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Old 13th February 2013, 11:33 AM   #1
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Unhappy 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.
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Old 13th February 2013, 11:55 AM   #2
Minion is offline Minion  Canada
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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 .....


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Old 13th February 2013, 11:59 AM   #3
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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
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Old 13th February 2013, 05:56 PM   #4
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thanks guys makes more sense now
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Old 13th February 2013, 06:14 PM   #5
Mooly is offline Mooly  United Kingdom
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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,
Need help with the NE5543N opamp
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Old 13th February 2013, 11:53 PM   #6
Minion is offline Minion  Canada
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Quote:
Originally Posted by jackinnj View Post
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
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Old 14th February 2013, 12:05 AM   #7
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Do you mean post #71emitter amp help?
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Old 14th February 2013, 06:43 AM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Bradford336 View Post
Do you mean post #71emitter amp help?
Yes, it was just for general examples of how simple an opamp based gain stage can be.
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Old 14th February 2013, 09:39 AM   #9
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ok thanks mooly
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Old 14th February 2013, 10:24 AM   #10
Mooly is offline Mooly  United Kingdom
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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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