Capacitors at 50v vs 25v on replacement in preamp power supply? - diyAudio
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Old 20th July 2013, 12:41 PM   #1
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Default Capacitors at 50v vs 25v on replacement in preamp power supply?

The electrolytics in my preamp are about 20 years old and due for replacement. In replacing the ones in the power supply I thought I'd move up from Philips to Nichicon KZ but the 1000uF 50V cans are a bit large for this space. I could attach via some hookup wire or could try 1000uF25v. Is the 25v likely to be too low a voltage? The Philips they are replacing are 1000uF 63v, but just after that are the opamps which are rated at only 15v. I've attached the PSU schematic for reference.

Classe-Model 30-PSU.pdf
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Old 20th July 2013, 01:04 PM   #2
Mooly is offline Mooly  United Kingdom
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Moved to line level.

I wouldn't use wire to extend leads because you lose much of the benefit of low ESR. If in doubt on voltage then measure what is actually present when its working.

(Can't open the PDF on this tablet so can't be more specific )
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Old 20th July 2013, 01:08 PM   #3
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The values used were pick either because they had a lot of them to use up or convenience. As the 78 and 79 series regulators will fry with more than 28Volts on their inputs, 35Volt types would be as low a voltage that I would be comfortable with.
Using flying leads introduces issues and I will not recommend that.
Why not look at Farnell, CPC or Mouser for replacements that are the correct size?
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Old 20th July 2013, 01:09 PM   #4
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Default Try this JPEG

This JPEG schematic may be easier to open.
Classe-Model 30-PSU.jpg
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Old 20th July 2013, 01:10 PM   #5
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First measure the voltage that is on the old caps and measure the AC secondary voltages please. Then we can give you sound advice. I think 63 V rating is over-dimensioned quite a bit but I could be wrong. Before the filter caps of the 7815 and 7915 there are series resistors probably to loose some voltage. Maybe the transformer they used had too high voltages ? Just measure and tell the results.

Do not use extended leads ! Belongs in the "failure category".

* I would replace the 4700 F/16 V caps for 10 to 100 F 25 V though. 4700 F is way too large if they're situated right after the regs. Don't use low ESR caps there. It is silly that they used larger filter caps after the regs than before the regs. Wrong thinking, the first filter caps and the regs will filter out low frequency ripple, not the caps after the regs. Larger values are OK if series resistors are used (which they didn't). Have seen quite a few commercial audio devices that had oscillation of the regs because of too large caps after the regs.
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Last edited by jean-paul; 20th July 2013 at 01:20 PM.
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Old 20th July 2013, 01:22 PM   #6
Mooly is offline Mooly  United Kingdom
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Thanks- OK, caps to the right of the regs need to be 25v or higher for the 15 volt regs and 16v or over for the 5v reg. Caps to the left, the input side, well I would measure the actual voltage but I suspect 35v is probably near the mark. If the voltage is around 22 then don't use 25v but go for 35v
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Old 20th July 2013, 01:34 PM   #7
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There's a secondary reason for picking larger caps. Check a datasheet and you will notice that rated ripple current is correlated to can size which in turn is correlated to max voltage.

The proper way to replace them would be to find the datasheet of the originals and find a replacement with similar or better max ripple current.
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Old 20th July 2013, 01:42 PM   #8
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Hmm, interesting. That schematic is not even what is in there! Can't believe I didn't notice before. The earlier caps on the pos and negative rails is 1000u 63v but measures 15vdc in use. The second cap in the chain is not 4700u 16v but another 1000u 63v, and it measures 28vdc in use!

So the main power rail schematic cannot be right, as the operating voltage is 28v which would fail a 16v cap. But it seems I could either:
  1. use a 1000u 25v on the earlier caps and 1000u 50v on the second caps on the power rails. OR
  2. use the 1000u 50v on the earlier caps and 470u 50v (or 100u 50v) on the second caps. The smaller secondary cap might free up enough physical room for the larger caps, though laid on their side.

BTW: On the 5V rail the spec shows first 4700u 63v and then 4700u 16v, but both are 4700 16v! The first measures 5vdc and the second 10.8vdc in use.
The first cap on Neg input and Pos input measure at 15VDC. But the second measures at 27.8VDC!

Thank you for reminding me to do the actual in-use measurements!

Last edited by brucew268; 20th July 2013 at 01:45 PM.
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Old 20th July 2013, 02:02 PM   #9
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Very often, manufacturers choose "by convenience of having fewer parts to stock, with a bit of an over-rating that lets them serve many different purposes interchangeably". Or... "you can buy 100,000 caps at 1000 uF/63V cheaper than 10,000 caps in 5 different values".

