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Dual
Class-A Line and Headphone Output Board Documents

Our build thread for DIY projects,
including the MS Mid Side Matrix.

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Dual

by
mediatechnology
» Wed Feb 08, 2012 4:44 pm

Roger and I decided to do a general purpose output PC board
using the current-boosted THAT1646 (or DRV134) I've been writing about since
2007.

The web page to order is here: http://www.ka-electronics.com/KAelectronics//Dual_Class-A/Dual_Class-A.htm

Production DRV134
Build

Dual Class-A Output using a
Current-boosted DRV134.

Brief
Description

The Dual Class-A Output uses the
THAT1646 or DRV134 with discrete BD139/BD140 outputs and a THAT1240 or THAT1246
for balanced input.
A second BD139 serves as a Vbe multiplier.
If desired,
the Dual Class-A can be used fully direct-coupled with low output offset typ
<5mV.
Gain is unity or -6dB depending upon the application and choice of
line receiver.
A low impedance open-loop output (<2 Ohms) permits driving
capacitive loads either with or without build-out resistance.
Inputs can be
paralleled or cross-coupled via jumper selection.
Phoenix connectors are
provided for input, output, power and optional level controls for headphone
use.

The Dual Class-A board can be used
as

1) A stereo headphone amp.
2) Dual
single-ended class-A line output.
3) Fully-balanced line output with
cross-coupled differential input.
4) A high-current line "re-balancer."
5)
Dual single-ended or differential transformer driver.
6) Dual output
distribution amp.
7) Parallel operation (without external ballast resistors)
for double output current.
8) Passive EQ network driver. (Pultec)
9)
Control Room monitor driver.
10) A few more things we haven't thought of
yet.

I'm going to be using a set of these to buffer the sound card output
to drive stuff on the bench and another set as a precision headphone
amp.

Schematic of the DRV134
build

Dual Class-A Output schematic using a
Current-boosted DRV134

Large file suitable for printing: http://www.ka-electronics.com/images/jpg/DCAO_173_DRV134_Schematic_Large.JPG

Dual Class-A Output Circuit
Description

Input for the dual Class-A is received
by a "0dB" gain THAT1240 (INA134) or "-6dB" gain THAT1246 (INA137). The choice
of line receiver gain is up to the user but generally-speaking 0 dB parts are
used for headphone amplifier applications or when the board is used as a dual
single-ended output. -6dB gain is the best choice for fully-balanced outputs
when the inputs are cross-coupled because it provides overall unity gain input
to output.

Input links allow (1) dual operation, (2) parallel input in
which both outputs are driven in the same polarity, or (3) cross-coupled which
drives the outputs in anti-polarity to provide a fully-balanced differential
output.

A Phoenix terminal block permits level trim or headphone volume
control to be inserted prior to the output stage.

The dual Class-A output
does not provide gain. For applications as a headphone amplifier with
professional line levels or with consumer level ("-10") equipment driving
moderately sensitive headphones, gain is not usually required. In the rare
application in which gain is required, an external stage providing both gain and
volume control can be inserted after the input stage.

The dual Class-A is
fully DC-coupled with the exception of C13. C13 is optional for headphone
applications where there might be concern about having DC in the wiper of the
potentiometer. The bias current of the THAT1646 or DRV134 is satisfied
internally but can return through the wiper creating excess noise. In practice
several boards have been built without C13 and have been quiet. R17 (0R) exists
only to provide a PC board jumper.

The THAT1646 or DRV134 are used in the
dual Class-A unconventionally to provide "common mode drive" and a differential
Vbe bias voltage for the output stage consisting of Q1 and
Q3.

Conventional applications of the THAT1646 or DRV134 as a balanced
line driver have pin 4 driven by input and pins 2 and 7 used to provide common
mode feedback and ground sensing.

In the dual Class-A circuit, the DRV134
pin 4, the "differential" input, is used to provide a Vbe reference for the
output devices that is "voltage mirrored" around the input signal appearing on
pins 2 and 7. If the input is held at ground, the Vbe reference voltage
appearing on pin 4 appears as +Vbe on pin 8 and -Vbe on pin 1. If +1 volt is
applied to pins 2 and 7, then the output on pin 8 is +1V + Vbe and on pin 1 +1V
- Vbe. (Note: The THAT1646 has a differential gain of 2, the DRV134 in this
configuration a gain of 4. The DRV134 requires R1 and R2 to provide a Vbe/2
reference.) The "voltage mirror" is actually a Vbe or bias mirror.

