Car Amp Caps

[zinda]

What I have is an amp that I was using and have switched it out as I get new amps (some to fix and others are working fine) it always worked fine and while I had it out I decided to look inside, I'm sure everyone who frequents this site has opened up most of their amps just to get a look at the quality of it.
I noticed right away that the 2 large Caps looked crowned and cut the cap off to make sure that they were actually domed or not. After I cut both off I saw that they were indeed very slightly domed. The amp still functions as it should but while it's out and I'm ready to replace them the problem I'm having is locating a suitable replacement. As always when it comes to car amps they always seem to find the strangest sizes and values possible!
These are 4700uF 80v and really short and fat I'm guessing 30mm x 30mm, my question is if I can use a higher voltage Cap as a replacement? I have piles of extra caps that might be candidates but not sure how much higher I can safely go. I'm thinking that these are not filtering voltage but rather supplying voltage that is provided to them as needed? So if I'm correct the manufacturer is going to go with the cheapest route they can which is going to be a value that is just higher than that needed for it to work.
So if I have some 100v or maybe 120v or 150v or 180v or 200v etc...
is it really going to make any difference as long as I stay with 4700uF? Or how low could I go 63V of a better brand? I was tempted to try out something higher but figured I'd see what others have done in this situation.
Last time I replaced these caps on an Alpine 750w amp I was super lucky and found 2 Mallory's on eBay from US seller that just barely fit. The cover was nearly touching them and I put a piece of insulating plastic with a rubber spacer in between the board and the Case to keep it from flexing and shorting if the bottom cover was pushed on. Also had to do a very tricky install since they were slightly larger in diameter and touched other components near them. Had to offset the leads 1/8" on 1 and 3/16" on the other lead to get the positioning correct to get them through the holes and stay tight to the board. They had to be tight as possible or the cover would not go back on without pushing them down.
That amp was working also, but, had to open it before I sold it and saw the same problems as this one. I had 3 of those amps and after I replaced the caps it was much louder and sounded way better than the others. I'll add that the old caps were ELNA, I only replaced the 1 set since I had sold the other 2 amps a few days earlier. I never opened up the other 2 but had to look inside that last one just to see what they looked like - just like I did with this one.
If I need to stay with the exact same voltage I can look on ebay to try and find some, I've already looked at other supply sites w/o any luck. Although I didn't dig super deep into the Aliexpress supply site.

[mariushm]

Re: Car Amp Caps

Yes, you can use capacitors rated for higher voltage than the ones you have now.

I suspect they use such high rated voltage capacitors in order to handle pulses from the battery or situations where the engine sends higher voltage for very short periods of time (transients and so on).

Also, I guess they use such large capacitors so that they would handle the currents needed (think about it, hundreds of watts from 12-14 volts) and to have enough energy to not stop your engine if you have some bass drops pulling lot of current from battery.

Search digikey, mouser, farnell , rs components for capacitors rated for 105c as your amp may be hot for long periods of time. Try to select some that can handle high current.
Others may add something to this thread.

[kaboom]

Re: Car Amp Caps

I suspect they use such high rated voltage capacitors in order to handle pulses from the battery or situations where the engine sends higher voltage for very short periods of time (transients and so on).

No.

There's a DC-DC converter that boosts battery voltage for the bipolar 35V to 65V rails, as in any amplifier. Except for those PWM "class D" things (Kicker BassStation), where the power supply is the amplifier.

Bet they don't talk about any of that over at EEVBlows/Krudkay's!

[mariushm]

Re: Car Amp Caps

Ah, ok. I just assumed they used a Class D amplifier for each channel, running straight from 12v.

I know about split rails, various amplifier types, boosting voltage... it just makes little sense to me to complicate things with a dc-dc converter in a small car amplifier.

[kaboom]

Re: Car Amp Caps

Ah, ok. I just assumed they used a Class D amplifier for each channel, running straight from 12v.

Consider a high frequency transformer between the PWM switchers and output filter network.

Those self-contained Kicker amp/subs have what appears to be a PWM-driven MOSFET H-bridge feeding a step-up transformer. The audio modulates the PWM directly, just as in those small PWM amps found in LCD TVs, or TPA3122s. These are unique, in that there's a transformer between the (audio) PWM and speaker- voltage step-up and audio "modulation" all in one shot.

Now, this Kicker I only examined quickly, since I was just curious about their implementation. The above appears to hold true, since they're getting about 100W RMS out of this, into a four-ohm load, from a 13.8V supply. The giveaway that the single transformer does double duty is that there's no seperate step-up TX in the usual sense, limited freq response (500Hz), and those four MOSFETS connect directly to the incoming supply (high- and low-side).

So the amp is actually feeding a load of about an ohm, as reflected by this little transformer, which of course steps the voltage up to the speaker- 20V AC or so from about 10. That's all I know- didn't trace it and there certainly aren't schematics, outside of Kicker anyway.

What's interesting is how larger "class D" car amps will have the booster for +/- 65V, which then feeds a conventional PWM "amplifier." These little Kickers combine both PWM functions into a single power stage, and use a single transformer.

I know about split rails, various amplifier types, boosting voltage... it just makes little sense to me to complicate things with a dc-dc converter in a small car amplifier.

About 20W is all you can expect into four ohms on a 14V supply. More voltage=more power. Car "subwoofers" tend to be horribly inefficient, 80-85 dB W/M typically. People jam 10 inch drivers into half a cubic foot, then turn up the EQ. "It still sounds good." Sure, but now we're talking 75 dB/W, as expected. Enter the 500W amplifier and blown woofer conundrum.

The cycle repeats, often weekly.

[999999999]

Re: Car Amp Caps

Try to stay at 80V to 100V, picking a lower ESR capacitor make/model rather than a higher voltage rating. Forget about original diameter and height, instead focusing on lead spacing and what the max diameter and height is that will fit within the available space.

Otherwise you may end up putting in capacitors with a lower service life rather than higher.

It is up to you where to source your capacitors, but more than half those I've bought off ebay were advertised as "new" but were old stock, sitting around for several years. If you're in the US you can get fairly inexpensive USPS shipping from places like Digikey, so long as the package stays under a certain # of ounces.