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Old 22nd October 2009, 05:52 AM   #1
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Location: melbourne
Default aleph power questions

hi all
im running a aleph 5 with double the output devices so 12 per channel instead of 6 and i,m running a 500va 30/0/30 toroid per channel instead of the normal aleph 5 voltage of 28/0/28

is this ok or do i need more voltage like 40/0/40 to run the extra output devices

also what is the resistor that controls the bias on the brian gt mini a boards when they are built as a aleph 5

id like to up the bias as i,m running 4x conrad 350 x 151 sinks per mono block and have heaps of heat to spare

regards sheafer
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Old 22nd October 2009, 07:24 AM   #2
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You dont need higher voltage.
4 conrad heatsinks per channel should easily dissipate 400 watts of heat per channel.
So you are looking at about +/- 40V dc and 5 Amps bias current.
If you want to do this then using Nelson's formula (2:1 ratio) you need an 800VA transformer per channel.
You may as well go the whole hog and go for 1.5kVA transformer per channel.

I always love to hear of realy ambitious people.

Last edited by Melon Head; 22nd October 2009 at 07:27 AM.
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Old 22nd October 2009, 07:27 AM   #3
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by thanh1973 View Post
You dont need higher voltage.
4 conrad heatsinks per channel should easily dissipate 400 watts of heat per channel.
So you are looking at about +/- 40V dc and 5 Amps bias current.
If you want to do this then using Nelson's formula (2:1 ratio) you need an 800VA transformer per channel.
You may as well go the whole hog and go for 1.5kVA transformer per channel.

Yep ...

Fully agree no substitute for cubic inches ...................
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Old 22nd October 2009, 10:13 AM   #4
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hi all
so am i correct in saying that if i double the outputs on the aleph 5 from 6 to 12 per channel (which i have) i can run the standard aleph 5 rail voltage no probs but the va rating of the transformer must be increased to supply the extra current ?

the 40vdc you refer to is this about what i will get after the powersupply and rectification using the 30/0/30 v ac voltage ?

im using a aussie amplifiers psu 3 powersupply per channel with 6x 18,000mf each

regards sheafer
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Old 22nd October 2009, 10:25 AM   #5
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Yes that is right. It could be as high as 42V, but you are guaranteed to have losses.
I am not sure what the PSRR of the amp is so you might need more capacitors.
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Old 22nd October 2009, 10:55 AM   #6
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Without doing any math, you should probably double the capacitance of the PS.

R13 on the Mini A attenuates the bias current, leaving it out will get you the junction voltage of Q5 across the source Rs so increasing the value above the A5 value will increase the bias. Don't go too far.

Check out the A60.

Best, Bill

Last edited by Bill Fuss; 22nd October 2009 at 11:07 AM.
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Old 22nd October 2009, 11:13 AM   #7
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hi guys
great so i understand (i think)

now re the capacitance ,so are you saying that the 108,000mf per rail i have is not enough ? i thought it would be fine
jeez looks like im going to need a chassis just for the caps themselves

re the toroid i cant find 30,0,30vac toroids in over 500va so im stuck there
i do have 2x 1kva 40/0/40 toroids but the voltage is too high too use unless i could cut it down to 30/0/30 somehow

hi bill so all i have too do to increase the bias is leave r13 out ?

regards sheafer
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Old 22nd October 2009, 11:16 AM   #8
AndrewT is offline AndrewT  Scotland
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500VA 30-0-30Vac transformer has a maximum continuous DC output current of ~4Adc, after the rectifier and smoothing. You cannot and should not ask for 5Adc bias for a ClassA amplifier.
It is usually recommended that the operational continuous DC current be limited to less than or equal to half the maximum continuous.
A 1200VA 30-0-30Vac transformer can just meet this 5Adc bias requirement.

Is Pass' advice to use a transformer that is rated @ 6 to 10times the maximum output power?
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Old 22nd October 2009, 11:41 AM   #9
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hi andrewt
so if i did run the 2x 500va 30/0/30/
with the above setup then what is the max bias i could run and be safe ?

or is the 2x 500va totaly unsuitable for this project

if i run higher power supply capacitance then can the va rating of the toroid be relaxed a little so that the 500va is ok

also i can get a the same 500va toroids in 25/0/25 if needed so im thinking by lowering the supply voltage a little i could still run a high bias with the 12x irfp240 per channel and stay safe with the 500va toroids

id love to be able to get some 25/0/25 in 1kva for the project but there is just nothinbg available in that voltage over 500va in australia

your oppinions are aleays appreciated
regards sheafer
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Old 22nd October 2009, 12:15 PM   #10
AndrewT is offline AndrewT  Scotland
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the continuous max is ~4Adc from 500VA 30-0-30Vac.
The recommended max continuous is 2Adc from 500VA 30-0-30Vac.
I thought this was clear in the previous post.
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