Parallel LM4780/3886: leave away those 0,2Ohm between the outputs?

[Bassterror]

Hi

On this shematic R11/12.
http://www.audio.webd.pl/pdf/lm4780_sch.pdf

What will happen if I use a piece of wire instead, to improve the amps output impedance.
Are the Chips going to fight?


Greetings,
Stephan

[Bazukaz]

I see no reason not to use the 0,2 ohm resistors. Without them , you are increasing risk of having excessive heating or stability problems of your amplifier.

Regards,
Lukas.

[Bassterror]

I'm going to try it when my boards arrive.

If anybody did it before, please let me know.

[pinkmouse]

Originally posted by Bassterror
I'm going to try it when my boards arrive.

If you're running it as two separate channels, you will most likely be ok, but if you're running it parallel, then buy some spare chips for when the ones you have go bang...

[Bassterror]

If you're running it as two separate channels, you will most likely be ok, but if you're running it parallel, then buy some spare chips for when the ones you have go bang...

What makes you think they bang?
Have you already tried?

[jackinnj]

Yes, they will go bang, or rather, a loud POP !

[paulb]

Originally posted by Bassterror


What makes you think they bang?
Have you already tried?

I think you would like an explanation. Even a very small difference in the voltage offset at the output of the amplifiers will result in a very large current running between them. The resistors help the amplifiers to share the load current.
Try it with the resistors and measure the voltage difference right between the output of the two amplifiers. It won't be zero. If you hook it up without the resistors it will certainly draw a lot of current (with protection circuitry I doubt it will go bang, but I wouldn't risk it).

[Bassterror]

Even a very small difference in the voltage offset at the output of the amplifiers will result in a very large current running between them. The resistors help the amplifiers to share the load current.

Yes, i'm aware of that.
I supposed the difference might be small enough, perhaps by using selected rersistors...

If you prevent the amp from delivering high current not just into each other, but also into the load, how can you ensure proper control over woofers movements?

In that case, i whould say that paralleling sucks, do u agree?

[macboy]

Originally posted by Bassterror
Hi

On this shematic R11/12.
http://www.audio.webd.pl/pdf/lm4780_sch.pdf

What will happen if I use a piece of wire instead, to improve the amps output impedance.
Are the Chips going to fight?


Greetings,
Stephan

Originally posted by Bassterror


Yes, i'm aware of that.
I supposed the difference might be small enough, perhaps by using selected rersistors...

If you prevent the amp from delivering high current not just into each other, but also into the load, how can you ensure proper control over woofers movements?

In that case, i whould say that paralleling sucks, do u agree?

No, the difference will never get "low enough" even if you carefully hand-pick the resistors. Each amp has negative feedback to ensure that it drives the output to the voltage that it thinks it should be. The open-loop gain is very high, and that means that the chips have a lot of ability to "correct" the output. Even if there is just a mV or two difference, each chip will fight the other to try to push the output voltage (which is now common, due to no resistors) its way, resulting in huge currents and, probably, destroyed chips. The resistors allow the output voltages to differ slightly. You need them.

If you want to have near-zero output impedance, then put the power amps (3886's) inside of the feedback loop of an opamp driver circuit. Because they are inside of the feeback loop, their output impedance is effectively null. Like the attached circuit. One note: the amps inside the loop must be faster than the outer loop. If necessary, you need to slow down the outer loop (add a cap in parallel with the feedback resistor, forming a LPF) to ensure this is the case. Otherwise the outer loop's opamp will oscillate.

[Bassterror]

Thanks for that suggestion macboy. I'm going to try it. Still waiting for the LMs...