Dc protection with gainclone?

[edpgc]

Hi I've got a LM4780 based amp and have implemented some relay driven DC protection for the speakers, however is it necessary? What failure modes are there for the LM4780 or similar that could cause it to output a DC dignal? How good is its built in protection?

If one of the rails potential was lost, even partially would it protect the speakers?

Im just wondering whether I should take it out or leave it there!?

Cheers

Ed

[Peter Daniel]

I'm not sure about 4780, but LM3875 has a protection against one rail being off.

When one rail fails, there is no DC on the output. You need to have a load present to observe it (high power resistor is fine for testing).

You might want to protect the input, from DC being passed from a previous stage, especially with minimized circuits when the amp hasn't unity gain at DC.

A simple coupling cap would do a job, and basically output protection circuit is not a neccessity.

[edpgc]

Hmm, well that was a wate of a few hours using spice. lol So is there any scenario whereby output protection would be necessary? I suspect only the failure of the chip itself in some rather specific way.

Thanks

[yldouright]

edpgc

As I understand it, the only failure danger with this chip is if it starts oscillating and even that is mitigated when you don't invert it but I'm not the authority on this chip here.

[analog_sa]

So is there any scenario whereby output protection would be necessary? I suspect only the failure of the chip itself in some rather specific way.

There is no doubt that a complete meltdown of the chip may result DC at output. It doesn't seem to happen often and most of us prefer to not have a relay along the signal line but if you want 100% protection a relay seems the only option

[yldouright]

ananlog_sa

I think I read in another thread that you run a battery gainclone. If this is so, which circuit did you use? I'm planning on gutting an old Kenwood integrated to put in a gainclone, DAC and phono stage to run on 4 12V/7Ah SLA batteries and could definitely use your help and experience in this adventure if you have any.

[analog_sa]

run a battery gainclone

No. I only tried batteries briefly. PD is your guy.

[moving_electron]

Originally posted by analog_sa


There is no doubt that a complete meltdown of the chip may result DC at output. It doesn't seem to happen often and most of us prefer to not have a relay along the signal line but if you want 100% protection a relay seems the only option

Fuses are an alternative.

I use fast blow fuses in series with the speaker +, especially during amp testing. I use 3/4 - 1 amps worth of fuse for every 25W of speaker continuous RMS rating.

Saved me when I did a bonehead thing related to making a change in an amp. It was not a DC problem but would have been a "to much output" problem for my speakers. I was glad to have the protection.

[usekgb]

I have seen quite a number of LM3886 chips fail, and pass 40 volts DC to the outputs, frying the compression driver the chip was powering. This happened to the first generation of the JBL EON G2 powered PA speakers. Basically, the chip got too hot, failed, and started passing DC. I believe this was happening because JBL was just trying to get too much power out of the 3886. In the next generation of amp boards, they switched to the TDA7293, and the problem was solved.

So, to make a long story short, these chips can fail and pass DC. But this will only occur in extreme circumstances.

Cheers,
Zach

[Nuuk]

If you want to 'play safe' have a look at these .

As already stated, in theory it is not a good idea to have a relay in the speaker line but in practice, it hardly notices at all and the peace of mind it gives you may even help you enjoy the music more.