lm3886-dc on the output

xplod1236 · 22nd January 2004, 03:03 AM

I have built my first chip amp today. I used the lm3886 chip. It worked fine, but I noticed that when I turned it up all the way, the speaker moved out a little. That means that I was getting dc on the output. Does anybody know what could be causing that? One more question: what's the smallest feedback resistor that I can use to maximize gain without the amp oscillating?

I was playing around with it so much, that I ended up frying the chip. Oh well....I have 4 more, and my next project is to parallel all 4 of them to drive a pair of subs.

AndersZ · 22nd January 2004, 06:30 AM

You're not using any input cap which means that you could get incoming DC from your preamp/source.

peranders · 22nd January 2004, 07:40 AM

xplod1236, how much DC do you get?

Nuuk · 22nd January 2004, 08:29 AM

A better way to discover how much DC offset you have on the output is to measure it! (Unless you have an unusually large number of speaker drivers that you wish to dispose of).

carlosfm · 22nd January 2004, 09:37 AM

68ohms from the mute pin to V-?

1uf from Ri to ground?

UrSv · 22nd January 2004, 09:50 AM

Quote:

Originally posted by carlosfm
68ohms from the mute pin to V-?

1uf from Ri to ground?

68 Ohms as per datasheet for the 0.5 mA.

1 uF for the HP at 88 Hz which might seem a bit high depending on what the initial test is for.

peranders · 22nd January 2004, 10:37 AM

Quote:

Originally posted by UrSv

68 Ohms as per datasheet for the 0.5 mA.

1 uF for the HP at 88 Hz which might seem a bit high depending on what the initial test is for.

Urban, how about multiply the 68 ohms with 1000? 68 kohms!

Jax · 22nd January 2004, 10:44 AM

Poor mute transistor in the chip...

Rm <= (Vee - 2.6)/Im where Im > 0.5 mA.

For my 3886's with -33.6V Rm would be <= 62 kohm. I have selected 18 kohm.

68 ohm?

carlosfm · 22nd January 2004, 10:58 AM

I use 10k, and no cap.

Jax · 22nd January 2004, 11:00 AM

I forgot to add that I'm not using the cap either.

22nd January 2004, 06:30 AM	#2
AndersZ diyAudio Member Join Date: Aug 2003 Location: Stockholm, Sweden	You're not using any input cap which means that you could get incoming DC from your preamp/source. __________________ visit The ZedEn projects http://www.t.kth.se/andzak54/

22nd January 2004, 07:40 AM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	xplod1236, how much DC do you get? __________________ /Per-Anders (my first name) or P-A as my friends call me

22nd January 2004, 08:29 AM	#4
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	A better way to discover how much DC offset you have on the output is to measure it! (Unless you have an unusually large number of speaker drivers that you wish to dispose of). __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

22nd January 2004, 10:44 AM	#8
Jax diyAudio Member Join Date: Nov 2003 Location: South Sweden	Poor mute transistor in the chip... Rm <= (Vee - 2.6)/Im where Im > 0.5 mA. For my 3886's with -33.6V Rm would be <= 62 kohm. I have selected 18 kohm. 68 ohm? __________________ Jan

22nd January 2004, 11:00 AM	#10
Jax diyAudio Member Join Date: Nov 2003 Location: South Sweden	I forgot to add that I'm not using the cap either. __________________ Jan


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22nd January 2004, 09:37 AM	#5
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	68ohms from the mute pin to V-? 1uf from Ri to ground?

22nd January 2004, 10:58 AM	#9
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	I use 10k, and no cap.

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