Author Topic: Do amplifiers lose gain over time?  (Read 350 times)

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Offline Arlen Moulton2

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Do amplifiers lose gain over time?
« on: June 01, 2020, 06:50:33 am »
Hi everyone,

I work at a car garage in the UK and we have a small toilet building at the bottom of the yard, the boss likes to have BBC Radio 4 playing in there and so there's a small AM/FM/CD boombox next to the sink, it's been playing this station for over a year at full volume but due to the damp atmosphere the volume pot has oxidised meaning that only the right channel was playing. The other day I adjusted the volume and the left channel kicked back in, but it is much, much louder than the right channel (neither are clipped or distorted) so my question is: do amplifiers lose gain over time or is the speaker itself worn out?

Thanks, hope you're all well, Arlen.
 

Online Kleinstein

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Re: Do amplifiers lose gain over time?
« Reply #1 on: June 01, 2020, 07:51:40 am »
There is no general tendency for amplifiers to lose gain. There may be slight tendency for speakers to get weaker, but that should not be much.

It is more like the pots that can get bad (can go either way). There is also the possibility the tuning could have changed getting of the center and thus less signal.
 
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Offline T3sl4co1l

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Re: Do amplifiers lose gain over time?
« Reply #2 on: June 01, 2020, 03:52:43 pm »
Heh, back in the days of vacuum tubes, yes: the device performance itself drops over time.  Circuit design can mitigate that to some extent (negative feedback), but eventually the tube just wheezes too little to be worth using, and it's time to buy new ones.  Typical commercial tubes lasted on the order of 2,000 to 10,000 hours, depending on how hard the circuit was running them, and on manufacturing quality.  (Premium quality tubes, ran under gentle conditions, last much longer; the record is perhaps some tube-centuries of collective lifetime, with no failures, in undersea cable amplifiers.  Specialty tubes also found use -- still to this day in fact -- in satellite communications; their excellent reliability record makes them ideal for aerospace qualifications.)

But with semiconductors, nah, it's much more mundane.  Either the circuit works or it's a puff of smoke.  Everything else is mechanical: crusty potentiometers, crumbling speakers, corroding connections, etc.

Tim
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Bringing a project to life?  Send me a message!
 
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Offline Arlen Moulton2

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Re: Do amplifiers lose gain over time?
« Reply #3 on: June 01, 2020, 04:47:17 pm »
There is no general tendency for amplifiers to lose gain. There may be slight tendency for speakers to get weaker, but that should not be much.

It is more like the pots that can get bad (can go either way). There is also the possibility the tuning could have changed getting of the center and thus less signal.

In that case, I think what could have happened is that the FM stereo circuitry has shifted its balance slightly (you can't turn this off on this specific radio) and that's making the right channel quieter than the left. Could another possibility be that the isolation electrolytic on the right output of the amplifier chip has become weak?
 

Offline langwadt

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Re: Do amplifiers lose gain over time?
« Reply #4 on: June 01, 2020, 05:11:57 pm »
likely a bad/dirty pot, try massaging it a bit vigorously turning the volume up and down
 
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Offline pigrew

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Re: Do amplifiers lose gain over time?
« Reply #5 on: June 01, 2020, 05:56:29 pm »
Yes, amplifiers do lose gain over time. Transistors (I'll focus on BJT) are composed of semiconductor, doped with various impurities. One degradation mechanism is these impurities diffusing away from their desired positions, reducing gain. Other effects also where something (e.g. "hot electrons" or cosmic rays) will hit knocking atoms of their proper place in the semiconductor crystal lattice causing higher resistance/lower gain. Another notable effect is that the metal wires in packages will become thinner (metal migration). These effects happen faster at higher temperatures.

In the case of (standard class AB/B/C) audio amplifiers, feedback networks are used to reduce the circuit's gain. The internal transistor gains (open-loop gain) won't have much of an effect on the circuit's gain, within the useful lifetime of the transistors. Only a minuscule fraction of the open-loop gain reduction will be expressed in the circuit's gain. Catastrophic failures would need to happen before the circuit's gain would be reduced. Distortion would likely be noticed if the open-loop gains were reduced.

As other's have mentioned, it's likely the feedback network that's degraded, by way of a dirty pot or a blown resistor. There are some cleaning solutions for potentometers available, or the part could be replaced.

Power supplies also commonly fail in amplifiers, but they wouldn't cause only a gain reduction.

Regarding speakers, yes, they can wear out (magnets will lose magnetism over time), but this is unlikely to be the cause of the issues being faced.
« Last Edit: June 01, 2020, 06:06:46 pm by pigrew »
 
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