Sunday, May 24, 2015

Cook that Krell

I was all worried that one channel of my FPB 300 was running hotter than the other.  I shouldn't have been, my Krell, which was sitting in the corner of the living room for 4 years (during my recent building boom) waiting for me to get around to prepare for getting fixed, was operating normally.

The FPB *has* to heat up to the normal max temperature to trigger something like recalibration.  It then sets itself back down to lower, usually in the 600W bias plateau, but if the amp has been driven hard enough it will get set back to the 300W minimum bias plateau.  (My wattages are the actual amplifier power consumption…what is available as Class A power is different and will require a long explanation later.  What has revolutionized my understanding of the FPB amplifier is watching it's power consumption from moment to moment over hours of listening.  Now I think I understand it.  It makes very good use of the ultimate limited resource--the heatsink area of the amplifier.  You could say it "optimizes" the resource for the requirements of the situation.  I am happy to be even more a fan of Sustained Plateau Bias.  It's a way to approximate a much larger non-adjusting Class A amplifier, compare my FPB 300 with the metal of a pair of Pass Labs XS300's, which have similar power into 2 ohms, but use about 6 times as much metal.)

As you are driving the amp hard, it will hit the top normal temperature, which appears to be about 165 degrees F, then get set back, and this may happen over and over.  Ultimately, when driven hard enough, the amplifier becomes a high bias Class AB, with 300W minimum bias, and there is no more setback, consumption just falls immediately after every peak.  Many people may not drive the amp that hard--I have the least sensitive loudspeakers ever sold, and with the most difficult to drive load.

Once both channels have hit max temperature once or twice and been set back, they begin to have fairly identically, so I am wondering if hitting max temperature is a necessary thing to keep the amp working right.  Moving the amplifier to a different location is going to cause differential air flows…which has a profound effect on passively cooled heatsinks, and a new recalibration--hitting max temperature--will be necessary before things are balanced again.

As I was using my amplifier for awhile, the left heatsink had it's bias setback, and before long it was the cool heat sink compared to the right, which took it's time to get to 165F, but after some harder playing it did too, and then the right channel got the bias setback.  The power consumption fell to no less than 600W, but during ever more brief pauses in the music it would ramp down to 300W.

I am just guessing that a recalibration occurs when the bias is set back.  It would make sense for the amp to do its recalibration on thermal setback, as then it is changing the bias in a big jump anyway.

165F is a fairly high temperature for audio amplifiers, but it is nothing in the world of power inverters, for example, which require similar parts.  The Krell was specifically designed to run this hot as part of it's normal behavior, and as a way to get more class A power from a limited amount of metal, even if it's not usually considered a limited amount of metal.

Update: I'm seeing 175F after a day of hard playing.  I think the ultimate setback point is 180F as it was for the KSA-x00s amplifiers.  It the sensors see that, the amp kicks down the bias.  When the heatsinks have been cooler, they don't measure the temperature as quickly as the sensors do, so the rise in setback point actually demonstrates the sensors read quicker than my measurement of heatsink temps, as well as the actual temperature being higher than I thought.

It's only to be expected that if there is a setback point, eventually things will hover around it.  So 180F is a normal Krell temperature.  If the amp shuts down, or if I see 200F, I should call Krell.

Power is in the 700's with more than a second silence dropping to 600W.  Still takes about 20 seconds to get pushed down to 300W, so the nominal bias level is 600W consumption now.  So this is a long term sustainable bias level.  Actual wattmeter readings are typically in the 700's.

That bias level is comparable to A pair of Pass Labs mono XA60.8's, each of which weighs about the same as the Krell, and continuously dissipates 375W per mono block chassis.  So Krell is made to cook at max temperature, Nelson Pass designs his amps to have a more moderate temperature--at the cost of twice as much metal for similar performance.  And of course the XA60.8 has no where near the voltage or current capability of the Krell either, even as a heavier pair of amps.

The first XA amp which beats the FPB voltage spec of 60V is the XA200.8, which has 62 volts.  Only the XA160.8 and XA200.8 (and XS300) have 50A current rating.  Krell doesn't give a current spec, but Martin Colloms measured the FPB 600 as having 117A and described it as arc welding.  The XA200.8 weighs 177 pounds (per mono block) and dissipates 750 watts per channel.  I pushed my FPB to 1400+ watts draw, but only for a dozen seconds as it was already warm.

Krell FPB amps are the poor man's Pass Labs…   The Eco-Freak Pass Labs…

People will say Pass Labs is fundamentally different, all about simplicity, etc.  The FPB amplifiers have a no-feedback output which is simply an array of emitter followers in parallel and differential push-pull.  That's simple and elegant. Stuff ahead of that is a bit complicated, of course, including but not limited to the bias system.  But nevertheless the FPB specs bandwidth at 240kHz and 0.1Hz.  The distortion is spec'd at 0.02% at 1kHz and 0.15% at 20kHz and that's for full power.  Pass typically specifies 100kHz and 1% distortion.  (Of course that means he's too indifferent to specs to bother to measure distortion just before it begins to rise near clipping.  But it won't out-do the Krell either.)

I keep forgetting this, but Krell specs the full output (300W) input level as 2.35V RMS, but I continue to believe that more input is possible to reach the full peak output implied by 60V, 450 watts into 8 ohms.

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