# Class-A Amps That Sit Unused For Years >>> How to Power-On Safely



## Tin_Ears (Aug 8, 2012)

Greetings technical gurus!!

I've read many times that it's best to bring the voltage up slowly on car audio amplifiers if they've been sitting unused for an extended time. This is done with the amps sitting idle with no output signal. It has to do with old capacitors.

I'm sure this procedure is fine and safe for all but Class-A amps. What worries me about Class-A is that, if they're truly pulling full current all the time, then this way of powering them on might do more harm than good. I don't know much about electronics but I do know that halving the voltage doubles the current demand which causes overheating.

*QUESTION:* With a true Class-A amp... will starting with 3v cause a 4x current demand in the amp thereby risking burning up the PS and/or output stages?


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## wheelieking71 (Dec 20, 2006)

and, what amps do you have that are "true class-A"? because there are very few, if any. perfect example: my beloved ADCOMs. advertised as "true class-A". reality: class-A topology up to a few watts, then class A/B just like every other amp of their time. they are just designed to run with a high bias. but, they are not "true class-A".

start with 12v. if it draws too much current the fuses are there for a reason. if it doesnt have onboard fuses, add some in-line. no signal, let it idle for a few hours. if you dont know what the idle current is supposed to be, google, beg, borrow, steal to figure it out. check it. as a matter of fact just go ahead and use the meter when you first power it up. most have a 10amp max. i am sure there are amps out there that draw more than 10amps at idle, but not many. after the amp is good and warm, check for DC at the outputs. 

but the guys who really know what they are doing (not me) are going to at least need to know what kind of amp you are dealing with. you cant just use the generalization of class-A (which doesnt really exist in the 12v world) and expect a good answer.


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## Tin_Ears (Aug 8, 2012)

Heh... that's why I used the term "true class-A". My beat-up old Soundstream Class-A 100 II is far from it. My question is hypothetical... just in case I ever buy a true class-A car audio amp. This pertains to home audio amps too which, I do know, are far more common. A slow warm-up of latter would require a variable A/C transformer which I don't currently own.


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## hirino (Aug 2, 2011)

ive never doen any thing different just hook them up and never had an issue .


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## [email protected] (Jun 12, 2008)

Just use a small fuse, alittle larger than the idle current requirements.


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## hirino (Aug 2, 2011)

BeatsDownLow said:


> Just use a small fuse, alittle larger than the idle current requirements.


how will that make a difference? unless the fuse blows it wont change the amount of current
imo if you want to slow the current u should use a light bulb to draw some current kinda like when chargeing a caraudio cap .


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## [email protected] (Jun 12, 2008)

hirino said:


> how will that make a difference? unless the fuse blows it wont change the amount of current


Its just a safe way to start it up. If something might be wrong and it tries to pull alot of juice, it will stop it. I was told to do this on old amps that sit for along time by a couple of well respected people that know more than I ever will about amps.


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## MarkZ (Dec 5, 2005)

Tin_Ears said:


> Greetings technical gurus!!
> 
> I've read many times that it's best to bring the voltage up slowly on car audio amplifiers if they've been sitting unused for an extended time. This is done with the amps sitting idle with no output signal. It has to do with old capacitors.
> 
> ...


Not usually. Class A doesn't mean that the amp strives to maintain constant output power. It's just a description of how the output transistors are configured.

The purpose of slowly increasing voltage is to see if you get any caps that pop. These caps will (usually) be located in the power supply section, which is largely independent from the output configuration.


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## hirino (Aug 2, 2011)

BeatsDownLow said:


> Its just a safe way to start it up. If something might be wrong and it tries to pull alot of juice, it will stop it. I was told to do this on old amps that sit for along time by a couple of well respected people that know more than I ever will about amps.


agreed, that is only to protect the amp which is what the fuse does all on its own no need to really under fuse . i was simply curious . i think adding a bulb between the amp and batt would draw some of the current . specificly one that are used to intially charge the audio caps


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## Tin_Ears (Aug 8, 2012)

I think the amp is going to draw as much current as it's designed to no matter what other devices are in series or parallel between it and the power supply. Only the voltage could be affected, if the other device is in series, unless the power supply can't keep up with total current demand in which case it should shut down anyway. Ohm's law, IIRC, dictates that current draw is inversely proportional to supplied voltage. In other words, if the voltage is halved then the current is doubled. The problem is the extra current also creates extra heat... maybe a lot more. This isn't an issue with typical class A/B amps due to the low bias voltage on the rails. My concern is with class A amps which, if truly class A, draw maximum current 100 percent of the time. What I don't know is how class a amps react to severe under-voltage. I suspect supplying voltage far below spec is a bad thing for "real" class A amps... but I don't really know.

As was stated above by others, the reason for a slow warm-up of supply voltage is to give old capacitors a chance to bleed instead of pop. This also gives them a chance to recondition... sort of.

Anyone have a true class A amp they want to lend me to use as a guinea pig?


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## MarkZ (Dec 5, 2005)

Tin_Ears said:


> Ohm's law, IIRC, dictates that current draw is inversely proportional to supplied voltage. In other words, if the voltage is halved then the current is doubled.


Nah, that's not Ohm's law. Ohm's law would predict that if voltage was halved, current draw would be halved too. I = V/R. That would be similar to how a linear power supply might operate.

Your amp probably doesn't have a linear power supply in it. So what you say is true, to an extent... the amp will probably attempt to increase its current draw in response to the lower supply voltage, but it only does so within pretty strict limits. For example, it may only draw an additional 10% current if you cut the voltage in half (a class A amp, probably even less). Or it may shut off. 

You can estimate heat based on the total amount of power a typical amp puts out at lower supply voltages (efficiency isn't going to change HUGELY). Most amps deliver less power with lower voltages, so I wouldn't worry about heat. Besides, you're just testing at this point, so it's not like you're going to run it balls out for long periods of time until you're sure everything is good.


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