Recapping your amp


I was wondering how you would know its time for a recap on your amp. Even when it sounds fine and the caps are not bulging or leaking. 
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Depends on age and thermal stressing. Ie, how hot the capacitors have been throughout their time on this planet.

Capacitor wear is primarily a time and temperature issue.

When used in pulse power supplies, the max ratings on current or amperage of the given capacitors, then those can come into play in the issue of aging. (besides the time and temperature issues). Capacitors have specs, like a max rating, and then age related data under those max conditions.

Capacitors in pulse power supplies, or in class d amps, are 'hit hard' continuously. Kinda like running a engine that has a 7,000 rpm 'red line'..at the max rpm of 7,000 rpm..continuously. Always. forever. the end won't be long coming. Maybe a day or two, for the engine. If it makes it that far.

Kinda like beer storage. Cool, dry, dark (no light/heat exposure)...and this leads to a long capacitor lifespan.

Units from the mid 2000’s can be bad from over-stressing and some 1980’s-late 1970’s capacitors CAN be fine, but 99% of them aren’t. That’s just vintage buyers and sellers fooling themselves that ’all original’ is a good thing.

Generally, anything over approx 12-15 years of age needs to be assessed. Depends. Depends on the specific example.

Ie, digital or class d amps, which are always over stressing their capacitors in their power supplies.

How many horror stories are there of 5-10 year old subwoofer amps that are broken? thousands and thousands.....

soon.....The electronics wear down ’butchers bill’ will begin coming due on all these new fangled class d amplifiers out there, these days....

conventional old school power supplies, known as 'linear power supplies', they age well. Those are built to last, in design and execution of the build.

Modern pulse power supplies are junk, in the realm of longevity.

Note that 99.99% of all class D amps and subwoofer (plate) amplifiers are built this way and are essentially ticking time bombs in the expected lifespan department. small, light, efficient..well..there is a price to pay. It's a case of quality drift being fast and high and a short lifespan. Which is perfect for the big corporate game, the end point and short lifespan is built right in, so you can get on to buying a new model in fairly short order.

Pulse power supplies have crept into so called 'high end' gear, and I'm not sure it's a good idea at all. But, what the heck, the speed of change in the digital end of the market is so fast, that some of that digital stuff is technologically disposable in 5-10 years, like the the $29 DVD player from Walmart.
Aluminum electrolytics can even go bad(via dielectric polarization), given enough time, on the shelf and unused(http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.823.6133&rep=rep1&type=pdf). Of course, heat and cycling, also take their tolls, but- if your component is older than 20 years, it’s virtually certain, the caps are due for replacement, even if(to the eye) they seem perfect.   Ears adjust to losses in fidelity, when that loss takes place over time and most won’t notice, like they would, were the change sudden.(http://www.elna.co.jp/en/capacitor/alumi/trust/) (https://www.illinoiscapacitor.com/pdf/Papers/Life%20expectancy%20of%20Aluminum%20electrolytic%20capa...) (http://www.powerguru.org/electrolytic-capacitor-lifetime-estimation/)
If you start hearing a 60-120hz hum when only the amplifier is on, then it is time for power supply caps to be replaced. Prior to that, you may hear a softening of the sound (transients and mid to lower bass).

If the amp is running and sounds normal, leave it alone.
If the amp is running and sounds normal, leave it alone.

I agree...I recapped some old speakers and never felt they sounded quite the same. 
Recapping loudspeakers is a thing that must be individually assessed in how it is done. Small bypass caps of a film type sometimes works (across electrolytic types), but if the crossover is originally all electrolytic, then the new should also be all electrolytic - unless one wants to get down to redesigning the crossover.
Let's assume;

You have a family car with timing belt, that according to the owners manual is supposed to be changed every 7 years or 100,000 miles.

Your car is 6 years 10 months old and has 70,000 miles. It was not a taxi, was cared for, and was not abused.

Timing belts give no warning signs before they go.

Are you going to get the timing belt changed in two months ?    

Hard to describe but if you hear a sound just like tiny/tight fart when you turn the amplifier off, that's usually a good indication that a recap might be required. My real old Adcom 545 did that and my 555 has started doing too. From the sound quality perspective, both amplifiers have much softer bass now as compared to when they were new which makes the overall sound rather flat and lifeless. Both have been used (actually abused) on a daily basis since1987 by me and friends who (still) borrow the 555 for parties.

Sounds like you are indeed close to that timing belt failure moment.
Yea but when to replace it before or after it goes.
A timing belt won’t degrade your engine’s performance, before it fails(completely). The point of my references was, that(electrolytic) capacitors do. Of course: to many, "...still working...", is good enough. Do you replace your racing tires, AFTER they blow out, or- as soon as they begin to lose traction?  One's course of action, will depend on how much they value performance(different strokes...).
As with timing belt, replace before it goes. A failed power supply capacitor can blow up a speaker, and a leaking one can mess up the amplifier’s board. Before that, you will have lost some dynamics, of course.
Teo - Regarding my speaker recap, the capacitors and resistors were approved by the original desiger.  Sonic caps and mills reisistors superior to originals used in the 80's and 90's but it did change the character of the sound for the worse for me anyway.