XA30.5 review on 6moons

[noisefree]

Hello Mr. Pass,

may I ask you what is the purpose for the two different elcap types?


Regards
Dirk

[Nelson Pass]

What different Elcap types are those?

[noisefree]

The "double pack" in the bias circuit (16V/220µF)
and the "quad pack" (100V?/1000µF?) near the ground connectors.

[Nelson Pass]

The 220's are across the bias network and the larger ones
are front end supply decoupling.

[noisefree]

Thanks a lot!!! :-)

[GRollins]

Originally posted by Nelson Pass


I already use Harris and Fairchild in addition to IR.

...Fewer devices favors bandwidth...

...More favors general distortion figures...

--I remember that you've mentioned having Harris and Fairchild on hand. Setting aside the use of IRF in followers, do you allocate specific MOSFET manufacturers' parts for particular products in your lineup?
--Yup, that's me. Guilty as charged.
--Well...yes, but no...you buy by the thousand, possibly even ten thousand, depending on the part. That gives you a larger pool to match parts from than someone who buys ten or twenty and has to take best available match rather than reach into a bin of .01V parts and pull out a fist full of closely related devices.
Part of the problem I've run into is that, even buying moderate quantities, the MOSFETs have at least two and often more series of stamped numbers. That kinda knocks the same-wafer, lower-distortion strategy into the weeds. I got lucky this past fall and received 100 devices in "only" two series .25V apart from one another. Hmmm. We takes what we can get.
Why things have gotten this way, I cannot say. I used to see a single series if I ordered twenty parts. No longer. From where I sit, it seems that the problem began about a year, maybe eighteen months ago. (Certain people...ahem, ahem...certain people have had their parts since the Jurassic and might not have this problem. [Okay, okay...so your Fairchild parts date from the Pleistocene...picky, picky, picky...])
So what to do? The obvious thing is to start matching at more than one current. That way, I no longer have to worry about wafers. If the boogers match, they match and I'm good to go. Yes, that will take more effort and more time and I confess that I haven't started yet--I'm just barely keeping my head above water time-wise at the moment. For the time being, I've still got some decent looking Vgs matches from the same batches and I'm getting by on those. The next time around, I'm going to at least two point matching and possibly more, because I'm a little frustrated with this broken series problem and (given my current approach of using as little feedback as possible) it'll give me lower distortion, anyway.
Using fewer devices gives me the chance to use devices that match closely, thus lowering distortion. To use more devices would necessitate loosening the matching criterion, raising the distortion.
Maybe others are having better luck with their parts buying than I am. I sure hope so.

Grey

[GRollins]

I took a moment and looked at Digikey and Mouser. Both report having the 2SC4793. Digikey has the 2SA1837; Mouser has it on order, but with no delivery date specified. A few other places had them as well but were more expensive. I'm in no position to buy anything at the moment and won't be for quite a while--just thought it would be interesting to see of they were available without too much hassle. They are.

Grey

[gl]

I buy unbroken tubes of MOSFETs from Mouser or Digikey. I ask for them that way and pay whatever it costs to round up to the nearest tube. The codes have been the same on a per-tube basis. Are you getting different lot codes in the same tube?

Graeme

[Ian Macmillan]

Originally posted by Nelson Pass
The 220's are across the bias network and the larger ones
are front end supply decoupling.

Does this mean that there are no capacitors used for bootstrapping and that we are all chasing our tails in the Aleph X builder's thread?

Ian.

[Nelson Pass]

Originally posted by Ian Macmillan
Does this mean that there are no capacitors used for bootstrapping and that we are all chasing our tails in the Aleph X builder's thread?

Perhaps you are.

There is no bootstrapping in the PL products. If I want a higher
load impedance for the UGS I suppose I would either bootstrap
the resistors which source current to the bias network or use
CCS's, but I don't want the higher load impedance - my resistors
are chosen for double duty.