When does my class Atoll IN200 A/B integrated switch from class A to class B?
For one when dynamic peaks occur ... generally one to three watts will drive 87 - 90 db plus peak draws equals crosses over to 'B' a lot and for brief duration (dependant upon content). Pass is a bit of an artist (topology sculpture) as are many designers and shows different ways to deliver audible qualities. |
https://positive-feedback.com/audio-discourse/leaving-class-a-2019-redux-a-biased-opinion-from-nelson-pass/ Unless the manufacturer specifies the transition point or some magazine has done the measurements, I don't think there's any way of knowing exactly. Manufacturers tend to boast about amps that run well into A before leaving and going into AB. So if a manufacturer doesn't specify, it's probably very low. Still, the difference in sound that you might hear between your Atoll and a Pass or a Sugden isn't all attributable to class AB vs class A operation. |
I know this the higher, the transition point, the hotter the box runs.. Normally until they hit that tipping point they start cooling off. Idling they would run hotter, why the bias is set higher, the tipping point..they cool off until you REALLY push them...they heat up again.. Get yourself a cheap thermal gun, you might be surprised what you can learn from one, and if the left and right are biased the same... Could be load induced, but interesting, none the less. Regards |
Thanks, oldhvymec. I think you all can see the direct interest in this question. Many people on this forum debate different classes of amp, including Class A vs. Class A/B. If, for a given set of speakers/usage, a Class A/B amp is running nearly all the time in Class A mode, then there would be little reason to investigate pure Class A amps, except for whatever interest their design might offer the music. As an aside, I know that certain old Adcom amps such as the GFA555 are still prized on the used market; one reason is the famous designer, but another is because this design supposedly ran longer in class A mode before switching to class B. |
I think that is very optimistic too, I have a Belles SA30 class A power amp that has been measure more that 50watts and I can tell you that it gets hot, not warm, but hot, one can just touch it for a second or too before it gets very uncomfortable. and it has big heat sinks and good ventilation holes on top plate and bottom plate. as it sounds fantastic I can live with it as a heater at the same time 😉 |
Yeah, there is no way that this amp is running 40w class A. To deliver 120w into 8 ohms, the amp probably has rail voltages of approximately +/- 46v. To deliver 40w RMS class A, you'd need a bias current of about 1.6A to allow the amp to deliver 3.2A peak while staying in class A (assuming a push-pull output stage which is the most efficient for class A). Across the two rails, that's an idle dissipation of almost 300W for the two channels. To support that, you'd need heatsinks probably 20 times the size of the ones in that amp, and it would still get toasty - probably too hot to hold your hand on it. A class A/B amp with heatsinks the size of the IN200 is probably biased just enough to prevent the output transistors from completely turning off near the zero crossing - maybe a bit higher. I'd be surprised if it would stay in class A beyond a few hundred milliwatts. |
If you want to get a flavor for what a class A amp sounds like, you should keep an eye out for a Pass Amp Camp Amp. These are sold in kit form in the DIY Audio store for about $330, but regularly show up on the used market in assembled form for $250 to $350. It only puts out about 8w, so you won't be able to play your current speakers very loud, but it should give you a good sense of what class A can provide. It's also possible to bridge the amp and use two as monoblocks if you find you really like it and want a bit more power. If you buy a used one, you should be able to sell it for what you bought it for after you've played with it for a while. |
I wrote to Atoll and conveyed the skepticism posted here about the Class A operation of the Atoll. They replied: From Atoll engineers: "Hello, Thank for for this information. This is one of the most current mistakes people make about our amplifiers; they compare the size of our heat-sink to a single one; this is nonsense regarding to thermal dissipation because when you use a single, big heat-sink you have to put in isolation components that increase (a lot) the thermal resistor. For that reason, and on that point they are correct -- in order to have the same dissipation you would require an almost 10 times bigger heat sink for each channel; but with our trick, the transistors are transferring about 100% of their heat to the heat-sink with no waste of energy. You'll find attached a measurement that shows the distortion when power is rising; as you can see (in the picture), the distortion is under 0,002% up to 50W on both channel. Best regards. ATOLL Team" I don't think I can post pictures here, but anyone who'd like to see it can P.M. me. |
Quick follow up. I discovered this comment about their more advanced amp and I sent it to Atoll: What I found said, “The high bias allows the IN400SE to operate in a Class A for the first ten watts before going into Class AB operation." http://www.theabsolutesound.com/articles/atoll-electronique-in400se-integrated-amplifier/ And I asked Atoll if they were claiming the my IN200SE operates to 40w in class A while a later and more expensive model only operates in Class A for the first 10 watts? If that is true, why did you not employ this technology in the later amp?"" They replied: "The IN400se use almost the same technology as the IN200se so both work in class A up to the third of the maximum power." So, they're in contradiction. Is it Class A up to 10w or to 40w? They seem to say both but insist (to me) that it's 40w in Class A. Despite the lack of heat, fins, etc. They're just jerking me around at this point, right? |
