XA30.5 review on 6moons

cviller

Member
2005-12-09 8:01 pm
Toronto
Great circuit porn... :D

How hard are those output devices biased? ~5W per device?

And how does this sound different to fewer devices biased harder?

I'm a bit puzzled... the mantra in this forum seems to be higher current equals lower distortion, but does this still apply when the current is shared between many devices... ?
 
So hard to find info and reviews on Pass Labs gear these days ...

Depends on the language the review should be in. Image Hifi (1/2008) in Germany has a nice review of the XA30.5 that also includes an "informal" review of the firstwatt F3. And the excellent LP Magazin (2/2008) - also in Germany - has a nice review of the new Pass Int 150. And as a bonus it has a picture of the latest (?) UGS modules that reveals the new cascoding devices :smash:
 
according to my secret Pass Labs connection :rofl: , they're 2SC4793/2SA1837 ........

seems that Papa choose (this time ) "theymustbeubercool" approach , maybe also he scavenge 158 Kilos of mentioned critters , for ~56 greens ........

:devilr:


ps. Jaccovitty is certainly better with augmenting pic details ......... than me .
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Actually they were chosen by Wayne.

There is a whole interesting area of conversation, which doesn't
particularly belong here, on the subject of what to use for
cascoding transistors and how to bias them. The answers were
roughly arrived at by trying all sorts of things and seeing what
really fit best. A very tedious process, but AFAIK, the most
reliable.
 
cviller said:


How hard are those output devices biased? ~5W per device?

And how does this sound different to fewer devices biased harder?

I'm a bit puzzled... the mantra in this forum seems to be higher current equals lower distortion, but does this still apply when the current is shared between many devices... ?



Now there's an interesting question. As with most things, you can hit it quickly and move on or take a deep breath and submerge yourself in things for a while...
Set aside the rail voltage question for the moment and assume that all other factors are held constant. The more current you can supply, the better. In a class A amp, you have to plan on delivering all the current from the quiescent bias--going to class B is cheating.
I glanced at the owner's manual and saw that the amp doubles into 4 Ohms. That means that you've got to double the 8 Ohm bias. In a 30W/ch amp, that's not really all that intimidating because the rails are fairly low and the heat dissipation is correspondingly less.
Okay, that's the quick answer. Now go a level deeper.
I believe Nelson has said that he feels that it's the overall level of bias that matters--not the bias per device. That's a slightly different way of saying the same thing I said above. Treat the amp as a classical Black Box and ignore the innards. Run the bias up as high as you dare and you don't have to worry about low impedance loads or dips in the impedance curve (which aren't a problem until you happen to hit that frequency, granted, but...).
Lotsa devices versus few devices is another question entirely. There are at least a dozen ways to look at this. Part of Nelson's view is--has to be, really, from a businessman's point of view--reliability. He puts a small army of devices in the output in order to make things more reliable in case of operator error. Even day-to-day reliability due to heat dissipation improves.
I've been thinking about this and I'm leaning the other way at the moment: Fewer output devices with more bias per device. It's not that hard to argue high bias per device, so I'll leave that alone. There's a more subtle point in that if you think about matching devices, there's always a little slop in the result. We, as DIYers, don't have the resources to buy as many devices as someone who does this for a living. If, as a result, you're limited to a matching pool of 20 units, you're not going to find perfect matches unless you've been a very, very good boy and the audio gods are smiling upon you. The practical implication of this is that you're building something like an Aleph 2, which uses six pairs of devices per channel and you've got each device going a little more or a little less while the music is playing. To some greater or lesser degree this causes distortion.
I can just see the next fifteen posts wailing about how they matched their devices to .00001V etc. etc. etc. Follow that with another fifteen posts from people worried that somehow their Alephs suddenly sound worse than they did yesterday. Not the case, guys. Your Alephs sound just as good as they always did.
What I'm talking about is the idea that if I can get two devices to match then I'm better off than having four or six...assuming that the devices can take the heat. So it's a balancing act. And since the devices are running harder, you have to consider the possibility that they will pop sooner rather than later. A not-so-inconsequential benefit is that the cumulative Gate capacitance seen by the front end drops.
Note also that the pictures also seem to show the output devices to be IRF parts. Given that the IRF P-ch devices have that odd frequency-related gain thing, then Nelson just might be rolling out his stock of IRF outputs with an eye towards using, say, Fairchild in future designs. What better way than to overbuild the outputs in current product? Mind you, I'm not saying that this is the case, just that it's a possibility.
There are a few other things I was going to throw in but I'm needed elsewhere. It's taken me three or four hours even to get this far. Bummer.

Grey
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Pretty improbable.

I already use Harris and Fairchild in addition to IR.

The costs of output devices as a percentage of total costs is
sufficiently low as to have very little economic influence.

In the context of performance and reliability, there is such a
thing as too few and too many. It's a balancing act between
bandwidth, distortion, and reliability. Fewer devices favors
bandwidth. More favors general distortion figures and reliability.
Of course all this is in a context. Every designer makes his
decision, and mine is here on the table.

:cool:
 

steenoe

Member
2004-03-23 10:43 am
Nelson, what a beauty:cool:
Does all the signals run through those flat-cables? Any special type?
Looks like there is sitting a TL431 in there. Is this a F4 like circuit with an UGS for voltage gain?
BTW. If I order one, do I get one of the Pass Labs T-shirts then:D

:)
 

steenoe

Member
2004-03-23 10:43 am
gl said:
I recall it being stated that the AX100.5 uses some new type of output device(s). Yet here we have the IRFP240 and 9240 again. I'm confused. Do the larger amps use the new and different output device types?

Graeme

I recall that the smaller members of the family, are different from their bigger brothers! I cant help thinking that the smaller ones are an ofshoot from the F4 (or rather the opposite, in fact), with an UGS piggy-backed!! Pretty logical development, to me.. So the smaller ones might still use the IRFP240/IRFP9240's!!

:)
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
So many questions.

steenoe said:
Does all the signals run through those flat-cables? Any special type? Looks like there is sitting a TL431 in there. Is this a F4 like circuit with an UGS for voltage gain? BTW. If I order one, do I get one of the Pass Labs T-shirts then:D

The drive cable to the output stage carries signal, but at low
current. The ribbon cable is ordinary, but we do parallel wires
for reliability. This circuit is not the same as the F4, in spite of
the presence of a TL431. Yes, I will send you a T shirt if you
buy one.

diy_Qui said:
I'm curious. Space does not seem to be an issue. Why PCBs are piggy-backed ? (Modular design for topology change in the future ?)

Mostly for ease of production and service. I am not anticipating
a topology change in these models.

gl said:
I recall it being stated that the AX100.5 uses some new type of output device(s). Yet here we have the IRFP240 and 9240 again. I'm confused. Do the larger amps use the new and different output device types?

I don't recall any new devices. We also use Harris and Fairchild
versions of the IR parts, depending on price and availability. The
"P channel anomaly" encourages us to use non-IR P channel
equivalents in Common Source applications, but not Common
Drain, where the difference is minimal.

steenoe said:
I recall that the smaller members of the family, are different from their bigger brothers! I cant help thinking that the smaller ones are an ofshoot from the F4 (or rather the opposite, in fact), with an UGS piggy-backed!!

No. They all share the same topology.

:cool: