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Old 25th March 2012, 05:01 PM   #1
AC439 is offline AC439  United States
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Smile I thought it was a myth...

I stopped by a local surplus shop few days ago and saw some new 4700uf 35v caps cheap. They are rated at 105C and have a relatively small size (about diameter of a quarter and less than 1" of height) compare to their rated capacity. But I couldn't stop the temptation to buy them so I did. I just built a 1996 JLH dual rails amp with only a pair of 6800uF caps. I do hear small hums when I am close to the speakers so I know I have to improve the power supply.

I then used four of them (two in parallel, making a pairs) and added a pair of 1 ohm resistors to the original caps and form a dual rails C-R-C circuit. After readjusting the bias and offset, I put the unit to test. The JLH magic is gone ! The amp sounded less powerful and bass/midrange was weak. I then tried to bypass the 1ohm resistors and just let the new caps parallel with the existing cap. Same thing. I ended up removing the new caps and the magic is back.

The original pair of 6800uF caps is ELNA that I bought over 25 years ago. Even I forgot where I bought them and how much I paid. But they sound great, warm and very round. I thought the pair of old Elna would have already dried up over these years.

I used to think caps and just caps to filter our ripples. I thought the bigger the better. Definitely I now think that's more than that.......
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Old 25th March 2012, 05:07 PM   #2
AndrewT is offline AndrewT  Scotland
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Can you work out which capacitor parameter/s are not up to specification?
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Old 25th March 2012, 05:14 PM   #3
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Hello!

Just a thought here, but perhaps your 25 years old ELNA's have lost most of their capacitance over the years. That's probably causing the slight hum you've experienced - you are listening to an amplifier with VERY small filter capacitance, but this is also responsible for the kind of sound you got used to.

Now, with proper capacitance in place, you might just hear how your amp "really" sounds.

There are a few topics discussing the influence of small vs. large capacitance of power supply filter caps, first and foremost the LM3875-GainClone kind of amp. They're said to sound magic only if the ps-smoothing capacitance is very small, in the range 1000uF per rail.

Maybe this is all wrong, but as said, just a thought...

Regards!
Martin
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Old 25th March 2012, 05:25 PM   #4
AC439 is offline AC439  United States
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Andrew...I don't think I have the knowledge to work out which is not up to spec. But I remember I came across your statement that the C closest to the amp is more important in terms of sound.

Martin....The 25 years old Elna caps were not really used. They were pretty much sitting in the storage. The hum is not bad, I cannot hear it when I'm more than 1 feet away from speakers, and there has to be no music to hear the hum. I don't know about if smaller caps are good to JLH but I have read enough here that people are using much bigger caps than I do for their JLHs. I know class A needs really good filtering.

I think I will save some money and buy better caps down the road. I think the good sound now is due to the JLH schematic and the ELNA. I heard people praising ELNA caps here.....

AC
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Old 25th March 2012, 05:31 PM   #5
AndrewT is offline AndrewT  Scotland
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You can fairly easily measure leakage current.
You can fairly easily measure discharge time at a fixed current.
With quite a bit of research you can measure ESR.
Quote:
I know class A needs really good filtering.
this is not strictly true.
A ClassAB amplifier draws little quiescent current when idling. This low current demand allows the smoothing capacitance to deliver a very low ripple DC to the amp.
The amplifier then attenuates the line ripple and with luck and good design you do not hear the hum when the music is not playing.
A ClassA amplifier draws a continuous quiescent current when idling. This high current demand results in a higher ripple on the DC supply to the amplifier. It's the job of the well designed amplifier to reduce the hum down to an inaudible level when no music is playing.

The ClassA quiescent current can easily be 100 times the ClassAB quiescent current. This makes the ripple ~40dB worse for the ClassA situation.
Either reduce the ripple or increase the hum attenuation or do a bit of both.
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Last edited by AndrewT; 25th March 2012 at 05:39 PM.
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Old 25th March 2012, 05:36 PM   #6
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Default Try Elna Silmic

I've had some VERY good results using Elna Silmic caps in power supplies. So far, they have sonically surpassed a bunch of other highly rated caps, Nichicon, Panasonic, Blackgates, etc. There is definitely something special about some of those Silmics. (Maybe not all of them, I haven't tried much beyond the 4700uF variety).

Yes, there are many here who would say "It only makes a difference if the specs are different"....but there are also others, like me, who do hear a difference, even when our regular testing procedures wouldn't point that way. Bottom line, you're not out to satisfy anyone other than yourself, so explore this more.

If, as you have found, you are hearing a difference, then you might want to look into the more recent Elna' offerings and compare. I have... and I like 'em. A lot.

Best Wishes on your continued exploration.
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Last edited by Jack Caldwell; 25th March 2012 at 05:44 PM.
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Old 26th March 2012, 01:48 PM   #7
AC439 is offline AC439  United States
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Thanks again Andrew. I will dig a little deeper about measuring those cheap caps. But I think I get what I paid for. The caps (4700uF 35v) has no brand name and only cost me 50 cents (US$) each.

Jack: I'd definitely like to try a more recent version of ELNAs. I went to mouser.com last night but can't find them. Can you suggest any reliable web parts stores ? I don't want to buy electronics from ebay.
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Old 26th March 2012, 06:15 PM   #8
Mooly is online now Mooly  United Kingdom
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How about a picture of the caps
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Old 26th March 2012, 06:21 PM   #9
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Try a simple experiment with the new caps to see if they are truly 4700uF.

Its not accurate but discharge them and then wire them across a battery with a known resistor.

Calculate t=0.7RC and measure them with a stopwatch to see how long they take to get to about 70% of Vcc.

As I said, its not perfect but it will indicate if the caps are about 4700uF.
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Old 26th March 2012, 06:45 PM   #10
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That method can be very accurate for "pure capacitance" (ignoring internal impedances) using a fairly high resistance that keeps the time between stopwatch points practically long to reduce error.
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