I would also be concerned about the normal signal voltage, although such a failure would make matters worse.
Yes, as a very worst-case approximation.
Edit: if both the amplifier and the listener are grounded and the insulation of both sides of the headphone doesn't work, you might get twice the current I calculated.
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yes, basic insulation is part of the normal function. But accounting for abnormal things like faults or damage often requires additional measures. Like 'double insulation' or an earthed shell, for instance.
That's exactly my point
After a large cup of coffee I looked at the schematics posted
Boy am I scared!
"All" of them use differential +/- 300-400 V supplies 😱
No fear of shorting capacitors since none are used; we can say they come "pre-shorted"🥴
No or very small DC at output only because output tubes are balanced which as we know will work fine ... until it fails.
Considering average consumer product non-failure lifespan, amps can practically be guaranteed to fail sometime during User life.
Specially in Tube equipment, where tubes constantly degrade just by being ON, no need for actual "failures" for that.
There's a reason tubes are easily removable without a soldering iron and plug into a socket instead.
Meaning even without "a failure", tubes can easily drift and apply significant DC voltage to output connectors.
Given the deadly voltages involved, I would NOT avoid galvanic isolation, either transformers or capacitors.
Even a 5k series resistor as mentioned will not make output safe.
DC current can be, say, 300V/5k=60mA, way above the established safe level.
Twice as much if failure makes one anode shoot up, the other shoot down.
No practical fuse will protect user.
You like electrostatic Earphones?
So be it, I guess sound must be superb.
Will fuses make them safe by themselves?
Not in my book.
Too crude devices for that delicate and vital job.
As I mentioned above: you CAN design a circuit which detects DC, also too high AC (saving you from deafness) and trigger a relay.
IF relay coil is triggered by an external circuit, applying enough current to coil when a problem is detected by a dedicated circuit, yes, it can open in milliseconds.
But to expect it to be triggered by audio signal itself, with no "help", is marginal at best.
And as was also mentioned above, coil can not differentiate between Audio signal or DC by itself.
How about this as an overall concept to protect users from high-voltage failures:
1. Use pull-down resistors as in the more receng estat amp designs. The resistors will work instantaneously to limit the current in case of a failure.
2. Add a current sensing that triggers a relay to disconnect the output if there is too much (DC) current flowing for too long (say 30 mA for 1 second?).
(1) is done all the time with current estat amps, and it seems good enough for Stax to get a CE certificate.
(2) could be achieved with a sensing coil wound around the wire carrying the output. Or an optocoupler sensing the voltage across the pull-down resistors. Or something else? It should be simple and reliable.
1. Use pull-down resistors as in the more receng estat amp designs. The resistors will work instantaneously to limit the current in case of a failure.
2. Add a current sensing that triggers a relay to disconnect the output if there is too much (DC) current flowing for too long (say 30 mA for 1 second?).
(1) is done all the time with current estat amps, and it seems good enough for Stax to get a CE certificate.
(2) could be achieved with a sensing coil wound around the wire carrying the output. Or an optocoupler sensing the voltage across the pull-down resistors. Or something else? It should be simple and reliable.
I believe that if there is a real risk of dying (just as it's seems) then it's seems we actually are in a cul de sac here.
Mind you, I would love it if some good designer could accommodate the OP's legitimate request with an original and dedicated project, but until now...
Anyway, since nothing is unbreakable not even the components of an eventual circuits to detect anomalies and therefore any solution of this type remains unacceptable. IMHO
Furthermore, even assuming that these hypothetical circuits to detect anomalies were infallible (and they are not) then they would affect the SQ.
This way they would nullify the main reason why you buy an electrostatic headset, the unparalleled SQ.
Hence the already mentioned cul de sac.
Mind you, I would love it if some good designer could accommodate the OP's legitimate request with an original and dedicated project, but until now...
Anyway, since nothing is unbreakable not even the components of an eventual circuits to detect anomalies and therefore any solution of this type remains unacceptable. IMHO
Furthermore, even assuming that these hypothetical circuits to detect anomalies were infallible (and they are not) then they would affect the SQ.
This way they would nullify the main reason why you buy an electrostatic headset, the unparalleled SQ.
Hence the already mentioned cul de sac.