Has anybody built an Aleph-x to this spec??? - Page 2 - diyAudio
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Old 28th April 2004, 03:11 PM   #11
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Ok Richard, as requested one power supply schematic for your amp. Now, I have assumed a few things when calculating the values for the components here:

First, the rails after rectification will be at 24.5V and you wold like them to be 20.5V (so we have to lose 4V in the supply some where)

Second, you will be drawing 8A bias curent (this is what you stated earlier and I assume you haven't changed your mind)

Third, you don't want your resistors in the supply getting too hot, so I have suitably over rated them for the job.

Ok, with all of that out of the way (and looking at the schematic below), you can see how the transformer will be wired to the +/-AC points on the rectifier PCB; with the two centre taps joined together and atached to your star grounding point. You will then need to run a line from this star ground point to the 0V section on the capacitor part of the power supply (you could use this point as your star ground, if you are going to use a large aluminum plate like a lot of people seem to do, that way you won't need as many cables in your amp).

Now, as you want to drop 4V accross the supply and you will have 8A flowing in the circuit, that will mean that we need a total of 0.5ohms in series with each supply rail. Now, I mentioned in the e-mail about tring to optimise these vaues and their placement, but for now, I will simply say that the total resistance should be split equaly between the two resistos on each rail (R1 and R3, or R2 and R4), so that you will have 0.25 ohms for each one. Also, with 2 volts accross each resistor and 8A flowing through them, each 0.25 ohm resistor will need to dissipate 16W.

Now, as 0.25ohms is not a standard value and 16W is quite a high rating, the best way to mke these resistors up, would be to parallel a number of resistors together to get the desired result. There are two main combinations which could be used for this, firstly, four 5W 1ohm resistors to get exactly 0.25 ohms and have a bit of spare dissipation. The second would be to use two 0.47ohm 10W resistors which will give a slightly lower 0.235 ohm resistance, but will still drop nearly 4V overall. The choice is up to you in terms of cost and space as to which combination you go for (you could even use 0.22 ohm 25W metal clad resistors if you were to mount them on a suitable heatsink surface), but all of the ones I have mentioned above should work fine in reducing your supply rails to close to what you want as well as helping to reduce the supply ripple you will get.

If you have any more questions though, feel free to ask and I will try and give some sort of useful answer.

andrew.

P.S., I will test the bridges in so much as they have the correct conections made and so forth, but I haven't got any real means of testing them at the 8A supply current you will be using them at, as I haven't got anywhere at all in putting my supply together yet to test it (the woes of only having one room to live in, and too much junk).
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Old 28th April 2004, 03:22 PM   #12
D3 is offline D3  England
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Thumbs up Thanks!!!

Thanks BP.........you really have helped I ideally want to get to arround 20v to get my IRFP044N back in a realistic operating zone.

Will this supply give good ripple results..........from the threads I have read it should work pretty well.

Cheers, cant wait to get the bridges.....by the way how much heat do you think will be emitted by the bridge???

Cheers
D3
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Old 28th April 2004, 03:29 PM   #13
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Hi, for the bridges, with your 8A draw, you should be looking at about another 16W in total from all four diodes, so you shouldn't get any problems with this.

As far as the ripple goes, I guess the best way would be to simulate the circuit in some software, however, I have never tried to do that before (I guess I should learn at some point) so you may need to find someone else to look at it for now, but I imagine it will be very low anyway.
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Old 28th April 2004, 04:14 PM   #14
Rarkov is offline Rarkov  England
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Default My Layout

A while back, Richard asked me for closeups of my Aleph boards. Here are all the images I have + my proposed layout. These are going to be collosal!
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Old 28th April 2004, 04:15 PM   #15
Rarkov is offline Rarkov  England
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The boards...(and the compulsary dog hair of course!)
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Old 28th April 2004, 04:19 PM   #16
Rarkov is offline Rarkov  England
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Luckily I made a swap with Cobra2 for ready built boards. He's used great soldering & components throughout so all credit should go to him. He also provided the extra large caps!
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Old 28th April 2004, 04:21 PM   #17
Rarkov is offline Rarkov  England
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Finally - the ammount of capacitance per channel?

2*(230000uF + 68000uF + 33000uF) = a whopping 662000uF
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Old 29th April 2004, 03:31 AM   #18
moe29 is offline moe29  United States
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It's hard to tell, but are the diff. pair FETS isolated from each other?

Is it that i just can't see the rubber inserts?

I haven't been able to locate nylon screws or other proper hardware
to isolate them...
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Old 29th April 2004, 03:58 AM   #19
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Use metal screws and the little nylon inserts that fit into the holes of the TO-220s. I've never cared for plastic screws, as they deform over time, especially when subjected to heat.
Looks like a SILPAD between the MOSFETs, but I can't tell about hardware.

Grey
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Old 29th April 2004, 03:14 PM   #20
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Default SILPAD

Mr Rollins

Should there be more thermal mass than just 'silpad' .....would it be a good idea to use a thickish section of aluminium....to give thermal mass......or do the two transistors in question....need to be directly...........

I seem to remember reading that this was one possible solution to thermal instability....
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