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Old 28th April 2008, 05:58 PM   #1
woodman is offline woodman  United States
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Default Coupling capacitor voltage question

Hi,

I'm trying to understand something about this circuit. This is part of the phono stage of my newly acquired preamp. I suspect that one of the coupling caps is drying out causing a slightly low output on one channel. An ordinary tech would simply order and install new 4.7uf 63V capacitors and be done with it. I, however, am an audiophile (and tech) who feels compelled to install audiophile components since I'm going to replace those capacitors. The capacitors I want to install (Elna Silmics) are not available in in 63V values. I can only seem to find 50V and 100V.

Of course, replacing the 63V caps with 100V caps will work, but what I don't understand is why the coupling caps are rated at 63V in the first place. AFAIK, one rule of thumb is for the cap to be rated for twice the supply voltage. These are single pole regulated supplies (10.4 & 15.2), so these caps are rated well more than twice the supply voltage. I have asked some of the engineers I work with if there was any reason I couldn't use 50V caps, and they said it 50V be fine. Still, I seek the advice of the audio community:

Is my thinking correct in that there is no way the coupling caps are going to see more than 50V even with a catastrophic failure?

What was the designer thinking when he used 63V caps? Maybe higher voltage caps sound better or there was a large stock on hand? The design of the preamp is overkill, so maybe the caps are too.

How does one spec a coupling cap for voltage? I cannot seem to find this info anywhere.

Thanks,
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Old 28th April 2008, 07:05 PM   #2
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Generally, higher the rated voltage on an electrolytic, the more linear it will be when the voltage swing across the electrodes traverses only a small portion of the range. For instance, if it is rated 63v and charges between 10v and 10.5v, it is likely to be more linear than a 16v cap subjected to the same range, other things being equal.

In practice, there's likely to be very little difference between 50v and 63v rated electrolytics for audio coupling usage.
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Old 28th April 2008, 11:02 PM   #3
gain is offline gain  United States
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thanks for that insight, linuxguru (awesome OS btw).
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Old 30th April 2008, 12:03 AM   #4
woodman is offline woodman  United States
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Thanks LinuxGuru!

Guess I'll go with the 100V caps for extra linearity I suspect the 100V caps will also be less prone to drying out.

BTW - I'm no guru, but I've been using Debian in my PCs since '98!

Cheers,
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Old 30th April 2008, 12:17 AM   #5
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The reality is that companies like to reduce their stock of parts to as few types and values as is reasonably practical. 63V covers a lot of territory without being excessive or expensive, so it's a good part to stock. It was probably used in lots of different products, and over a wide voltage range. Electrolytics like to operate at a reasonable percentage of their rated voltage, so it wouldn't be desirable to put a 63V part in say a 5V circuit. OTOH, even though they have a surge rating, you also like a bit of safety factor- I wouldn't use one in an unregulated 60V circuit. IMO, select the voltage using common sense, knowing what the circuit voltages are, and could possibly be. Sound quality will probably be more affected by the brand and type (series) of cap you use, if it's affected at all, rather than any general rule concerning voltages. Personally, if I could fit a decent 4.7uF film cap, even a Mylar, instead of an electrolytic, I'd go that route.
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Old 30th April 2008, 11:54 AM   #6
AndrewT is offline AndrewT  Scotland
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4u7F electrolytic? NO!!!!
go to a metallised plastic film type.
or better still metal foil.
But not electrolytic.
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Old 1st May 2008, 04:14 PM   #7
woodman is offline woodman  United States
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Quote:
Originally posted by Conrad Hoffman
The reality is that companies like to reduce their stock of parts to as few types and values as is reasonably practical. 63V covers a lot of territory without being excessive or expensive, so it's a good part to stock. It was probably used in lots of different products, and over a wide voltage range. Electrolytics like to operate at a reasonable percentage of their rated voltage, so it wouldn't be desirable to put a 63V part in say a 5V circuit. OTOH, even though they have a surge rating, you also like a bit of safety factor- I wouldn't use one in an unregulated 60V circuit. IMO, select the voltage using common sense, knowing what the circuit voltages are, and could possibly be. Sound quality will probably be more affected by the brand and type (series) of cap you use, if it's affected at all, rather than any general rule concerning voltages. Personally, if I could fit a decent 4.7uF film cap, even a Mylar, instead of an electrolytic, I'd go that route.

Quote:
Originally posted by AndrewT
4u7F electrolytic? NO!!!!
go to a metallised plastic film type.
or better still metal foil.
But not electrolytic.
Yes, my first reaction was to use film caps as I "know" electrolytic coupling caps are not as good for the purpose. However, I have three considerations: 1) I may not keep the preamp and do not want to spend the money on film caps, 2) there is not enough room for film caps in the phonostage, and 3) the most perplexing - I have not owned a nicer sounding phonostage! Since I do like the sound despite the electorlytics and number of gain stages, I don't want to mess with it too much.

I simply wanted to find the best electrolytics for coupling. I have looked for Elna Silmic & Tonorex, Nichicon Muse ES, and Black Gate N, and all of these are available in 50V ratings.

I must admit to a certain amount of self-doubt in replacing 63V caps with 50V caps - from day one at school, it was drilled into my head that one should never replace a capacitor with one of a lower voltage rating. My first concern is safety so I wanted to be absolutely certain that 50V caps will work and I now feel pretty confident about it. Still, I have been between thinking and over-thinking this circuit for a little while now and have a more concise question:

What would be the minimum safe capacitor voltage rating in this circuit?
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Old 1st May 2008, 04:30 PM   #8
djk is online now djk
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"What would be the minimum safe capacitor voltage rating in this circuit?"

15V would be fine.

These are nice, and about $1.50 each in small quantity (4.7uf at 50V). They also make them in 16V.

http://www.wima.com/EN/mks2.htm
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Old 1st May 2008, 05:39 PM   #9
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Well, IMO your schooling was aimed at a repair technician, but when you start pooging a design, you become the designer. You are responsible for safety, service life, sound quality, and everything else. The easy (and often preferable) route is to just replace what's there with an equally rated part. Get a good Panasonic cap and be done with it. OTOH, if you measure the supply and conclude the factory cap was over-rated, apply a 20% or so safety factor to the measured voltage and choose a new cap. The (probably) lower voltage might let you use a reasonable size film cap, and it would be hard for the worst film cap not to be better than the best electrolytic. Cost isn't much of an issue- stop looking at high priced snake oil and buy real world components. There are some nice cheap Panasonic polypropylene caps from Digikey for a buck or so.
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Old 2nd May 2008, 05:54 AM   #10
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I know that people say you have to operate a cap at a good fraction of the rated voltage, but in my experience, the capacity measures with my meter the same, and acts the same with higher voltage caps.

I used 100V 10uf cap in the input of my subwoofer amplifier, and even though it gets just mV's, it works fine.

Also the higher voltage caps are less crucial about accidental reverse polarity at lower voltages.

*I've noticed in a lot of consumer electronics, that a lot of 50V and sometimes 63V caps are used for the lower uf values . Usually it's when the caps are the same physical size as other higher uf, lower voltage caps in the circuit. Like you may see a bunch of 50V,1uf or 10V,10uf next to each other of the same brand and are all the same size. I notice mostly 1uf - 10uf you may see a lot of 50V and 63V caps in circuits that may run less than 12V. It's very common.

Also IMO the higher voltage caps last longer and die less, so I'm always a fan for getting the highest voltage caps I can get when building circuits. Only sometimes I may push the voltage rating close if they are good caps and I know its safe.
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