Capacitors, Burn-in and mythology

Although I accept that there could be measurable differences between capacitors, I question whether these differences are audible. If, for example, distortion goes from 0.001% to 0.01%, that's a tenfold increase, but neither is audible.

Furthermore, electrolytic capacitors do take time to reform, I measured the leakage of one electrolytic that had laid in my drawer for over 30 years and this reduced substantially over a 48 hour period, but then electrolytics are generally used in circuit positions where leakage isn't important, like decoupling.

I suggest that just about everything makes a measurable difference (even cables!) but very little makes an audible difference that isn't very easily explained.

S

 

Electrolytic capacitors are rated at a certain number of hours at a rated voltage and temperature. A rule-of-thumb is that lifetime doubles for every 10° temperature difference, so a 85° capacitor with a rated life of 5000 hours, will, in a normal domestic environment, say at 40° internal temperature, will last 80000 hours, or over 100 years, used day in day out for two hours a day. I have a number of vintage amplifiers, all over 40 years old working to their original specification and with original capacitors.

In most cases, recapping amplifiers 20 years old is pointless. Yes, some amplifiers might have been abused, overheated and left on 24/7 for 20 years, but most domestic HiFi isn't.

S

 

I can vouch for this, tested a load of old large caps that were thrown from a repair shop that closed down. They were still within tolerance despite their age.

 

When i service old kit the caps get thrown into a large box - must be many hundreds in there over the last few years.
Everything from 1uf couplers to 10kuf PSU caps. I recently pulled 50 at random and measured them for ESR, capacitance and leakage, Of the 50, only 1 was outside spec!

This isn't always the case and I've noted some brands of axial non-polar caps used in old crossovers can drift a fair bit, but generally I'm finding quality branded polarised caps from the 70s and 80s kit I tend to work on are perfectly fine.

Heat, high ripple and running caps near the voltage limits do the most damage.

 

Non-polar electrolytics as used in most crossovers are normally made from two conventional electrolytics back-to-back inside one envelope. This means that none of the electrolytics in the pair have any polarising voltage across them, which isn't how electrolytics are meant to work. That may account for why they lose value over time, whereas normal polarised electrolytics that spend their life with their design DC voltage across then last very much longer.

Non-electrolytic large value non polarised capacitors are a lot bigger and more expensive which is why they're rarely used in commercial crossovers.

S.

 

Interesting to know, thanks Serge! I didn't realise a lack of DC would make them die quicker.

To be fair, a lot of speakers use film caps. At least the high-end stuff people here have. Only when a really large value is needed say more than 47uf will electrolytic usually be found.

 

Interestingly, I've found in case that where old non-polar caps in crossovers have drifted its been upwards - often 50-100% high uf than spec.
One of the reasons many old loudspeakers can sound 'off'.

 

I haven't seen any properly conducted test results on electrolytic capacitor performance Vs voltage, so my suggestion above is based on supposition, knowing how electrolytic capacitors work. In particular, electrolytic capacitors take a small amount of time to form, and will only form in the presence of a DC potential across them. In back-to-back non-polar electrolytics, there is never that polarising voltage, so they will never form properly. That will also make leakage higher, but as loudspeaker crossovers are inherently low impedance devices, the fact that there's leakage in the kohms region is pretty irrelevant.

Why the capacitance drifts upwards, I can't suggest a mechanism, but certainly, if the capacitance doubles, then the crossover point will be way off the design frequency. Film caps are a lot larger and more expensive than non-polar electrolytics, so whilst I can see them being used in larger, more expensive loudspeakers, at the budget end, which also tend to be smaller, electrolytics get used.

S.

 

Hello, is anybody here ? :0

 

NP electrolytics tend not to follow the expected filter function either, IME. If I model a 2nd order xover and use an electrolytic, the filter tends to go okay for the first ~20dB then the slope either becomes more or less steep. It messes with the phase so the result needs tweaking without the aid of simulation. Film caps are a lot easier, lol.

As you say though, the size and expense means there are not always alternatives. I have some 100uF polyprop caps here that are the size of a Coke can!