(Maybe - sometimes there are "deals")

This is why you see that your system actually has a 1000 uF/63VDC cap where the 4700 uF is specified. "Because it is enough - whether 1000 uF or 4700 uF" (after all there ARE those regulators there, soaking up all the humpty-dumpty noise of rectification. The output-side caps are just electron reservoirs. And you're talking "preamp" here, where mostly balanced-current amplification stages are used.)

OK - so - like other recommendations, here's mine: 35VDC minimum. Choose the can using Mouser or one of the other distributers that has all dimensions ... to fit the same as the original caps. Don't over-worry about 1000, 2200, 4700 uF ... clearly the manufacturer wasn't worried. On the pre-regulation side, you can even go to 330 uF or 470 uF. It is a mistake to put too-large a cap on the input side, as that stresses out those poor little 1N4003 diodes during charge-up. As many of us learn "the hard way" in tube-amps, smaller is often better for the first stage of electron reservoirs. On the post-regulation side, anything from 470 uF to 10,000 uF is fine. The regulators won't allow themselves to "fry" if you use larger values. However ... again: larger values aren't necessarily your friends.

Good preamp circuitry (repeating myself) often uses balanced-current or "constant current" amplification stages. This puts a uniquely low "reservoir requirement" on the filter caps.

Have fun! That should be rule 1.

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Old 20th July 2013, 02:22 PM   #10
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Quote:
Originally Posted by brucew268 View Post
Hmm, interesting. That schematic is not even what is in there! Can't believe I didn't notice before. The earlier caps on the pos and negative rails is 1000u 63v but measures 15vdc in use. The second cap in the chain is not 4700u 16v but another 1000u 63v, and it measures 28vdc in use!

So the main power rail schematic cannot be right, as the operating voltage is 28v which would fail a 16v cap. But it seems I could either:
  1. use a 1000u 25v on the earlier caps and 1000u 50v on the second caps on the power rails. OR
  2. use the 1000u 50v on the earlier caps and 470u 50v (or 100u 50v) on the second caps. The smaller secondary cap might free up enough physical room for the larger caps, though laid on their side.

BTW: On the 5V rail the spec shows first 4700u 63v and then 4700u 16v, but both are 4700 16v! The first measures 5vdc and the second 10.8vdc in use.
The first cap on Neg input and Pos input measure at 15VDC. But the second measures at 27.8VDC!

Thank you for reminding me to do the actual in-use measurements!
There is something very wrong with that values. I think it is the other way around. You can not have higher voltage after the regs than before them ! The first filter caps are for filtering ripple and the voltage on them should be at least 3 V higher than the voltage number of the reg. So an 7815 will have at least 18 V on pin 1 and the 7915 will have at least -18 V at the input. If they work like they should you will see +15 V at the output of 7815 and -15 V at the output of 7915. Can't tell it much simpler....these are classic text book design rules.

Maybe you can draw a new schematic with the right values (as built) ?

And please take a picture of the board. If you want to change parts just do it right in one time. Saves time and the PCB.

Interpreting the given values and putting them in the (probable) right context I would:

- use the largest possible value (try if 2200 F 35 V fits physically) for the primary filter caps of 7815/7915 and I guess you measured 28 V on those so I would choose at least 35 V rated low ESR caps like Panasonic FC/FM. Lots of choice there. For the primary cap before the 7805 I would use at least a 16 V rated cap. It is wise to choose caps that have the same dimensions as the old ones. Better still: measure AC voltages of the secondary windings to check of you are safe with 35 V rated caps for the 7815/7915. The series resistors are there for a reason.

- 10 to 100 F 25 V standard electrolytic caps at the outputs of the regs and definitely not larger (if you like the regs to behave well that is). Check 78XX datasheets and you will read why. The regs don't fry or something like that but they will not perform any better and they possibly even oscillate at a low frequency with very large caps.

Don't worry about the 1N4003 diodes. They are not afraid of 2200 F caps. While you are at it you can replace them for the better 11DQ10 diodes. Since it is your device you don't have to "get away with it" like manufacturers do. You now have the choice to make it a good PSU. The difference in price is marginal.

When you get the hang of it you might want to look how well decoupled the rails are locally at the active circuitry. You might win some points there !
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Last edited by jean-paul; 20th July 2013 at 02:48 PM.
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