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Old 4th November 2003, 01:48 AM   #1
pundip is offline pundip  Australia
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Angry Powerfull preamp needed for lm3886

I have just finished construction the lm3886 in non inverting mode. My problem is that I cannot get enough output from it by using the standard audio sources. The amp is operating at gain of 20 (Rin = 1k Rf = 20k) 47k from +in to gnd. My computer speaker headphone jack can generate 4v ac . I used this source to feed the amp with 80hz sine and managed to get 27vac on the outputs. I need a preamp that can generate this level of signal. I used the TL072 with the same gain of 20 (Rin = 1k Rf = 20k) 47k from +in to gnd (power supply is -+15vdc). I am having the same issue with the output too weak. I halfed the Rin to double gain and no change at all. Can standard op amp like the TL072, LM6172 etc generate this level and if so what circuit should I be using. I cannot mess with the lm3886 modules because of the way they were constructed. Any advise or suggestion welcome.
Thanks in advance.
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Old 4th November 2003, 02:03 AM   #2
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Your story doesn't sound right. If your opamp stage really has again of x20 you'd be having no problem saturating the 3886 output. You should probably check your wiring.
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Old 4th November 2003, 02:05 AM   #3
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odd, are you sure you have everything setup correctly. you should only need a gain of less then 10, and you have a gain of 400...

an opamp with a high GBP and slew-rate would be needed for high-gain at 20khz. for 80hz, i can't see where the problem is.

are you useing inverting or non-inverting amplifers? if your using a non-inverting amplifer you may have the resistors backwards. giving gains of 1.1 and 1.01 on the preamp. this seems the most likely canidate for error IMO.
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Old 4th November 2003, 02:34 AM   #4
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Default here is the circuit

here is the circuit used. I bypassed the Rin with no audiable effect whatsoever. A am using a protable minidisc as source at the moment.
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Old 4th November 2003, 02:38 AM   #5
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So you don't get any gain.
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Old 4th November 2003, 04:17 AM   #6
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you need 1 more resistor, from inverting input to ground. also, the 1k resistor is uneeded unless otherwised noted.

that circuit actaully attenuates the signal! look, opamp analysis is easy when you realize that the output of the amplifer is such that both input terminals are equal in voltage. for this, the input to non-inverting is k*Vi (where k < 1). so if the output is k*Vi, the inverting terminal will be at k*Vi as well. if you add a resistor from inverting terminal to ground, you make a voltage devider. the output must now be larger so that the voltage at the inverting terminal will be the same, and thus you get gain.
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Old 4th November 2003, 04:20 AM   #7
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Quote:
Originally posted by theChris
you need 1 more resistor, from inverting input to ground. also, the 1k resistor is uneeded unless otherwised noted.
Maybe that's where the 1k should go.
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Old 4th November 2003, 04:23 AM   #8
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yeah, that would give a gain of 21.
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Old 4th November 2003, 07:46 PM   #9
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Default thanks

adding the 1k resistor from -in to gnd has increased gain but... I am getting a lot of hum his and noise. The sound quality also suffers a bit. THe powersupply is using 7815,7915 regulators. The whole thing is not sheilded at the moment. Could it be becuase the gain is too high? Or is it a grounding problem. Also I am told that you can make a lin 100k pot into a log volume control. Does anyone know how to do this.
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Old 12th November 2003, 02:10 AM   #10
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Heres the volume control circuit. Connect + out to the capacitor and - out to the - circuit.

LM7815 and 7915 regulators are not a good choice. They can only deliver 1A max, limiting the output power to 4W. Try using no regulator, the 3886 doesn't need one.

Take a look at the datasheet for the LM3886 at www.national.com. They suggest a ciruit on the first page on a typical application. That circuit is the one I used to build my first LM3886 amp, which now powers a subwoofer. Ignore the switch and replace it with a direct connection unless you need mute function. Also ignore the caps on the power supply pins, since your PS should have them on it, right? If you need any other resistor or cap values then post them here. Make sure you calculate the resistor that goes from - supply to the mute pin, to make sure the mute isn't stuck on or so you don't fry your amp. If you decide to remove your regulators, then post the exact voltage output here and i can calculate a resistor for you. Otherwise, let me know and I can post the values for the +-15V supply.

The noise is often caused by either a bad solder connection or the power supply wires being too close to the input wires. Is your amp built onto a board, or does it use p2p wiring? Is the power supply built onto the board? Always twist power supply wires together. Some pictures would be helpful. Is the heatsink touching anything or is any other chips attached to it? finally, another cause could be too long of a wire from the minidisc to the amp. How long is the wire and how is it connected?

Hope this helps, Mike
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