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birdyman 1st October 2010 02:53 PM

AM I Planning for disaster??
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I am all pumped up for Aleph x . But I have small problem with Transformer. I have single primary, 60-0-60 800VA transformer and 70-0-70, 1000va trans former from one of my dead P.A. I am panning to use one side of 60-0-60 for each channel.(see diagram). Am I planning for disaster??

nar 1st October 2010 06:15 PM

You won't get your 36V from the transformer you show. Plus, you don't need no bridges, 1 is sufficient, and you'll get around 83V symmetric PSU

birdyman 1st October 2010 07:10 PM

(correction: Rectified voltage should be 84 and with virtual ground + -42 rail.)

Is 83v too high for Aleph amps or 800VA too low for that voltage to do class A operation?
I am creating virtual ground by splitting filter caps(it will be few pairs of caps).
By having virtual ground, am I introducing any unstabilities to amp?

Zen Mod 1st October 2010 07:39 PM

you can do that same way as Quad does in his 606 , 909 and simillar amps - with two bjt net.

but - yes - you're chasing a disaster

sell those xformers and buy adequate ones

BobEllis 1st October 2010 07:42 PM

You'll end up with +/- 83V. (subtract the diode voltage drops.) Depending on the board you use, you may be able to get away with a virtual ground or none at all.

No such thing as too much voltage, only not enough heat sink. :clown: The AJ-X has some voltage limits, though. That said, search for the AXE-1-2 spreadsheet to look at your power and heat dissipation requirements. To be not current limited, bias @ 10A, you'll get 340W into 8R, dissipate close to 900W per channel, and a 3 KVA transformer per channel is recommended. Not to mention the fun of matching the 60 output devices needed to keep the junction temperatures reasonable.

For either of those transformers, look at class ab. If you want an X amp, scale the X-100 up to 80V rails, although this isn't really a beginner project. It can be very similar to an AX with a push pull Class AB Source follower output stage.

For a straight AX, look for different transformers, 18-20VAC.

birdyman 1st October 2010 09:21 PM

Thank you for the reply. Quad 606 information was what I needed. This is DIY project after all. If I burn my amp and transformer , my wife will be happy at least. I will build PS first tonight and stress test with light bulbs(may of them) to see if it can hold its voltage.

Zen Mod 1st October 2010 09:39 PM

you must divide that split secondary to 2 independent secondaries

then go to route 606


BobEllis 1st October 2010 09:42 PM

Are you going to use a refrigerated cooler to keep the output devices junction temperatures down?

Zen Mod 1st October 2010 09:55 PM

good ole 6L6GC tossed in bucket of oil is waaaay smarter approach :clown:

kropf 1st October 2010 10:01 PM

One minor problem I see is that your supply is very unlikely to be a symmetrical +/-40V, since that depends on the capacitance on each side of the virtual ground being exactly equal all of the time.

The much bigger problem, as I see it, is that the virtual grounds from each half of the PS will see about 60VAC between them. That will cause all kinds of trouble, and especially if they ever were both connected to an external ground at the same time. Very dangerous.


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