lm3875 problem

jsteigs · 23rd November 2004, 03:23 AM

I'm using the lm3875 circuit off of decibel dungeon and i get the circuit to work but it doesn't sound like 50W. Any ideas why this could be? this is my first GC but i have built other stuffs.

rampage101 · 23rd November 2004, 04:07 AM

Have you checked all resistors are correct? What source are you running from?

jsteigs · 24th November 2004, 09:24 AM

yes, all the resistors are correct and i have a 40VCT (40-0-40) supply.

UrSv · 24th November 2004, 09:29 AM

40 VCT (40-0-40), what exactly is that? A transformer with a 80 VAC transformer with a center tap i.e. two 40 VAC secondaries or do you mean that you have +/- 40 VDC?

If it doesn't sound like 50 W are you sure you are driving it to the level where it would?

carlmart · 24th November 2004, 09:35 AM

Sounding is a very vague term.

It will depend on how efficient is the speaker system you are using, what type of speaker it is, what source you are using, the room the speakers are in, your perception on what is loud, etc.

There's an exact way to quantify that, which is using a load (4 or 8 ohm) and an oscilloscope, inject an AC signal and see how many volts you get before clipping. As simple as that.

Carlos

mcs · 24th November 2004, 11:39 AM

Quote:

Originally posted by jsteigs
but it doesn't sound like 50W.

How does 50W sound? I have tested the output power on my amp (see the results here: http://stiftsbogtrykkeriet.dk/~mcs/LM3875/index.html), and I get 37W with a 2*22V transformer. So 50W should be possible to get if you have a 2*40V DC power supply.

Best regards,

Mikkel C. Simonsen

jackinnj · 24th November 2004, 02:03 PM

Quote:

Originally posted by mcs

How does 50W sound? I have tested the output power on my amp (see the results here: http://stiftsbogtrykkeriet.dk/~mcs/LM3875/index.html), and I get 37W with a 2*22V transformer. So 50W should be possible to get if you have a 2*40V DC power supply.

Best regards,

Mikkel C. Simonsen

+/- 40VDC has too little headroom for the LM3875 -- you may be causing the chip to go into its protection mode, and if its a 40-0-40 transformer you are way over the bounds for the chip as this will (should) produce +/-56VDC.

soundNERD · 24th November 2004, 02:08 PM

It can't be a 40-0-40 transformer.

There is no dobut 112VDC would fry the chip. The absolute max is 84VDC.

peranders · 24th November 2004, 02:10 PM

Quote:

Originally posted by jsteigs
I'm using the lm3875 circuit off of decibel dungeon and i get the circuit to work but it doesn't sound like 50W. Any ideas why this could be? this is my first GC but i have built other stuffs.

Max voltage is 2 x 28 VAC and 2 x 24 V seems to be "normal".

50W is more or less peak power so you have very little headroom. If you are using a 4 ohm speaker and high voltage the poor LM3875 has a hard time to deliver max power.

Notice also that the LM3886 has substantially more current capability which is good for the sound quality especially if you have heavy speakers.

soundNERD · 24th November 2004, 04:12 PM

peranders,

max voltage is 84V, so +/-42VDC.

How do you get +/-28?

23rd November 2004, 03:23 AM	#1
jsteigs diyAudio Member Join Date: Oct 2004	lm3875 problem I'm using the lm3875 circuit off of decibel dungeon and i get the circuit to work but it doesn't sound like 50W. Any ideas why this could be? this is my first GC but i have built other stuffs.

23rd November 2004, 04:07 AM	#2
rampage101 diyAudio Member Join Date: Apr 2004 Location: NSW Australia	Have you checked all resistors are correct? What source are you running from? __________________ What's a signiture?

24th November 2004, 09:29 AM	#4
UrSv diyAudio Member Join Date: Jan 2002 Location: Sweden	40 VCT (40-0-40), what exactly is that? A transformer with a 80 VAC transformer with a center tap i.e. two 40 VAC secondaries or do you mean that you have +/- 40 VDC? If it doesn't sound like 50 W are you sure you are driving it to the level where it would? __________________ UrSv Those who say it can't be done should not stop those who are doing it.

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24th November 2004, 09:24 AM	#3
jsteigs diyAudio Member Join Date: Oct 2004	yes, all the resistors are correct and i have a 40VCT (40-0-40) supply.

24th November 2004, 09:35 AM	#5
carlmart diyAudio Member Join Date: Mar 2001 Location: Brazil	Sounding is a very vague term. It will depend on how efficient is the speaker system you are using, what type of speaker it is, what source you are using, the room the speakers are in, your perception on what is loud, etc. There's an exact way to quantify that, which is using a load (4 or 8 ohm) and an oscilloscope, inject an AC signal and see how many volts you get before clipping. As simple as that. Carlos

24th November 2004, 02:08 PM	#8
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	It can't be a 40-0-40 transformer. There is no dobut 112VDC would fry the chip. The absolute max is 84VDC.

24th November 2004, 04:12 PM	#10
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	peranders, max voltage is 84V, so +/-42VDC. How do you get +/-28?

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