just built a lm3886 amp

Gaston

Member

• Jan 31, 2007

• #1

i just built an amp useing the lm3886. it sounds amazing. it starts to clip at 32 volts. so i figure e=i*r... 32=i*8...32/8=4......p=v*i.... so .....p= 32*4.....that means 128 watts? thats a lot of power for that little amp. my question i why on the data sheet does it say 38 watts at 8 ohms and 28 volts? i come up with 98 watts

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 31, 2007

• #2

You need to calculate the RMS output voltage of the chip, and use that - NOT the peak to peak output. For a sinewave you divide the p-p output by 2.828 to get RMS.

This gives about 15/16W in to 8 ohms.

 

Gaston

Member

• Jan 31, 2007

• #3

do you have to use p-p or can you use one peak *.707?

 

Gaston

Member

• Jan 31, 2007

• #4

never mind i just answered my question. sorry

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 31, 2007

• #5

Gaston said:

never mind i just answered my question. sorry

Click to expand...

Yes, it's the same thing!

 

audioguru

Well-Known Member

Most Helpful Member

• Jan 31, 2007

• #6

The LM3886 is rated to deliver 68W RMS to a 4 ohm load when it has a positive 28VDC and a negative 28VDC supply. The total supply voltage is 56VDC and the sine-wave output is 16.5V RMS which is 46.6V peak-to-peak. So it has a loss of 56V- 46.6V= 9.4VDC.

It is rated to deliver 38W RMS to an 8 ohm load when it has a positive 28VDC and a negative 28VDC supply. Again the total supply voltage is 56VDC. The sine-wave output is 17.4V RMS which is 49.3V peak-to-peak.
So its loss is reduced to 56V-49.3V= 6.7VDC because the current is lower.

It gets very hot when its output is 68W. Any higher output would fry it even if it has a huge heatsink with a fan.

What is your "32V"? Is it 32V peak-to-peak across 4 ohms? Then the power is only 32W RMS.

Look in Google for The Gain Clone Amplifier. It is made with the LM3886 and LM3875 power amps. One guy makes and sells a stereo amp for $4000.00US!

 

Gaston

Member

• Jan 31, 2007

• #7

i have two transformers.after being rectfied and filtered they each put out 38volts. i use on for positive and one for negative. the output starts to clip at about 32 volts peak. i have had it spike up to just about 38 without clipping though. iv'e run it wide open just under clipping for a while and it is loud as hell. it went past the threashold of pain for me. i had to put my ear protection on. and i am used to loud music.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 31, 2007

• #8

As you've now changed your signal to peak, then the output power will be 60/64W in to 8 ohms.

Bear in mind you need ten times the power to be twice as loud, so it's twice as loud as a 6W amplifier!, and only half as loud as a 600W amplifier.

I seem to remember reading that the 'gain clone' chips are very sensitive about their supply voltages, and exceeding it soon kills the chip, I'm not sure what the spec is?, have you checked it?.

 

Gaston

Member

• Jan 31, 2007

• #9

spec sheet says 84 volts max. why do they call it a gainclone?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 31, 2007

• #10

Gaston said:

spec sheet says 84 volts max. why do they call it a gainclone?

Click to expand...

OK - I've no idea, but never been bothered enough to try and find out!.

 

Gaston

Member

• Jan 31, 2007

• #11

iv'e got a problem now. when i take the scope lead off of the output the sound goes away and all i get is a loud hum. i can put a single lead on the negative but if i put a single lead on the positive side it almost blew my brain out of my skull. it sounded like an extremely loud pop..pop..pop.............. any ideas as to what could be causing this?

 

Gaston

Member

• Jan 31, 2007

• #12

if i put a .1ufcap to earth ground it goes away. does this sound correct? ground loop mabey?

 

Sceadwian

Banned

• Jan 31, 2007

• #13

It's called a gainclone because the orignal design was called Gaincard, the parts were about 100 bucks, but they sold them for like 3 grand, and people bought them (audiophiles are a different breed) or at least obsessed with numbers that don't mean anything and willing to shell out a buck or two for those numbers.

 

Gaston

Member

• Jan 31, 2007

• #14

im useing a .1uf but how would you figure the ideal value for this.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 1, 2007

• #15

How can we know?, you've never even shown us the circuit? - but it sounds like you've built it incorrectly!.

 

Gaston

Member

• Feb 1, 2007

• #16

https://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf here is the link to the board and schematic.the only difference is that c5 and c6 are on the power supply board and r1 is 100k. and the pot is on the case not the board

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 1, 2007

• #17

Can you take a picture of how it's assembled?.

How far away are C5 and C6?.

BTW, I would suggest you place C5 and C6 where they should be, close to the chip, they aren't really the main smoothers.

 

Gaston

Member

• Feb 1, 2007

• #18

i can take a pic this evening when i get home. so you think c5 and six could be the problem? i didn't have any caps that could take the voltage. i might have to pull one off of a scrap board if i can find one. what time is it in england right now. im at american central time and it 10:40am. is it nightime over there?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 1, 2007

• #19

Gaston said:

i can take a pic this evening when i get home. so you think c5 and six could be the problem? i didn't have any caps that could take the voltage. i might have to pull one off of a scrap board if i can find one. what time is it in england right now. im at american central time and it 10:40am. is it nightime over there?

Click to expand...

You posted at 4:42PM UK time.

 

Gaston

Member

• Feb 1, 2007

• #20

here is a pic