Today I'm doing a little shuffling around with the HT setup. I decided to try using my 840A in my HT set-up, using one of the inputs as fixed, so I'm essentially using it as power amp.
The first thing I noticed is that I had to adjust the gain on the fixed input to -00 dB - full out. The second thing I noticed is that amp gets HOT. Much hotter than when I used it as an integrated. I could crank it for an hour and it would barely get warm to the touch.
Interesting thing is that in stereo mode in my HT setup it actually sounds quite a bit better than my old Bryston 3B, more dynamic and lively. Even my wife noticed.
Finally, does anyone know what the heck this actually means?
"This unique design gives pure Class A operation at low levels, moving smoothly into an enhanced version of Class B at higher levels. This system should not be confused with Class AB, which inherently generates greater distortion at high levels than a pure Class B design.
XD technology feeds a controlled current into the output stage in a new way so that the usual Class B crossover point no longer occurs at zero volume - the worst possible position in terms of distortion -- but at a significant output level. "
Awesome David. It doesnt surprise me at all that the 840A beats the 3B NRB. The NRB series were strictly designed for studio usage and probably would sound near as full as the 840A. I just text messaged my dealer to see what price he can get an 840A for as I would like to test drive it at least.
XD technology feeds a controlled current into the output stage in a new way so that the usual Class B crossover point no longer occurs at zero volume - the worst possible position in terms of distortion -- but at a significant output level. "
I beg your pardon?
I don't think zero volume really means a lot in distortion characteristics, does it? I do not see where the transition is supposed to happen on this amp, nor do I understand the volume at WOT for the thing to work. This seems like a strange place to put a 'signal' to the input switching section.
I guess you could consider it more marketing mumbo-jumbo than anything else. But, if you own the amp, you probably don't want to think if it as "mumbo-jumbo". However, IMO that's still what it boils down to, just another marketing idea and another anagram to sell it, neither of which will probably be around in another two years.
Class A operation has always possessed a certain "Gold Standard" status in consumer audio. Sounds good but has obvious drawbacks. Not many consumer audio designers are willing to admit their products run in class B operation as the notch or crossover distortion can be rather high and obvious to most listeners. Long before I got involved in audio, manufacturers were trying to design an amplifier with class A sonics and class B efficiency. Class D, G, H and so on all claimed to have the answer but most have faded into the history of consumer audio with no one really missing them. Sliding bias has long been a favorite of the consumer audio designer but it too has had its day and gone. Some derivation of sliding bias though is how most of these designs have operated.
"I don't think zero volume really means a lot in distortion characteristics, does it?"
Yeah, it does if that "zero volume" is the point where the output device turns on. And that's how class B operates which is why no one really uses class B for home audio. In class B the outputs turn off and on just a bit ahead of and just a bit behind the 0° transistion from positive to negative waveform. This is what you hear as notch distortion in class B at low volumes, the high distortion of the output switching from dead off to full on. At high volumes class B sounds kind of OK since at high volumes your critical listening takes a back seat to other priorities. So class B is most often found in commercial amps used for sound reinforcement.
In a class AB amp (what you see in 95% of consumer audio) the crossover from positive to negative is less abrupt than in class B as the AB outputs are switched off slightly behind the 0° point and switched on slighty ahead. This provides an area where both outputs - in a transistor design the PNP and the NPN - are operating simultaneously for a brief moment which tends to cover up notch distortion making it less measurable and oftentimes less noticeable. Increasing the bias in an AB amp will make this transistion range wider and push the amp into one of the sub-categories of class A/AB. You'll find most class AB amps operate in a quasi-class A mode for the first few watts of output and then make a transition to full class AB to reach full power. This is where sliding bias comes into the picture for some designs and how most manufacturers have manipulated their designs to achieve good low level performance and then switch to a straight class AB operation to manage high output. If you read the Cambridge White Paper on the design principle, you'll see they go into a few of the various methods of how switching can be accomplished in such designs. How it's finessed and what it's going to be called - what anagram will be used - change from time to time and manufacturer to manufacturer but the whole idea has been pretty much the same for the last fifty years.
