LM3886 parallel output resistors

[jmar]

Been in electronics a long time actually but sometimes my memory fades on certain items in circuit design (what design? I write software these days) -- and something that I need a refresher on is the purpose of the small value power resistors on the outputs of these amplifiers when running in parallel.

Some sort of isolation? (but what?) can't be AC and at one tenth of an ohm wouldn't have much effect on DC either. ?

Is it a protection circuit? If an output is shorted to ground does it initiate some sort of internal shutdown and at the same time provide a buffer or something?

What is going on with these? --and thanks

[russo]

I think they are load sharing resistors, to balance te current that each lm3886 puts out.

[jmar]

that makes sense -- to help stabilize the load I suppose.

[abraxalito]

Originally Posted by jmar

that makes sense -- to help stabilize the load I suppose.

The resistors are there because the amplifiers are in effect pure voltage sources at their outputs. Their output impedance is very low - of the order of milliohms.

If you connect two voltage sources together and their voltages are even slightly different, large currents will flow between them. So the resistors are there to try to limit the currents that flow between the amps because ideally we want all the current to flow into the speaker, not into the other paralleled amplifier.

Any clearer now?

[jmar]

Sort of the "path of least resistance thing". If one source is lower, lookout.

But because the voltage difference between the 2 is inevitably not that much,
only a tenth of an ohm will suffice.

As a buffer really.

Right? (close enough?)

[abraxalito]

Think of the two amps without any speaker connected - maybe that will help. If there's a gain mismatch, one amp 'sees' a load of 0.2ohm into the other one. That 0.2ohm load is more than an order of magnitude lower than the speaker impedance - even a 1% gain mismatch at an output voltage of 30V is 300mV - giving 1.5A into the lower gain amplifier.

[jmar]

Gotcha.

You know my head has been out of analog audio for some time and playing around with the chip amps will be a good tutorial.

I'll get out the test gear and get this module aligned better for parallel operation.

Funny, I haven't even HEARD an LM3886 before -- bought this module for the convenience of quick audio and value. thanks.

[AndrewT]

Originally Posted by jmar

Funny, I haven't even HEARD an LM3886 before --

you may have "heard" this or one of it's brethern in a TV or table top radio or in computer speakers or a multitude of other worse than useless implementations.

[jmar]

Now you tell me.

So, the LM3886 on a scale of 1 - 10?

Are you talking the about the chip itself or as you say, the implementation?

Lousy speakers etc.

[abraxalito]

I'll put in my tuppence worth here. Its mostly about implementation, not the chip itself.

I started playing with the LM3886 shortly after RS components introduced it into its UK catalogue. I don't recall exactly when this was, but sometime in the mid 1990s. First up, I built the application circuit straight out of the datasheet and found it sounded terrible - a really, really harsh top end.

It took me a while to figure out why the sound was so lousy - in those early days, the recommended application circuit didn't look like fig1 in the current (dated 2003) datasheet, it looked more like the AC test circuit shown on p5. Notice that circuit has a 220pF between the inputs of the chip. I found when I removed that capacitor, amazingly all that harshness at the top end disappeared. From then on, I was hooked on chip amps.

The only problem I found with the sound of the LM3886 was when it ran into clipping - the protection circuits would activate and the waveform looked a mess. It also didn't sound great. But keep the beast out of clipping and it sounded fine. In the end I went for the TDA7294 in my final design because this part didn't have the messy clipping behaviour I found in the LM3886.

The greatest advantage of chip amps I found is the ease of building an active speaker. The sound quality improvement from ditching the passive crossover and putting the amps right up next to the drive units has to be heard to be believed. So much so that I pretty much have given up listening to passive speakers now. An active speaker driven by LM3886s to me has always sounded better than the same speaker with a passive XO driven by an amplifier of any hi-end credentials, provided the implementation is done right and its not allowed to clip.