Quote:
Originally Posted by kvant  During turning on the device, not necessarily. The transients can be slow, subsonic. You could hear just about nothing, while your phones are still taking abuse. I have experienced this with some experiments with a servo. This is one of the dangers of designs with DC coupled outputs. SOHA is AC coupled in between the tube and the buffer, but I guess the time constant (C times the input impedance of the buffer) there is pretty high, so that a lot of low frequency mess passes through. And the powerful buffer will happily supply as much current as the voltage requires. With a cap on output, the time constant is typically smaller, so that slow transients associated with heating up the tube are blocked and everybody is happy.
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Interesting ... and I once got slammed for claiming "the coupling caps on the SOHA weren't big enough to block all of the DC on startup." I probably should've clarified it by stating headphones - or even some DMM's - don't much care if the transient is 2Hz AC instead of 0Hz DC,
but you've stated it much better. The point is, either one will potentially damage headphones on startup with the SOHA - and it lasts quite a few seconds longer than one would think it takes to charge a couple of very small 0.1uf film caps. As you also state, the buffer is amplifying those transients all the while.
In your reference about DC coupling: on the Millett and MAX, the output sits right at the halfway point of the supply voltage difference (24-27VDC, typically). Without the large electrolytic caps, the headphones will see a full 12VDC or more. On startup, the critical time is in fully charging the large caps, which takes some 5-10 seconds (closer to 5 seconds, thankfully), depending on various factors. So if they're plugged in, the headphones are exposed to that full voltage on startup, then it steadily decreases as the caps charge.
Note that I am not stating a preference either way - there are advantages and disadvantages with either method. It's just best to have eyes wide open about the potential consequences of each.
(Plus, I saw an opportunity to redeem myself slightly on an earlier posting incident.
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I have tested the transient spike on various el cheapo cans but I have ( as yet ) not exposed my decent cans to such abuse..dB
