The real question is what frequency should a 6dB or 12dB filter be set, what time constant will protect your speakers but not react to bass. The threshold voltage is usually one diode junction or perhaps two (0.65 or 1.3V), ie whatever is handy given the input circuit. Any DC is a problem but a couple volts is tolerable. No one bothers with a precision voltage reference and comparators. That would just be wasteful when 3 or 4 transistors does a great job. There is a chip that integrates the usual circuit, ie uses the natural diode threshold voltage of the transistors.
The DC offset is the average voltage over a reasonable period of time, longer than one wave at the lowest frequency you expect.
Since most failure modes will peg the output at a large offset or all the way to a rail, triggering an RC type circuit quickly, the exact threshold shouldn't matter too much (as others have commented)
Since most failure modes will peg the output at a large offset or all the way to a rail, triggering an RC type circuit quickly, the exact threshold shouldn't matter too much (as others have commented)
So lets do some crude math. If your amp has a 50V rail and you will allow 10Hz at that voltage, your filter has to attenuate 10Hz down to the protection threshold, ie a bit less than 100 to one. That implies that from a 1Hz filter, we need 1/f^2 or a two stage filter. We could use a single stage filter, ie 1/10 per decade from 0.1Hz but that would be a slow 10 second protection. We could raise the allowable frequency to 20Hz, or raise the threshold to 5V, etc. The threshold can be raised with a DC attenuator. I like the idea of a higher threshold but I would keep the two stage filter at no less than 1Hz.
D. Self has a good section on RC filters in his audio book. He shows how the single stage filter works better than a two stage filter. It's quite easy to sim. In the very first ms, the single stage rises faster to the 1 or 2V needed to trigger a detector (when faced with the output going to rail).
This article on esp audio is quite good too (and free) : Loudspeaker Protection and Muting
This article on esp audio is quite good too (and free) : Loudspeaker Protection and Muting
I use about 4 volts DC plus or minus as the threshold over 250mS.
If it stays high or low longer than that protection cuts in.
Worked on my test rig for about 10 years with no problems.
It based around an 8 pin PIC microcontroller and a solid state relay.
The SSR is opto coupled.
The PIC also gives 4 second power up delay.
One of my later models also does mains failure detect.
My all singing, all dancing unit also does over current detect.
If it stays high or low longer than that protection cuts in.
Worked on my test rig for about 10 years with no problems.
It based around an 8 pin PIC microcontroller and a solid state relay.
The SSR is opto coupled.
The PIC also gives 4 second power up delay.
One of my later models also does mains failure detect.
My all singing, all dancing unit also does over current detect.
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