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Old 11th September 2010, 07:34 PM   #1
crash42 is offline crash42  United States
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Default Cheap LM3886 PCB I probably shouldn't have bought.

My electronics experience is mostly in years of Synth DIY. I felt confident about building a LM3886 amp to drive my monitor speakers, but after reading several threads about chipamp builds I believe there may be some black magic and voodoo involved that I do not totally understand.

I was originally going to build my amp on vector board using one of the many designs available, but while ordering the chips I noticed I could buy a set of PCBS for it for an additional $4. Why not?

There's a couple of things on the schematic that came with the boards that I'm not quite sure about. There's a 22uF 500v cap on the input right after the pot. Seems a little excessive. Is there some sort of magic that comes from hugely over rated DC blocking capacitors (I'm assuming this is the purpose of this cap)? How should I gauge a substitutions?

There is also a 1ohm resistor shown between the output and the speaker. Seems weird to me. Not sure what it's doing there. I've seen resistors to ground off of the output on other designs, but not a resistor in the signal path. I assume this would need to be a 3 - 5 watt resistor as well?

The power supply board seems to be the CarlosFM design that you've all answered plenty of questions about, so all my questions about that I've found in other threads.

I can scan the schematic if necessary. Thanks for any help. I hate asking questions, so I promise I won't make it a habit.
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Old 11th September 2010, 08:16 PM   #2
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Quote:
Originally Posted by crash42 View Post
There's a 22uF 500v cap on the input right after the pot.
No voodoo, It is probable a film cap and they are often only available in such a high voltage ratings with this capacitance. Personally, I dislike huge components in the signal-flow. It only add's more inductance.

With kind regards,
Bas
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Old 11th September 2010, 08:52 PM   #3
crash42 is offline crash42  United States
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Thanks for the response.

Ah, so I could do without it? I've seen input caps everywhere from .22uf to 22uF, what's the difference? I'm actually debating that, but since I'm using this to power monitor speakers for a modular synth DC blocking might be a good idea.

So then what do you think this 1ohm resistor is for?
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Old 11th September 2010, 10:11 PM   #4
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The input capacitor forms a highpass filter with the input resistor. The higher the value of the capacitor, the lower the frequency cutoff, and the more deep bass you get. With a standard 10k input resistor, the capacitor stops being important past 2.2uF (10k/2.2uF has roughly a -3dB point of 7Hz).

Unnecessarily large input cap can cause audible transients (popping) at turn on. As for the 1ohm resistor, i don't think that was supposed to be there. Or if it was... it should probably be 0.1ohm if it's in series with the speaker.
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Old 12th September 2010, 12:54 AM   #5
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Are you sure that resistor isn't supposed to be more like 10R with a wire wrapped around it to form an inductor in parallel?

Like in this schematic.....

Click the image to open in full size.
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Old 12th September 2010, 04:01 AM   #6
crash42 is offline crash42  United States
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If it had looked like that I'd be less confused. The schematic actually looks just like this.
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File Type: jpg lm3886.jpg (32.3 KB, 530 views)
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Old 12th September 2010, 04:11 AM   #7
T in AZ is offline T in AZ  United States
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2.2uF/ 50V seems more reasonable to me.
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Old 12th September 2010, 08:15 AM   #8
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The 1 ohm resistor is there for if you want to parralel the boards. If you are not going to and only use the boards in "single" mode. you can easily rather populate a wire link in the place of the 1ohm oputput resistor.
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Old 12th September 2010, 11:01 AM   #9
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Quote:
Originally Posted by tangmonster View Post
The 1 ohm resistor is there for if you want to parralel the boards.
Explainable, but 1 ohm still seems a bit excessive to me. Or is the LM3886 different than transistor amps?
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Old 12th September 2010, 11:23 AM   #10
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Just build it already.
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