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-   -   Are These Caps Too Good to be True? (http://www.diyaudio.com/forums/parts/202254-these-caps-too-good-true.html)

techbiker 10th December 2011 09:02 PM

Are These Caps Too Good to be True?
 
I've been looking for a good deal on a pair of 6300uf+, 150v+ capacitors for a while and I recently stumbled across a few of these 6800uf "Itelcond" capacitors on Ebay. The price that these are selling at seems suspiciously low and I haven't had much experience with "Itelcond" capacitors. The company does exist (it is Italian) and I pulled up a spec sheet for this model of capacitor. The ripple current seems decent for these specifications, however one would think that some corners were cut in order to get the price this low!

2 Itelcond Capacitor 6800uF 350VDC | eBay

How do I know if these are fake? The guy that is selling these is based in the US and his reviews have been good since 1999.... What do you guys think? Should I take a chance? I've been wanting to replace the large filter capacitors in my Crown Comtech 400 since they have probably been used quite a bit since 1996.

Thanks

manamanam 10th December 2011 09:31 PM

They look genuine.
I have seen these caps pulled out from a 100 KVA UPS unit and kept one for souvenir to decorate my office.
Have in mind that these are huge!

techbiker 10th December 2011 10:03 PM

Thanks for the information! I double-checked the space in my amplifier and I don't think that these will fit without extensive modifications. Crown just didn't leave much room for larger capacitors (a couple transistors are in the way). :(

That's too bad. These seem like a great deal!

manamanam 10th December 2011 10:14 PM

Are you sure they have enough capacitance fou your amp? These are only 6800μF.

techbiker 10th December 2011 10:22 PM

Well, my amp uses 2 6300uf ~200 volt filter capacitors by default (one for each channel). The tough part is finding capacitors that are designed for this voltage and capacitance. I've heard that I can go as low as 100v (I believe) for the filter capacitors if I never use the 70v option (these amps can be switched to drive 70v PA systems). Do you think this would make sense?

thanks

manamanam 10th December 2011 10:39 PM

Not sure, you could take some real messurments, if not, better stay within specs.
I did a quick search, these are really expensive!

techbiker 10th December 2011 11:03 PM

Yeah that makes sense. I just looked through my previous threads and saw that DJK mentioned this in regards to my Comtech:

"A 150V cap will do for 70V mode, a 75V cap will do if you only run it in 8/4Ω mode (and never run it in 70V mode). I wouldn't replace these unless they're all dried up. I would tack a 22F in parallel with each."

The problem is that I have no idea if my capacitors are dried up. I don't know if there is a way to test them (besides cracking open the caps). These filter capacitors are already ~16 years old so if they are not dried out yet, they will probably dry out soon. I'm never going to use my amplifier in 70v mode, so I wonder if it would make sense to see if I get an improvement in sound quality after installing higher capacitance 75v caps...

Here are a couple that look ok:

2 CDE 75v 10000uf computer grade bus capacitors new! | eBay

Is it true that there isn't much of a downside to installing higher-capacitance capacitors? I've heard that higher capacitors are more stressful to the power supply at turn-on, however I usually just leave my amplifier running 24/7.

What amount of capacitance do people who build amplifier power supplies usually shoot for here?

Thanks!

manamanam 11th December 2011 09:05 AM

Maybe the caps are still good, if anybody in this forum knows if there's a way to measure the capacitance of a multi-thousand μF capacitor without expensive equipment, it would be helpful.
I guess you could go with 10000μF caps without problems, but going beyond manufacturer specs, always have a risk. There's not a usual capacitance, it depends of the amp design.
There's an article you could read: Solid State Power Amplifier Supply Part 1
On part 3, there are 3 tables that can give you some examples.
Whatever you decide, be sure that the capacitors are manufactured recently. A cap that is stored for 10-15 or more years is an old cap. Try to choose a capacitor type that is new or buy from a supplier that sells many pieces and refreshes stock frequently and directly from the factories.

RJM1 11th December 2011 09:21 AM

"a way to measure the capacitance of a multi-thousand μF capacitor without expensive equipment"

See post #6
http://www.diyaudio.com/forums/parts...ml#post2785117

AmpliFire 11th December 2011 09:41 AM

Also, leakage current will be an indication. Simply connect a (>10A) diode, paralleled with a 10kohm resistor, in series with the cap (in between the DC source and the positive terminal of the cap). For best leakage measurements, the DC source should be close (~90%) to rated voltage (or simply: do the test with the amp's power supply). The diode will allow quick charging, but will be cut off when the cap voltage becomes less than 0.6V difference with the DC source, in which case the remaining charging will be handled by the resistor, until a steady state occurs. Then, leakage current remains, but note that leakage current is normally quite high in the first seconds/minutes, and might take several minutes to reach steady state. By measuring the volatge across the resistor you can work out the steady state leakage current, and if it is within normal values. If not: replace caps. I normally monitor the voltage drop across the resistor for 5 to 30 minutes, or one hour for aging large caps.


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