[pftrvlr]

Looking to build one tube headphone amp. Most of the schematics I see use 220uf/400v electrolytic capacitor in PSU. I find in ebay some 2200uf/400v for sale. Is 2200uf a overkill for phone amp? How big is too big for the application?

Yeah, 2200uf/400v capacitors are big - 50mm x 155mm!

 

[nikongod]

quality before quantity.

for a headphone amp, you should not need more than about 100uf.

with so few farads, they should all be of very good quality. might i suggest film?

it is my opinion that tube amps with mega-huge capacitor banks and no separate filament supply (turns heaters on before HV, and leaves them on after...) only serve to INSURE that the tubes will have HV on the plates well after the filaments cool off.

my speaker amp (a 2*4W jobbie with about 220uf of film) will play till the heaters cool off.

 

[splaz]

It depends a lot on the application.

Say if it's a filter cap for the power supply, it should be fine but may be overkill. Just make sure anything in front of it is suited to the inrush from it.

Other applications, depends...

I'd post up the schematic so where it's used can be seen and perhaps more knowledgeable advice can be given, probably from others.

That's not that big though, in my books big is beer can or bigger, although you can of course get ones like 44 gallon drum sized caps, not that I'll probably ever own one.

 

[AndrewFischer]

Quote:

Originally Posted by pftrvlr /img/forum/go_quote.gif How big is too big for the application? Yeah, 2200uf/400v capacitors are big - 50mm x 155mm!

2200µF 400V stores a lot of energy. U = 1/2C*V^2 or 176J
That much stored energy makes for a decent firecracker if something fails. A Chinese firecracker is about 30J.

With a correct design it will work.

You need to make sure that won't overload your rectifier or transformer. If everything is low impedance the inrush current will be large. You may need to use an inrush limiter.



What are you building? For a class A headphone amp, you don't need that much reserve power. More important is the caps ability to filter out noise. A pi filter with smaller low impedance caps is the standard high quality B+ supply.

 

[pftrvlr]

Thanks for the help. I had EE background, but never built a tube amp before.

I might go with a pi filter, either CRC or CLC. I will see if I can do some PSPICE sim to get a feeling of it.

What are the good smaller low impedance caps?

 

[Roam]

2200uf is just far too large and will actually cause several problems. Inrush has already been mentioned so I'll skip that. The big problem is a large capacitor will have massive charging current spikes when used as the first element of a filter. At the peak of each AC cycle where the capacitor "tops up", there will be a huge 20A-100A current depending on the ESR of the capacitor and the output impedance of the transformer. This spike creates massive amounts of noise which is then radiated from all the wires & through the air, contaminating every single part of the circuit. Not only that, if the impedance of the transformer is low enough it will take a chunk out the top & bottom of the entire AC circuit it's plugged into, you now have bad power in an entire branch of your home's power. This is what I call a bad thing.

The proper solution is a choke or resistor input filter, followed by a small to moderately sized capacitor and then further stages of RC or LC filtering as required to reduce ripple to your satisfaction. This keeps the charging currents smaller and increases the charging time, which means much less noise will be produced and coupled into the rest of the circuit. It also helps keep the AC power in the line clean so your other components can perform at their best.

 

[AndrewFischer]

Quote:

Originally Posted by pftrvlr /img/forum/go_quote.gif Thanks for the help. I had EE background, but never built a tube amp before. I might go with a pi filter, either CRC or CLC. I will see if I can do some PSPICE sim to get a feeling of it. What are the good smaller low impedance caps?

I'm using Nichicon UPW. There are better though. Panasonic but I can't remember the line.

With a CLC filter you want your input cap to be <50µF.