Q2, a
BD139 and an identical device to NPN output Q3, provides a temperature-dependent
Vbe reference to set output idle current and provide thermal feedback for
temperature regulation. Q2, which has identical geometry to the output devices
it measures and a mounting hole, is located between output devices Q1 and Q3 on
the heat sink. R3 provides a current source for Q2. R5, R6 and trim pot R35 set
the output stage idle current and create Vbe multiplication. C13 provides noise
filtering and turn-on delay. When the THAT1646 is used, R1 is not installed and
R2 is 0R. The DRV134, when configured for common mode drive, has a differential
gain of 4. R1 and R2 scale the collector voltage of Q2 by 1/2 when a DRV134 is
used. The voltage at the collector of Q2 appears on pin 8 and its inverted "Vbe
mirror" image on pin 1.

Q1 and Q3 form an open loop output stage. D1 and
D2 provide reverse Vbe protection for Q1 and Q3. R4 and R7 provide emitter
degeneration and provide ballasting when both channels' outputs are
paralleled.

R35 sets the output stage current which is measured
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
mA, 280 mV.

R8 is an optional build-out resistor and is not essential to
maintain stability with highly-capacitive loads. The selection of R8's value
depends on a number of factors including short-circuit protection capability and
optimum matching to the load. Generally, for headphone applications, R8 can
range from 0 to 33 Ohms or more. For balanced line driving applications, which
are usually terminated in 100-120 Ohm shielded twisted pair, 45-50 Ohms per leg
provides maximally-flat transient response, reduced DIM distortion and reduced
peak current requirements.

The output connector for each channel has two
ground connections. The "GP" (ground plane) connections may be used to connect
cable shields. The "PG" (power ground) connection has a dedicated return trace
to the power connector ground and should be used for headphone driving where
there is significant current in the return.

D9 and D10 provide reverse
polarity protection. R31 and R32, which are "defined interruption" (fusible)
resistors, serve to limit current in the event of reverse polarity. In
applications where R8 is made low, e.g. 0 Ohms, the builder might want to
consider making R31 and R32 larger to provide short circuit protection. C17 and
C18 provide bulk bypass capacitance. R19, R22 and C1-C6 (100 nF) provide local
bypass.

Distortion performance

30
Ohm load at 100 mW. Rbuildout = 0R.
Green is output, Red generator
monitor.
The driver adds ~ 0.0017% THD.

Dual Class-A Headphone Amplifier with
30 Ohm load at 100 mW.

600 Ohm load, single-ended, +20 dBu.
Rbuildout 0R.
Green is output, Red generator monitor.
Note that the output
THD is essentially identical to the generator.
To fully see what the THD is a
null test is required.
The driver subtracts ~ 0.0002% THD.

Dual Class-A Line Driver with 600 Ohm
load at +20 dBu.

Dual
Class-A DRV134 Bill of Materials

(Mouser part
numbers shown.)

4 IC Socket, 8P

4 Diode, 1N4148
4 Diode,
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
Resistor, 1R* 1W MF, 594-5073NW1R000J
4 Resistor, 3R3 1W MF,
594-5073NW3R300J
2 Resistor, 47R* 1W MF, 594-5073NW47R00J

12
Capacitor, 100nF 100V, 581-SR201C104KAR
2 Capacitor, 47uF 35V,
647-UVR1V470MDD
4 Capacitor, 220uF 25V, 647-UVR1E221MPD

1 Header, 4
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
Hfe) 512-BD13916S
2 Transistor BD140-16. (160 Hfe) 512-BD14016S
2
Heatsink, CTS 7-342-1PP-BA, 10 degC/W, Digi-Key 294-1085-ND or
equivalent.*s
6 each 4-40 x 1/2" screw, lockwasher and nut.

