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Old 5th October 2011, 12:00 AM   #1
nige838 is offline nige838  United States
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Default LM3886 loud pop when turned off

Hello all,
I just completed my first gainclone. I gutted the main amplifiers on a old realistic stereo and implemented 2 LM3875s. I have a problem where when I turn the stereo off, it makes a POP and the speaker cone moves A LOT.
I'm running it a 55 volts, using this schematic
but instead of 5600uf caps on the + and -, I'm using 10000uf caps...

Why is the amplifier doing this? What can I do to fix the problem?

Thanks
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Old 5th October 2011, 12:22 AM   #2
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What schematic?

Mike
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Old 5th October 2011, 03:42 AM   #3
nige838 is offline nige838  United States
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Quote:
Originally Posted by Michael Bean View Post
What schematic?

Mike
Here it is, sorry...
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Old 5th October 2011, 04:08 AM   #4
djoffe is offline djoffe  United States
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Default See Data Sheet, Figure 2

The schematic you have is too simplified! Unfortunately, National encourages this by a not so well written data sheet...but if you're going to have something stable, it must be a lot closer to Figure 2...Output LR network, Zobel network, compensating caps on the feedback to shape the loopgain, small bypass caps across the rails as well as the big ones...

Your design is marginally stable, so just the noise from the interruption of the AC supply at turn off is amplified horrendously, overdriving the amp, and making a huge pop at the output.

There was a similar thread about a similar problem a while ago, but I haven't tried to search for it.
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Old 5th October 2011, 05:14 AM   #5
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There is no ground reference path on the '+' input of the amp AND the gain is 30 dB down to DC. National says the capacitor is optional but it's very good to include it. There is no harm increasing the value over the 22uF shown. It just moves the low end -3dB frequency lower.

G
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Old 5th October 2011, 11:57 AM   #6
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That schematic is terrible, the parts that National calls "optional" really aren't. This is what I'd recommend you build to get the most reliable and best performance.

30X AMP-2.JPG

Mike
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Old 5th October 2011, 12:31 PM   #7
AndrewT is offline AndrewT  Scotland
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I would ALWAYS recommend that Input RF attenuation be added to Michael's example schematic.
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Old 5th October 2011, 04:03 PM   #8
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55 volts is too much
Quote:
Originally Posted by nige838 View Post
Hello all,
I just completed my first gainclone. I gutted the main amplifiers on a old realistic stereo and implemented 2 LM3875s. I have a problem where when I turn the stereo off, it makes a POP and the speaker cone moves A LOT.
I'm running it a 55 volts, using this schematic
but instead of 5600uf caps on the + and -, I'm using 10000uf caps...

Why is the amplifier doing this? What can I do to fix the problem?

Thanks
55 volts is too much, you need 25-27 volts on each arm. In LM3875 no mute function, and you need delay of loudspeakers connection, something like this. Or you can use upc1237 if you want speakers protection and delay.
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Old 5th October 2011, 04:16 PM   #9
AndrewT is offline AndrewT  Scotland
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55Vdc is just perfect for a 3875.
It can be single polarity supply 0Vdc to +55Vdc, or 0Vdc to -55Vdc, or dual polarity supply, +27.5Vdc, 0Vdc -27.5Vdc, or dual polarity with the centre voltage not at 0Vdc, eg. +35Vdc, 7.5Vdc, 20Vdc. It can even have the centre voltage not at the arithmetic mean between the two supply polarities.
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Old 5th October 2011, 07:16 PM   #10
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Quote:
Originally Posted by AndrewT View Post
55Vdc is just perfect for a 3875.
It can be single polarity supply 0Vdc to +55Vdc, or 0Vdc to -55Vdc, or dual polarity supply, +27.5Vdc, 0Vdc -27.5Vdc, or dual polarity with the centre voltage not at 0Vdc, eg. +35Vdc, 7.5Vdc, 20Vdc. It can even have the centre voltage not at the arithmetic mean between the two supply polarities.
He hasn't specified what power supply he uses. Maybe it +55v and -55v, how i can understand that the 55volts it is the sum of voltages?
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