How expensive is it to run a Class-A amp?

[PENG]

I'm not educated or versed on amplifier design, but I found this in the user's guide about the design:

Krell Current Mode A proprietary Krell circuit topology in which the audio gain stages of a component operate in the current rather than voltage domain. This unique technology provides the component with exceptional speed and a wide bandwidth.
Krell Sustained Plateau Bias III A patented microprocessor control system that maintains Class A bias operation regardless of musical demand. Class A bias is the most accurate method used to amplify musical signals. The new Sustained Plateau Bias III software is featured in the Full Power Balanced X Series amplifiers and was developed from the algorithm utilized in the Master Reference Amplifier. Sustained Plateau Bias III maximizes an amplifier’s efficiency both in power consumption and heat generation.

Not sure what all this means.

The technology used for the bias control is impressive, if works well it is something nice to have for achieving class A without over powering the A/C system in the summer. As far as being the "most accurate.....", that's just marketing hype though could be true to a point back in year 2000 or at launch. Measurements showed that, while beating the specs, this amp cannot be considered most accurate if compared to some of those Hypex N core amps, and/or Benchmark's class AB amp, just as examples. Regardless, it would have been a great choice for audiophiles who could afford the money and room 15-20 years ago.

Below are the THD+N vs Output at 2 kHz for Monoblock version. The stereo ones should be similar.

FPB350 MC Monblock

Compared that to the Benchmark, bridged into 4 ohms, at 1 kHz continuous:

Benchmark AHB2

 

[PENG]

So this is unusual for a class A design?

I am not aware of any other class A amp that has such varying bias scheme. So in that sense it may be unique, but I don't know if there were/are others that have something similar.

 

[mtrot]

That's because, as it claimed, the bias level was concluded by a microprocessor in such a way that it would vary with the music signal. Even at very high input level, the amp would only consumed 3000 W as needed, that is, not continuous. A dedicated 20 A 120 V line should be able to meet such demand for a few seconds without tripping the breaker.

Interesting. I will say, I'm now using it on a standard household 15A outlet and whenever I turn it on from the front button, it will dim all the lights on that circuit. Unfortunately, it appears that whoever originally wired this house back in 1987 put a lot of outlets and recessed lights on the same circuits. Also, a big bass transient will sometime dim the lights. I have wondered if this could be causing a loss of bass "slam" and impact. However, the amp has never triggered the breaker. Bear in mind, my speakers are 93dB sensitive and like I stated, I don't really play it all that loud.

I recently got an estimate for running a dedicated 20 amp line and they came back with $740, which I thought was too high for a less than 100 foot run. I do plan to check with a couple more electricians once we can get out of this virus lock down situation.

 

[PENG]

Interesting. I will say, I'm now using it on a standard household 15A outlet and whenever I turn it on from the front button, it will dim all the lights on that circuit. Unfortunately, it appears that whoever originally wired this house back in 1987 put a lot of outlets and recessed lights on the same circuits. Also, a big bass transient will sometime dim the lights. I have wondered if this could be causing a loss of bass "slam" and impact. However, the amp has never triggered the breaker. Bear in mind, my speakers are 93dB sensitive and like I stated, I don't really play it all that loud.

I recently got an estimate for running a dedicated 20 amp line and they came back with $740, which I thought was too high for a less than 100 foot run. I do plan to check with a couple more electricians once we can get out of this virus lock down situation.

You really should use the linked peak spl calculator to figure out what your peak demand is. If it is well below the amp's rated output then you should be fine. If the lights dim only momentarily during the bass "slam", you won't lose impact because those big (being Krell, I would assume) storage caps on the dc side will have you covered for those moments. Obviously the lights dim due to the voltage drop on the ac side.

If you sit 12 ft from the speakers, even your AVR can drive it (just one speaker) to 80 dB average, 100 dB peak, not quite reference level but if you don't listen loud right now, don't be surprise if you are listening to around 70-75 dB average most of the time.

 

[mtrot]

You really should use the linked peak spl calculator to figure out what your peak demand is. If it is well below the amp's rated output then you should be fine. If the lights dim only momentarily during the bass "slam", you won't lose impact because those big (being Krell, I would assume) storage caps on the dc side will have you covered for those moments. Obviously the lights dim due to the voltage drop on the ac side.

If you sit 12 ft from the speakers, even your AVR can drive it (just one speaker) to 80 dB average, 100 dB peak, not quite reference level but if you don't listen loud right now, don't be surprise if you are listening to around 70-75 dB average most of the time.

Thanks very much!

 

[3db]

First of all, the NAD 973 has differential Class A input circuits, but that does not imply that the power amps operate in Class A. They most likely operate in Class AB as with most currently produced AVR's. If they do operate in Class AB, then the AC power drawn when outputting a steady 1000 watts would be around 1500 watts, not much more.

However, in all practical situations, that would not happen. When playing a surround HT system, unless you were using the all channel stereo feature, most of the power used comes from the 3 front channels, and a lot less is demanded for all of the surround channels.

In my opinion, the cost of repairing is too high. I would suggest the Monoprice Monolith 7 channel power amp, which is very affordable, is built by ATI which is a reliable US company, and has excellent specifications. It's solidly built and the only negative aspect is its total weight of 93 lbs.

