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Summary: National
Semiconductor’s “Overture Series”
of single-chip high power operational amplifiers have proven to be very
popular for the DIY audio community.
The chips are available in stereo and mono configurations, with
mute capability, insulated and non-insulated packages and power ratings
up to 60 watts per amplifier section.
National makes implementation of their chips even easier by
providing the user with interactive design tools on their website,
allowing the builder to determine the correct power supply and heat sink
for each of the chips. To maximize performance I intrepidly used a power supply which exceed the maximum suggested from National’s interactive Overture Series Design Guide: (http://www.national.com/appinfo/audio/files/Overture_Design_Guide15.xls), with +/- 26 volts on each rail. I found that any less than this the distortion climbed rapidly. Boards for this project are available here: CHIPAMPS
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The Amplifier Section: The schematic of the amplifier section is shown below. One half of the LM4780 is designed as an inverting amplifier, the other non-inverting.
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recommendations cited by National are used in this design we allow some
flexibility in the printed circuit board design for the substitution of
different coupling capacitors, i.e. you may use two 4.7μF
polypropylene capacitors, or a pair of “back to back” 100μF
electrolytics bypassed with a 1μF or 100nF polypropylene capacitor
(shown in the dashed boxes). The
4.7μF coupling capacitor seems to be more than adequate for my
purposes with a -3dB point of approximately 7 Hz. The alternate components will lower the –3dB point to
approximately 3 Hz. With the alternate
coupling system (electrolytics bypassed with a 100nF polypropylene)
distortion was slightly lower from 10Hz to 20Hz (below the audible
range), i.e. by about 0.001%. Above 4000Hz, however, distortion
was slightly worse.
Decoupling the chip from the power supply is important, as the Overture series are susceptible to high frequency oscillation. The 100nF ceramic capacitors tackle this problem at the high end. The 10uF/50V electrolytics, on the other hand, will reduce the likelihood of “motorboating”. I view the 1,000uF/63 V electrolytics as a localized reservoir, limiting supply droop or I2R losses from the power supply to the operational amplifier. The mute function can be selectively enabled, coupling pins 14 and 20 to the negative supply through the RC network as shown on the schematic, or by grounding and thus muting the amplifier. The mute function can be bypassed by omitting C12 (10uF/50V electrolytic) and connecting pins 14 and 20 to the negative supply rail at the junction of R9 and R10.
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Choice of Heat Sink: PDMAX is a term which describes the power which must be dissipated by the device into the heatsink, not into the load. A bridged amplifier using the
LM4780, has a calculated PDMAX
4 times that of a single
amplifier ! The heat sink
is going to be quite large than we are ordinarily used to and a
fan is necessary. The
LM4780 has a relatively small surface area to thermally connect the case
and heatsink.
If the chip isn’t cooled sufficiently the SPIKE!tm
thermal protection will kick in. I
found that without a fan the heat sink temperature rose to nearly 200 Fo
when the amplifier ran for a few hours. With the fan, however, the
temperature rose to only 110 Fo .
(The heat sink I used measured 11” x 4.1” with ten, 1.3”
fins.)
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A thermal impedance of 0.82 Co/W is a very big heat sink, and using a 150 Co junction temperature is at the high end. For this reason a fan is recommended. I tapped the heat sink for a 6-32nd screws and mount the device to the heat sink using the tensioning bar, 6-32nd screw and a star lock-washer. To provide electrical isolation, the LM4780’s case is connected to the negative supply rail, I used a Berq Sil-Pad cut to fit. (Note further, if you apply too much torque to the screws you can crack the case of the LM4780.) Tensioning Bar The tensioning bar is made from 1/8"aluminum stock. You can mill or file the edges so that they are slightly chamfered as shown below:
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The Power Supply: The power supply is a straightforward full wave bridge with 10,000 mF filtration. An A.C. input socket was “liberated” from a Dell server power supply. In case your network administrator disapproves, we have furnished the equivalent circuit below consisting of a bifilar-wound 2 x 200 mH choke, and “line rated” A. C. polypropylene and ceramic capacitors. These will serve to limit radio frequency interference into the power supply. The circuit is protected with a 5 amp slow-blow 3AG fuse. I used a Thermometrics inrush current limiter rated for 8 amps to limit the surge current although you may find these unnecessary. The value of the RC snubbers for the MUR860 diodes were calculated using the stated diode capacitance (100 pF at 30 VDC), the inductance of the transformer secondary windings (12 mH) and the interwinding capacitance (560 pF) of the transformer. The supply schematic appears below:
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Printed Circuit Boards :I would urge anyone building the bridged LM4780 amplifier to use a printed circuit board. An XRay image of the PCB is shown below:
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Performance Characteristics: The bridged version of the LM4780 amplifier shows response comparable to that contained in the National Semiconductor specification sheets. Average distortion is 0.01% across the audible spectrum.
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