| muralman1 |
I just bought anX150 for my Apogee Duetta Signatures. A buddy bought a Llano Trinity 200 (200 watts/side) the same time. At an apogee site <http://audioworld.com/cgibin/sw/for...+Group&number=1> there has been an ongoing hot discussion whether the Llano is truly Class A throughout its rated power using moderate heat sinks that don't get hot. Randy White, the designer insists it is. After a phone conversation with Randy, one member came back with this to report:
Just got off the phone with Randy White. His amps run pure class A to rated power. They run cooler because the new MOSFETs he is using have a much lower impedence than usual. Even so the Amp idles at 300watts/Ch. The Trinity 200 that is. These Amps have an efficiency of 66% at full power. Not bad at all. Of course it stinks at Idle but what the hey, how many of us listen at Idle.
What say you? |
|
|
| grataku |
me sez,
now I know why I don't hang @audioworld.com.
66% eff. is too high for class A. |
|
|
| AudioFreak |
unless of course it uses dynamic bias.....
my guess, It aint class A @ rated output. |
|
|
| Lisandro_P |
| quote: | Originally posted by AudioFreak
unless of course it uses dynamic bias.....
my guess, It aint class A @ rated output. |
This increases efficiency, but nowhere up to 60%! I agree, it's probably class A at lower output levels and AB at full output. |
|
|
| muralman1 |
| What about Randy White's assertion that highly efficient Mosfets are largely responsible for the Class A rating? |
|
|
| AudioFreak |
| quote: | Originally posted by Lisandro_P
This increases efficiency, but nowhere up to 60%! I agree, it's probably class A at lower output levels and AB at full output. |
Well push pull can be just under 50% so i wouldnt right it off totally.
| quote: | Originally posted by muralman1
What about Randy White's assertion that highly efficient Mosfets are largely responsible for the Class A rating? |
Has almost nothing 2 do with it sorry. Pure Constant Bias Class A cannot be over 50%.
I guess althought unlikely, it's also possible Randy's amp uses a tracking power supply. |
|
|
| Lisandro_P |
50% is the theoretical maximum. But i dunno, actually i never got to think how much the dynamic bias thing improves efficiency, though i'm almost sure it's not 10%.
A schematic for that amp would be nice... anyone? :D |
|
|
| hagtech |
The lower impedance of the FETs has nothing to do with the amount of heat dissipated in a class A or class AB amplifier. It's only marginally meaningful in switching class D types.
Even if you meant "thermal" impedance. The answer is the same. Hogwash!
If dynamic biasing is used, then it is possible to achieve higher efficiency, but FET impedance is irrelevant.
Jim Hagerman |
|
|
| muralman1 |
You can take a look at the Trinity 200 at:
http://www.whiteaudio.com/page2.html
I will be listening to the Trinity in comparison with my Pass at my place very soon. I will post my findings. |
|
|
| grataku |
Muralman,
don't bother. Noboby here cares. |
|
|
| Jakeh |
"The 1/2 inch thick front panel is a welcome addition to any audiophiles system"
snort. |
|
|
| AudioFreak |
| quote: | Originally posted by grataku
Muralman,
don't bother. Noboby here cares. |
I wouldnt say that grataku, I would like to see that muralman come to understand why the accusations made about this amplifier are most likely false. |
|
|
| muralman1 |
I don't care one way or another. The builder insists his amp is Class A. This assertion has met up with a lot of resistance elsewhere. I was hoping for some insight here on how the 200 watt Llano could be made that way. Since The Llano is the only amp around that goes class A all the way without getting hot and with what looks like less cooling apparatus than the 20 watt Class A Pass 150, It seems to me, a builder of class A amps would be curious. this amp is a single triode leading to solid state Mosfets. Someone has said it is push pull. The builder gives credit to advanced Mosfets.
Has anyone looked at it yet? |
|
|
| Joe Berry |
muralman1,
I haven't seen (or heard) the Llano, but here is one possibility. Note that the driver stage contributes some output current and could therefore technically be considered part of the output stage. As long as the driver stays in class A -- and it will do so even while driving the outputs in class AB -- one could claim that the output stage works class A to full output.
