solid state amp warm-up time, myth or fact?

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I once rebuilt a 200W Mark Levinson amplifier. Every transistor was replace in both channels. Every capacitor. The bias was identical. The amplifier warmed up faster on one channel than the other. One side sounded fine after 10 minutes, the other took about 20 minutes. The channels sounded identical after warmup.

After close examination it was found that one channel had a different brand of resistors for the 1/4W sized parts. Same 1% values, just a different brand.

Go figure.
 
djk said:
I once rebuilt a 200W Mark Levinson amplifier. Every transistor was replace in both channels. Every capacitor. The bias was identical. The amplifier warmed up faster on one channel than the other. One side sounded fine after 10 minutes, the other took about 20 minutes. The channels sounded identical after warmup.

After close examination it was found that one channel had a different brand of resistors for the 1/4W sized parts. Same 1% values, just a different brand.

Go figure.


Unfortunately manufacturers use different definitions for the
power ratings. One brand may be reasonably cool at the
rated power while another brand may be close to overheating.


As for the warm-up time, most transistors are likely to be
operating well within their margins, so they will probably
heat up slowly to the steady state operating temperature.
 
djk said:
The bias was identical.
You mean the setting or the bias current, the value? Apperently not (the value), because of the different heating up times.

My contribution to this thread is that I suspect that a "cool" amp is less sensitive compared to a "hot" one.

Leaving it on, I think a "hot" amp has nothing to gain if it's left on 24/7. It will probably break down much sooner compared to an amp which is switched on when it's actually used. Costs money also to have it on, except for houses with electrical heating.
 
Turning it off and on.

Leaving it on, I think a "hot" amp has nothing to gain if it's left on 24/7. It will probably break down much sooner compared to an amp which is switched on when it's actually used.

Actually turning it off and on many times is worse than leaving it on all the time. This is because turning it on is the most stressfull time for the components of the amp. Especially the power supply and power switches and relays.

Most research organisations leave their equipment on most of the time. On off periods are minimised. Apart from other reasons reliability is one reason to reduce on off cycles.

So leaving it on all the time is a good practice . Amps also take time to break in - may be months ! Leaving it on also stabilises the operating point. So when you want to listen, it is ready. The only negative thing about this is that in countries where the power fluctuation is very bad it might kill the amp if you get spikes or high voltage often. So you will have to do this judiciously.
People with power hungry amplifiers may be forced to turn them off due to their idle power consumption! That I guess probably covers most Class A users except Nelson's amps and similar types.
Cheers.
 
Circlotron said:
Are we to conclude then, that monster heatsinks are bad for sound then because they stop the amp from warming up very much???? I dare everyone to sidestep THAT question. :att'n:

Real men don't use heatsinks. :)


Another interesting conclusion might then be that amps should
sound better in warm climates than in cold ones. Perhaps I
should consider moving south? :)
 
The term 'warm up' when applied to an amp has nothing to do with equating high temperature to better sound.

Allowing an amp to warm up is just letting it reach thermal and electrical stability.

Critical operating conditions like bias and DC offset are/should always be adjusted when the amp is fully warmed up and for a big class-a amp this could take several hours. When the amp is cold, bias current could be a long way from its correct warmed-up value hence the amp may not sound so good.
 
I would say that the majority of power amps still use the traditional transistor and voltage divider approach to set the bias.

But even if bias is not an issue there may be matched differential pairs of transistors at the input stage that will warm up at different rates due to differences in power dissipation leading to higher distortion until they settle at similar temperatures and characteristics.

Things change a lot when an amplifier is warming-up, transistors are very sensitive to even quite small changes in temperature and it seems reasonable to expect that an amplifier may sound better once its operating points have stopped moving.

Or maybe it's just my ears that need time to warm-up:hot:
 
Since transistors have a thermal coefficient it would be surprising if there was NEVER any change in sound for ANY amp when operating temperature changes. However, it does not follow that this is true for EVERY amp since there are a number of thermal tracking schemes in use to try to control this.

Thus, the initial question posted is slightly flawed in that it sought generalized advice regarding solid state amps in general rather than specific models or at least specific design topologies. I've not made a study of this. Has anyone? But I would expect that some benefit considerably from warm up while other exhibit little if any audible change. Most likely it depends on thermal efficiency (I would expect class A to be more suseptible - but who know?) and on the the success and accuracy of the thermal tracking technique used in specific layout.
 
The one and only
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It's a fact. As I point out in the adjacent thread, devices such
as Mosfets reach higher transconductance figures at higher
temperatures, so it's not just a matter of getting the bias to
stabilize. Also, the higher the bias for Mosfets, the more
temperature stable, so there's your big heat sinks...
 
Hi Nelson,
how long is the warm-up time then, for bjt's or mosfets? i guess we average users never noticed this as solid state amps can operate almost instantaneously....thanks and more power...

Hi sam9,
your comments makes a lot of sense to me, can you elaborate some more? ...thanks
 
is there such an amp?

Hi RichardC,
you said, "Things change a lot when an amplifier is warming-up, transistors are very sensitive to even quite small changes in temperature and it seems reasonable to expect that an amplifier may sound better once its operating points have stopped moving."

i was wondering if there is such an amp....
 
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