What Cambridge seems to be saying here is they have designed their amplifier to operate in quasi-class A at low volumes (nothing new there) and then switch to class B for high efficiency at some (unstated?) output power. They are using current into a sink as their method for making this switch. Rather than making the transistion from positive to negative waveform (and PNP to NPN) around the 0° point they have made the transition higher in the waveform. In doing so they've provided more overlap between the positive output and the negative output though both eventually switch out of handling the signal on the opposite side of the wave. So, once again, "quasi-class A" at low levels is what they are selling along with the ability of the amp to produce reasonably high RMS wattage for its size and cost.
What would appear to make the design "novel" (to use Cambridge's own word) is the use of a third output device in a sub-system they call "The Displacer". I don't see any information regarding just where the amplifier makes the switch from this quasi-class A to what appears to be pure class B operation but I assume it's at a reasonably high output - maybe somewhere around ten watts RMS but everything else reads as if the amp is running in straight class B (not AB) at high volumes.
Class B operation would certainly explain why the amp ran cool to the touch in the other set up.
"The first thing I noticed is that I had to adjust the gain on the fixed input to -00 dB - full out."
I'm not understanding what you did here. If the gain is "fixed", then it's not adjustable - "fixed" meaning a set value which is not user adjustable. Anyway, I would think this "quasi-class A" operation and where the amp transistions to class B has something to do with the heat in your new set up.
I think the wide-open-throttle VC is used as a signal or switch to run as a power amp. That position would not be used otherwise. Pure supposition, but it seems a very intentional setting for the user to setup as CA intends. What happens if you back off the VC in operation, Dave?
To answer your question Jan, the way the 840A works in "fixed" mode is that you flag one of the inputs to operate in fixed mode, but then you have to set the gain for that input using the VC. After that, everytime you turn the amp on the gain setting will be saved.
I was playing around with it last night a bit with my system in HT mode, and I turned the gain down to -15dB to match the rest of the speakers. It works fine. So it seems the VC being set to 00 is not a switch to operate it in power amp mode.
So you're running this still as an integrated amplifier by running through a line input and not a "main in"? I take it there are no main in jacks which eliminate the pre amp section from the signal path. Used as an integrated in this situation the vc should be at full open rotation so as not to load down the input with its resistance. This wouldn't matter if the volume is actually set by an active circuit rather than a resistive pot but I have no idea how your amplifier operates.
"So you're running this still as an integrated amplifier by running through a line input and not a "main in"? I take it there are no main in jacks which eliminate the pre amp section from the signal path."
Not many people using a pre amp/power amp combination would think to run two preamps, one after the other. There should be no need to do so in most instances, though it's certainly not uncommon for a MC cartridge to feed a (step up) pre amp which feeds a second (control) pre amp. And most straight power amps have a "pre amp" gain stage to accomodate the outputs. The issue is always how much gain is required and generally what is the least amount of circuitry the signal must pass through to obtain that amount of gain. Each level of additional circuitry beyond that point introduces noise, distortions and some characteristic deviations from neutrality. To answer your question directly, two pre amps would be neither good nor bad just on the face of it. You would need to know the situation, why it was being employed and what benefits/disadvantages are coming into play. In HT there are very few real standards for neutrality so you have quite a bit of wiggle room and mostly your own expectations of how a car wreck should sound if you placed your ear exactly at the point of impact. Listen for noise and distortions being produced by the additional gain stage and, if you find nothing objectionable, then use the system as you please.
This was more of an experiment than anything else. I'm pretty happy with the result actually. This is my HT system, but we often use it for listening to music in the family room/dining area. Even at loud volumes there is no distortion, and it actually sounds quite a bit better than the previous system using the 3B driving the main speakers.