Please, I am not putting words in anyone's mouth -- just wanted to offer one possible explanation. BTW, this approach has been used in the past by other manufacturers of "class A" amps. |
|
|
| muralman1 |
| Thankyou very much, Joe. You are right about manufactures using the accolade Class A loosely, to say the least. Way Back When, I had a Denon that claimed to be "optically" class A. I bought it. :eek: |
|
|
| Super_BQ |
Thanks for your input Joe Berry,
Another popular high-end brand that seems to **** ME off is Plinius.
Last year I picked up a brochure of their SA-250 amp and the specs say when bridged, the sucker can do 900 watts into an 8 ohm load! In big red lettering across the brochure it says "CLASS A".
Taking a quick look at the SA-250 manual which is downloadable at http://www.pliniusaudio.com/dow/dow.htm
Looking at Page 7 it talks about Bias switching (those familiar with Plinius their SA amps can be switchable between Class A and Class AB modes). Taken from page 7:
"The current required from the mains supply in BIAS A is approximately 1,000 watts, similar to a
small electric heater. The amplifier should not be connected to a wall outlet that is shared with.... "
So right from the sheep's mouth... we're talking about a transformer that draws approximately 1 KVA.
With some simple math we take 900 / 1000 and we get 90% - so what we have is a 90% efficient amplifier? Give me a break Plinius! Even sheeps don't poop out more poop than a male cow.
Even worse, the manual quotes on page 20 explains, "Why Class A is Better".
It's sad to see how hifi manufacturers have (and still today) get away with feeding the public with so much BS. Just as bad as the hifi magazine reviewers, even the definitions of electronic terms BECOME misleading.
BQ |
|
|
| Joe Berry |
BQ,
Plinius' claim actually may be accurate, at least for normal (unbridged) operation. Assuming that it draws 1,000W from the wall at idle in class A mode, each channel is probably dissipating about 400W, implying a true class A operating range of about 200W, and perhaps the full 250, depending on the bias circuit.
Just eyeballing it, I'd guess that the heat sinking for each channel is at about .1 degree C/W, which would give a 40 degree C rise in temperature for class A operation -- probably still safe for the output devices, but way too hot to touch comfortably.
It's true that bridged operation in itself doesn't increase the range of class A operation beyond what each channel can deliver on its own. If Plinius (or anyone else) suggests that their amp works class A at levels beyond half the amp's idle dissipation, they really ought to offer some defense for the claim.
BTW, I tried earlier to locate the idle dissipation spec for the Llano, but was unable to find it on their web site. Is it there somewhere? |
|
|
| Dustin Haug |
| quote: | | It's true that bridged operation in itself doesn't increase the range of class A operation beyond what each channel can deliver on its own. |
Why is this? I'm not arguing just curious. |
|
|
| Joe Berry |
Dustin,
The range of class A operation is mainly determined by the amp's idle current. While bridging allows twice the output voltage swing across the load, each channel in the bridge still transitions from class A to AB operation at the same point in its output current swing.
Take for example a 100W push-pull class A stereo amp in which each channel idles at 2.5A and can develop 40V peak across an 8 ohm load. The amp is class A because the current needed for 40V across 8 ohms is 5A, which is twice the idle current. Given linear operation, each side of a push-pull output stage can deliver twice its idle current before the other side shuts off.
If we now bridge the two channels, we can get up to 80V peak across an 8 ohm load. This requires up to 10A output from each channel, but since each channel still idles at 2.5A, it still transitions from class A to AB at 5A output, at which point one side of the output stage shuts off while the other side delivers the 10A to the load.
Hope this is clear -- let me know! |
|
|
| Joe Berry |
Reviewing the info on the Llano web site, I noticed mention of a passive bias circuit. This suggests that Llano is not (any longer) using Hexfets or other trench-type mosfets that need some kind of temperatrure compensated bias circuit, such as the one shown in my schematic above.
Llano may instead be using lateral mosfets, which not only don't need a temperature compensated bias, but also tend to turn off very slowly, giving them an inherent quasi-class A behavior when used in complementary push-pull stages. It may be that the Llano design is leveraging this behavior in making its claim. |
|
|
| Nelson Pass |
I have a nice rule of thumb evaluating claims of "pure
class A operation":
Take the continuous idle power draw from the wall and
divide it by the total rated continuous output per channel.
