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I'm in the process of servicing my Carlsbro Stingray Super combo amplifier.

While disassembling it, I noticed that one of large capacitors (I assume smoothing) has started bulging and leaking:

Whole PSU

Leaking Cap

Cap Markings

I've tried searching the brand and markings etc. to get a datasheet with no success. The closest I can find is this eBay listing: ITT 4700uF Industrial Cap. This shows 10A ripple at 100Hz 85C.

I've found three new contenders for replacing my capacitors:

Option 1 - nichicon

Option 2 - TDK

Option 3 Vishay

All of these seem to have ripple current capabilities in the 1-2A range and it seems rare to find anything with the 10A capability of that other capacitor in new stock.

Without the datasheet for the ITT capacitors in my amplifier, how can I determine a suitable replacement aside from the usual voltage rating >= old capacitor rating and new capacitance = old capacitance?

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    \$\begingroup\$ What wattage is the amp and what voltage is on the cap? \$\endgroup\$ Commented Aug 22, 2019 at 8:00
  • \$\begingroup\$ Amp is 150Watt 2x12 solid state, you should be able to see on the image that the cap is rated 50VDC \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 8:01
  • \$\begingroup\$ No, what voltage is on the capacitor - measure it. \$\endgroup\$
    – Andy aka
    Commented Aug 22, 2019 at 8:25
  • \$\begingroup\$ Ah sorry, can do when i'm home but that wont be until this evening, although im slightly nervous of powering it back on with the bulging cap as the bulge is by the leads at the bottom, looks like its ready to pop! \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 8:43
  • \$\begingroup\$ Once I do measure the voltage over the cap what is it that you're looking for? \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 8:49

2 Answers 2

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You measured the loudspeakers' DC resistance at 12 ohms each, which means they're "16 ohms" speakers. Note the speakers' impedance varies with frequency, it includes an inductive part and reactive parts due to mechanical resonances, cabinet, etc. So, it is normal you don't get 16 ohms on your multimeter for a "16 ohms" speaker. Here's a "8 ohms" speaker impedance vs frequency as an example (source):

enter image description here

There are two "16 ohms" speakers in parallel, so that makes it a "8 ohms" speaker.

If the amp is rated for "150W RMS into 8 ohms" then its peak output voltage will be around 49 volts. Allowing voltage headroom for the power transistors, this means the power supply voltage will be higher, which means 50V caps were a bad choice and it would be a very good idea to bump the voltage rating up to 63V.

If the amp isn't really "150W" supply voltage could be lower. Who knows.

You can't measure the supply voltage on the old caps with a voltmeter: electrolyte has leaked out, so they will no longer smooth the voltage properly, there will be a lot of ripple, and the voltmeter will not give the correct voltage. You can measure peak voltage with a scope though... or you could disconnect the transformer and measure its AC output voltage, and calculate the peak value... It's simpler and safer to just use 63V caps.

Note a "50V" cap won't pop as soon as it sees 51 Volts. It'll work fine at 55V, or even 60V, but its lifetime will be shortened drastically. If you use the amp 2 hours every weekend, hey, it will probably last through the 1 year warranty!

Likewise, derating (like using a 63V cap at 55V) increases life time.

Now, the type of cap: either a snap-in or screw terminal cap. Snap-in is cheaper.

You can use 4700µF or more. I'd just pick one that fits in the mounting hardware you got. For a 150W amp I'd use more capacitance, like 6800 or 10000µF. Back in the day, when the amp was made, they were expensive, this is no longer the case.

Ripple current rating: at peak power (48V peak into 8R) is the amp is 150W, we'd have 6A peak if the loudspeaker was a resistive load. It is a reactive load though, so current will depend on frequency.

I'm sure you don't play at full power all the time. So you don't really need a cap with 6A ripple current. In fact, most likely any snap-in cap will do, they usually have 3-4A ripple current rating for this voltage and capacitance spec. It'll be fine.

If you want to feel good about yourself, you can use this one, it says "Disigned for high grade audio equipment, giving priority to high fidelity sound quality" ... (ie, pay extra for marketing). But really, any snap-in cap will do.

Note the wiring you got isn't optimal for hum reduction. Wires from the rectifier to the caps carry 100Hz current pulses, and wires from the cap to the amp carry a halfwave rectified version of the amp's output current. So it makes sense to minimize the loop area of these current paths to minimize radiated magnetic field and hum. So you can wire the input wire bundle tight together, and the same with the output wire bundle, like this crummy drawing:

enter image description here

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    \$\begingroup\$ Exactly the type of answer I was looking for! Could you explain in slightly more detail about the phase inversion as I cant understand why phase inverting one output of the amp then wiring the speakers in anti-phase reduces the current by half \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 10:43
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    \$\begingroup\$ Thanks! I added a note in the answer. \$\endgroup\$
    – bobflux
    Commented Aug 22, 2019 at 14:46
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    \$\begingroup\$ The note makes sense, however I was under the impression that this is a single amplifier powering 2 speakers, especially as the speakers are wired in parallel. What made you think there are multiple amps? \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 14:49
  • \$\begingroup\$ In this case, I made the mistake of assuming it meant "2x150W" lol. So forget the bit about having 2 amps. What is the speaker impedance? Two 8 ohms in parallel thus 4 ohms? \$\endgroup\$
    – bobflux
    Commented Aug 22, 2019 at 15:18
  • \$\begingroup\$ I, too, was initially thrown off by the "150Watt 12x2" comment above, as I could clearly see by examining the circuit layout and wiring in the photo that this is obviously a single channel amplifier. Only after I googled the name did I realize that the "Combo" model includes two 12" speakers, thus the 12x2(which are wired in parallel). There is no phase inversion, and it would seem quite odd to me if there were, even though it could be done with a two channel amp... \$\endgroup\$
    – Hitek
    Commented Aug 23, 2019 at 5:15
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Found someone with the exact same issue here EEVBlog

Caps suggested

RS 871-2748

CPC MAL205858472E3

I would still be interested if anyone has comments about how to determine the specs for a replacement

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  • \$\begingroup\$ As a side-note I've ordered the option from RS, will report back when its fitted \$\endgroup\$
    – andowt
    Commented Aug 22, 2019 at 9:20

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