Parallel capacitance

[glen65]

Hello guys, (I need memory jolt)
I have seen in amp power supplies where they connect
caps in parallel to increase capacitance where they would
have one smaller cap in parallel with the larger one. The idea
was obviously to increase overall capacitance to a certain level.
But it seems to me there is another reason I just can’t remember
exactly what it is. It seems to me it had something to do with
charge and discharge time but I’m not sure.


Thanks

[richie00boy]

Big caps have poorer high frequency performance than small ones.

[vynuhl.addict]

Small bypasses .01-10uf are mainly for bypassing a larger cap at higher frequencies due to its poorer ESR, impedence path to ground for AC at these frequencies. But with everything there is of course always a catch, you can very easily go overboard with bypassing and cause more problems than without bypassing. Many will say stager them in 1/10th values downward but even that can very easily be overkilll. Typically an audio grade capacitor will have datasheets which list their ESR at high frequencies, with bypassing you dont want to cover too much of the frequencies already taken care of by the large caps but frequencies above at which it will be weakest and drop out of the circuit at as a decent rule of thumb


Colin.

[glen65]

Thanks for the reply,
I have seen cases including some of the diy amps
where for example they would have a 10,000uf cap
coupled with a smaller cap "say 8200uf or smaller".
I knew there was a reason other than the obvious
increase in capacitance. I just couldnt remember what
the advantage of doing this was

[sandyK]

If they have used 8,200uF in parallel with 10,000 uF , it is most likely because they wanted 20,000uF but did not have any other 10,000uF available. Using 2 in parallel like this to achieve the higher capacitance also results in lower impedance at HF. If space permits, this may be far more cost effective than a larger , more expensive type with low ESR.

SandyK

[FastEddy]

" .. But it seems to me there is another reason I just can’t remember exactly what it is. It seems to me it had something to do with charge and discharge time ..."

There is a relatively new trick with audio op-amps involving parallel caps of differing types. Take a look at Page 15 of this: http://www.analog.com/UploadedFiles/...D8065_8066.pdf Note the parallel caps bypassing the power rails. These are usually speced out as a combination regular electrolytic and plastic (polypropylene, polystyrene, MKT type). The plastic caps are usually placed very close to the op-amp (close coupled), but the electrolytics can be anywhere on the smaller boards.

The results are much better specifications for the op-amps (better THD, less phase shifting, better CRMM & PSRR, etc.).

(SandyK is also correct ... parrallel caps are "additive" [C + C + C = 3C], serial caps not [1/C + 1/C + 1/C = 3/3C] ... http://en.wikipedia.org/wiki/Capacitor .)

[sandyK]

The parallel 4.7uF shown is a tantalum capacitor, as mentioned in a later page. I have seen this Data sheet before, as I am using both AD8065 and AD8066 in some equipment. I do agree with a 4.7uF electro used as stated.
BTW, I hate tantalum capacitors in audio circuitry. I have also had several tantalum bypass capacitors, that were well within their ratings, go short circuit. Where tantalums can be worthwhile, is on much larger PCBs with long circuit traces. at the power supply end. e.g. DAT players, Cassette decks etc.
SandyK

[FastEddy]

" ... agree with a 4.7uF electro used as stated. ... BTW, I hate tantalum capacitors in audio circuitry ... I have also had several tantalum bypass capacitors ... go short circuit. ... "

Likewise, and I've also seen 'em go zzzzzttt ... smoke and stink

The electro / plastic combo rules, dudes.

There are several interesting articles on this very subject ... ask Bob Pease.

[john65b]

I built a UCD400AD a while back and have a 56,000uF cap per rail with no bypass...some may say way too much if not a Class A, but I had them already.. Sounds just fine on both upper and lower end.

Anyway, been thinking of bypassing them with two .047uF Polystyrene caps that I have, totaling just under .1uF - don't really have a problem with sound, but what the hey... and I heard Polystyrene cap is the next best cap after no cap at all...

[FastEddy]

" ... been thinking of bypassing them [56,000uF x 2] with two .047uF Polystyrene caps that I have, totaling just under .1uF ..."

Good choice ... you may discover a slight improvement with results of spectrum analysis, but probably not too much of a "golden ear" difference. You might consider putting the plastic caps as close as possible in parallel with the onboard electrolytics on the main board(s), which may require four instead of two. ( Close to the tall electrolytics in back, close to the output FETs & heat sink(s) = http://www.hypex.nl/pics/products/UcD400.gif )

I use 0.33 uF yellow MKT type rated to 100 volts ... I buy 'em by the hand full = ~US$0.25 each, but any value above about 0.01 uF will work. The basic idea is just to smooth out the higher frequency noise of any power supply, that is trying to get past the fat electrolytics on the PS rails and interconnecting wiring. Mounting them close to the output MOSFETs or Bi-Polars works best and covers a world of sins ...

IMOP: another place where this quick and easy mod sometimes does a whole lot of good is on CD and DVD players that have analog output circuits (like a built in DAC). A couple of those 0.033 uF yellow MKT types right across the +/- internal power rails leading to the op-amp(s) on the DAC board(s). ... The switching supplies on the cheaper players are notoriuosly noisy and this helps a lot = Example: Oppodigital.com model 981 has such a DAC for 24bit / 96K / dolby 5.1 output. I just popped a couple of these plastic caps into the bus / power connections ... and immediately noticed a significant difference in clarity of the analog audio. Even my old ears could tell the difference.

(FYI: the above is a common DIY fix and is documented elsewhere on the 'Net = works great.)