LM3886 very quiet

x51 · 30th January 2011, 08:57 PM

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.

Ar4 · 30th January 2011, 09:20 PM

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

nigelwright7557 · 30th January 2011, 09:31 PM

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.

AndrewT · 30th January 2011, 09:44 PM

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.

x51 · 31st January 2011, 08:38 AM

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

Bone · 31st January 2011, 09:27 AM

You definitely need a preamp. There is not enough output from any guitar to drive the 3886 to full output.

AndrewT · 31st January 2011, 09:59 AM

I'll guess and suggest that setting Rin to 1M0 and gain to 200times (+46dB) will allow a guitar to overload a 3886.

x51 · 31st January 2011, 01:00 PM

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

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

mastodon · 01:23 PM

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

AndrewT · 31st January 2011, 01:13 PM

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?

30th January 2011, 08:57 PM	#1
x51 diyAudio Member Join Date: Nov 2010	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.

30th January 2011, 09:31 PM	#3
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

30th January 2011, 09:44 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

31st January 2011, 09:59 AM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	I'll guess and suggest that setting Rin to 1M0 and gain to 200times (+46dB) will allow a guitar to overload a 3886. __________________ regards Andrew T.

31st January 2011, 01:12 PM	#9
mastodon diyAudio Member Join Date: Jan 2011	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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30th January 2011, 09:20 PM	#2
Ar4 diyAudio Member Join Date: Apr 2010 Location: Latvia	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

31st January 2011, 08:38 AM	#5
x51 diyAudio Member Join Date: Nov 2010	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

31st January 2011, 09:27 AM	#6
Bone diyAudio Member Join Date: Jun 2004 Location: Dorset, UK	You definitely need a preamp. There is not enough output from any guitar to drive the 3886 to full output.

31st January 2011, 01:13 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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? __________________ regards Andrew T.

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