Saturday, May 23, 2015

Thinking about Sustained Plateau Bias

I am a big fan of the Krell Amplifiers with Sustained Plateau Bias, especially the later models which implemented fully regulated power, such as my FPB 300 and on up.

But how useful is this SPB, really?  It's not clear how much this has been actually copied (of course, SPB and other acronyms are Krell trademarks, and different from the underlying patent, which might be licensed so long as it is not named the same…and could be in widespread use as far as I know).  It does seem possible that many big audiophile transistor amps use a managed bias scheme of some sort, not unlike how the FPB does it with a microprocessor, whether they fall within the claims of the patent or not.

It's incredible in my FPB 300!  Reasonably intelligent.  It gets standby and idle bias perfect from the get go.  It holds on to the 300W consumption idle for low background music.  Then the next bump up is at least 450…if both channels 600W consumption.  Then consumption seems to go up in about 100W increments as needed up to 1400.  I can't test any higher than 1400.  It jumps up quickly, takes longer to go back down.

If the amp is cold, it may hang on to 600W.  If amp is warm, it gets assertive about pushing consumption back down.  It amp gets too hot--above 165C--it pushes down consumption fast even if the loudness hasn't changed.  But in my testing, it has never pushed consumption down below 600 during loud passages for this reason…only during low levels or silence.  During silence when it wasn't too hot, bias lowers back to idle bias in less than a minute, how fast depending on how hot it was.

It is precisely during episodes of the amp pushing down bias due to high temp that it tends to make the chassis ping noise.  Or sometimes if amp is just left to cool off after hard playing.  But it has been quite quiet playing tonight.

So while it's not "adaptive" (so says Krell in owners manual) it is actually somewhat adaptive.  There are limits to what you get…and they are not necessarily preset limits such as buying a 30W Class A amplifier and having 30W into 8 ohms, 15W into 4 ohms, etc., of Class A power.  No with the KPB 300 you can get, momentarily, tons of Class A power.  But if it heats up too much, you get less and less Class A margin, and ultimately you may be forced to endure Class AB.

This is a pretty good deal, actually.  In fact it makes a lot of sense for the amp to work this way, it gives you all the Class A power the heat sinks can support, and all the Class AB power the transformer can support.

By letting the bias go lower during less demand, and at idle, heat sink margin is saved for the louder passages for the amplifier to remain in Class A.

It's a great idea I think.  It's only a disappointment if you believed every word of the hype, that you were getting 300W of Class A power into 8 ohms (and 1200W of Class A power into 2 ohms), and as much as you want at any time, always Class A power.

That would be impossible, without the actual old fashioned Class A amplifier that dissipates 3-4 times the Class A power and has no more power at all than that (because if it has more Class AB power above that, the 3-4 multiplier becomes even larger).

So 1200W in two channels is 2400W means the amplifier would have to dissipate at least 7200W to be this hype dream amplifier.

Actually the FPB 300 heatsinks seem pretty comfortable in my air conditioned room dissipating about 1000W continuously (with normal breaks).  Above that too much, and I get the thermal system cutting back the dissipation to 600W or so.  Then it would gradually climb back up to 1000 again.

So with the thermal setback…it actually is adaptive, and with many levels on tap, it adapts to any room conditions, giving you the maximum Class A power within the limits of the situation.  It doesn't happen to be close to 1200W per channel for sure…but it's not chopped liver either.

It will be interesting to do that calculation…what actual Class A power into 8 ohms (and half as much into 4 ohms) from the apparent comfortable maximum of 1000W continuous dissipation.

Earlier today, I was enjoying mine right now at about 305W AC consumption, FM radio station KPAC playing local artists, level set to 70 (-33.8dB but other reductions apply in home distribution), heat sinks r/l 113/116F.  At idle power consumption was 75W, heat sinks r/l 87/83, ambient 76 with AC running (blowing right over amp).  Basically now it's a high bias AB amp with likely less than 10W Class A power into 4 ohms, but if power exceeds something it rises to higher levels of class A power as much as it needs to and can sustain.  I'd be consuming more power with a pair of XA30's I believe, and though they have more Class A power than I do now continuously, I can have way more on demand, and way more power overall.  With a low level playing it can be very stable at 300W consumption.

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