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Help selecting smoothing caps...!

N

ntom

pfm Member
Having made a start on a home build HiCap replacement, I am in a quandary as to what capacitors to use as the smoothing caps.

I am starting with a dual 16v transformer so after rectification I should presumably be able to get around 30v. The hicap has dual 16000 uF caps (as the hicap use dual half wave rectifiers -if that is the right term).

What value should I start off using? This is an experimental project & first foray into homebuild.... so i dont want to go over the top & want to bring the project in if not for minimal outlay then for not too great an outlay!!! I intend to have half a dozen regs attached to it to power an 82 pre.

Is it necessary to provide as much as 30000 to 40000 uF as per the HiCap?

If so any suggestions for best VFM caps?

Is it worth stringing together say 8 or 10 4700uF caps?

For longevity what rating do the caps have to be? Would 35V caps be sufficient? The hicap has 63v caps but then if I remember correctly the transformer is providing a higher voltage than I have....

I dont mind experimenting but having checked some of the sites there are so many makes / sizes / prices of caps available, some guidance from people who have been there would be appreciated.....

Thanks in advance

Neale
 
I would recommend you talk to LesW at Avondale and get a couple of Kendial caps from him. They are very reasonable in price, and they sound great.
 
There is some sonically discernable correlation between transformer rating and capacitor value. Put simply, ot works ilke this: We have fixed in our brains that a large capacitor value - equals large bass. Not so, in fact over a certain value, the bass becomes slow, overblown and plodding. A similar equation is attached to transformer size, a throwback to more machismo days perhaps. Upping the VA rating of a transformer will decrease the perceived bass extension until at a certain point, the bass will become so light as to uninteresting. These are generalisations of course, there are exceptions to every rule.

The use of toroidal transformers is now widespread as they have a low profile and a low external field. They are however, so wideband in their passing of mains borne hash that the regulators have some difficulty in providing a clean DC supply. This means that regulators have had to become more and more sophisticated over the years having to eliminate this mush and prevent the sound from being degraded.

The large number of mains conditioners etc. now on the market is testimony to the poor quality of our mains supply.

To cut to the chase:

If you want the best performance from a power supply, it's a good idea to start with a decent transformer. A so called 'EI' laminated is the place to begin and around 150 - 250 VA for a HiCap clone will be about right. Use a pair - one for each rail for the ultimate in intermodulation rejection. Following on, rectifier by either Schottky or fast recovery diodes neither of which produce much in the way of artifacts on the raw DC again, giving the regs a fighting chance. The HiCap uses only a pair of 15000uF caps and it's best not to increase this figure by more than say fifty percent. This leaves open the choice of regulators and as I have to declare an interest, I'll leave others to give you the griff on what's available and how they stack up in sound quality.

Finally, enjoy what you've built and use your own ears to evaluate what you've achieved - don't take any flak over 'non authorised mods'.

I hope the project gives you what you're looking for.
 
Les,

Reading an article on the TNT site regarding power amp PSU's, unless I got the wrong end of the stick the auther mentioned putting some caps between the transformer and rectifier to help clean up the mains first. Any thoughts?

Paul.
 
I'm not sure how you got the figure of 30V.

If you are using a 16 - 0 - 16 Volt transformer, with centre-tapped full wave regulation (ie. 2 diodes, as done in a Hicap), you will get a single output of

16 x root(2) - 0.6 = 22V DC.

Alternatively, if you use full wave rectification (ie. 4 diodes), with the centre-tap unconnected, then you will get a single output of

32 x root(2) - 2 x 0.6 = 44V DC.

It is better to use the same rectification scheme as in the Hicap.

BobMax
 
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