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100W
LM3886 Power Amplifier
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This
is 100W LM3886 power amplifier is based on the PA100 parallel amplifier
detailed in National Semiconductor's application note - AN1192. Since my
DIY speaker is 4-ohm and somewhat difficult to drive, I want to have a
more powerful amplifier to match with it. Therefore I designed this
amplifier which uses two LM3886 per channel, in parallel circuit. This
amp can deliver about 50W into a 8-ohm speaker and 100W into a 4-ohm
speaker. This is a stereo amplifier and therefore 4 LM3886s are used. The
LM3886 circuit is in a non-inverted configuration, so the input impedance
is determined by the input resistor R1, i.e. 47k. The 680 ohm and 470pF
resistor capacitor filter network is used to filter out the high
frequency noise at the RCA input. The 220pF C4 and C8 capacitors are used
to shot out the high frequency noise at the LM3886 input pins. I used
high quality audio grade capacitors at several locations: 1uF Auricap at
the input for DC blocking, 100uF Blackgate for C2 and C6, and 1000uF
Blackgate at the supply filter.
The PCB is designed in a
way that the power ground is separated from the signal ground, as you can
see from the below layout. The signal ground is located in the middle and
surrounded by the power ground. There is a thin trace near C5 connecting
them. The PCB layout is done by using PADS PowerPCB 5.0. I think it is a
powerful layout software. After finished the layout, I send it to a PCB
manufacturer in HK to make the PCB. After the PCB is made, I found some
of the drill holes are not large enough.... I have to make it bigger
manually.
Finished PCB.
The 20k and 1k resistors are hand matched to 0.1%. For the output
resistors, I used six 0.5W 1% 1ohm resistors in parallel per 3886 output
instead of one 3W resistor because 3W 1% resistors are hard to find.
I used the insulated version - LM3886 TF, so that I can mount it directly
to the case and heat sink with thermal compound.
Coupling capacitor is Auricap 1uF 450V. I used this high quality
capacitor because it is in the main signal path.
The HF filter capacitors are Silver Mica 47pF and 220pF.
The power supply filter used is Blackgate 1000uF 50V.
C2 and C6 are Blackgate 100uF 50V. For better result, the bi-polar
version should be used. However I am not using it because the bi-polar
blackgate is too big to fit into my PCB.
The 680 ohm + 470pF filter network is installed at the RCA. This help
filtering the high frequency noise before it gets onto the board.
The WIMA 0.1uF supply decoupling capacitors are soldered directly on the
pins of LM3886 at the back of the PCB. This helped to remove some high
frequency noise.
The 3886 are mounted on a 3/8 inch aluminum plate then to the case.
Outside of the case I used 3 PC CPU heatsinks. I used Arctic Silver
thermal compound between the aluminum layers to improve heat
conductivity.
With all these big heatsinks, it only get slightly warm when listen at
normal volume.
The power supply used is a regulated power supply. I used 10000uF per
rail before the LT1083 regulator. After the regulator, I have 100uF on the
regulator board. The advantage of using regulator is that the power
supply ripple voltage is removed. If power regulation is not used, I can
hear very little 50/100Hz hum from the speaker.
Power Supply Schematic :
The high current MUR860 diode is used to ensure high current flow.
The voltage regulator used is LT1083, it can provide about 8A of current.
Transformer used here is a 500VA 2x 25V. The power supply is then
regulated by 2 LT1083, after the regulation, the voltage is 30V.
Will consider to use this supply circuit instead, the TIP2955 is capable
of providing 15A of current:
I did some DC measurement and the result is quite good, I got 7 mV of DC
offset at the speaker terminal. The voltage difference between the output
of the 2 chips is less then 1 mV.
The sound of this amplifier is similar to my LM3875 amplifier, which is
very clean and detail. It has no hum, no hiss and no noise. Compared to
the LM3875 Gainclone, this amp can deliver twice the power to my 4-ohm
speaker, and it improves the dynamics and bass punch a lot.
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