2 Trim
Pot, 1K, 15T, 594-64W102

2 Line Driver IC, DRV134PA
2 Line Receiver
IC, THAT124X or INA13X.*

C15, C16, C21-C24, D5-D8, R18, R20, R21, R23,
R25, R27, R28, R30, IC3-IC6 not used.*

A picture
of the stuffed Dual Class-A board

Dual Class-A board stuffing
diagram

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1771

Joined: Sat Aug 11, 2007 2:34 pm

Location: Dallas, Texas

Website

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Re:
Dual Class-A Line and Headphone Output Board

by
emrr
» Wed Feb 08, 2012 5:21 pm

Looks good.

Best,

Doug
Williams
Electromagnetic Radiation Recorders

emrr

Posts: 302

Joined: Sun Mar 02, 2008 11:27 pm

Location: NC, USA

Website

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Re:
Dual Class-A Line and Headphone Output Board

by
mediatechnology
» Wed Feb 08, 2012 6:30 pm

Thanks Doug.

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1771

Joined: Sat Aug 11, 2007 2:34 pm

Location: Dallas, Texas

Website

Top

Re:
Dual Class-A Line and Headphone Output Board

by
tmuikku
» Mon Apr 16, 2012 5:52 am

Hi!

Are you going to manufacture / sell the boards
sometime near future?

I'm planning a monitoring unit for my mobile
recording rig with four stereo inputs, four headphone outputs and a monitor
output. All five outputs have volume control and switch to select one of the
four stereo inputs.

I see there is couple of transistors / fets on the
boards (next to the output connectors) that are not in the headphone amp
schematics here viewtopic.php?f=6&t=23
, are those buffers or something for the volume pot? I'm not sure if the input
chip could drive five volume pots if all the outputs have selected same input.
If these are buffers the boards would be perfect for my scenario.

Would
be a huge time saver if these boards were available since they would provide all
I need for my monitoring unit!:) Thanks!

tmuikku

Posts: 11

Joined: Fri Apr 06, 2012 8:33 am

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Re:
Dual Class-A Line and Headphone Output Board

by
mediatechnology
» Mon Apr 16, 2012 6:58 am

Hi tmuikku -

Thank you for joining us and
posting.

Are you going to manufacture / sell the boards sometime near
future?

That's the plan. I'm about five mouse-clicks
away from that with maybe a 10-14 day delivery. I've been waiting on Roger to
finish his build of it and any feedback he might have.

I'm not sure if the input chip could drive five volume pots if all the
outputs have selected same input. If these are buffers the boards would be
perfect for my scenario.

Sounds like they would be
perfect other than the combined input impedance issue.
The THAT1240 can only
drive 2K loads.
What you could do however is switch the 1240 inputs to the
various sources (balanced) with each pot connected between the 1240 and 1646 HP
driver section.
The 1240/1246 has an 18K (+/-20%) common mode or differential
input impedance. If all five channels happened to be switched to the same input
the impedance would be about 3K6.
That might be a workable solution.

I see there is couple of transistors / fets on the boards (next to the
output connectors) that are not in the headphone amp schematics

Those TO-92 parts are actually LM317/LM337s to
provide a tracking sub-supply for the 1240/1246.
The board can be built using
either DRV134s or THAT1646s.

(When 1646s are used, the sub-supply softens
the 1646 hard clipping behavior by allowing the 124X to clip slightly before the
1646 sense inputs. This is a characteristic of the 1646 only when it's used in
the common mode drive configuration.)

If I don't here something back from
Roger and his eval PM or e-mail me and we'll get something going.

PS
radiance (posting the results here) and some other people over at group-diy have
built these for HP amps and have really liked them. The protoype boards have
done really well as line drivers too so the monitor output mode ought to work
pretty well for you.

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1771

Joined: Sat Aug 11, 2007 2:34 pm

Location: Dallas, Texas

Website

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Re:
Dual Class-A Line and Headphone Output Board

by
tmuikku
» Tue Apr 17, 2012 10:42 am

mediatechnology wrote:Thank you for joining us and
posting.

Thanks for keeping up very good
forum!:)

mediatechnology wrote:Sounds like they would be perfect
other than the combined input impedance issue.
The THAT1240 can only drive
2K loads.
What you could do however is switch the 1240 inputs to the
various sources (balanced) with each pot connected between the 1240 and 1646
HP driver section.
The 1240/1246 has an 18K (+/-20%) common mode or
differential input impedance. If all five channels happened to be switched to
the same input the impedance would be about 3K6.
That might be a workable
solution.

