What is happening when an amp "warms up"?

[Deke609]

Hi all - I'm hoping someone or mutiple someones can help me understand the basics of "warm up". I assume it involves more than just the tubes and other components reaching a temperature range at which they perform within spec.  Do the OTs, PT, caps and any other things need to "warm up"?  And what does this mean in each case?

Take three scenarios:

(1) Amp is cold and turned on.

(2) Amp has been playing music for 2 hours.  It is momentarily turned off, and then on again within a minute.  Does it still need to "warm up"?

(3) Amp has been playing for 2 hours configured for 16 ohms output impedance.  It is turned off, quickly switched to 2 ohms and turned on again within 1 minute. Do the OTs, which are still quite warm, need to "warm up"?  And would it be any different if I went from 2 ohms to 16 ohms?

And just in case it is relevant: let's say the amp in question is the Stereomour 2.

many thanks,

Derek

 

[Paul Joppa]

Excellent question, with regrettably few answers!  :^)

I personally expect that we know less than 10% of the significant things that go on during warm-up. So take the following with many large grains of salt ...

The one that I do believe in is transformer magnetization, specifically parafeed outputs, just because we have some experience with them. The turn-on and/or turn-off transient currents will magnetize the core of such a transformer. The extent depends on several factors, but it is sometimes enough to be audible. Music itself is a good-enough demagnetizing signal; I can't recall hearing a case where 30 minutes or so is not enough to mostly restore performance. It's not really a temperature thing - you have to run some music through the transformer.

Transistors are much more sensitive to temperature, both gain and operating point will shift significantly as the devices reach operating temperature. This is usually counteracted by the use of heavy negative feedback, which introduces its own problems, making it pretty hard to be sure what change causes which audible audible effect. And there's no automatic guarantee that warmer is better - just that it's stable and repeatable.

There are probably similar things going on in the speaker driver damping material - that goop on the surround (or the exotic rubbbery stuff it's made of) for instance. The stuff is surely sensitive to temperature, and has hysteresis (takes a "set") when not used.

Well, those are the examples that come to mind right now.

 

[Deke609]

Many thanks, PJ. That gives me something new to read up on.

Regarding transients from turn-on or turn-off - is that the same as or similar to "inrush"? And is the resulting magnetization "hysteresis"?  I read somewhere last night that you can rid iron of hysteresis by hitting it repeatedly -- which is not a practical solution in the case of audio amps, unless gently tapping also suffices.  Probably best to stick to running music through the transformer!

Derek