make this really quick:
There is basically no danger plugging/unplugging a live headphone amp if the volume is muted.
There is danger turning on/shutting off SOME amps if the headphones are plugged in. There are a few variables here, and the position of the volume knob (in my experience) has NOTHING to do with whether you are safe or not.
OTL amps (tube or solid-state) can produce output voltage spikes on power on/off as high as B+, although usually about half (only 75-125V) these dont last long, but the "body contact time" from a bullet is only about 1/1000 of a second....
Direct coupled SS amps can have DC offsets at the output of as high as a few volts (sometimes even more) for a few seconds. thats the same as plugging your headphones into an old 9V battery.
Transformer coupled amps are the least apt to destroy a headphone that is carelessly left plugged in on power on, but they pretty much all sound like transformer coupled amps. If thats the sound you are after, get to it: if not just be careful.
It is good to be respectful of your equipment and know its limitations and dangers. Probably not good to be so scared that you buy something you dont like the sound of because it is failure proof.
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Originally Posted by manofmathematics /img/forum/go_quote.gif
There is always a lot of talk of the risks surrounding leaving your headphones plugged in to a tube amp during power on/off. My question is this: What are the associated risks, if any, with plugging your headphones in to a tube amp that is already powered on, and does removing the load from a tube amp pose any risks of its own?
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On a tube amp there are very few risks associated with plugging in/out when the amp is on. These risks are about the same (although less likely to be problematic) as associated with a SS amp.
The LARGEST risk is that of shorting a live output. In the case of a direct coupled SS amp with j-fet outputs, this can blow a transistor. mosfet output amps are less susceptible to this damage because of the differences in mosfets and j-fets. In the case of either SS or tube with a coupling cap on the output, there is a SMALLLL risk that the amp will oscilate after seeing a pure capacitive load or blow a transistor. In reality this is exceptionally rare. In either of these cases, if the volume/source is muted the dangers from plugging while on are basically eliminated.
There have been comments that running certain OTL tube designs unloaded can cause problems with the tubes, but the drain resistors across the output side of the caps in these amps provides enough load to make this a non-issue.
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| My questions are more towards the OTL side of things, but would be interested to know if there are any differences in risk between OTL and TCO. |
As applied to an output transformer coupled headphone amp, the results are largely the same.
The transformer in a transformer coupled headphone amp does eliminate the possibility of passing DC, and generally reduces the magnitude of the power-on-spike to an acceptably low level.
There are some speaker amps which REQUIRE a load across the output terminals at all times for stability, however this requirement is not seen in production headphone amps.
