80W @ 6 ohm

Anarion

Member
2008-02-13 10:44 pm
Hi there, I'm new here. I want to make myself an amplifier with the LM3886(TF), the thing is that i own a pair of loud-speakers (Aiwa SK-810) from an old stereo.
I was wondering should i make any kind of modification to get 80W @ 6 ohm?
Or what is the max power i could get from this chip??
What PSU should I use?



Well, that is the first thing i wanted to discuss. The second is that I was willing to do some kind of "logic" input so i could use de switch "mute" to toggle it from the computer. I want to use something really simple like a PAL or a GAL, i know how to make it work in paper, but i don't know how to program them! Any help here??

Thanks in advance to you all.


Anarion
 
Dear Anarion,

You can get ~83W into 6R load with LM3886T.. You'll need 2°C/W heatsinks for each chip. The power supply must be 35v symmetrical (35+35) at least. And the current need is 5.25A for each rail. That means you will use 26+26 v secondary 275VA transformer, seperated rectifier bridges and 20,000uF reservoir capacitors for each rail.
And if u use Rf=20K, Ri=680R then your input sensivity will be ~1v RMS for 83W output...

PS: With LM3886TF you must use 1°C/W heatsink which is really big one. And even with this heatsink, chip will be still very hot!
 
Concerning PALs and GALs I can give you support. If you're looking for programming them you need to look for a "PAL/GAL programmer" (i.e. google for it). Many (E)EPROM programmer are also capable of burning those devices. So watch out what they are able to program and what not.

In addition to the hardware you need a PAL/GAL assembler to get the JEDEC-file which a programmer definately needs to program the device successful.

For a burner hardware you might try this one: http://elm-chan.org/works/pgal/report_e.html

For the PAL/GAL assembler running under windows I have no clue since I got my ATARI ST/TT/Falcon for that purpose with a special build hardware and a matching assembler.

Good luck.
 
Anarion said:
Hi there, I'm new here. I want to make myself an amplifier with the LM3886(TF), the thing is that i own a pair of loud-speakers (Aiwa SK-810) from an old stereo.
I was wondering should i make any kind of modification to get 80W @ 6 ohm?
Or what is the max power i could get from this chip??
What PSU should I use?



Well, that is the first thing i wanted to discuss. The second is that I was willing to do some kind of "logic" input so i could use de switch "mute" to toggle it from the computer. I want to use something really simple like a PAL or a GAL, i know how to make it work in paper, but i don't know how to program them! Any help here??

Thanks in advance to you all.


Anarion

What kind of sound card?
Howabout a multimedia keyboard with a handy mute button?
Or what about a remote control pot kit with the handy 4 source inbuilt selector?
The LM3886T(F) could blow those speakers to bits, so what about using the LM3875TF? The 56w amp runs cooler, and there's a few other things about it that makes it easier, in my opinion.
 

Anarion

Member
2008-02-13 10:44 pm
Re: Re: 80W @ 6 ohm

danielwritesbac said:


What kind of sound card?
Howabout a multimedia keyboard with a handy mute button?
Or what about a remote control pot kit with the handy 4 source inbuilt selector?

There is a HUGE difference between a "software" and a "hardware" mute. First of all because if you turn off the computer before the amp. you would hear a very loud and horrible sound, and second it would work as small "on/off" since you can make some code run in the init script of your linux distro to turn the amp. on when the sistem starts and off before the system powers off.


The LM3886T(F) could blow those speakers to bits, so what about using the LM3875TF? The 56w amp runs cooler, and there's a few other things about it that makes it easier, in my opinion.

Then what's the diference between LM3886's 56W and LM3875's 56W? because i don't get it
 
The LM3886 can make about 80 watts while the LM3875 can make about 56 watts. This varies depending on voltage of supply and load. There are more examples for voicing LM3875 to personal taste (more examples = easier), and since its slightly less powerful, the sound is cleaner (in proportion).

See the documents at National Semiconductor's website, and also see their free application spreadsheet.

At end result, the difference in decibel output at the speakers isn't noticable.

However, with less power hitting the speakers, the monstor thump (never heard it yet) isn't really a concern.