I looked at the manual and it says nothing about the amplifier class at its output. My guess is that the NAD operates in ClassA up to a certain power output and then switches to class AB. It certainly is a heavy unit and a very capable amplifier. Can you shop around for different repair costs?

 

[PENG]

I looked at the manual and it says nothing about the amplifier class at its output. My guess is that the NAD operates in ClassA up to a certain power output and then switches to class AB. It certainly is a heavy unit and a very capable amplifier. Can you shop around for different repair costs?

I went a little further by reading the service manual. The idling current for the final stage is only 40 mA so my guess is that it would leave class A very early, likely less than 0.5 W, practically a real class AB amp.

 

[Ciaran]

I went a little further by reading the service manual. The idling current for the final stage is only 40 mA so my guess is that it would leave class A very early, likely less than 0.5 W, practically a real class AB amp.

I'm shocked but very grateful for your thoroughness!

 

[Ciaran]

I looked at the manual and it says nothing about the amplifier class at its output. My guess is that the NAD operates in ClassA up to a certain power output and then switches to class AB. It certainly is a heavy unit and a very capable amplifier. Can you shop around for different repair costs?

I would shop around - there are a few high-end AV stores in my area that do repairs - but the problem is just carrying the damn thing in for them to have a look!

As soon as the forum told me it was A/B I was happy to repair it and bring some old gear back to life - even if it a costs a little.

 

[ski2xblack]

So this is unusual for a class A design?

Seems like class g taken to the next level (taken a similar approach to active biasing as class g does with power supplies). I suppose it increases efficiency, but kind of at the expense of simplicity.

 

[PENG]

Seems like class g taken to the next level (taken a similar approach to active biasing as class g does with power supplies). I suppose it increases efficiency, but kind of at the expense of simplicity.

If it is, it would be just class AB with multiple (minimum 2 if "G", or infinitely variable if "H") rail voltages. NAD called its "PowerDrive circuit topology" whatever it really is..

 

[ski2xblack]

If it is, it would be just class AB with multiple (minimum 2 if "G", or infinitely variable if "H") rail voltages. NAD called its "PowerDrive circuit topology" whatever it really is..

Correct, and that's exactly what NAD has done over the years.

The adaptive bias of the krell is new to me, but my initial impression is of a complex solution to an insignificant problem.

 

[PENG]

Correct, and that's exactly what NAD has done over the years.

The adaptive bias of the krell is new to me, but my initial impression is of a complex solution to an insignificant problem.

Agreed, imo it is for audiophiles, not for practically minded people.

 

[panteragstk]

Yup, all 2 weeks of the winter.....this is TX!

You guys got two weeks? This "winter" felt like 15 minutes.

 

[Mikado463]

I am not aware of any other class A amp that has such varying bias scheme. So in that sense it may be unique, but I don't know if there were/are others that have something similar.

Plinius, in their SA-103 has a variation of this whereby a switch allows the listener to toggle between a Class A or AB output.

 

[PENG]

Plinius, in their SA-103 has a variation of this whereby a switch allows the listener to toggle between a Class A or AB output.

Thanks, read it and understood that its like the XPA-1 except if switched to class A it is still rated 125 W, same as if it is switched to AB. The Emo amp would be limited to 60 W in class A and 500 W in AB.

The Krell is different, in that it vary the bias based on the input signal, i.e. the actual need, a much more efficient full time class A design. Again, the Plinius, like the Emo, simply give the user the choice to run them in A or AB.

 

[Kvn_Walker]

I never got a chance to use it since it was damaged in shipping, but the Vincent SP-331 tube hybrid amp (first 10 watts Class A) is claimed to draw 270 watts sitting idle!

 

[PENG]

I never got a chance to use it since it was damaged in shipping, but the Vincent SP-331 tube hybrid amp (first 10 watts Class A) is claimed to draw 270 watts sitting idle!

Kvn, that review is fine on the subjective part but he got the some of the important facts wrong, examples: transformer rating, class etc.. To be fair though, even some of the info on the Vincent website are technically incorrect. Just for example, transformers output power rating should be in VA not Watt, and since there are no standards for the power consumption specs so it they claimed, say 800 W, they should specify the conditions (such as, half load, full load, and load impedance (i.e. including phase angle), but they didn't. What a shame, that even at such price point, the specs are so lacking in details.

 

[Kvn_Walker]

Kvn, that review is fine on the subjective part but he got the some of the important facts wrong, examples: transformer rating, class etc.. To be fair though, even some of the info on the Vincent website are technically incorrect. Just for example, transformers output power rating should be in VA not Watt, and since there are no standards for the power consumption specs so it they claimed, say 800 W, they should specify the conditions (such as, half load, full load, and load impedance (i.e. including phase angle), but they didn't. What a shame, that even at such price point, the specs are so lacking in details.

Yeah I couldn't remember where I had read about that idle power usage and was surprised that the website, and the owner's manual itself, provided so little info. I had to google reviews to find it again.

The one I bought off ebay was $540 + S/H second hand. Long run, I'm glad Fedex did destroy it, because I couldn't be happier with my Parasound.

 

[lovinthehd]

Does bragging about excessive power consumption and heat dissipation work as a sales point?