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Old 20th December 2002, 08:11 PM   #31
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Default Re: Re: Images... ...1 of 2

Quote:
Originally posted by peranders

Very dangerous to drive the amp at very high frequency with no or little load, even dangerous with load. I have made the same mistake myself.
Not really true. I've been running the amp at HF for a couple of days now, load and no load. As long as voltages do not go over specified ratings of parts, no harm is done. Under certian situations, the lead inductance can generate large voltage spikes, but I hardly suspect this was the problem at 100Khz.

In my case, the problem was not the frequency, but rather the heat generated. I was dumping about 70W into an 8 ohm load for an extended period of time with poor heatsinking. I knew heat was a problem, so kept high power tests to short bursts. (All, but the last one, of course)

-Dan
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Old 20th December 2002, 10:16 PM   #32
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Exclamation Peranders is right

Operating an audio amplifier at high frequency and max power for long periods can result in destruction of the output transistors.

This is caused by cross conduction of output transistors, because of the relatively long time needed to evacuate charge carriers in the base-emitter region.

Amplifiers with modified output drive like this one (R56, C51) are more tolerant, because of the accelerated evacuation of the charge carriers by the RC network. This is also depending on particular characteristics of the output transistors, high Ft and low Ccb are essential.

Regards, Pierre Lacombe.
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Old 21st December 2002, 12:05 AM   #33
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Default Re: Peranders is right

Quote:
Originally posted by P.Lacombe
This is caused by cross conduction of output transistors, because of the relatively long time needed to evacuate charge carriers in the base-emitter region.
Regards, Pierre Lacombe.
Hi,

I realize that cross conduction can be a problem, but I'm pretty sure that this is not so in my particular case. The output devices get just as hot at 1Khz as 100Khz. under similar loads. (Again because of the small temporary heatsink). Under no load situations HF and LF the devices stay cool.

However, I guess, if you consider that the fets may need more VGS to conduct current into the load than under no load, there is more charge in the gate, and thus more to remove. Not sure how more current affects the levels of gate charge with the input/output capacitance.
I would suspect that with the gate charge and input/output capacitance, the problem should also be as apparent on the rising egde of the waveform as well as the falling one, showing itself as a limiting of rising as well as falling slew rate.

Overall, my gut feeling is this isn't the case, but I'm open to the suggestion. (And more comments)


-Dan
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Old 21st December 2002, 12:08 AM   #34
Rambi is offline Rambi  Netherlands
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Quote:
Originally posted by Jeff R
I have a question about the servo op amp, U1. There is only a capacitor in the feedback loop. If the op amp has a dc output offset voltage (and they all do) it is possible for a dc voltage to develop across the integration capacitor and ultimately build up so large as to saturate the op amp. Typically, a large value resistor is paralled across the capacitor to provide a path for the dc to discharge. You have a pretty large Rin right now, and the LF411 is not exactly a low offset voltage op amp, so I am concerned about this charge buildup.
In this case that won't matter since the (inverting) integrator feeds back to the input of the amplifier; It's a closed loop. A dc buildup by the integrator will be amplified (by the amplifier) and fed back to the input of the integrator, negating the buildup (since the integrator itself is inverting).

Of course an offset of the opamp might cause the output of the amplifier to be not exactly 0V. But I'm not interested in 'absolute zero', I just want to compensate for drift by temperature changes and imbalances in the circuit (although it probably won't compensate for totally unmatched components).

Although there's no connection drawn in the schematic, it is connected by the label 'IN', which is also placed at the top of C61. I used labels here to keep the schematic relatively clean. Same holds for the feedback (NFB labels, between R54 & R55 and at the base of Q4).

Remco
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Old 21st December 2002, 02:22 AM   #35
Jeff R is offline Jeff R  United States
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Quote:
Originally posted by Rambi


In this case that won't matter since the (inverting) integrator feeds back to the input of the amplifier; It's a closed loop.
Remco
While true, the general rule in audio design seems be to use local feedback rather than overall global feedback. By using a paralleled resistor you can negate most of the charge build up right there without having to go through the feedback. I have no idea if the effects are audible - probably not - but then who would have thought that different brands of metal film resistors would sound different?

Just something to consider. Good luck!
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Old 21st December 2002, 10:48 AM   #36
Rambi is offline Rambi  Netherlands
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Quote:
Originally posted by Jeff R

.. I have no idea if the effects are audible - probably not - but then who would have thought that different brands of metal film resistors would sound different?

Just something to consider. Good luck!
Thanks!
And yet another option to include on v2.0


Remco
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