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Bradford336 13th February 2013 11:33 AM

lm3886 help
 
Hey again guys. OK I purchased this amp kit Chip Amplifier Sales, LM3886 Audio Chip Amplifiers Representatives, Sound System Supplies
It has a very clear great sound. The only problem being that with a 35-0-35 vdc supply and a 8ohm speaker the electric guitar is louder without it. I keep reading about how great these chips are and how loud they can be. I just DO NOT SEE IT! I wrote the manufacturer of the kit I purchased and they simply wrote back "preamp needed". I see all of these posts and nobody mentions a NEED to preamp to get an audible sound out of a 8ohm speaker. What am I missing?
Any cheap easy preamp circuits?
My supply power is from a 25-0-25VAC 4A transformer running through a rectifier and buffer circuit. My final output voltage measures about 35-0-35 it has been a few days and cant remember exact number. I do remember it was not the 25X1.41 I was expecting. This is my first project and it is confusing me to death trying to figure this out.

Minion 13th February 2013 11:55 AM

The input of a LM3886 amp expects a line level audio signal of about 0.7v-1v , a passive electric guitar puts out a low level instrument signal of maybe 200mv .....

so yes for a guitar you would need a high impedance preamp to get the instrument level signal up to line level ...... You will also need a preamp for shaping the tone as a guitar without a proper guitar preamp doesn't sound very good .....


Cheers

jackinnj 13th February 2013 11:59 AM

One of the problems you have "preamplifying" a chip-amp is the rail voltages are higher than those which most opamps can sustain. You've got to be a little creative.

You can use an operational amplifier like the OPA2604 in "suspended supply" configuration -- and drive it off the power rails of the LM3886. See Linear Technology Magazine, pages 20-23: http://cds.linear.com/docs/LT%20Jour...04N2_Jun94.pdf

Bradford336 13th February 2013 05:56 PM

thanks guys makes more sense now

Mooly 13th February 2013 06:14 PM

Remember that the supply voltage of the power amp (or any amp) has no relation to the gain which stays the same as the supply is altered.

The preamp can dead simple. Post #35 here should get you started,
http://www.diyaudio.com/forums/analo...ml#post2989964

Minion 13th February 2013 11:53 PM

Quote:

Originally Posted by jackinnj (Post 3368013)
One of the problems you have "preamplifying" a chip-amp is the rail voltages are higher than those which most opamps can sustain. You've got to be a little creative.

You can use an operational amplifier like the OPA2604 in "suspended supply" configuration -- and drive it off the power rails of the LM3886. See Linear Technology Magazine, pages 20-23: http://cds.linear.com/docs/LT%20Jour...04N2_Jun94.pdf

I generally overcome this by running the preamp in a single supply configuration then just use the +V from the power amp for the preamp , it negates having to regulate the PSU for the preamp and many opamps will run on 36v DC ......

Cheers

Bradford336 14th February 2013 12:05 AM

Do you mean post #71emitter amp help?

Mooly 14th February 2013 06:43 AM

Quote:

Originally Posted by Bradford336 (Post 3368985)
Do you mean post #71emitter amp help?

Yes, it was just for general examples of how simple an opamp based gain stage can be.

Bradford336 14th February 2013 09:39 AM

ok thanks mooly

Mooly 14th February 2013 10:24 AM

If your not sure then just ask :)

I'm sure there are plenty of kits available or you could just knock one up on veroboard very quickly and cheaply. The power supply is no problem, just two zeners and two resistors to run it off the LM3886 supply.


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