XA30.5 review on 6moons

[Nelson Pass]

Originally posted by GRollins
--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?

I use either Harris or Fairchild for the P channel Source applications,
I have no doubt that there are others equally qualified with
comparable parts. I have tried Samsung in the past with good
results and expect that Toshiba parts would also be fine.

Originally posted by 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.

It's a fairly ordinary transistor - you can sub comparable bipolars.

Originally posted by 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.

There is a slight tendency for hand wringing and sleepless nights
over matching Mosfets. The primary concern is for equal current
sharing, and once the Vgs is reasonably matched at operating
temp, you depend on the Source resistors to enforce the
sharing at higher currents. I use same lot codes and tight Vgs
tolerances because I can do so easily. This should not prevent
you guys from going ahead with different lot codes if that's what
you've got.

[noisefree]

Mr.Pass, is there a prefered voltage type for the front end MOSFETs?
I'm searching for an alternative to the hard to find fairchild 200V parts in Germany. :-( Because the gain devices are common source the easy to get IR devices will not be my first choice.
Also I would like to ask, if you can give me a hint again what is the purpose of the 3W resistor between relay and meter-led-switch connector and of the 2,2kOhm / 3W type? Perhaps both for LED or coil circuitry?

Dirk

[Nelson Pass]

We prefer the voltage which will not break. Since I use these
modules in X1000.5's, that would be 160V or more. For an
XA30.5, it can be considerably less, but the 200V parts are the
most common.

[GRollins]

Originally posted by gl


Are you getting different lot codes in the same tube?

Yes.

Let me pound on this one again in case anyone's confused.
1) MOFETs are poorly controlled when it comes to Vgs. It's the nature of the beast. Bipolars vary, JFETs vary, MOSFETs vary...if you want something tightly matched, you'll need to look at tubes--they're much more uniform in their characteristics than any of the solid state competition.
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.
3) This leads to one device doing more work than the other (aka "current hogging").
4) Nelson has long suggested that--when possible--DIY folks use a simple Vgs match on MOSFETs manufactured at the same time, on the same wafer of silicon. There is usually (not always!) a tiny embossed number on the body of the device. This tells the tale.
5) If you can get devices manufactured at the same time, they will generally have similar characteristics.
6) What characteristics? Well, this is where things begin to get annoying. Suppose you were to supply two MOSFETs (matched for Vsg, but from different lots) with a voltage at their Gates. If you use the voltage that gives you the current you matched for, then the devices will both give you that same current. Marvelous! Just what we wanted! Er...not so fast, bub...what happens if you change that voltage at the Gate to something higher or lower? Yikes! The bloody things don't act the same--one will give higher current than the other. But I thought they were matched, you say. Well, yes they were...at the one current you chose to match them at. This does not necessarily mean that they're matched at any other current.

(We will now pause until the rioting in the streets calms down.)

7) This is why Nelson suggests using devices from the same batch. There is a high probability that the devices from the same batch will react similarly to an increase or decrease in voltage at the Gate.
8) But what do you do if you only have devices from different batches? First off...DON'T PANIC...relax, have a home brew...it's going to be okay. You simply match the devices in question at more than one current. The more data points you have, the more accurate the match. If you want to get really into it, you can drop some money and buy or build a curve tracer.
9) If you really want to go nuts, you can match for noise or most any other parameter you might be able to think of. I used to know a couple of guys who decided to go into business building head amps. They bought an entire batch of Siliconix parts and matched them for six different parameters. Don't worry, nobody's going to ask you to do that. For the vast majority of DIY uses, just nailing the Vgs for a couple of currents will do the trick.

Grey

[gl]

Egads man! Calm down. You answered my question with eloquence and completeness in line one. And I was only mildly interested anyway. Sheesh.

Graeme

[GRollins]

I didn't mean you, specifically, but there are any number of other people reading this thread. As Nelson noted above, there's a lot of angst out there about part matching. Note the number of threads started because someone built amplifier X, Y, or Z, measured the voltage across each Source resistor...then had a mild case of hysteria because the voltage wasn't equal across all the resistors. "But I matched all the MOSFETs to within .00001V!" they cry. "What have I done wrong?"
Actually...nothing. The amp is fine. Sit down and listen and quit worrying. If you really want to get serious about evening up that discrepancy, then match you devices at the idle current you intend to use instead of at 10mA. If you really, really want to go whole hawg, match at more than one current.
Since I've gone and opened this particular can of worms, I want to make sure that no one comes back three months from now, having taken something out of context, and wants to know why I said that it's okay to run MOSFETs from different batches when their amp smoked the first night because one device was doing all the work. (I get e-mail from people who seem to have some pretty free-form interpretations of things I've said.) I hope to forestall some of that by flogging the idea that you've got to do a little more work during the matching stage of the game if you want to make it come together properly.
I'll still get the e-mails, just hopefully not as many.

Grey

[Ian Macmillan]

Originally posted by Nelson Pass

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.

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

Ian.

[moe29]

(i know this thread isn't about matching...) but,

when i built my Aleph 2, i matched via VGS to get similar fets, then i
observed the fets chosen in the working amp. Taking VGS readings
again while all warmed up and running in the circuit, i swapped out
until i got good reading all around... within .01 or something like that.

(the Aleph 2 still sounds good today)

[Babowana]

Originally posted by GRollins


3) This leads to one device doing more work than the other (aka "current hogging").

We have Rs . . . the device of less work will be compensated by (Vgs-Vgs_th)^2 . . . even if could never reach to the same value as the upper one.

[noisefree]

OK, thanks Mr. Pass,

will go first with the well known IR parts, but hold my eyes open for 200V fairchild devices!
...one fine day I will get them! :-)

Dirk