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I'm trying to design a circuit in which a microcontroller produces a signal and then using an amplifier drives an 8ohm speaker. I've so far used LM386, but it cannot go above 1W, which turned out to be insufficient. Besides that, I wish to add an (second order) anti aliasing LPF at the output of the microcontroller.

The general schematic is below.

If I wish to feed the speaker with 2W, then I would need 4v, 500mA on the speaker. Is it possible for me to do it this way, or is there a better circuit for my needs? Is there an op amp that can comply with these requirements?

enter image description here

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    \$\begingroup\$ While there may be some high power output opamps available, it may be easier and cheaper to use a transistor buffer stage with the LM386 that you already have. The transistor can be included in the feedback loop of a voltage follower opamp to improve its output fidelity. \$\endgroup\$ – Chintalagiri Shashank Jan 27 '13 at 13:36
  • \$\begingroup\$ What level of audio quality do you need? \$\endgroup\$ – Phil Frost Jan 27 '13 at 14:00
  • \$\begingroup\$ @Phill Frost, Since the main goal is DTMF (in high frequencies, around 17kHz) I need very low harmonic distortion. that's pretty much the only requirement I guess... \$\endgroup\$ – Daniel Jan 27 '13 at 14:33
  • \$\begingroup\$ DTMF at 2W? It would be interesting to know what the intended usage model is. \$\endgroup\$ – Michael Karas Jan 27 '13 at 17:36
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There are several power op amps, and also audio amplifier ICs, which would meet the stated requirements. However, this approach may be overkill for the kind of fidelity likely from the signal generation source.

As mentioned in a comment to the question, adding a transistor buffer stage is perhaps the best way to achieve the desired results.

Also, do note that a dual-rail (+/- x Volts) supply at a relatively high voltage is needed for these op-amps / chip-amps. While the dual-rail requirement can be circumvented with a suitable virtual ground implementation, voltages involved are still not your run-of-the-mill logic voltage range.

In any case, to answer the question...

Some suitable Op Amps:

  • PA85: High voltage power OpAmp - Obsolete, I think, but still sold. This used to be a classic.
  • OPA541: High Power Monolithic OpAmp
  • OPA549: High-Voltage, High-Current OpAmp
  • OPA2544: High-Voltage, High-Current dual OpAmp
  • NE5532: Low noise dual OpAmp

Some suitable Audio Amps:

  • LM3886: 68 W continuous average output power into 4Ω at VCC = ±28V
  • LM3875: 56 W continuous average into 8Ω
  • LM4780: Stereo 60W, Mono 120W (parallel-able) Audio Power Amplifier
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To expand on the comment about a "transistor buffer stage", here is one way to do it:

class B buffer

This is a class B output stage and will introduce some distortion as it switches from sourcing current to sinking current, but it's sufficient for many needs. The current demand on the op-amp is divided by the transistor's current gain \$h_{fe}\$, and at your power levels there's probably not a general purpose op-amp or power transistor that wouldn't work here.

This schematic is from OPERATIONAL AMPLIFIER BASICS by Harry Lythall which has a section on audio power amplifiers with some additional designs for different needs, lower distortion, etc.

There is also a great amount of audio circuit information at Elliott Sound Products.

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I have actually just recently built a small prototype around a dsPIC33FJ64GP802 that does exactly what you are looking to do.

I used a MCP6021 opamp (not an official "audiophile" opamp but it has a THD quoted as 0.00053% (typical, G = 1 V/V) rail to rail in/out and it's a pretty good all round part - it also has a mid-supply reference which is handy for single rail apps) driven differentially from the dsPICs DAC, and a TDA1517P 2 x 6W Amplifier IC to drive a smallish 8 ohm speaker (this is the weak link THD-wise - you may want to use a discrete transistor stage after the opamp, or a better quality audio IC)
Results were very good for my purposes, here's a picture (the board has since been hacked for other purposes but the three main components remain as shown):

dsPIC Audio proto

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  • \$\begingroup\$ I like that the datasheet claims "No switch-on/switch-off plop." Is "plop" the preferred technical nomenclature? :-) \$\endgroup\$ – Phil Frost Jan 27 '13 at 15:01
  • \$\begingroup\$ Hehe, I take it you are referring to the "excellent" TDA1517P datasheet - I had a chuckle at that too. I think they are correct though, I cannot recall hearing any on/off "plops" ;-) \$\endgroup\$ – Oli Glaser Jan 27 '13 at 15:03

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