Saturday, May 23, 2015

Krell and it's bias

My Krell FBP 300 seemed to be approaching a stable 114 degrees F or so at idle in both channels over a couple hours.  That didn't seem to change playing KPAC at Tact level 60 (about -44dB).  When I cranked the level up to 80, one channel rose to about 122 but the left channel shot up above 150 degrees F.

I rolled the volume back but the hot channel didn't seem to cool down much.

I reversed both input connections and speaker connections.  The problem was same as before and in the same channel of the amplifier (left).  This time, when I turned the level down, at first the left seemed to get lower, down to about 140.  But then later it shot up to 155 at the hottest point.

This is probably wrong, there shouldn't be that imbalance.  But what is the correct temperature?   Apparently many Class A amps run their heatsinks around 120-130F.  IIRC Nelson Pass advises the low end of that for the homebuilder (45-50C…the F5 is designed to run 25 above ambient or typically 50C, while Pass Labs amps are spec'd to run at 53C, where obviously everything has been carefully engineered for that and higher).  My Aragon 8008 BB used to idle in around 132 until I put the Super feet on it, now it idles about 115F, and it's not even considered a Class A amplifier.

But lookee here!  The Krell KSA-100S manual describes Sustained Plateau Bias and says that if the heatsinks go above 80C (!) the amplifier cuts out the two highest bias levels until it falls back to "normal" temperature (not defined).

So that's saying in a way that 80C is a kind of "normal" condition that the amplifier adapts to (I believe many other amplifier would shut down at 80C or even lower).  Well we see at least one way Dan the Man demanded special versions of his power transistors from Motorola.

If a 100S can rock with 80C so can my 300 I believe.  So this isn't any reason to panic, but nor is it reason not to get the amp fixed asap.  The temperature difference shouldn't be there.  Actually the cold channel might even be better a bit hotter…but that was specifically the channel repaired last time (IIRC) for distortion.  It's probably optimized and so should the other channel be.  When optimal the FPB amplifiers have amazing low distortion at all levels and all audible frequencies.  The FPB 300 was measured by Martin Colloms as having a 300kHz power bandwidth.

On this blog here I was posting about the temperature imbalance in September 2010, after having had it repaired in 2009.   The same blog mentions microprocessors on each channel's main board, and how they need to be reset, and how the procedure is simple if only someone would reveal it.   The consensus seems to be Get the Amp Fixed!  I wasn't actually the original poster, but chimed in with my own similar problems.

On this blog here a person reports 79.6F in standby mode, idling/background music at 97F, 30 minutes serious listening as 132F for the FPB 300.  Lots of other interesting comments, including the guy who says the normal range for inverters is up to 80C, and the unit shuts down at 90C.

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