Halo A51

[Jon SS]

I just bought the C2 and A51 combo. After almost a year of research I am very happy with my decision.

Some quick questions about the A51. It gets very hot (which I realise is normal), and I was wondering how the heat sinks work. Are they supposed to get very hot so that the compenents in the amp stay cool?

Also I was planning of putting the system in a enclosed wood custom entertainment centre when I move into my new house next year, where the doors can remain open when in use. How much room do you guys have above the amp to allow for cooling etc? Has anybody put their amp in a entertainment centre.

Thanks

[David Meek]

Jon, that's exactly how the heat sinks (fins, etc) work. They draw off the heat from the components and because of their large surface area can dissipate the heat into the air surrounding them. However, if you don't have GOOD ventilation that constantly allows replacement of the hot air with cool, you'll run into problems with your A51 and possibly your other components. If you are going to enclose the A51 in a cabinet, I'd plan on building in some exhaust fans (very quiet ones, that is) for the equipment area. In this case, it's fortunate that the A51 is a Class A/B amp and not a straight Class A.

:hb




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[starford]

Regarding heat: I think you're seeing the results of five amplifiers heavily biased towards Class A operation in one chassis.

I remember reading a post from John Curl on AudioAsylum.com where he states that he likes to run his designs warm (a reference to using a lot of Class A bias). It's telling that is signature amp (before the JC-1 for Parasound) was called the BBQ.

The idea is that your A51 is running Class A most of the time while in use, and therefore is consuming all of that Class A bias (times five channels) all the time. The specs say "higher pure Class A power than many amplifiers selling for twice or three times it's price", so I'd guess the Class A bias is around 20-25 watts, so that's 100-125 watts of power at work, all the time, no matter what.

The amp is designed for this, natually, so the heat sinks will heat right up, and stay that way. The components used are all rated for the heat ranges they operate in, and there's a thermal sensor to tell you when things get too hot. In fact, I think it shuts down automatically.

As for storing it in a cabinet, I'd take a few precautions (see the manual for similar recommendations):

1. Put it on the top shelf
   Keep the doors open (or off) while in use
   Use the 12v trigger to switch it on only when the processor is on
   If possible, use vented side panel and rear (a la Salamander Synergy)
   Use a large, low speed fan to circulate the air (look for 80mm computer fans that spin slowly on PC parts web sites, like Directron.com)

Bottom line is, you've got five (mostly) Class A amps, and the trade off is pure music for lots of wasted electricity and heat.

[David Meek]

The input stage is Class A, comprising JFET transistor devices. For each channel, the driver stage, also Class A, employs two MOSFET devices. Finally the output stage comprises a total of eight bipolar transistors per channel (rated at 15 amps, 60 MHz). The output stage operates at Class A up to about 6 watts per channel, beyond which it employs Class B operation.

Perhaps more significantly, each of the amplification stages for the A 51 operate in differential mode, which means that for each channel, the positive and negative phases of the input signal are handled separately. The potential advantage of this approach is that common mode distortion (noise that results from unwanted voltage in both the positive and negative phases) can be avoided

This is from Widescreen Review's website - a review of the A-51. Switches at 6 watts? Kind of early?




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[nicholtl]

could someone explain the class A/B operation a little further? This has me quite confused. So class A (meaning what?) operates when the amp is pushing 25 watts of its 250 watts of power - assuming we're talking about the A51. Then class B (not as good as A?) kicks in? Why?

[starford]

could someone explain the class A/B operation a little further? This has me quite confused. So class A (meaning what?) operates when the amp is pushing 25 watts of its 250 watts of power - assuming we're talking about the A51. Then class B (not as good as A?) kicks in? Why?

From Robert Harley's "The Complete Guide To High-End Audio":

Class A - Mode of amplifier operation in which a transistor or tube amplifies the entire audio signal

Class B - Mode of amplifier operation in which one tube or transistor amplifies the positive half of an audio signal, and a second tube or transistor amplifies the negative half.

Mr. Harley explains (see page 183) that Class B amplifiers split the workload of reproducing the audio signal between complimentary pairs of transistors (in the case of solid state amps such as the A51), thus allowing one set of transistors to rest while the opposite swing of the signal is being produced, an arrangement referred to as 'push-pull'. This is an efficient and economical way to reproduce an AC waveform.

The drawbacks mainly include the fact that there must be a transition between where one transistor (or bank of them) stops working and the other one (or bank) takes over. This is done by a phase splitter. This introduces an unpure modification, however small, to the signal reproduction chain.

In Class A, a single output device is asked to produce the entire waveform, both positive and negative swings. This makes the output device work the entire time, keeping it 'on' and creating a lot of heat as a by-product, but it also keeps the output in a constant thermal state, providing more linear results.

Many of the great amps of our day are pure Class A designs, like Pass Labs and many single-ended tube amps. You could fry an egg on them. By the same token, many other great amps are Class A/B designs where the designer has gone to great lengths to minimize the effects of Class B operation. Witness Krell, Mark Levinson (influenced by John Curl) and others.

In practical terms, modern semiconductors are well made and very fast in their switching; so much so that Class B operation has little to no real drawbacks in everyday use. The vast majority of power amps are Class A/B designs, where the market demands hundreds of watts of output and the designer gives you some Class A operation (the purest form) at low listening levels and transitions that to Class B operation as the volume goes up.

[nicholtl]

So that means we're only getting the purest form of sonic reproduction at low listening levels? That kinda sucks, since nobody really buys seperate amps for "comfortable" listening right? At least, I know I don't.

[starford]

Well, low levels aren't necessarily the same thing as low volume.

During music reproduction of many music types (Classical, Jazz and soundtrack vocals, for example) the amp is essentially idling. Music with a stong beat or bass line works an amp harder.

All that reserve power is there to service dynamic peaks, usually few and far between. The primary reason to have large amplification in use is to produce those dynamic peaks a cleanly as possible, and huge reserves accomplish this. Low power amps run out of steam because their reserves are tapped and they begin to clip the signal.

So, the idea is that the designer has given you a pure amplification sweet spot that will be in use more often that not, and a lot of muscle in the form of A/B operation to service those dynamic peaks when they come.

It's interesting to note that while audio amplifiers are designed for broadband output (20 Hz- 20K Hz) at their rated power, things change when you have a variable load that dips down below 4 Ohms or are only amplifying a narrower part of the signal, say between 80 Hz - 20k Hz. One way to keep an amp working in the sweet spot is to ask it to amplify less of the signal, and splitting that job between two devices. This is the idea behind bi-amping and powered subwoofers.

[nicholtl]

Ahhh, very interesting and informative reply. Why is it that more often than not, people say they hear no difference when bi-wiring (is this the same as bi-amping?) their speakers, despite using premium speaker wiring and terminations and such?