Amplifier main capacitor replacement - voltage question

[Tino]

Hi guys, I want to replace a pair of large main caps in an amp thats over 20 years old. The removal and insertion looks to be quit straight forward but my question is of voltage ratings on the caps.

The existing caps are 80V and 29000uF rating
The caps I found are 75V and 33000uF rating

Now I measured on the voltage across each cap's positive and negative terminal and get about 32.5V. I'm not an expert but the caps look like they are in series so should I be concerned at all at the 5V difference? I'm very far from the max voltage on the caps anyway...

Not sure if this pic will help but any suggestions regarding the swap will be welcome.



PS anything wrong with CDE brand caps? Originals are Mallory.

[es44]

I don't think the caps are put in series, one is for positive, and the other is for negative voltage.
If the voltage is lower than ~70 volt at each rail, as it sounds like, i wouldn't hesitate changing them.

Have fun.
Ebbe

[Tino]

Yes you're very right. I inspected the amp closer and indeed on the back there is a fuse for the positve and negative rails. I guess I didnt look closely when I took that picture.

I think I will definitly try and source the parts. I mean the amp works but I have a very slight hum even with no input signal (nothing connected). The hum is barely audible in the speakers (a little more noticable on one channel than the other). I can hear the same slight hum from the transformer if I put my ear nearly right up on it.

Anyway I figured the caps are old so maybe start there

[es44]

There's no harm in changing 20 years old capacitors anyway. You could try to measure the ESR of them, just for fun, if You know someone with an ESR-meter.
I've become "addicted" to Bob Parkers ESR-meter, nice little piece of equipment

Best regards
Ebbe

[Mooly]

Hi,
I would be surprised if the cause of the hum are the caps. Is that the main reason you want to change them. Have you measured the ripple on them.
The killer of electroylitics is mainly heat. The bridge rectifier is in a strange place bad heat wise, good physically. Does it get hot ?
Most causes of hum are down to wiring errors and induced hum from the magnetic field of the amp.

[GRollins]

There's one cap each for the positive and negative rails. The plate in the middle (doubling as heat sink for the bridge) is--or should be--at ground potential. Measure from that plate to the two other terminals. The one with the red wires should give you a positive reading. The one with the blue wires should give you a negative reading.
If you have 30-something volt rails, then the 75V caps are quite safe. In fact, you could quite easily do with lower voltage caps, possibly as low as 35V ratings if you're running 32V rails.
The thing to watch out for is over-doing the capacitance. If you put something like 100,000uF caps in the amp, then you'll put a great deal of stress on the bridge diode, particularly at turn-on. Your 33,000uF caps will not be a problem.
Which Threshold is that?

Grey

[Zero Cool]

Lets be real careful here!!! I find it hard to believe that the rails in that amp are really that low! I would double check that reading.

Caps are expensive. and i doubt a MFG would use caps priced nearly double what a cap would cost for 30-ish Volt rails. It just doesn't make sense. I would make very very certain of the rail voltages before suggesting or connecting a lower voltage cap. a 35-40v cap across 70 volt rails could easily turn into an explosive situation!! So lets make double doubly sure of those rails first.

I would suspect that the rails are actually 70-75V rails. thats pretty normal for caps rated at 80V to see rails at that voltage. If that amp is a 150-200 watt per channel amp, 70V rails are about right. 32 volt rails would maybe be a 50 watt amp.

if the rails are 70V, you could use 75V rated caps, but your safety margin is less and the caps will run warmer, not last as long. Using 100V rated caps would be better, they will run cooler and last longer.

Please double check your meter settings, make sure your on DC Volts and re-measure those rails from the large flat ground plate to the outer terminals and see what you come up with.

I just don't want to see anyone get hurt by an exploding cap or something. I have had it happen to me and its not fun to have to dig shrapnel out of your face! So please double check and be careful.



ZC

[es44]

Originally posted by Tino
Now I measured on the voltage across each cap's positive and negative terminal and get about 32.5V.

I must admit, that i took for granted, that Tino did measure the voltage, writing that way.
Of course it would help to know which amp we are talking about here. I'm not really sure it's a Pass labs, or is it ?

Best regards
Ebbe

[GRollins]

It's not a Pass Labs piece, it's a Threshold (note the logo on the sticker on the power transformer). Plus the mechanical construction and heat sink profile are consistent with Threshold product.
I doubt the rails are anywhere near the 80V rating of the caps. So why use 80V caps? Because if you buy things in sufficient quantity you get a very good price. Rather than stock, say, six different voltage ratings, you buy the highest voltage rating you'll need and get them cheap, then use the big part in every amp.
I'm guessing that it's one of the smaller Threshold Stasis amps. With a little digging, I could probably even give you a reasonable guess as to which model, but I've been out sawing down trees that have succumbed to the ongoing drought, and I'm absolutely beat. Shooting from the hip, I say the 75W version.

Grey

[Tino]

Hi Zero Cool,

You're exactly right. It is indeed a 50wpc amp. Its a Threshold SA3. So that means that mid 30's should be accurate for that wattage amp then?

Originally posted by Zero Cool
Lets be real careful here!!! I find it hard to believe that the rails in that amp are really that low! I would double check that reading.

Caps are expensive. and i doubt a MFG would use caps priced nearly double what a cap would cost for 30-ish Volt rails. It just doesn't make sense. I would make very very certain of the rail voltages before suggesting or connecting a lower voltage cap. a 35-40v cap across 70 volt rails could easily turn into an explosive situation!! So lets make double doubly sure of those rails first.

I would suspect that the rails are actually 70-75V rails. thats pretty normal for caps rated at 80V to see rails at that voltage. If that amp is a 150-200 watt per channel amp, 70V rails are about right. 32 volt rails would maybe be a 50 watt amp.

if the rails are 70V, you could use 75V rated caps, but your safety margin is less and the caps will run warmer, not last as long. Using 100V rated caps would be better, they will run cooler and last longer.

Please double check your meter settings, make sure your on DC Volts and re-measure those rails from the large flat ground plate to the outer terminals and see what you come up with.

I just don't want to see anyone get hurt by an exploding cap or something. I have had it happen to me and its not fun to have to dig shrapnel out of your face! So please double check and be careful.



ZC