bleeding off a few volts from PSU

[cowanrg]

well, ive finally resumed progress on my big aleph2's.

my transformers will arrive tomorrow (1.7kva per mono). they have 45v secondaries. i am having a hard time finding caps that are large and rated above 50v. the ones i want to use are only 50v.

my question, after rectifiers, im sure i will be over 50v... with current draw, i should be right at that, or a bit under. ive heard of using a pi filter to lower power a bit? can i do something to make sure i stay under 50v?

or better yet, can somoene tell me where to get some reasonably priced caps that are 63 or 75v rated? im going to do about 250,000uf per channel, so they need to be BIG.

thanks! im ordering the metal here in a few days and will be re-designing the case. the new case will be 0.25" top, bottom and back, and 0.5" sides. its all going to be CNC milled to fit together like a glove. i didnt like the last one, looked too much like sheet metal.

[MikeW]

This is a good price.
http://www.apexjr.com/new.htm

[AuroraB]

You will need at least 70V cap's.
45V secondaries will give you 60V++ after rectifiers, so a bleeder will have to take out at least 15V to be on the safe side!
If your trafo is a toroid, at least here in Europe, toroids are spec'ed at full load, which means that your idle voltage will be even higher. You should also have enough safety margin to handle slight overvoltage,- also within the allowed +10% from your power company.
I think I would settle for 80-100 V myself................

[byteboy]

If you have the space available for it, maybe using cheaper 50 Volt types in series (double number of capacitors, because capacity would be halved) would be a cheaper solution than buying (much-) more expensive single 80V-100V types?..........

What types did you plan to use and what is the price / source?
(I' am looking myself too for a good/cheaper source for capacitors)

[jackinnj]

your power company is going to love you. what do you pay for a KWH out there in Colorado (it's about $0.12 here in NJ).

63.6 volts rectified and filtered -- plus a 15% margin for line voltage variation -- so you might as well breathe in deeply and order 80V (Panasonic TSHA) or 100V caps.

If you are going to load up a quarter of a farad, you should put a slow-start circuit on the power supply line -- I use a 100 ohm 50 watt resistor which is shorted out after a couple seconds -- this reduces the surge current -- remember that 1/4 of a farad looks like a dead short for some milliseconds. 50 watts seems to be OK since the resistor is only heating for a couple seconds. There are resistors in this flavor available in TO-220 packages == a little more expensive than the surplus store, but a lot smaller and they can be heatsinked.

Lastly, excercise extreme car when "tooling" around this much capacitance. I've stated before that you want the power supply voltage to decline by 90% in the time it takes you to unscrew the cabinet -- this may not be conservative enough in the case of so much capacitance since the reservoir will still be capable of supplying amps of current. It's time for "belt and suspenders".

When the music stops you will want some form of "bleeding" to kick in -- you can work the math -- if it were my project I would use the reservoir caps to drive a fan on an active load.

[cowanrg]

thanks everyone for your input!

to start, my power only costs $0.071 per KWH. its a bit cheaper. lost month's power bill, for the WHOLE house with taxes (3 bedroom house with AC, and all electric appliances), was under $45. we never leave on lights that we dont need, etc... so power isnt an issue

as for bleeding the cap bank before working around it, im used to it. my aleph3's have 208,000uf per channel. this is almost 1/4 farad. as SOON as i opened up a case to work on it, or just turned it off to check a problem, i reached for this motor i have. its a pretty big one, but will operate on as low as a volt, but not burn up with 50v... so i just touched it to the cap bank, and it runs for a half a minute or so, and when it stops, i KNOW the bank is dead. it will still barely turn at 1v. i could bleed each rail down to about 0.7v or so. i did this every time i would play around.

i have no problem loading my aleph3's up. they are operated by a 12v trigger from my pre-amp, and they both kick on at the same time. i have a thermistor on them and it seems to work well. the lights dont really dim all that much when BOTH kick on at once.

i think the best solutions so far would be to use a TON of the little ones MikeW suggested (i would want like 30 or so per channel), OR do what byteboy suggested and just double up. this would effectively get me 100v from a 50v cap, which would be playing it safest.

my xformer is a toroid. loaded it will most likely be a bit lower, but i would still be running close to 60v if i wasnt bleeding off a few volts. so, i would need 63-70 or higher. so, let me see if i have room inside the case for 50 small caps, or a few bigger ones.

byteboy,

this is what i was planning on using before i found that 50v wouldnt work. they are 50v, but are 68,000uf, so you dont need many:

http://www.apexjr.com/capacitorsR.html

scroll down to computer grade. at that price, i can get several.

[GRollins]

Burning off 15V or so per rail at ca. 3A is an awful lot of waste heat. If you're serious about building Aleph 2s using those transformers, I see three choices:
--Regulated rails.
--CRCRCRCRC...
--Variac
If anything else occurs to me, I'll check back in, but I'd recommend biting the bullet and building 1.2s.

Grey

[cowanrg]

hum, ok.

none of those are options i really want to do.

i think my best bet for now is to keep the rails at their voltage, and get twice as many 50v caps as i had thought, wire them in series, and then have 100v caps. that would allow me higher rail voltages, AND keep my capacitance.

i was thinking of doing 8 of the 68,000 uf caps though. so now i need 16 per channel. thats a whole lot.

[byteboy]

Maybe instead of CRC you could better use RCRC....etc. (or LCLC.... )?

It could lower the voltage at the first cap(s) enough, if correctly chosen, to a voltage you could get away with using 63 Volts caps?

[cowanrg]

true, but i havent really seen any 63v caps out there either. PLUS, i do like the idea of actually having extra power on the rails. i plan to run these with a little more bias than normal, and always at 4 ohm, so the extra voltage cant hurt...

im seeing if i can fit twice as many caps in my case. looks like i can.