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Old 3rd April 2015, 06:47 PM   #1
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Default Picking caps and paralleling low esr with high esr caps

What should I look for when picking caps to recap my mcintosh mc2205? I don't want to spend money on worthless things, but I also don't want to cheap out on an upgrade that is going to hopefully last another 40 years. Do I look for low esr? I've heard a higher operating temp can last longer but idk on that. Just looking for what criteria and ratings I should be keeping an eye out for when selecting caps.

Also I've been told that when using big electrolytic caps, like the 39000 uf filtering caps in the power supply it can be beneficial to put a low esr ceramic cap I think in parallel to improve sq, but I'm currently an EE student and I don't see how the low value, low esr cap would really do anything at all until you get to really high frequencies well above the audible range.

Also just curious, what limits the power output on an amp like my mcintosh? I know power guard kicks in with distortion, but what causes the distortion at high power? And what leads to clipping? Is that when the transistor voltage reaches the voltage of the rails?

Thanks!
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Old 3rd April 2015, 07:10 PM   #2
jcx is offline jcx  United States
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wet electrolytic caps eventually dry out as their electrolyte solvent evaporates, diffuses through the seal
higher temperature rated caps have better seals and diffusion rates are a strong function of temperature so derating - using say a 105 C rated cap at 60 C will give over an order of magnitude longer expected life

small caps are essentially useless across the terminals of huge caps - which are connected by wiring and traces that add inductance swamping the small cap high frequency possible advantage

put small caps physically at the locations where they are needed so that wiring effects don't waste their utility

put snubbers across bridge diodes, local decoupling at power Q terminals
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Old 3rd April 2015, 07:15 PM   #3
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There is no need for low ESR in your application.

It is generally a bad idea and totally useless anyway to parallel small cap over a remote large PSU cap.

Yes, clipping is when the output hits the rail.
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Old 4th April 2015, 08:24 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
I'm currently an EE student and I don't see how the low value, low esr cap would really do anything
indeed !
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put small caps physically at the locations where they are needed so that wiring effects don't waste their utility

put snubbers across bridge diodes, local decoupling at power Q terminals
Locate the capacitors where they can do some good.
At each location, choose the type that suits that location duty.

A capacitor alone is not a snubber.
A capacitor will resonate with an inductance if there is a step change in current.
To attenuate the resonance one must add in resistance to dissipate the resonant energy.
A snubber is a series combination of RESISTOR plus capacitor to reduce the DC dissipation.
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Old 4th April 2015, 09:00 AM   #5
CBS240 is offline CBS240  United States
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Indeed.
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Old 4th April 2015, 09:59 AM   #6
DF96 is offline DF96  England
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Originally Posted by maggiesnmacs
Also I've been told that when using big electrolytic caps, like the 39000 uf filtering caps in the power supply it can be beneficial to put a low esr ceramic cap I think in parallel to improve sq, but I'm currently an EE student and I don't see how the low value, low esr cap would really do anything at all until you get to really high frequencies well above the audible range.
You will have been told that by people who have never been EE students, and so would not recognise a parallel resonance even if it blew up in their face. Much to the surprise (or denial) of some people, audio circuits obey the laws of physics.


Stay with the EE studies. Then you can become a better audio designer than many 'audio designers'.
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Old 4th April 2015, 11:58 AM   #7
wwenze is offline wwenze  Singapore
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To add to above, read datasheets too, the parallel ceramic/film cap myth comes from some mythical thinking that bigger caps tend to have high ESR. This isn't true; comparing capacitors of different values from the same product line the bigger ones will always have lower ESR. It's not a proportional difference (e.g. 2200uF doesn't get half the ESR of 1000uF) but it's still a noticeable decrease.
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Old 4th April 2015, 01:42 PM   #8
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