Friday, November 4, 2011

Up and down and up with Aragon 8008 bias level

I've now watched the Aragon bias level over more than an hour.  It may be on the high side of normal for this amplifier, possibly due to long term drift of some kind, but the bias circuit seems to be working and the high bias may be part of why the amp sounds so good.

Within a minute, the outer channel bias is at 21 mv (NOTE: I mean the voltage measured across the 0.33 ohm emitter resistors, I use the term "bias" for that measurement in this post.)  20mv is specified for a Palladium amp, which has the same size heatsinks; 8mv is specified for the 8008 by Klipsch tech support, but I'm wondering if these specifications aren't deliberately conservative.  At two minutes, it's roughly up to 25mV.   At that time, there is about 160 watts total quiescent power draw, same as I measured after leaving on overnight (though, based on later data, I believe that corresponds to 26mV long term average bias).  24mV is the specified value for the inner channel on a Palladium, so it can't be too bad, if actually being used the amp will dissipate much more power than that.

From there, the bias keeps rising until it reaches a peak of 30.3mV at 6 minutes, with total power draw of 192 watts.  It slowly starts falling, reaching 29.8 at 10 minutes with 187 watt draw.  At 20 minutes, it'd down to 26.45 with 168 watt draw.  At 30 minutes, it's down to 26.1 with 158 watt draw.  Here my notes stopped, but I recall it got down to about 24mV (140W or so) and started rising again, peaking just over 26mV and falling again.

Apparently there is some oscillation caused by the thermal mass of the heatsink between the power transistors and the bias regulator transistor.  Over many hours, as every part of the amp rises to its long term most uniform temperature, the oscillations should largely damp out, theough I recall still seeing a few watts fluctuation around roughly 160 watts quiescent after 10 hours of idling.  There may also be some effect caused by airflow from my HVAC system cycling on and off.

When the amp is actually operated, as I discovered last night, the bias cuts back.  I observed 14 mV bias after playing music at moderately high level for 30 minutes, at which point quiescent power draw was below 140 watts.  That must help the heatsinks from getting too hot.

All in al, I think this is about right for me, about the highest bias possible for this amp, but probably enough downward bias regulation and heatsink capacity to handle any loads I will actually throw at it.

If it becomes a problem, say if I can't party all weekend without tripping thermal cutout, I can adjust bias downward to the specs suggested by tech support, which must have been intended to handle exactly that sort of situation.  I do have an above average line voltage and far below average efficiency speaker.  But I don't tend to listen with average levels above 90dB, and even continuous use on my part is not likely to cause rise to cutout temperatures, especially because I run the bass below 84 Hz on my subwoofers and not the panels.  (When, years ago, I caused my Parasound HCA-1000A to thermally shut down, I was not only running the bass on the panels I was attempting to increase the highs with some equalization.)

Clearly the amp should not be left on all the time, it wastes a lot of power, shortens lifespan of parts, and serves little purpose because the amp warms up to full bias anyway within 2 minutes.  But my fear that amp still hadn't reached full bias after 30 minutes of operation yesterday, would rise to 160W overnight, and then just keep on going upward relentlessly were unfounded.  Actually, the amp reaches full bias after about 2 minutes of operation, overshoots by 25%, and then has some damped oscillation staying within 10% of full (26mV) bias, and cuts back bias by 50% under heavy use until the heat sinks cool down.  These are indications of well thought out design, not failing circuitry.

Now I can also calculate the amount of linear Class A operation.  26mV across 0.33 ohms is 0.0788 amps.  The total current across 6 pairs of transistors is 0.473 amps.  Twice that can be delivered to speaker in Class A: 0.946.  Peak power into 8 ohms is 7.16.  RMS is 3.58.  While the amp is playing, it heats up and the bias, and therefore the Class A power, drops a bit.  If I cut the bias back to tech support specs, I'd get less than a watt in Class A.

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