Saturday, November 12, 2011

My Aragon 8008 BB Warmup

Time, total amplifier power consumption, emitter resistor voltage
1min, 140w, 21mV
2min, 160w, 25mV
3min, 175w, 28mV
4min, 186w, 29.5mV
6min, 192w, 30.3mV
8min, 190w, 30.1mV
10min, 187w, 29.8mV
?, 175w, 27.9mV
?, 171w, 27mV
20:30, 168w, 26.5mV
30, 158, 26.1mV

All measurements taken from right channel (what Klipsch calls "outside" channel).  While Klipsch information suggests inside channel is to be biased higher, I measure higher temperatures (by about 3 degrees F) on the outside channel, therefore I suspect my early production unit has the same bias (much higher than Klipsch specified) in both channels.

Beyond 30 minutes, the voltage continues to fall to about 24.5mV, the starts rising again toward 26.5mV peak.  After 10 hours of idling (with covers on) there is still a little oscillation, but it's close to 160w and therefore presumably 26mV.  The temperatures in the middle of the heat sink measure between 130 and 136 degrees F, it gets cooler toward the edges, down to 125 or so (but possibly also measurement error, given that my IR probe has some width function.

Thus, there appears to be a damped oscillation, the "mass" effect probably coming from thermal mass, and the "loss" coming from convection.

When playing music at moderate level, the heat sinks do not appear to get much beyond 136 degrees, and in fact cool down with music at moderately low level.  I haven't yet seen temperature above 137 degrees.

Now Klipsch called for 8mV for inside channel, and 12mV for outside channel, but that doesn't fit with my numbers at all.  And in fact it doesn't seem to fit with the Klipsch specification of 120w power consumption for 8008 mkII (same as BB) amplifier.  Looking at my numbers, an easy extrapolation shows that 120w would correspond to emitter voltage of 17mV.

From that number, or any of these numbers, given the assumption of equal biasing in both channels, we can figure out how much power the output transistors are dissipating at idle and therefore how much the rest of the amplifier is dissipating.

I'm going to use one of my best numbers, the 30 minute reading of 158W and 26.1mV.

26.1mv across 0.33 ohms is 79.1mA
79.1mA across 140V (+/- 70V rails) is 11.1w  (Note: I didn't measure rails. AC was 123V)
11.1w for 6 transistor pairs is 66.6w
66.6w for 2 channels is 133W
That means the rest of amplifier must have been consuming 25W, quite plausible.

Now that I can estimate the actual factor emitter voltage (17mV), I can calculate the maximum class A power for an amplifier with that level of bias current.

17mV across 0.33 ohms is 51.5 mA
51.5mA for 6 transistor pairs is 309mA
Maximum Class A average power into 8 ohms is 2RIb^2 1.53w (or 3.06w peak)
Maximum Class A peak power at any impedance is Vr(2Ib) 43.3w  (Vr is 70v)
Maximum Class A average power at any impedance is 21.6w
The impedance for maximum Class A power is V/I  113.3 ohms

That would seem to be the Class A specifications for a factory biased 8008 BB or 8008 Mk 2 operating at 120W idle.  It would have less than half of the 8 ohm Class A power as mine, but still well exceeds 1w.

On the other hand, an amp biased according the the Klipsch memo at 12mV inner channel and 8mV outer channel would do this:

12mV across 0.33 ohms is 

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