Speaker Protection

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chris661
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#16 Re: Speaker Protection

Post by chris661 »

I was thinking of going for a hard voltage limit instead, by using a long chain of diodes which would conduct at, say, 50V. That way, any voltage exceeding 50V at the output terminals would be pulled down. Might be worth adding a couple of ohms somewhere to keep the caps and diodes happy.

It's crude, I admit, but it'd work near-instantaneously and reliably to prevent the PSU caps discharging directly into the speaker by providing a short until the caps have less than 50v in them. At that point, I don't think there'd be much charge left and the speaker alone could safely dissipate the rest.

Combine with a fuse in the PSU, and I feel like this would catch any fault condition that might pose a threat to the speakers.

Chris
Wolfgang
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#17 Re: Speaker Protection

Post by Wolfgang »

If B+ was connected to the speaker, the current would depend entirely on the output impedance of the power supply.

This happened not with a HV psu but with the SLB (active bridge rectifier, cap multiplier)at 38V which can deliver 5A max /2x15kuF for the dual rail version. The SLB has a very low output impedance.
Was there a post-mortem on your Lowther driver?
The driver was still working but made a scratchy sound. The nice people at Lowther's said the voice-coil needed to be replaced.


I was thinking of going for a hard voltage limit instead, by using a long chain of diodes which would conduct at, say, 50V. That way, any voltage exceeding 50V at the output terminals would be pulled down. Might be worth adding a couple of ohms somewhere to keep the caps and diodes happy.
That's basically what BR does in the Pinnacle. TVS diode, output to ground (5KP20CA/breakdown voltage between 22-24V), with additional fuse in the psu. I used originally a TVS diode but took it out again - unfortunately before the short happened -because it really dulled the sound of the InvOTL.
What would happen mechanically would be interesting -
The cone moved an inch or so into its final position accompanied by an ugly noise. Part of the voice coil was burnt but still working.
I don't even think the typical protection circuits used for PA amplifiers would catch a signal that might go from 0 to 300+V almost instantly.
The xrk DC protection circuit has a 25ms reaction time at 48VDC which the Lowther would have survived.
B.Rosenblit was asked this question about speaker protection and OTLs in one of the rare interviews he gave to audio magazines very early on and he replied that it's all about how much the voltage can rise before the fuse would disconnect the B+ psu. It's not about how high the voltage in the psu is but how high it can rise before it's disconnected. So people shouldn't think in terms of the full B+ psu which can be some hundreds of volts but in terms of maybe 20V that could hit their speakers for a very short time.
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Nick
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#18 Re: Speaker Protection

Post by Nick »

So people shouldn't think in terms of the full B+ psu which can be some hundreds of volts but in terms of maybe 20V that could hit their speakers for a very short time.
Ok, but what if I want to listen to a 25W 20Hz signal (just to be awkward), the peak value of that every 25ms is 20v.

I guess things become worst with no crossover in the way.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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