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Old 22nd March 2002, 12:53 AM   #1
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Default Pass Labs XA200 amplifier

Feel like speculating in aluminum futures?
There are pictures and some basic specifications of the XA200 on the Pass Labs site.
No, I haven't built the Aleph-X prototype yet--hope to start on that this weekend or perhaps next week. My fingers are crossed that nothing goes wrong in the meantime to eat up my free time. Don't need no more computer viruses 'round here...

Grey
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Old 22nd March 2002, 05:40 AM   #2
Apogee is offline Apogee  United States
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Grey,

Please post your results as you work on this....

I am going to be trying the same thing within a couple of months except with more outputs than the Aleph...

I would be very interested in knowing about the hurdles...

Thanks,

Steve
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Old 22nd March 2002, 06:10 AM   #3
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Well, the easiest part for me to guess is the supply, which would need to be about +/-35VDC at 8.5A idle.

As for the actual gain circuit, my best guess fow now is that it's basically two standard Aleph channels bridged together at their inverting inputs. I've tried this in simulations and it seems to work, although I can't tell whether to simply ground both of the noninverting inputs or tie them together and float them off ground by 10K ohms or so.

In the model, floating this junction gives increased second order distortion when measuring one side of the load to ground (third order distortion is not much affected), as compared to grounding it. Of course, you really need to measure these effects *across* the load, but I've yet to figure out how to do that in the simulator.

Maybe I'll just have to make a real one.
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Old 22nd March 2002, 03:29 PM   #4
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Rethinking the above post, it seems that my terms may have been incorrect, or at least confusing. What I mean to say is that the two channels are bridged together AND DRIVEN via their inverting inputs, using a balanced input signal.

What actually ties the two channels together in this scenario is the interconnection of their two NONinverting inputs. Thinking more about this, I believe that this point must be allowed to float for true s-s operation to occur. Of course, I'd be interested in any light that NP is willing to shed on this!
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Old 22nd March 2002, 04:45 PM   #5
jam is offline jam  United States
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god has spoken!
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Old 22nd March 2002, 06:20 PM   #6
BrianGT is offline BrianGT  United States
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Don't forget this picture. I love the finished appearance.

Click the image to open in full size.
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Old 24th March 2002, 11:06 AM   #7
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Quote:
Well, the easiest part for me to guess is the supply, which would need to be about +/-35VDC at 8.5A idle
I'm not so sure about this value!
Quote:
two channels are bridged together AND DRIVEN via their inverting inputs, using a balanced
Not sure about this either. Nelson hinted that one of the major "hurdles" in running the X-Front into a SE back was the DC offset, which sort-of implies to my simple non-EE mind that he isn't running the Aleph backs in a balanced mirror arrangement.

What say some of the clever folks?
(excluding NP, as I don't think he should be expected to answer this thread ..... unless he feels inclined )

cheers, mark
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Old 24th March 2002, 01:48 PM   #8
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mark,

For a 200W/8 ohm amplifier you need a +/- 57V peak output swing, which in a bridge design means just over 28V from each side of the output stage. This implies at least +/- 29V rail voltage; I spec'd it at 35V to allow some margin and because it's about what you'd get from a standard 2X25V transformer. Also, 2 x 35V x 8.5A = about 600W dissipation, which is the rating for the X1000 chassis. Also note that 8.5A is "more than double" the idle current used in the X1000, so at least in this regard it more or less matches the description given on the Pass Labs site.

As for the topology, I admit that I don't know exactly what NP is doing, but I do believe that bridging two Aleph channels together would take care of the DC offset problem. However, I must also apologize for again having inadvertently given an incomplete description of the setup. What I said (the second time) above is, I believe, correct as far as it goes, but there is just a bit more to it. I may post more details later if I get permission.
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Old 24th March 2002, 10:49 PM   #9
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Smile Its gynormous

If you are going to try this out, you're going to need a huge case like the commercial version. At 600W idle, its going to be like a large tube amp for heat production. This is not an undertaking for the faint at heart, and it will be expensive to build, especially for a single pair of amps.

Having said all that, I wish you a lot of luck, I'd like to see you succeed spectacularly.
__________________
Regards,
W.
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Old 25th March 2002, 03:02 AM   #10
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Default not too beaucoup

Big as it is, the XA isn't any larger relative to its power output than the Alephs. So, for example, you could build a 100W XA on the same chassis as an Aleph 2, if you wanted to.
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