Power supply Capacitors

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Power Supply Caps

I think it will help. I think it will tighten up the bass. When the amp needs more current, usually in the bass freq., it will have it on reserve.
There are formulas out there to estimate what you might need, but I think the more the better. But, on start-up, those Caps will put a strain on your bridge.
I saw your post and thought I'd give it a try myself. I had some 37,000 ufd caps for a future project so for kicks I swapped them with some 11,000 ufd in a Soundcraftsman amp I have. WOW there is a pretty big difference in the bass, with much cleaner bass at low levels and quite a bit more power before the onset of clipping. Now I just have to figure out how to FIT these in the case!
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Power Supply Caps

It does make a difference, doesn't it??

The caps will not drawl much more voltage (there may be a peak voltage...I'm not 100%, most likely!), but they will need larger amounts of energy to fill to capacity. This is a problem at start up. Ever hear the thump in your speakers when you power up some amps? That's where the problem starts, then it goes back to normal. All that energy gets drawn to fill up those large resevoir caps. (This is the argument for leaving you amp on all the time. But some think that leaving it on will degrade parts just as well, so which is it? I don't know. That's why it's an argument!!) The break down of the rectifiers happens over time. The bridge should be at lease twice the secondary voltage coming out of the transformer, and maybe 5 times the current drawl.

I keep refering to Nelson Pass and Pass Labs. They have a great article on his "ARTICLES" web page about power supplies. It explains power supplies pretty well. Also, read the A75 article from cover to cover. I learned so much from it.

Oh, one last thing!! Be real careful when working with Power supply CAPS!! I've seen them melt tips of screw drivers!! They can store a voltage long after the power is cut from them!!! It's their job! Find out how dissipate the stored voltage. Anyone know a good value and wattage of resistor to do it??

Great to hear the upgrade helped!!
Well I normally leave the thing on. As a matter of fact I have been running this particular amplifier constantly for about the past 3 years. Nothing has ever failed in it. So I am not worried about the rectifier failing. As for discharging big capacitors it is really quite simple, just take a screwdriver, grab the rubber handle and use the metal part to short out the capacitor! It is dangerous I am sure, but so is sky-diving.
No, that's a good way of destroying the capacitor, your screwdriver and possibly yourself.

The way to discharge a big capacitor is to connect a resistor across the terminals. The resister should be big enough to not exceed the current rating of the capacitor and rated at high enough wattage to withstand the power it will be dissipating. A half-watt 10k resistor should work fine for 50V rails. You need to leave it there long enough to discharge the cap, which is given by 5RC, where R is the resistance and C is the capacitance.

You can use smaller, higher power resistors to discharge it more quickly: ie a 500R resistor would discharge the same cap in 25s, but would need to be at least a 5W job for 50V rails, better if it were 10W.

A better solution is to wire large leak resistors (10k-100k) across the power supply capacitors. Then when you turn the equipment off, the powersupply will automatically discharge after an hour or so. It's a lot safer than sticking metal objects into high power electronics. You need to choose the resistors to be large enough so that the current leaking across them is insignificant (factor 10) compared to the current drawn by the circuit the caps are powering.

As Jon has pointed out shorting the terminals of the capacitor to discharge it is most dangerous.Charged capacitors are ticking bombs.I have tried it when I started out as a DIYer and I have a melted screwdriver.Fortunately I was not hurt.So play it safe.

[Edited by Vivek on 12-13-2000 at 02:46 AM]
Bigger caps will generally improve the bass in an amplifier by increasing the damping factor but beware that larger caps usally have more internal inductance and that will decrease the damping factor in the mid and high frequencies. To solve this connect a 10-20uf polypropylene Capacitor accross the big fliter caps. I have done this with many consumer amps and the sound has improved quite a lot and made the owners very happy.
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By-Passing large caps


Say you have four 30,000 mF caps in a full bridge config. Would you by-pass every cap, or just the closest to the circuit?
Also, should the by-pass caps be 10% of the value of the 30,000 mF caps? Or more?

(Actually 10% sounds like too much.)



[Edited by vdi_nenna on 01-11-2001 at 07:20 PM]
Many engineering sources say to bypass with 10% and this may be a good idea with such large filter caps. Use a low ESR type cap at 5,000u and then bypass that with the poly with a value of 500u. place these as close to the wire that supplies the current to the amp circuit. I don't thing you would gain to use a smaller polly to bypass the 500u.
Bigger caps give better bass that is true because you have more power stored. So when needed the amplifier can supply more current. At low frequencies the current needed is large. Bypassing the caps is a good idea. What also helps very much is putting some caps on the diodes of the rectifying bridge. It filters the RF emition of the diodes making the sound of the amp alot clearer.
Things in parallel with Filter Caps

Ok, so we're collecting quite a few components near the power filter/reservoir caps now...

Is it OK to wire the following items all in parallel (point to point) with the terminals of the main caps:
1) Main Filter/Reservoir Caps
2) 10% of main caps for mid/high damping
3) 0.01uF caps for filtering out remaining AC noise
4) 5w resistor for draining main caps on power off

What I thinking is crimping/soldering one end of 2,3,&4 above to one lug, and the other end of 2,3,&4 to a second lug and screwing them across the terminals of my Main Filter/Reservoir Caps. Any problem with this?
I just came across this thread whilst searching for information about safely discharging large caps. I noticed it was quite old so refreshed it for the benefit of other new members to the forum.

A few minutes ago I did what many people have done and placed a screwdriver across the terminals of a meaty cap to check for charge. The screwdriver nearly welded itself to the points and it scared the life out of me, can't imagine what would've happened if i'd put my fingers there!! or even worse if the cap had exploded.

I will try the resistor method (outside, rather than 12" from my face on the kitchen table!), guess it's just a resistor held in pliers across the terminals. How will I know when the board is safe to handle? as I want to take some of the components off (I am dismantling a broken amp to us the chassis and maybe some other parts for a chip amp)? The board is now removed from the chassis so there is no earth connection of any kind.

Does the 'screwdriver method' pose a real risk to causing the capacitor to explode? or just a big shock? I certainly won't be trying it again anytime soon.

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