Yep, that would work but I need to check out
how much the switching system is going to cost. I was hoping to get by with
cheap rotary switches by switching the unbalanced signal. Well, this is not a
big issue since it is rather easy to breadboard a buffer stage and try out.
Also, changing requirements is no problem I still have some planning to do. I
could reduce the switching to change between one common input and "channel
input". I'm not too familiar with the pcb and all the possibilities and all the
ingenuity like the hard clipping behaviour you mentioned so lets not get too far
ahead yet

Anyway,
I'll definitely buy some of these when you are set. I'll check out the thread,
if no update from Roger I'll email you!

Thanks!

tmuikku

Posts: 11

Joined: Fri Apr 06, 2012 8:33 am

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Re:
Dual Class-A Line and Headphone Output Board

by
raf
» Tue Apr 17, 2012 2:29 pm

This has been on the "near front burner" for quite a
while...

It's been crazy here, have had zero time to play with new
deisgns, and today I am scraping up cash for the tax man... But in the next
couple of days I intend to hook up a Class A board as a transformer driver and
give it a thorough listening in the studio...

I have been using this
circuit as a reference headphone amp for a long time, using it for final
listening tests on the P3S ME compressor product we make. Definately the best
sounding headphone amp in the house, and there are some good ones
here.

Best
Roger

Americans Against The Tea Party
We are
the 99%!

raf

Posts: 679

Joined: Sat Aug 11, 2007 4:04 pm

Location: California

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Re:
Dual Class-A Line and Headphone Output Board

by
mediatechnology
» Tue Apr 17, 2012 3:08 pm

Thanks for keeping up very good forum!:)

Thank
you for that tmuikku.
I wished I had more time to experiment and add
content.
I'm thinking I will soon...

I was hoping to get by with cheap rotary switches by switching the
unbalanced signal.

Well, you would need 2 pole
switches if it were balanced.
A 2 pole 6 pos is usually about the same price
as 1 pole 12 pos.

Or, you might also look at using the MS matrix board
without the MS.
It has four balanced inputs (1240/1246) and four 1646
outputs.
You could use the balanced inputs and then use the 1646, with one
leg grounded to make it SE, to provide your SE switching.
The ~75 mA output
stage of the 1646 could drive many pots; your switching would be 1 pole 5
position.
The pot outputs could then drive the HP amp/monitor output.
You
might have to bootleg the 5th input somehow.

There's a thread on
"alternative apps" here: viewtopic.php?f=6&t=450

Thanks
Roger for checking in.
Forgot it was "April 15th."
I filed about a week
ago, cut the check, and am doing my best to forget it.

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1771

Joined: Sat Aug 11, 2007 2:34 pm

Location: Dallas, Texas

Website

Top

Re:
Dual Class-A Line and Headphone Output Board

by
raf
» Thu Apr 19, 2012 6:56 pm

Yup, tax time... Didn't want to tie up production money, so
I waited to cut the check until due day.

Anyway, fired up one of 2 units
today, a dual transformer balanced output stage and I can tell you, this thing
ROCKS!
Bass is delivered effortlessly thanks in part to P/P operation and
partly because of the transformer, a Carnhill (big for modern
transformers)

Drums sounded closer, more real which I didn't expect...
Bass is BIG and effortless.

Going back and listen to some more tunes,
Think I will skip bass practice

Oh, yeah.. Had a little 1" X 0.5" strip of .063" Al for a heatsink,
shared by all 6 transistors and it is definately way too small. I removed it and
installed 2 of the anodized 3 holers (3X TO220) I had here and all is
well...

More soon.
rf

Americans Against The Tea Party
We are
the 99%!

raf

Posts: 679

Joined: Sat Aug 11, 2007 4:04 pm

Location: California

Top

Re:
Dual Class-A Line and Headphone Output Board

by
raf
» Thu Apr 19, 2012 6:59 pm

I will be experimenting with overdriving it
too.

rf

Americans Against The Tea Party
We are
the 99%!

raf

Posts: 679

Joined: Sat Aug 11, 2007 4:04 pm

Location: California

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