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Old 30th January 2011, 08:57 PM   #1
x51 is offline x51  Bulgaria
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Default LM3886 very quiet

I recently built a guitar amp based on the LM3886 according to the instructions here: http://www.generalguitargadgets.com/...lm3886_amp.pdf.
I am using a 150 VA 24-0 24-0 toroid and 2 4 ohm speakers wired in series. The amp works but its very quiet. When turn it all the way up its probably as loud a 1W-2W amp.
Any idea why?
Thanks in advance.
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Old 30th January 2011, 09:20 PM   #2
Ar4 is offline Ar4  Latvia
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1) Have you double checked all component values?
2) Are you using any preamp?
3) Try one speaker, as chip is running around at +-32VDC
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Old 30th January 2011, 09:31 PM   #3
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You definitely need a preamp with a signal from a guitar.
Some effects boxes boost the signal a bit, try one and see if things improve.
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Old 30th January 2011, 09:44 PM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
the pre-amp questions are the clue.

The "system" gain must be designed to suit the duty.
What is the output of your guitar?
What load does the Guitar need to see?
What is the sensitivity of the speakers?
What signal do they need to see.
From what the Guitar sends out to what the speakers need to see, you can design the system gain.
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Old 31st January 2011, 08:38 AM   #5
x51 is offline x51  Bulgaria
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I dont have a preamp. Its just the LM3886. The load on the guitar must be al least 1Mohm as far as i know. The sensitivity of the speakers is 88 dB/W/m. In the spec it says Rated power: 15W, Maximum power: 50W. BTW whats system gain and how can I design it. Also, I tried a guitar effect and it louder but it is still not as loud as what i imagine 50W would be. I have a 25W fender amp and its much louder than the LM3886.
Thanks
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Old 31st January 2011, 09:27 AM   #6
Bone is offline Bone  United Kingdom
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You definitely need a preamp. There is not enough output from any guitar to drive the 3886 to full output.
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Old 31st January 2011, 09:59 AM   #7
AndrewT is online now AndrewT  Scotland
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I'll guess and suggest that setting Rin to 1M0 and gain to 200times (+46dB) will allow a guitar to overload a 3886.
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Old 31st January 2011, 01:00 PM   #8
x51 is offline x51  Bulgaria
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Thank you for all the replies. But even when I connect it to my computer its still not that loud. I also double checked all the components and they seem correct.
Quote:
Originally Posted by AndrewT View Post
I'll guess and suggest that setting Rin to 1M0 and gain to 200times (+46dB) will allow a guitar to overload a 3886.
So all I have to do is change R3 with a 1M resistor.
Thanks
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File Type: jpg LM3886 Schematic.jpg (68.2 KB, 554 views)
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Old 31st January 2011, 01:12 PM   #9
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No, swap the 10k pot for a 1m log pot.

/edit: ignore that remark; that won't solve it. See the datasheet pages 20-21 for dimensioning an amp using the 3886: http://www.national.com/ds/LM/LM3886.pdf

Last edited by mastodon; 31st January 2011 at 01:23 PM.
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Old 31st January 2011, 01:13 PM   #10
AndrewT is online now AndrewT  Scotland
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no.
You have to find out what load the guitar expects to see.
You have to find out what signal level the guitar is capable of sending to that specfied load.
You have to determine the maximum signal you expect the speakers to need.
From there you can (or with our help) determine the system gain.
You have to make your self informed, before you can take decisions, otherwise it's all guesswork and where did that get you first off?
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