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Old 26th May 2002, 04:26 PM   #1
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Default amplifier oscillations

hey all,
i hava a surface understanding of ac circuits. what is "amp oscillation"?
what are the characteristics?
what causes it?
what are the resulting problems?
and how do you generally get rid of it?

thanks for any input,
cheers,
scott
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Old 26th May 2002, 04:42 PM   #2
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If an amplifier is "unstable" it will oscillate. The characteristics is a very high frequency at the output of the amplifier, in the MHz often. This frequency is dependent on the speed of the feedback loop. Oscillation is very undesirable. Even though it is inaudible, the transistors are switching at that speed and waste a ton of energy and will get very hot. The transfer function of the amplifier must not have "poles" on the right half plane.

BeanZ
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Old 26th May 2002, 04:48 PM   #3
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i just got an aleph 5 up and measuring okay, without R21 loop. the fellow that was helping me, suggested that because the amp worked only when the R21 loop was omitted, that i may have oscillations. are there any other possibilities? all the key measurements are within spec. from the service manual and there is only between 27 and 51 milivolts dc at the outputs.

any thoughts?
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Old 26th May 2002, 09:53 PM   #4
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Hmmm...this sounds suspect...the amp functions when the current souce is disabled (AC-wise), but not when it's working? Can you be a little more specific?
What is it doing or not doing?

Grey

P.S.: Seein' as how this is more along the Pass Labs sort of thing, I'm moving the thread over to that forum.
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Old 26th May 2002, 10:02 PM   #5
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i'll try to be more specific, but not sure how much i can add.

when i have R21 hooked up, VR14 is too low to turn on the input pair, or whatever it's supposed to turn on. when i disable it, boom, well i guess that's not the correct word. how bout voila, outputs turn on and everything measures fine.

hope that helps.
thanks,
scott
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Old 26th May 2002, 10:22 PM   #6
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Lightbulb amplifier oscillations

Try reading http://www.engin.umich.edu/group/ctm/freq/freq.html

H.H.
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Old 26th May 2002, 11:51 PM   #7
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H.H.
whoah!!! i tried reading that article, but it's gonna take a few or a hundred passes for it to sink in. i've never seen this material before...way over my head.
i guess i need a scope to really find out if my amp is unstable in open loop, which would lead to instability in closed loop, eh?
what are the implications of me not having that current source loop in my amp? and why on God's green earth would my amp work without it in?
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Old 27th May 2002, 12:02 AM   #8
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Harry's idea of humor is to hand a drowning man an anchor. We don't need a lecture on stability, we need an oscilloscope. I'm assuming that you don't have one, or you would have already tried that.
You said at first that all the voltages read properly according to the schematic, but then you said that the voltage across R14 is low...
With R21 in place, what voltages are you reading across the back end of the amp, meaning R40-42, R64-66, and across Q5. And just for fun, is there any DC offset when that's hooked up?

Grey
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Old 27th May 2002, 12:23 AM   #9
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You replied while I was typing...
The amp will work either with or without R21. The traditional way to run a single-ended amp is <i>not</i> to have R22-25, R21, C10, and R19. Those parts are the ones that make it an "Aleph," remove them, and you've got what amounts to a Zen amp with a front end. Granted, the rest of the amp might could stand a bit of retuning if you disable that part, but it will work. Sometime later--after you get the amp running properly--read Nelson's patent for the Aleph current source (it's actually pretty readable). The objective is to get more power out of a single-ended design with less heat dissipation...i.e. make it more efficient. Compare the original Zen schematic (not the recent version) with the Aleph output stage, and you'll see what I'm talking about.
But, like I said, it will work, just not as well.

Grey
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Old 27th May 2002, 12:32 AM   #10
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Unhappy Harry's idea of humor is to hand a drowning man an anchor

And your idea of humor is to post schematics of amps that have no high frequency compensation. Tell us what you plan to tell this guy to do about it when he finds out with a scope that his amp is oscillating, and who is doing the greater diservice to beginers. You seem to imply that it is fine to leave caps out of Nelson Pass's origional designs without the basis of knowing what constitutes an amplifier with adequate high frequency stablity. There are plenty of tested and working designs by Nelson Pass for beginers to copy (with all the neccesary caps hopefully). Those who want to design amps are well advised to know a few fundamentals. Phase margin is not that tough to understand or measure. Spice models can give good indication before building the amp.

H.H.
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