Fuses are mostly used in mains lines or in power supplies. Why are they hardly used in the audio output? Do they affect sound quality? Do they introduce distortion of the audio? How?
I am asking because I was thinking of using fuses in the output of an electrostatic headphone amplifier, which uses several hundred volts at the audio output. You don't want accidents with high voltage pushing unhealthy amounts of current through the listener.
I am asking because I was thinking of using fuses in the output of an electrostatic headphone amplifier, which uses several hundred volts at the audio output. You don't want accidents with high voltage pushing unhealthy amounts of current through the listener.
'Several hundred volts' might be over the voltage limit of many fuses.
The problem with fuses for audio are that they introduce a small resistance in series with the load and that resistance is non linear, it varies with temperature and that means it varies in response to the current passing through it. So it can add amplitude distortion rather than harmonic distortion.
It is current that does you harm rather than just voltage and the current needed to kill you is quite small, just a few milliamps in some cases but to get that current to flow through your body requires a high (ish) voltage. How high depends on how conductive you are. Even just a few tens of volts between damp hands will make you think twice about getting hold again.
You would need fuses of such small a current value and also with a high voltage rating that I don't really think fuses would be suitable for what you describe.
The problem with fuses for audio are that they introduce a small resistance in series with the load and that resistance is non linear, it varies with temperature and that means it varies in response to the current passing through it. So it can add amplitude distortion rather than harmonic distortion.
It is current that does you harm rather than just voltage and the current needed to kill you is quite small, just a few milliamps in some cases but to get that current to flow through your body requires a high (ish) voltage. How high depends on how conductive you are. Even just a few tens of volts between damp hands will make you think twice about getting hold again.
You would need fuses of such small a current value and also with a high voltage rating that I don't really think fuses would be suitable for what you describe.
It will most likely create more problems than it solves. You’ve got to monitor the current which will put more parts in the signal path that can hurt the sound quality. In most cases, simple is better when it comes to audio. Every component adds or removes something from the sound that really shouldn’t be there. I use parts that are very overrated for what they will see so they are much less likely to fail. In general, resistors are cheap and often the case of failure so I use at least 1watt units where a half or quarter watt will do. Use well made transformers that are capable of double the needed current and wattage. The PT will run much cooler that way and won’t sag in voltage under full power. Just build it to last and you’ll never be wishing for more. All you’ll need is an input slow blow fuse if done right.
Hmm, not sure I understand this. Or maybe mg question was not clear.
The amp runs with a bipolar PSU (B+ and B-), and it has a bipolar audio output to the stators of the electrostatic headphone. The impedance of the headphone is approx. equivalent to a 100 pF capacitor. The fuse(es) would be inserted between the amp output and the headphone.
During normal operation, the peak current into each stator would be about 10 mA or less. With an assumed fuse resistance of 20 Ohm, the peak voltage across the fuse would be 0.2 V.
If something goes wrong inside the amp, one (or both) output(s) might get shorted to B+ or B-. There will be a very short current pulse to charge the 100 pF capacitance, but the fuse will likely not blow.
If at the same time something else goes wrong outside of the amp (e.g. in the the headphone), the B+ or B- might get shorted to ground. This would cause a DC current running through the fuse, so the fuse would break and disconnect the faulty amp output, helping to prevent bad things.
What's wrong with using fuses in this way (both in terms of safety and audio performace)?
The amp runs with a bipolar PSU (B+ and B-), and it has a bipolar audio output to the stators of the electrostatic headphone. The impedance of the headphone is approx. equivalent to a 100 pF capacitor. The fuse(es) would be inserted between the amp output and the headphone.
During normal operation, the peak current into each stator would be about 10 mA or less. With an assumed fuse resistance of 20 Ohm, the peak voltage across the fuse would be 0.2 V.
If something goes wrong inside the amp, one (or both) output(s) might get shorted to B+ or B-. There will be a very short current pulse to charge the 100 pF capacitance, but the fuse will likely not blow.
If at the same time something else goes wrong outside of the amp (e.g. in the the headphone), the B+ or B- might get shorted to ground. This would cause a DC current running through the fuse, so the fuse would break and disconnect the faulty amp output, helping to prevent bad things.
What's wrong with using fuses in this way (both in terms of safety and audio performace)?
You mentioned in post #1 of having 'several hundred volts' at the output. Fuses have voltage limits as well as the breaking current. Would that voltage limit be exceeded if the fuse ruptured?
So a really low current fuse might do some good in that scenario from preventing a rail short but you still have the non linear resistance aspect. A 50ma fuse has a volt drop of about 1 volt at its rated current. Only you can decide if that is going to be an issue.
In principle output fuses are poor protectors, that's why they stopped being used.
1) if too small, they will nuisance blow very often and drive you crazy (don't ask)
If too large they will always blow too late and protect nothing.
Meaning they will reasonably protect the power supply.
The amplifier power devices? ... not so much.
A fuse subject to, say, 100% overload, will take a couple seconds to blow open.
It is in the datasheets and you can check that yourself.
https://en.m.wikipedia.org/wiki/Fuse_(electrical)
From Littelfuse themselves:
Semiconductors, specially Bipolars who show second breakdown, will die in milliseconds.
Here we have a nice powerful 16A 220V 170W power transistor, which might be a good choice for, say, a 60 to 100W amplifier.
Thermal damage looks like we expect, meaning we expect it to fail if dissipation is exceeded but not before.
BUT that holds only up to meager 50V 😲
Good enough for, say, a 25-40W amplifier 😫
How come?
Notice acceptable power dissipation curve holds well ... up to 50V that is, and then bends down catastrophically.
