Leaving DAC on all the time OK?

Imin2u -- I know we're not talking about power tubes (in DAC's) but I thought generally tubes were like cars -- "It's not the years, it's the mileage" So that every hour powered up is an hour of useful life gone. Not so you say?
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Tubes last longer if turned off when not in use. The cathode has a limited lifetime where it is capable of full emission. There may be some turn-on stress, but that is outweighed by burning the life out of a tube.

On turn on, the filament is cold and conducts current with little resistance. The high in-rush current causes very rapid heating and thermal stress. Also, if the unit utilizes solid state rectification, the power applied to the plate come on almost immediately while the cathode may take some time warming up. This could lead to a condition called cathode stripping.

These issues are mostly non-existent if the unit uses tube rectification (the rugged rectifier takes all the abuse and slowly turns on the rest of the unit. In units with solid state rectification, the turn on can be rapid, but a lot of experts think that the "damage" is relatively minor.

It is possible to put a thermistor or other device in the power supply to slowly bring up the power in tube gear if that is a concern.

In response to your initial question I would agree with Jond, leave the DAC and the transport on.

However, since the topic has gone a bit further into the value of warm-up from cold start vs a constant on condition I've made an observation with a SS CDP left on 24/7.

When the rest of the system has been operating well past any reasonable warm up point (all tubes) when I use this CDP (Wadia) initially the upper frequencies are definitely overbearing in the highs. Dissapears within about 30 minutes or less (replayed the disc to verify). Thereafter, in that session there is no problem whatsoever.

Anyone else observed this in a different, or same, devices?

CDPs or DAC/Transport combinations draw very little power and usually don't put out much heat (high heat is what ages most components) so leaving them on is not a problem. With my CDP, if I turn it off, it can take several hours to get back on song. The manufacturer recommends leaving it on all the time. They claim that it takes quite a while for the capacitors to get back to optimum performance.

My Bel Canto dac doesn't even have an on/off switch. It's been going about 7 years now. It may be that your dac will last longer leaving it on all the time.

Tubes are the only issue here.