LM3886 amplifier problem

mbates14 · 11th August 2004, 03:07 AM

I was building an LM3886 amplifier (mono) for driving a subwoofer.

well, i used a schematic that I found here, and I changed it from inverting to non-inverting.

I got about +/-35VDC for the supplies, and I built the amp.

This particular schematic didnt have a output coupling capacitor, and I checked the output for DC, and I have a full 35vdc on the output. Something is wrong.

is there supposed to be an output coupler cap? Whats it for.

UPDATE: I tried to use a coupling cap, no difference. ALL DC.

anyway, I put the scale over to AC, and the audio is getting through and being amplified. just got tons of DC.

Any idea why this may be happining.

meat · 11th August 2004, 03:57 AM

I had the same problem the other day and it turned out that my feedback resistor was soldered to the wrong pin of my chip, so I had an open loop. That's probably not it, but you never know. You could also have some sort of a grounding issue.

mbates14 · 11th August 2004, 04:06 AM

the schematic I have called for a 1meg feedback resistor.

i doublechecked the wiring, and its not messed up.

mbates14 · 11th August 2004, 04:24 AM

oops. hehe. I found my problem. The search button is good for these kinda things DOH!

anyway, the problem was since im using non-inverting, I needed a resistor from the input to ground. Well, I didnt have that.

I put a 10k there, and my DC offset is just below 2mv

mbates14 · 11th August 2004, 04:58 PM

CRAP!!!!

I went to hook it up today, its no sound, all hum. its a low level hum, but its enough for my speaker to pop all the way out.

I need help!!!

Netlist · 11th August 2004, 05:03 PM

Hi Mbates14

Hum, speaker popping out?
Sounds like DC again.
What schematic are you using? Any link?
Don't worry, we'll get you out of the mess

/Hugo

mbates14 · 11th August 2004, 05:35 PM

I used this schematic and tried to make it non inverting.

basically, I took in the schematic, pins 9 and 10 and switched them, leaving the feedback loop on -, and putting another 22k resistor from + to ground for non inverting.

mbates14 · 11th August 2004, 07:12 PM

jee. i need to have a little more patients in what im doing.

Well, after scowering and scowering the circuit, OOPS! the 100uf cap on the mute pin8 was backwards. oops. I put that in the right way, and then for the hell of it, i made it back to inverting, and bingo. It works. no DC, and the sound pounds.

Netlist · 11th August 2004, 07:17 PM

Glad you found it.
Two things come to mind:
1) I would put a new cap in there.
2) The cap won't have been the cause of the problem IMHO.

/Hugo

mbates14 · 11th August 2004, 08:42 PM

OMG 1 chip starts an earthquake, I wonder what 2 or 3 would do parallel. Blow the roof off?

im scared to see what you guys have done with 5 or 10 chips a channel

11th August 2004, 03:07 AM	#1
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	LM3886 amplifier problem I was building an LM3886 amplifier (mono) for driving a subwoofer. well, i used a schematic that I found here, and I changed it from inverting to non-inverting. I got about +/-35VDC for the supplies, and I built the amp. This particular schematic didnt have a output coupling capacitor, and I checked the output for DC, and I have a full 35vdc on the output. Something is wrong. is there supposed to be an output coupler cap? Whats it for. UPDATE: I tried to use a coupling cap, no difference. ALL DC. anyway, I put the scale over to AC, and the audio is getting through and being amplified. just got tons of DC. Any idea why this may be happining.

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11th August 2004, 03:57 AM	#2
meat diyAudio Member Join Date: Jan 2004 Location: AZ	I had the same problem the other day and it turned out that my feedback resistor was soldered to the wrong pin of my chip, so I had an open loop. That's probably not it, but you never know. You could also have some sort of a grounding issue.

11th August 2004, 04:06 AM	#3
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	the schematic I have called for a 1meg feedback resistor. i doublechecked the wiring, and its not messed up.

11th August 2004, 04:24 AM	#4
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	oops. hehe. I found my problem. The search button is good for these kinda things DOH! anyway, the problem was since im using non-inverting, I needed a resistor from the input to ground. Well, I didnt have that. I put a 10k there, and my DC offset is just below 2mv

11th August 2004, 04:58 PM	#5
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	CRAP!!!! I went to hook it up today, its no sound, all hum. its a low level hum, but its enough for my speaker to pop all the way out. I need help!!!

11th August 2004, 05:03 PM	#6
Netlist diyAudio Moderator Join Date: Jan 2003	Hi Mbates14 Hum, speaker popping out? Sounds like DC again. What schematic are you using? Any link? Don't worry, we'll get you out of the mess /Hugo

11th August 2004, 07:12 PM	#8
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	jee. i need to have a little more patients in what im doing. Well, after scowering and scowering the circuit, OOPS! the 100uf cap on the mute pin8 was backwards. oops. I put that in the right way, and then for the hell of it, i made it back to inverting, and bingo. It works. no DC, and the sound pounds.

11th August 2004, 07:17 PM	#9
Netlist diyAudio Moderator Join Date: Jan 2003	Glad you found it. Two things come to mind: 1) I would put a new cap in there. 2) The cap won't have been the cause of the problem IMHO. /Hugo

11th August 2004, 08:42 PM	#10
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	OMG 1 chip starts an earthquake, I wonder what 2 or 3 would do parallel. Blow the roof off? im scared to see what you guys have done with 5 or 10 chips a channel

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