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enter image description hereenter image description hereI am novice to capacitors and need some help.

My amplifier has two large can capacitors rated 9000 uF and 125 V. These capacitors are no longer available. The closest one that can fit in the place/size that I could find is 10000 uF and 100 V.

  • Would it be OK with replace with higher capacitance and lower voltage?
  • What are other options I can explore? The amplifier is Carver Model M 4.0t.

enter image description here

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  • \$\begingroup\$ If the supply voltage can exceed 100V (e.g. with +10% incoming mains) ... no. \$\endgroup\$ Jun 8 at 13:04
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    \$\begingroup\$ No - unless you know for sure that the voltage actually applied to the capacitor never exceeds the rated capacitor voltage. \$\endgroup\$
    – Justme
    Jun 8 at 13:07
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    \$\begingroup\$ I would consider using something like: eu.mouser.com/ProductDetail/KEMET/… Which is a 9100 uF, 200 V capacitor. More suitable caps can be found on the same site. I also would want to be sure how the capactor is used. Probably it is a smoothing cap in a supply. I would want to be sure that the capacitor is suitable for that application. \$\endgroup\$ Jun 8 at 13:08
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    \$\begingroup\$ Digikey stocks 9000uF and 9100uF >= 125V capacitors screw terminal, so no reason to buy anything but a replacement. If you want to save a few bucks by soldering, there is also these which are a little cheaper: digikey.com/en/products/detail/kemet/ALF70G912KP200/12716863 \$\endgroup\$ Jun 8 at 17:21
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This is not a question that can be answered without more information. Information like the model of the amplifier means we can search out the schematic and hopefully determine what the actual working voltage is and then we can advise whether a lower voltage part would be a safe replacement.

Generally, it is bad juju to replace with a lower voltage part. The working voltage might be 100V and the 125V rating is for some safety margin. Substitute a 100V part, the safety margin has gone so the new part might blow up (literally). You could tolerate a lower capacitance part.

Why do you think you need to replace the capacitors? Have you tested your capacitors to see if they are still in spec? There's a lot of old wives tales out on the interwebs regarding electrolytic capacitors. Unfortunately, you need some electronics background to make sense of what is claimed.

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  • \$\begingroup\$ Thank you for feedback. The amplifier is carver M 4.0t. \$\endgroup\$
    – Iak
    Jun 8 at 14:49
  • \$\begingroup\$ I had suspected it was an American amp. I was reading the history of Mr Carver earlier this week. \$\endgroup\$
    – Kartman
    Jun 9 at 7:30
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Here's the service manual

That was not hard to find

https://usermanual.wiki/Collections/magnet/files/9365%20Audio%20Schematics%20and%20Service%20Manuals/Carver%20M-4.0t,%20TFM-4.0,%2042,%2045%20Service%20Manual.pdf.html

On page 12 they say with a 120VAC input, you should have 97V on the DC rail.

Next notice the spec "1000W bridged at 8 ohms". From P = V^2 / R...

V = sqrt ( P * R ) = sqrt (1000 * 8) = 89V

To bridge the outputs, those caps are effectively in series in bridged mode, means they have around HALF that voltage on them (plus some for headroom)

This is also suggested by the spec 375 W / Channel into 8 ohms

V = sqrt ( 375 * 8) = about 50 volts.

Have a DC voltmeter? This is SUPER EASY to check. Those caps are sitting in open air, easily accessed. While it's powered up, very carefully measure the DC voltage on each. I bet they're no more than about 75V, probably less. If so, you are def good to make the substitution IMO

I didn't look up a datasheet for those caps, and probably can't. But note sometimes there is a bleed resistor internal to them. This is there to drain the caps (make them safe) after you power off and to help 'balance' them in certain circuit configurations (like tube amplifiers with high voltage DC rails and high output power). If those caps have resistors and your new ones don't, you'll be missing those functions. Place about 470k 1W resistor across each cap will not harm anything and will ensure those functions (if originally present) are still there.

Some will say if they're not showing signs of bulging or leaking, they're still good. You need special equipment to really know. After almost 40 years I would def replace them even if it still works. At a minimum, with fresh power supply caps, you should find an audible difference - punchier bass notes and maybe less 120Hz hum (if there is any now).

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  • \$\begingroup\$ Perfect. I will perform suggested checks. Thanks again. \$\endgroup\$
    – Iak
    Jun 8 at 16:23
  • \$\begingroup\$ BTW -- Please do wear eye safety glasses --- Those monster caps, and other stuff in there for that matter --- can blow up at any time! Especially maybe when you're poking around if you slip ;) Don't risk your eyes. And don't probe a live circuit with two hands --- One hand in your pocket is the rule of thumb. These caps can be tested with aligator clips I think, makes that even easier. \$\endgroup\$
    – Kyle B
    Jun 8 at 16:27
  • \$\begingroup\$ These capacitors are bridged with a rectifier. I uploaded another picture in the original question. Each measure 111V, perhaps they are bridged? \$\endgroup\$
    – Iak
    Jun 8 at 17:04
  • \$\begingroup\$ Where exactly are you making that measurement? You should touch the capacitor pins directly, not the rectifier. The rectifier feeds both of these.. If you measure 110 there, you'll probably see 55V on the caps. I'd be very surprised if the original design only had 15V headroom (125-110=15). Yellow wires AC in. Measure from the bare wire (rectifier '-') to the purple wire (rectifier '+') you'll be measuring the voltage on both caps added together. The black wire between the caps is 'ground'. One cap has + voltage relative to ground, the other is - . Rectifier - is NOT ground. \$\endgroup\$
    – Kyle B
    Jun 8 at 18:56
  • \$\begingroup\$ Measurement at naked wire purple wire are 226V. \$\endgroup\$
    – Iak
    Jun 8 at 21:30

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