Also if your amplifier has a potentiometer or the fixed version (a voltage divider/fixed lpad), then the load at input will attract most of the "thump" so. . . so there's little reason to create a solution for a problem that need not exist. ;)

You can also use a dac, if you want, so you can control the quality of the output from the computer (instead of running your audio through the cheapest possible op amps inside the computer). There's a problem that's worth some effort.

Anyway, the thump, if any, is well below what a bit of dance music can do to a speaker. So, perhaps this is why I never worry about a satisfying little (tiny) thump that pales in comparison to one of my exuberant moods. ;)

A turn-on thump is possible with no load at amplifier input; however, that's quickly solved with a resistor or pot.
A motor pot is especially effective, because of the remote control feature, and anything running through it will only be at the loudness of its last use.

There's plenty of things to worry about, like quality of source, dc blocking, dc bias, impedance matching, cooling, voltage, power. . . that all need to come before adding complexities to the amplifier.
So, I'd suggest to start at the starting place. ;)

EDIT: And LM3875 makes more power for a given amount of input. This is quite helpful if running directly from a computer sound card source.
 
danielwritesbac said:


EDIT: And LM3875 makes more power for a given amount of input. This is quite helpful if running directly from a computer sound card source.


Dude , that doesn't make sense!!!?????

simple fact is ,the lm3886 is designed to be able to handle more current , since your speakers are 6 ohms.

I would get between 22Vx 2 to 25V x 2 220VA transformer.
2 x lm3886
get nice big heatsinks.

Experiment, look at the many photos in the picture thread.

For a pc sound card i stuck to the "standard" national application circuit on their lm3886 pdf.

use about about 10000uF filtering caps per voltage rail.

And after that experiment with different components , cap sizes , input caps.

Many people seems to prefer the sound of the lm chips with far less power supply filter caps.

don't worry too much about wattage.

You will enjoy the amp.
 
tangmonster said:



Dude , that doesn't make sense!!!?????

simple fact is ,the lm3886 is designed to be able to handle more current , since your speakers are 6 ohms.

I would get between 22Vx 2 to 25V x 2 220VA transformer.
2 x lm3886
get nice big heatsinks.

Experiment, look at the many photos in the picture thread.

For a pc sound card i stuck to the "standard" national application circuit on their lm3886 pdf.

use about about 10000uF filtering caps per voltage rail.

And after that experiment with different components , cap sizes , input caps.

Many people seems to prefer the sound of the lm chips with far less power supply filter caps.

don't worry too much about wattage.

You will enjoy the amp.

For 2 x LM3886 (you mean stereo) and with 22 to 25v AC (that makes 30 to 34v after rectifying) 220VA transformer power will not be enough absolutely!

I made a calculation above! Pls have a look at my posting.
 
Hi,
getting 60W into 8r0 is easy. It needs 31Vpk and 3.9Apk into the load.
Both are easily achievable with the 3886T or TF.

Substitute a reactive speaker load with typical severe loading and the Vpk remains the same, but current increases enormously.
Ipk<=Vpk/R/0.35~=11Apk.
That level of transient current will probably not pass the Spike protection, if the chip is hotter than 25degC.
You are virtually guaranteed to trigger the protection on high level transients, if you try to use the full resistive load ability as the design criteria for a power amplifier.
Using low impedance speakers makes this even worse.
Using very efficient speakers avoids the need to approach Vpk and thus keeps away from the triggering levels of Spike if you keep the chip cool enough. That's part of the problem with the TF version.
 
Re: 80W

tiltedhalo said:
Andrew T is 100% correct to achieve 80W into a 6 ohm load will put the chip into spike period the dissipation is to high given the supply voltage and current needed, facts are facts.
Simple bridge two 3886's at a SAO voltage and you have your 80W and a little head room.
Hi Tilted,
thanks for the undeserved compliment.
But, I'm going to disagree with your recommendation.
A bridged pair of chipamps supplying 80W into 6ohms is just the same as a single amp supplying 40W into 3ohms.
Now let's look at the voltages and currents needed for that reactive severe speaker load.
Vpk=15.5V
Ipk(resistive)=5.2Apk
Ipk(reactive)<=15Apk.

This result is even worse than the 60W into 8ohm example I gave earlier.
Using low impedance speakers makes this even worse.