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Old 8th March 2002, 11:29 PM   #1
Gabster is offline Gabster  Canada
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Question Power Supply for Aleph 4

Hi folks, I'm building an Aleph 4 but I want to use a single 1000VA transformer. To minimize interaction between channels I want to use 2 bridges and two sets of caps (see schematics below).

My questions:
1) Is the value of thermistors TH1 & TH3 is affected?
2) What would be a suitable Digikey's part number for thermistor TH2?
3) Is a 25A SPST power switch enough? Could it be less?
4) Are there other values that would require a replacement ?

regards,
Gabriel
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Old 9th March 2002, 11:16 PM   #2
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1. NO
2. KC006L-ND (CL-60 )
3. YES, and I don't see why you should. Higher ratings lasts
longer.
4. Looks ok to me....
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Old 9th March 2002, 11:29 PM   #3
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If you are going to do this, why not go the last yard and use 2x 500VA toroids? The increase in cost will be marginal, since most of the extra cost for split supplies is in the caps .... OUCH!

Though .... I did do this myself for my A40

cheers, mark
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Old 10th March 2002, 05:07 AM   #4
Gabster is offline Gabster  Canada
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KC006L-ND is the part number I had figured out for TH1 & TH3 but I still don't know what to pick for TH2. Sorry, I'm mostly a digital boy

Is the following thermal switch suitable (75C): 317-1085-ND, Datasheet. I would say yes, but it is rated at 15A which is not a big overkill. Anyway, I'm thinking of using two in parallel (one per channel)

What is the voltage rating of capicitors in the design (not PS' caps)? My guess is 63V.

What is the power rating of the 9.1V zeners? My guess is 1W.

Finally, the reason why I'm using a single transformer is to make my amplifier smaller. I'm trying to fit the Aleph 4 in a "regular" casing shape. Not a cube. Therefore, a good amount of the space must be used for heatsinks.

thanks, very appreciated help,
Gabriel

P.S.: What are your favorite online "audiophile grade" resistors/capacitors suppliers ?
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Old 10th March 2002, 09:40 AM   #5
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I think the thermal swith is ok. Thats what I would use.

The voltage rating of 63V for the caps is also ok.

The zeners are I think also 1-1,3 W is ok.

Another way to use 2x500VA toroids is to install them vertical.
You gain a lot of space like this.
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Old 10th March 2002, 10:40 AM   #6
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Hi all, better if using transformer with separate secondarys, not center tapped - reason is big circulating earth currents - best to keep these curents in the power supply stage, and not in the transformer.
You will need large busbar for earth common to have chance of conserving star earthing arrangement at the currents involved.
If you can run seperate transformers is best, but you still need to apply the above info.

Good luck with it, regards, Eric.
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Old 10th March 2002, 01:50 PM   #7
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Default Power Supply for Aleph 4

Eric is right,

If you are using one transformer with one secondary winding the earth system is critical for minimal induced hum from circulating ground loops via the input earth for the left and right driver boards and the common star ground.

( remember the input will also to joined at the input source end from your pre amp, so there will be a circulating loop from input earths which are joined at the star bus system and the preamp outputs)

I have this problem with my stereo Aleph 5 using a common transformer because I am using seperate driver boards.

I suggest the only way to solve this is to use a single board for both channels of driver circuits to ensure a symetrical common earth system at the input earth and have star earth on the common earth bus bar for the supply filter caps.

If you are using Mark Finnis boards this should be simple to arrange, just place the input ends together with a 20 mill pad for the common earth and run a single earth wire from there to the start ground connection.

The only other wires on the star ground will be the signal input ground, the centre tap from the transformer and the common for each speaker.

I have read that it is also wise to run a short sub (say 5 cm ) from the centre of the main filter capacitor bus bar for the star earth point.

This is supposed to minimise the infux of high charging currents running across the bus bar into the input earth which will degrade low frequency distortion (ref Doug Self)

The above should solve the problem and I recall on the service manual for the Aleph 4 Pass uses a common board for both channels driver stages and joining the upper and lower power stages.

If this approach or a similar alternative not used you may be disappointed with residual background hum from your speakers and it will be impossible to fix.

The other approach is stacked dual transformers (like Mark Finnis Aleph 4 )with independant rectifiers and earthing bus.


I hope this helps

regards

macka
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Old 10th March 2002, 02:20 PM   #8
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Gabriel,
The part for TH2 will be similar with TH1 and TH3.
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Old 10th March 2002, 03:51 PM   #9
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Its a big NO No to use the center connection bar of the big capacitors as a star earth reference. It coses too many problems.
The best is to take this connection point from the big caps with a short as possible wire to the metal case and earth the chassis and use that point as reference earth point. You have to get away from the big caps.
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Old 10th March 2002, 08:50 PM   #10
Gabster is offline Gabster  Canada
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What I was planning to do is to have my bridges/filter caps and drivers on the same circuit board (see diagram below). The center ground is located at the middle of this board (the pcb is drilled and a bolt emerges from the bottom of the chassis). The star earth reference is easily wired at this point.

Is this topology will ensure that I won't have any hum?

regards,
Gabriel

P.S.: cp642, could you point me at a digikey part number?
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