If it is a mono amplifier, divide the number by 2. If it is
a stereo amplifier, divide it by 4.
Now square the number and multiply it by the rated output
per channel. This gives the most generous estimate for
"pure" Class A operation, and twice this number is the
instantaneous point at which it leaves Class A.
Example:
A stereo amplifier draws 200 watts continuously
from the wall and is rated at 100 watts per channel:
200/100 = 2
2/4 = .5
.5 squared = .25
.25 times 100 watts = 25 watts. This is the rms Class A figure.
2 times 25 watts = 50 watts. The amp leaves Class A at
the instant of 50 watts, and probably operates AB to a 200
watt peak (the peak of a 100 watt rms sine wave).
All this assumes no weird circuits, and also assumes no
other losses. In real life, the numbers will be somewhat less. |
|
|
| Dustin Haug |
| Thanks for the explanation Joe, that makes sense. |
|
|
| Lisandro_P |
| quote: | Originally posted by Nelson Pass
I have a nice rule of thumb evaluating claims of "pure
class A operation" (...) |
Very good. I completely agree! |
|
|
| muralman1 |
My amp, the X 150, is rated at 150 watts per side into 8 ohms and 300 into 4 ohms. Its power consumption
is 200 watts idle, 600 watts max. Pass claims it runs Class A to 20 watts output then
switches to AB. Does the math work out, or is that not enough info? Doing Nelson's math as I
read it, I get Class A to 16.67 watts.
A friend, who owns the Llano Trinity 200 wrote this:
When I got into audio nobody told me there would be math involved, when I did the
math I got 112.5 wpc RMS and 225 wpc max class A into 8 ohms vs the claim of 200. If I
understand correctly, the power draw from the wall is the total and since the Llano draws 300wpc
at idle, the total would be 600watts. That is the first number in the equation. If that is wrong
then every step after that will be wrong.
Whereupon another aquaintance said this:
Don, your AC outlets carry 110 volts. No mater what you plug in it will always be 110. If you plug
something in with an impedance of say 100 ohms so much current will flow. If you plug in 50
ohms twice as much current will flow. Voltage remains the same. The amps power supply is
similar to your AC outlet. It supplies so much voltage in DC. Current flow now depends on load
impedance. As long as the power supply can provide the current flow it will provide what ever is
needed beyond Idle. Bias voltage remains the same regardless. How that translates into power
depends on the load.
Mike
:confused: |
|
|
| AudioFreak |
| quote: | Originally posted by muralman1
My amp, the X 150, is rated at 150 watts per side into 8 ohms and 300 into 4 ohms. Its power consumption
is 200 watts idle, 600 watts max. Pass claims it runs Class A to 20 watts output then
switches to AB. Does the math work out, or is that not enough info? Doing Nelson's math as I
read it, I get Class A to 16.67 watts.
|
Pass Labs rates the X150 @ 15W class A |
|
|
| Nelson Pass |
Looking at the Trinity, if that's 300 watts idle a channel,
then I would expect to be able to cook a steak on them. |
|
|
| muralman1 |
| It gets warm with moderate volume play. Less so than with the X150. Something to do with very efficient Mosfets, or so its been said. Thank you for your reply. I'm still looking for the right front end to feed the X, something clean, detailed and musical. |
|
|
| Nelson Pass |
| Oh! Efficient Mosfets! :) |
|
|
| sonnya |
I think they use a secret package with a looooow!! thermal resistance and a special coating on their heatsinks.
Back to reality : you can not get pass the thermal resistance who exists from the "die" through the package .. thermal compound .. and at last the heatsink...
High effiency MOSFET are only high effiency when shifted "on" where it is possible to use their low RDS... Like they do in Class D amps.
In a "linear" amp there is no miracle!!
Sonny |
|
|
| jduncan |
This is hilarious....I can't believe (well, given what some other manufacturers do, I guess I can believe.....) what this guy is saying. That's absolutely hilarious to use "high efficiency" mosfets as the reason a class A (not switching) amp doesn't dissipate very much power.
You've got to be kidding. As Sonnya just stated, this attribute of FET's only applies when they're fully on, not while they're switching. |
|
|
|