At 50V it stands 3.5A for a dissipation of about 175W 👍🏻
At maximum 120Vce it "should* stand 175/120=1.5 A.
BUT graph shows it stands only pitiful 0.2A 😲😫
No reasonable fuse will protect such a weakling
Couple seconds delay will protect power Transformers, mains wiring, electrical generators, stalled electric motors, etc. , all of which:
Again shown in datasheets.
1) if too small, they will nuisance blow very often and drive you crazy (don't ask)
If too large they will always blow too late and protect nothing.
Meaning they will reasonably protect the power supply.
The amplifier power devices? ... not so much.
A fuse subject to, say, 100% overload, will take a couple seconds to blow open.
It is in the datasheets and you can check that yourself.
https://en.m.wikipedia.org/wiki/Fuse_(electrical)
From Littelfuse themselves:
Semiconductors, specially Bipolars who show second breakdown, will die in milliseconds.
Here we have a nice powerful 16A 220V 170W power transistor, which might be a good choice for, say, a 60 to 100W amplifier.
Thermal damage looks like we expect, meaning we expect it to fail if dissipation is exceeded but not before.
BUT that holds only up to meager 50V 😲
Good enough for, say, a 25-40W amplifier 😫
How come?
Notice acceptable power dissipation curve holds well ... up to 50V that is, and then bends down catastrophically.
At 50V it stands 3.5A for a dissipation of about 175W 👍🏻
At maximum 120Vce it "should* stand 175/120=1.5 A.
BUT graph shows it stands only pitiful 0.2A 😲😫
No reasonable fuse will protect such a weakling
Couple seconds delay will protect power Transformers, mains wiring, electrical generators, stalled electric motors, etc. , all of which:
- Fail thermally
- Have significant thermal mass so it will take at least a few seconds to reach dangerous temperature.
- Semiconductor chips/dies have very low mass (a couple grams or less) so they reach danger temperature in seconds or fractions of a second
Again shown in datasheets.
Years ago I build an AB amp and wanted no fuse in the signal path. My solution was to place the fuse between the capacitors of the conventional power supply, making it an C-R-C filter. Their resistance removes some noise without limiting the power. For the case of any unwanted, constant high current draw, the fuses blow. Has been working without any problems since build.
PS this was a MOSfet amp, these do not blow that easy as bipolar do.
PS this was a MOSfet amp, these do not blow that easy as bipolar do.
My reason for asking is to protect things (incl. myself) from accidental high voltage DC showing up on the output of an electrostatic headphone amp. The conventional wisdom is to use some 5.1 kOhm resistors in between the amp output and the headphone connector. If there is any accidental high voltage (DC) at the output that wants to be shorted towards ground, the current running through these resistors will drop much of the voltage across the resistors, not the poor guy holding touching the headphone jack with his fingers (or other body parts). The pull-down resistors will thereby nicely limit the voltage and current goint into the finger, and they do not introduce any non-linearity to the audio signal. However, they don't disconnect the output from any accidental high voltage.
So at typical audio output current of maybe 1...10 mA, the voltage drop across the fuse would be 0.02...0.2 V. That's very little compared to a 500 V peak voltage signal. The voltage drop across the usual pull-down resistors is more than 200x higher, and no-one is worried about this. The difference is that the pull-down resistors are linear parts, so they don't distort the audio. Fuses may not be as linear, but their resistance is much lower. So... how bad are they? Is there any data on the non-linearity of typical fuses out there?
@JMFahey provides a great explanation. Most people don't understand fuses and their rating curve. The detail in his post is fantastic.
All of the Ampzilla models have fuses in the audio output (along with power supply fuses). The Son and Grandson do not. I can't recall if the SAE 2400 / 2600 have fuses in the audio chain. In my Ampzilla, I kept the fuses.
Only once did I blow a fuse in my Magneplanars. The ribbon has a fuse and somehow I managed to blow it.
All of the Ampzilla models have fuses in the audio output (along with power supply fuses). The Son and Grandson do not. I can't recall if the SAE 2400 / 2600 have fuses in the audio chain. In my Ampzilla, I kept the fuses.
Only once did I blow a fuse in my Magneplanars. The ribbon has a fuse and somehow I managed to blow it.
That's a good point. Looking at Mouser, there are some fuses with a 500 VDC rating (which might be suitable for an estat amp), but they all have a current rating of 0.5A or higher. That's about 10x more current than the limit I had in mind.
Not exactly relevant with estat amps.
I wonder how much. Would be nice to see some data. If there is none, I might add a fuse to an estat amp and compare the distortion spectra.
As I wrote before, I care more about the life of the listener than some transistor.
I thought your quesetion was more something of a joke, but now I realize you are taking it serious. There is no fuse that will protect the user from electrocution if your amp fails. Just tatoo that on your right hand.
I don't know your design, but there are options to keep the user safe: Use a capacitor or a transformer and design the inside into a high voltage and a low voltage part following regular rules. Every USB wall wart has this separation and people tend to stay alive. Galvanic separation is a nice thing.
I don't know your design, but there are options to keep the user safe: Use a capacitor or a transformer and design the inside into a high voltage and a low voltage part following regular rules. Every USB wall wart has this separation and people tend to stay alive. Galvanic separation is a nice thing.
A noble objective.
Sadly any kind of circuit breaker, be it a melting wire (fuse) or a thermomagnetic one will NOT protect a user.
Unless, say, it activates with 1 or 2 mA within 2 seconds.
Tough design goal.
That's why GFCI were invented, and they work on a different principle.
Its amplitude distortion (like compression) rather than thd. It's like putting a PTC thermistor in series with the load. fwiw I suspect the effect would be very small if the load appears as just a capacitive loading of a few hundred pF.