XA30.5 review on 6moons

[Nelson Pass]

Originally posted by Ian Macmillan
Many thanks for the answer, I will stop pursuing bootstrapping as a way to achieve "... approximately 20% greater power output for a given supply voltage" (taken from the XA.5 literature). I couldn't really see how bootstrapping was going to do it for me anyway but then I dont' (yet) see another way either. Time to get the thinking cap on

I don't recall using the word bootstrapping in the XA literature.
If I did, it is an error.

[Nelson Pass]

Originally posted by GRollins
2) In the real world, this means that if you supply two MOSFETs with the same voltage at their Gates, one will turn on harder than the other if they're not perfectly matched.

In the real world, I find that variations in transconductance
between like devices of different lot codes is better matched
than Vgs.

If you're going to get persnickety about it, you match the devices
in the operating amplifier, warming it up, measuring and listening.

A perfect DIY activity.

[GRollins]

I would like to spend more time on device selection than I do currently but given my time constraints, it's either that or build circuits. I choose building circuits. For the time being I'm still using the simple one-shot Vgs match, although I do match at the intended bias current.
A couple of months ago I went back through a batch of devices I had matched at a lower current and redid the matching for a higher current. There was some shifting of places back and forth, but nothing too Earth shattering. This was pretty much as expected, but it did make some minor differences in which devices I pulled. I didn't do any deep studying on it, but off the cuff, I'd say that something like one in four devices, maybe one in five, were different from the ones I would have pulled if I'd just gone with the original matching order. The devices that were left out of the new groupings were still fairly close to the groups I pulled; I could have used them without fear. Just figured I'd gild the lily a bit.

Grey

[Manu]

I would like to ask which metal is used to pot the transformer.
Is it to shield the output stage or front end?
And ... why on the other hand Firstwatt amps for ex. obviously don't need such shieldings.

Manu

[Nelson Pass]

Originally posted by Manu
I would like to ask which metal is used to pot the transformer.
Is it to shield the output stage or front end?
And ... why on the other hand Firstwatt amps for ex. obviously don't need such shieldings.

It is not potted in this case, it is banded with mu-metal, and the
reason is for low magnetic noise, mostly to avoid driving ground
wiring.

The FW amps have about 1/5 the transformer in them and at
that size, stray field is not nearly as much a problem.

[tinitus]

I have found that the shielding does lessen noise, but also that sound is better without shielding...seems trafos doesnt like metals, or it may be a case of disturbing the grounding...I have discouvered that sound improves when "lifting" the trafo from cabinet...and I have even found that the use of a brass bolt improves on sound...and decoupling the bolt from the cabinet is even better...I prefer wood fore my amps

[Manu]

Thanks Mr. Pass.

manu

[Ian Macmillan]

Originally posted by Nelson Pass


I don't recall using the word bootstrapping in the XA literature.
If I did, it is an error.

You didn't It was my (incorrect) assumption that some form of bootstrapping must be used to achieve the stated benefit.

Ian.

[radule]

Mr. Pass, refering to XA.5 literature, is it output stage now something like "first produced Aleph"?

[PW]

Originally posted by noisefree ..., but hold my eyes open for 200V fairchild devices!
...one fine day I will get them! :-)

Dirk [/B]

Try Reichelt. I ordered 9610, and got Fairchilds. But it's better to ask them befor ordering.

Peter