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Old 29th February 2008, 05:38 PM   #1621
gl is offline gl  United States
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The decision needs to be made whether to base your design on 1) what Nelson states clearly and unambiguously, 2) what Nelson states ambiguously, and 3) what others come up with as supposition and through the use of tenuous logic. I think there is too much of 2) and 3) going on. Just MHO.

I really don't want to rain on the parade but there should be more of a focus on essentials rather than minutiae. My comments should be taken as constructive advice.

Graeme
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Old 29th February 2008, 06:16 PM   #1622
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There's always option 4--Come up with something reasonable and try it anyway...
Option 5 is to try something unreasonable, simply because you feel in your gut that it's worth exploring...even though the simulations say it won't work. You never know, it might be just the ticket.

Grey
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Old 1st March 2008, 01:40 PM   #1623
400 is offline 400  Germany
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Default Aleph X-pcb

Hello,
I´m interested in building a Aleph x or an XA-clone.
But I think the group buy is closed a long timeago.
Is there any pcb-layout online or can someone send me such an layout? Then I will try to etch pcbs myself.

Marcel
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Old 4th March 2008, 09:18 AM   #1624
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Question Relative DC offset change...

Relative DC offset changed to 100mV from no transformer at input 50mV.
Why this happen?
I want more gain...so I changed input resister 10K to 100 ohms.
The result was increased relative DC offset to 1V.
I dont knnow why.
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Old 4th March 2008, 05:18 PM   #1625
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Quote:
Originally posted by Nelson Pass

It's really pretty simple. By driving the center point of the bias
voltage, the front end can effectively swing about 4 more volts
on each peak. The resistors from each supply function to provide
current to the bias voltage generator and load the front
end with a known resistance.

On a circuit like this, that 4 extra volts can be the difference
between 100 watts and 144 watts.

Also, keep in mind that the output devices want some extra
volts left over between Drain and Source on peaks, something
on the order of 5 volts or so.
It occurs to me that this is only necessary if a single supply is used for both FE and output stage. Is there a sonic advantage to using a single supply (as opposed to other considerations such as lower cost, weight and complexity)?

Ian.
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Old 4th March 2008, 05:23 PM   #1626
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Quote:
Originally posted by Nelson Pass

Also, keep in mind that the output devices want some extra
volts left over between Drain and Source on peaks, something
on the order of 5 volts or so.
I forget to ask about this in my last post...

If we allow an additional 5 volts for the output MOSFETs, doesn't this mean that if the amplifier is driven into clipping (ignoring for a moment the desirability of doing this), it is the FE that will run out of voltage first? Is this a better scenario that having the output clip? Or is it simply a case of clipping is bad however it occurs?

Ian.
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Old 4th March 2008, 09:21 PM   #1627
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For a circuit like this, I don't particularly care. The idea is simply
to leave some extra voltage on the table so that the performance
doesn't suffer too much at the rated wattage.
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Old 5th March 2008, 09:22 AM   #1628
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Fair point - clipping is clearly something to avoid anyway.

I'm still curious though - in your experience is there a sonic or performance advantage to using a single supply (as opposed to other considerations such as lower cost, weight and complexity)? I can see there is no direct need for a seperate supply if 5v is held in reserve at the output, but migh not a higher FE supply provide greater linearity? Or maybe this effect is swamped by non-linearity in the output.

Ian.
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Old 5th March 2008, 05:50 PM   #1629
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It depends on the design. Some circuits, like the Aleph 1-5 and
30 and J have to retain the same rails.

But as a rule, it doesn't hurt to have some extra volts on the
front end. The question is only whether they can really take
advantage of it. If you have to figure on throwing away 5 volts
for the output devices, then you only need to deliver Gate voltages
within a volt of supply. In our case, we drive the center point
of the bias circuit, so our front end only needs to swing within
5 volts of supply. It's still pretty linear at that point, and in truth
we make larger allowances than that.
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Old 5th March 2008, 05:53 PM   #1630
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Thank you - very helpful

Ian.
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