
Ohm load at 100 mW. Rbuildout =3D 0R.
Green is output, Red generator=20
monitor.
The driver adds ~ 0.0017% THD.
![3D"Image"](3D_http_/www.ka-electronics.com/images/jpg/DCAO_173_30R_100mW.html)
<=
BR>
Dual Class-A Headphone =
Amplifier with=20
30 Ohm load at 100 mW.600 Ohm load, single-ended, =
+20 dBu.=20
Rbuildout 0R.
Green is output, Red generator monitor.
Note that =
the output=20
THD is essentially identical to the generator.
To fully see what the =
THD is a=20
null test is required.
The driver
subtracts ~ 0.0002% =
THD.
Dual Class-A Line Driver =
with 600 Ohm=20
load at +20 dBu.Dual=20
Class-A DRV134 Bill of Materials(Mouser =
part=20
numbers shown.)
4 IC Socket, 8P
4 Diode, 1N4148
4 =
Diode,=20
1N4004
8 Resistor, 470R 0.25W CF
2 Resistor, 4K7 0.25W CF
4 =
Resistor, 10R 0.25W CF
2 Resistor, 0R* 0.25W MF Jumper, =
71-FRJ-50-0
4=20
Resistor, 1R* 1W MF, 594-5073NW1R000J
4 Resistor, 3R3 1W MF,=20
594-5073NW3R300J
2 Resistor, 47R* 1W MF, 594-5073NW47R00J
12=20
Capacitor, 100nF 100V, 581-SR201C104KAR
2 Capacitor, 47uF 35V,=20
647-UVR1V470MDD
4 Capacitor, 220uF 25V, 647-UVR1E221MPD
1 =
Header, 4=20
Pos, 538-90131-0122
1 Terminal Block, 3 Pos, 5.08mm, 651-1715734
6 =
Terminal Block, 3 Pos, 2.54mm, 651-1725669
4 Transistor BD139-16, =
(160=20
Hfe) 512-BD13916S
2 Transistor BD140-16. (160 Hfe) 512-BD14016S
2=20
Heatsink, CTS 7-342-1PP-BA, 10 degC/W, Digi-Key 294-1085-ND or=20
equivalent.*s
6 each 4-40 x 1/2" screw, lockwasher and nut.
2 =
Trim=20
Pot, 1K, 15T, 594-64W102
2 Line Driver IC, DRV134PA
2 Line =
Receiver=20
IC, THAT124X or INA13X.*
C15, C16, C21-C24, D5-D8, R18, R20, R21, =
R23,=20
R25, R27, R28, R30, IC3-IC6 not used.*
A picture=20
of the stuffed Dual Class-A board
Dual Class-A board stuffing=20
diagram![3D"Image"](3D_http_/www.ka-electronics.com/images/gif/DCAO_173.html)