lots of parallel caps, why?

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JojoD818

JojoD818

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#1
Hi everyone,

I just listened to a Carver AV-64 and was pleased with it's sound. Looking through the chassis, there seems to be nothing special about the power amp's circuit but I did noticed an array of 1,000uf capacitors, 10 per rail, for a total of 20 pieces.

My question is why use 10 pieces of 1,000uf caps when you can use a single 10,000uf capacitor? Does it have to do with ESR? Or whatever it is called? Any advantage, disadvantage?

Best regards,
JojoD
 
#6
Hi,
besides capacitance, every electrolyte cap has resistance and inductance :bawling: . Bigger electrolytes tend to have higher inductances, smaller caps lower inductances.

By paralleling similar caps you add the capacitances, while the over all inducance equals cap inductance / number of caps.

So by paralleling 10 caps you only have 1/10 of the inductance.
Also the cap resistance is reduced. Hence the parallel setup may deliver more current faster. :devilr:

Cheers
KlausB
 
#8
I suspect the more prosaic motivations: cost and space.

From my own diy experiece I conclude that sometimes many smaller caps cost less than fewer big ones, this may be even more the case when you buy in industrial quantities.

Secondly, many small caps can sometimes be packed in to the volume of an enclosure more easily than a few large ones. The opposite can be true depending on the dimensions of the enclosure and the other components.
 
#10
It is true that you divide the inductance by the number of caps by paralelling them.

Im though sure that the inductance of the caps in a powersupply is not an important factor....ESR is though.

As stated before...i agree (unfortunately) that its for prosaic reasons that many smaller caps are used instead of one big cap.

I use big caps with a film cap in paralel.....works as good as anyone could ever ask for. If you want something better than that, use a regulated power supply.


Magura
 
#11
Re: be Wary Of Setting Up Resonances Though.........

mrfeedback said:
An ME amplifier model (850?) runs 100 x 3300uF caps in the psu (330,000uF total).
Limitless psu current capability and it sounds like it too.

Eric.
Click to expand...


Anything with 330.000 uF of reasonable quality capacitance has unlimited current capbility.....it is simply just a matter of overkill in this case, then you end up with a battery powered amp instead 🙂



Magura
 
#12
I'm sceptical about the advantages of many parallel small caps, and tend to think that this approach is based on a false assumption.

For instance 10x100uF will have only slightly lower esr than a single 1000uF of the same type but 2x1000uF will have even lower esr, twice the capacitance and take up far less board space. Also many small caps will probably have higher lead and pcb track inductance.
 
#14
Richard C said:
For instance 10x100uF will have only slightly lower esr than a single 1000uF of the same type but 2x1000uF will have even lower esr, twice the capacitance and take up far less board space.
Click to expand...

that's true. the advocates of paralleling caps assume that ESR for all caps, big or small, is the same. however, if you look at datasheets for caps, you will notice that bigger caps tend to have smaller ESR.

another benefit for parralleling amps is that you spread the resonance frequencies over a much wider range (thus lower Q for the combined "cap"). This can be achieved by parralleling different caps tho. decoupling is such an example, and others have used on OTL output capacitors.
 
#15
Hello,

No one had mentioned dialectric absorbtion.

I believe you may have more 'speed' in your power supply but there must also be a negative or more manufactures would be doing it.

Having too many caps may 'smear' some transients in your music.

I like the sound of (1) good quality big cap paralled with a high quality film cap.

Any comments?

Thanks

Kevin Lee
 
#16
Magura said:
Ripple capacity for a cap is the current you can draw from them, naturally limited by the esr as well.

MAgura
Click to expand...

Magura,

Are you referring to the load current pulses? Normally ripple current is understood the charging currents during the diode opening cycle, which give rise to the ripple voltage.
It is true that larger capacitors have larger puls load current specs than small ones, so possibly there is very little difference in this respect between a single 10.000uF or 10 x 1.000uF. But I'm not sure about that, should look up the specs.

Jan Didden
 
#17
Another possibility is capacitance matching between pos and neg rails. Most electrolytic caps have tolerances of about 20%, which is pretty crummy if you think about it. If you are making lots of psus and you want a specific capacitance and you want it the same on both rails you can make use of a variety of values by selectively combining them. This could be cheaper than using just two caps and asking the capacitor supplier to select parts to, say, 1% tolerance.
 
#18
traderbam said:
Another possibility is capacitance matching between pos and neg rails. Most electrolytic caps have tolerances of about 20%, which is pretty crummy if you think about it. If you are making lots of psus and you want a specific capacitance and you want it the same on both rails you can make use of a variety of values by selectively combining them. This could be cheaper than using just two caps and asking the capacitor supplier to select parts to, say, 1% tolerance.
Click to expand...

Or you don't even measure them but just trust the statistics
to even out the sum of values. One potential problem, perhaps,
is that they individual caps may age at a different rate, eventually
leading to the same problems as with a single cap for each rail.
It will measure and sound well when brand new off the shelf
though, which is what sells it. 🙂
 
#19
millwood said:


that's true. the advocates of paralleling caps assume that ESR for all caps, big or small, is the same. however, if you look at datasheets for caps, you will notice that bigger caps tend to have smaller ESR.

another benefit for parralleling amps is that you spread the resonance frequencies over a much wider range (thus lower Q for the combined "cap"). This can be achieved by parralleling different caps tho. decoupling is such an example, and others have used on OTL output capacitors.
Click to expand...


Interesting assumption about other peoples ability to read data sheets 🙂

When paralleling capacitors there are several advantages over the big cans. First of all, the self resonant frequency of small caps is higher. This is of course a function of the lower series inductance. This is pretty much true for most applications even small caps for digital bypassing.

The disadvantage of smaller caps is lower expected life due to smaller volume of dielectric fluid.

Also, for those who claim big caps + film is the best - consider trying to make the big caps without film bypass better before applying the film bypass.

A final thought about really huge reservoirs - when using them, you typically need a higher rated transformer and diode bridge to be able to take advantage of the increased capacity. The reason for this is that the charge pulses get progressively smaller unless you are willing to add series impedance (such as inductance or resistance) prior to the capacitor banks.

Petter
 
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