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Replacing input coupling caps
Replacing input coupling caps
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Old 8th May 2019, 08:06 AM   #1
Learnincurve is offline Learnincurve  Norway
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Default Replacing input coupling caps

Apologies as I know this subject is discussed often, but I am in doubt about the specific implementation in my amp.

I have a Cambridge Audio 640R surround receiver, which doubles as a surprisingly good stereo amp for music, especially when fed through the "Direct" analog inputs from an external dac. The amp is over 10 years old and in need of replacement of all the electrolytics (at least in the analog circuits).

All line level inputs are coupled using 10U/16V polar electrolytics

There are similar polar electrolytics after the first stage op amps (rated at 25V).

The safe option would be to replace them with similar rated, "audio" types, such as silmic II's or nichicon "MUSE", but many discussions might recommend replacing them with bulkier film types. There seems to be plenty of room on the boards, so I'm guessing that I could find room for film caps if that would improve the sound.

The question is whether I should mess with the original design. I have heard and have no reason to doubt that the designer of the time for Cambridge Audio knew what he was doing and chose caps for a reason...on the other hand, that reason (especially in a 7 channel AV receiver) may well have been economics.

What is the consensus? If film would be better what type would be the best balance between money and sound? Would WIMA MKS2 10uF 50V 5% be suitable?

Last edited by Learnincurve; 8th May 2019 at 08:22 AM.
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Old 8th May 2019, 08:32 AM   #2
Learnincurve is offline Learnincurve  Norway
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One solution (if there is any real sonic advantage), might be to use the much more expensive film caps only where they matter (ie in the Front Left, Front Right channels only), and replace the other channels with cheaper electrolytics). It's a horrible hack for consistency, but I only use the two main channels for serious audio. - actually not true, as I have a couple of SACD's but hardly ever play them in multi-channel mode and anyway feed them from the built-in dac of a low-end DVD/SACD player.
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Old 8th May 2019, 09:44 AM   #3
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Learnincurve View Post
The amp is over 10 years old and in need of replacement of all the electrolytics (at least in the analog circuits).
I doubt that they need replacing, what makes you say that?
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Old 8th May 2019, 10:03 AM   #4
Learnincurve is offline Learnincurve  Norway
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Originally Posted by scottjoplin View Post
I doubt that they need replacing, what makes you say that?
Just that the amp is in daily use and it has been warm in there for over 10 years. The main power caps have been bulging for some time and will be replaced and I thought a full electrolytic recap of the analog circuits would be good maintenance at this point.

I did originally contact a workshop in the UK, who agreed that it was time, but logistically it won't work to visit them, so I thought I'd do it myself.

I have read that electrolytics in warm environments generally degrade over time and 10 years is a ballpark figure for replacement.
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Old 8th May 2019, 10:11 AM   #5
scottjoplin is offline scottjoplin  Wales
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From your description the power supply caps need replacing. Coupling caps have a very easy life with almost no voltage across them.
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Old 8th May 2019, 10:19 AM   #6
Learnincurve is offline Learnincurve  Norway
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Quote:
Originally Posted by scottjoplin View Post
From your description the power supply caps need replacing. Coupling caps have a very easy life with almost no voltage across them.
Thanks for the advice, which I appreciate. Back to the original question (if i were to replace them), would there be any advantage in going for pricier, non-polar caps, at least for critical channels?

BR.
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Old 8th May 2019, 10:54 AM   #7
sangram is offline sangram  India
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Depending on your DAC and whether you plan to sell the amplifier at some point, it may be more advantageous to explore whether you need any caps at all.

In line level application I doubt you would notice a difference between film and good non-polar electrolytic. The amp's inherent distortion and noise floor would be significantly higher than any contribution from series capacitors.

Also keep in mind that a 10uF film cap is significantly larger than an electro of the same value.
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Old 8th May 2019, 11:09 AM   #8
Learnincurve is offline Learnincurve  Norway
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Originally Posted by sangram View Post
Depending on your DAC and whether you plan to sell the amplifier at some point, it may be more advantageous to explore whether you need any caps at all.

In line level application I doubt you would notice a difference between film and good non-polar electrolytic. The amp's inherent distortion and noise floor would be significantly higher than any contribution from series capacitors.

Also keep in mind that a 10uF film cap is significantly larger than an electro of the same value.
I don't think I would dare leave them out, as replacing any DC-damaged parts would far outweigh the cost of even the most expensive caps. I've heard conflicting views on the use of non-polar electrolytics, as the back-to-back nature of them effectively doubles any noise and colouration that a polar electrolytic might introduce. I'm guessing that the guys at Cambridge decision to use polar types was deliberate.

Space-wise I'm pretty sure there is room for the 10V Wima's
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Old 8th May 2019, 02:11 PM   #9
indianajo is offline indianajo  United States
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I don't like taking the case off, the board out, and putting back. When I have to replace one e-cap, I do them all. Having them fail one at a time makes a real temptation to junk the appliance and buy a new one. Always going bad sounding on you as caps fail one at a time. Besides the little ones are $.08 apiece, and the freight for the box from the distributor is $8 up. Buying from a local store allows fraudulent parts to creep in. Your local guy can find a bargain, he doesn't have in incoming inspection department to sort out the trash. Authorized distributors buy the stuff direct from a manufacturer that produces a datasheet. I've gotten good long life e-caps from panasonic, nichicon, rubicon, vishay. Crummier end of life spec from CDE, united chemicon, multicomp(farnell) although those latter brands haven't failed yet.
Replace two caps at a time between function tests if it can be arranged. Bad solder joints and other errors are a whole lot easier to find if you have just changed two parts. Sound got worse, you made an error or the part was not correct.
I find industrial long life caps to be adequate for all the 300 or so I have done since I quit working. You buy a 10000 hour cap, you are not getting trash. The premium is a lot less than the "audiophile" caps. Plus I may not have to do that cap again in my lifetime. I've had to replace e-caps in my ST70 4 times since 1970.
Polyprophylene caps for input may work fine, also the extra size can cause hum or RF pickup. Worth an experiment. The advantage is not so much sound, as lifetime. plastic film caps don't fail unless voltage stressed. I've found mylar a little weird sounding when I replaced paper caps in a PAS2 preamp.
I don't know of any long life bipolar electrolytic caps. They come in one grade, life unspecified. The higher cost long life sealant is a rare feature, somewhat going out of fashion to look at the stock lists of distributors. The advantage of bipolar is it can handle any sort of DC coming out of the signal source. I think the typical unipolar cap use assumes the source is built of npn parts, which might not be true in the case of IC's. A DC rated cap will take up to 10% of its rated voltage backwards without damage.
Wear safety glasses when desoldering, solder can splash. Mark the polarity of the cap on the board with a sharpie before removing the old one.
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Last edited by indianajo; 8th May 2019 at 02:15 PM.
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Old 8th May 2019, 02:27 PM   #10
Learnincurve is offline Learnincurve  Norway
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Thank you for your detailed reply.

I'm going for the full replacement and will use Wima's in the signal path. They aren't big enough at the specified voltage to cause problems. Maybe false economy, but I'm leaving the digital boards out of the job unless visual inspection shows obvious degradation.

Just getting the amp out of the shelf and back in again is a major project as fishing around the back to disconnect and connect all the leads in the right order will take ages and cause considerable pain, so I agree on your first point.
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