I think they are jerking you around. Eliminating the electrical insulating material between the transistor and heatsink will reduce the thermal impedance, allowing a bit more of the transistors heat to be transferred to the heatsink, but this doesn't change the laws of thermodynamics. The heat still has to go somewhere. This would allow the heatsinks to run a little hotter without risk of damaging the transistors - say 60 or 65 degrees C. But even so, heatsinks this small would not allow nearly enough heat to be dissipated for 40W class A operation. And your case would be so hot that you wouldn't be able to touch it for more than a second or two. Take a look at the Pass Int-60. This is rated at only 60w/ch into 8 ohm (although that's pretty conservative) and has rail voltages considerably less than your amp, which means the transistors need to dissipate less power at the same idle current. The Int-60 will do class A up to 30w/ch. Now take a look at the size of the heatsinks on this amp. |
Coda uses three different taps on the transformer to achieve their versions. I have the CSiB integrated in V1 which is 150 watts into 8 ohms but with class A up to 18 watts, the V3 is 400 watts into 8 ohms and class A to 8 watt. So a unit can be adjusted to one of three versions, the correct transformer tap soldered in place and the bias set, all performed at the factory, not a user adjustment. I also have a Pass INT-60 which is class A to 30 watts, both wonderful units. Weighs in at 90 + lbs. I agree with what most have said that your unit is likely class A to 10 watts or less. The most important thing is how it sounds to you! |
hilde45 Looking at the size of the heat sinks https://ibb.co/1LXDhKt and the rail volts needed for Class-B 120w output, your lucky to have a couple of watts of Class-A Because of the high rail volts needed for 120w of class-B, the heat dissipation of Class-A goes up exponentially as the rails volts rise for Class-B. Think this way, if you have a 10w Class-A "only" amp (no B) the heat is X If that same amp is still 10w Class-A ((but 120w of B) then the heat is far far higher. Like I said your lucky to have a couple of watts of Class-A (sorry probably not what you wanted to hear) Cheers George |
Thank you George and others. You have told me what I wanted to hear, namely the truth with explanations. I did not buy this amplifier with the expectation that it would be class a for very many watts. I think this inquiry has taught me something also about what this company is willing to say even when they know that it is being repeated into a forum. If I have more dialogue with them I may just share it with you because it's kind of amazing how long they are willing to stick with this line of bs. |
"The IN200 works in Class A up to about 40W under 8 Ohm. The switch to Class AB is quite slow."BTW this is BS, that they claim for a 120w rms class-B they say it is. If it were true the thing would huge with external heat sinks and weigh 30kg+ not 12kg https://www.atoll-electronique.com/en/products/integrated/integrated-in200sig/ Cheers George |
It seems proved as BS. What I’m amazed at is that I confronted them via email with the evidence it couldn’t be true and they just repeated the BS.If your 100% sure they said "The IN200 works in Class A up to about 40W under 8 Ohm" Ask them to come on here and prove it, and they'll get a new one ripped. But if they said "The heat dissipation of all the heatsinks is 40w" this is probably correct. But this is a very different thing and has nothing to do about how high the Class-A ability is you’ll get into 8ohms. Cheers George |
Hi @hilde45, Maybe you’ve already read this interesting article about class A by Nelson Pass: https://positive-feedback.com/audio-discourse/leaving-class-a-2019-redux-a-biased-opinion-from-nelson-pass/ So, if you know the idle power draw of your Atoll IN200, you could infer somewhat where the class A bias is set. (Idle power draw is with the amp fully powered up and ready to play music, NOT “standby” power which might only be enough circuitry to receive a remote power-on command.) For example, the Luxman L-590AXII is rated at 30 WPC class A, and burns 260 watts at idle. Assuming 40 watts of the 260 is the PreAmp section, that leaves 220 watts idling to drive 60 watts for 2 channels of class A. In this case the power amp idle draw is about 3.7 x the class A power rating. Since Atoll does not appear to publish their idle power draw, you could use something like an inexpensive Kill-a-Watt power meter to get you in the ballpark. If it only draws 30 watts at idle then you probably have trivial class A after accounting for the PreAmp. I built a little Pass designed Amp Camp Amp, but found it wasn’t up to the sound quality I’m after, and I would not recommend judging any other class A amps based on this small amp’s performance. But it was fun to make, and I’ll probably never sell it. I find this stuff interesting. Happy holidays! |
@rockrider Thanks for the suggestions! The relevant Atoll technical specs I can find are: Power Wrms/channel/8Ω = 120 W Power Wrms/channel/4Ω = 200 W Impulse Power = 200 W Power supply (VA) = 670 Total of capacitors (µF) = 62 000 Input Impedance (kΩ) = 220 Sensibility (mV) = 100 Rising Time (µs) = 2,5 Signal/Noise Ratio (dBA) = 100 dB Bandwidth = 5 Hz – 100 kHz Weight (kg) = 12 If those aren't enough to answer the question (I presume they are not) maybe I can do some measurements. |
