My Subwoofer Amplifier, The Rowland SW150
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For my subwoofer amplifier, I got a couple of free samples of
the National Semiconducter LM3886 high power audio amplifer. From
a scrap pile at work I was able to find power transformers, large
electrolytic caps, and a case with power input filter, fuse, and
switch. I was able to buy a very large heat sink at a good price
on Ebay.
The National LM3886 is designed to used as a high power audio
amplifier. With the right power supply and heat sink, it can
deliver over 75 watts into an 8 ohm load. Since the Shiva driver
I used in my Subwoofer has dual
voice coils, I used two LM3886 amplifer chips, each driving one
voice coil. Each LM3886 is built onto an amplifier module. The
module is made from a piece of fiberglass PC board material.
Holes were drilled to mount the amp chip and components. A screw
terminal strip is used for the input and output connections on
each module. Both of the amp modules are mounted to a single
large heatsink. Note that the mounting tab on the LM3886 is
connected to the -40V supply inside the chip. I used plastic
insulators to mount the heatsink to the chassis to prevent
shorting the supply to chassis ground.
The design is based on the National Semiconductor application
in the LM3886 spec sheet. Click here
to see the schematic for the amplifier module. With both
amplifier modules connected to the dual voice coils in my Subwoofer I can get an output of
over 150 Watts.
The Rowland SW150 Subwoofer Amplifier
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This is a top view of one of the LM3886 amplifier
modules. The LM3886 is at top center. L1 is made by
wrapping some insulated wire around R6. R6 is a 10 Ohm, 2
Watt power resistor. The overall size of the module is 3
by 5 inches. |
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This is a bottom view of one of the LM3886 amplifier
modules. The heavy wire down the middle is the ground. |
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This shows the two amplifier modules attached to the
large heat sink. Wakefield Type 120 Thermal Joint
Compound is used to improve the heat transfer from the
LM3886 amp chips to the heat sink. Remember that the
tab of the LM3886 is connected to the negative supply
lead inside of the chip. The heat sink will have the -40V
supply on it so plastic insulators are used to isolate
the heatsink from the chassis.
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This is one of the LM3886 amplifier modules after
connection. The components are mounted on a piece of
fiberglass PC board material. It is hard wired on the
bottom. The LM3886 is in a 11 lead package with a tab for
heat sinking. The entire board is mounted directly to the
heatsink. A screw terminal strip is used for connections.
The design is based on reference design in the LM3886
spec sheet. I tested the module down to 10 Hz into a
dummy load and it had no dropoff from the output level at
1Khz. |
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This shows the power transformers, rectifier bridge,
and electrolytic caps. The caps are 47,000 mfd at 40V.
The transformers each have dual 28 VAC outputs. These are
connected in a series/parallel manner to provide 56VCT.
The center taps are connected to the common at the middle
of the two caps. This common is a piece of 12 guage solid
copper house wire. The bridge is rated at 35A and 400V. I
paralleled several conductors to connect the transformers
to the bridge and caps since I did not have any heavy
guage stranded wire. |
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This shows the input BNC connector, level
potentiometer, and DC blocking cap for the amp module
inputs. The upper part of the terminal strip has a 1
Megohm resistor and 1000V caps that connect signal ground
to the chassis ground. The BNC jack is isolated from the
chassis. The input to each amplifer module is twisted
with a ground wire. |
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This is another internal view. |
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The rear view shows the speaker connection binding
posts, input level control and BNC connector, Power input
jack with 4A fuse, and the two large slots cut near the
heat sink for convection cooling air flow. |
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The finished product. The front panel was coverd with
a piece of textured black craft paper. The paper was
18" wide allowing no seams. The amplier is about
17" wide, 7 1/2" high, and 10 1/2" deep.
The rocker power switch has a neon pilot light to
indicate power is on. The logo features a drawing of
our dog Benny. It was drawn by my wife. Benny was a great
little terrier mix that we adopted as a full grown adult
from the animal shelter in the Summer of 1984. He died on
February 9, 2000.
The Rowland SW150 Subwoofer Amplifier is dedicated to
Benny.
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Benny
Summer 1984 - Feb 9, 2000
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