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Old 9th February 2006, 08:28 PM   #1
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Default audible or noticeable symptoms of oscillation?

Hi all.
I have no serious test equipment yet -with the exception of a LCR multimeter-; over the last years, I have made some line, RIAA and headphone preamps with relative success, but there is always a shade of doubt about the objective performance of the devices: they ¨sound¨ -some of them sound very good, some not so good-but not always is easy to discern if the differences in overall sound quality are related to the design per se, or are related to a suboptimal PCB layout, poor bypassing, misplaced components etc.

I wish to know if any of you has consistently detected audible -or noticeable by any other mean, i.e. excessive warming of a device- symptoms of subtle oscillation in a circuit.

Thank you very much in advance.
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Old 9th February 2006, 08:52 PM   #2
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Subtle oscillation - difficult without an oscilloscope. More severe, yes quite easy by heat and sonics.
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Old 9th February 2006, 09:55 PM   #3
Eva is offline Eva  Spain
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When serious oscillation issues are present, random AM or even FM radio stations may be clearly heard in the output or in some nodes of the circuit. Less severe oscillation issues may be perceived as an increase in the noise floor and may be wrongly identified as 'humm', however, when that noise is amplified it tends to sound much like the 'buzz' from an AM radio not tuned to any station. The noise produced by op-amps suffering from mild oscillation at several tens of Mhz may sound just like white noise instead.
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Old 9th February 2006, 10:05 PM   #4
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you do need a scope -- a decent analog scope isn't that expensive and is a worthwhile investment -- good probes help -- but one thing you can do in the absence of a scope is measure the current in your circuit -- if it is greater than you anticipate from the design you 've got a problem. you can also touch the IC's with your finger and if they are roasting -- well enough said.

take a look a the application notes on bypassing on Analog Devices website -- they are clear and to the point -- (just put bypass into their search engine) Texas Instruments has a good application note on "situating" opamps, ground planes etc.
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Old 9th February 2006, 10:08 PM   #5
AJT is offline AJT  Philippines
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you can put a small am radio very close to the pcb and you will know if its oscillating or not.
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Old 9th February 2006, 11:54 PM   #6
lineup is offline lineup  Sweden
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I would guess in many cases
there will large DC-offset across output.
And this offset will not be very stable, constant.

This is easy to measure with a DC Voltage meter.
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Old 10th February 2006, 12:41 AM   #7
clem_o is offline clem_o  Philippines
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Why not build a high-speed active rectifier. AC-couple into it, so you can test various points in the circuit. Any DC offset at the output of the rectifier circuit will tell you if there's any hidden oscillation. Small-signal diodes are good to several MHz.

Cheers!
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Old 10th February 2006, 05:17 AM   #8
Enzo is offline Enzo  United States
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My experience is that you will NOT see any DC on the outputs from oscillation. When a power amp goes into HF oscillation - going RF as we say - it will usually still pass audio, but it does have a certain weak, washed out sound to it, and it is noisy in a hummy sort of way.

SOme non-scope methods would be:

Use the meter on AC current and monitor its draw from the mains. If that seems excessive, the unit might be cranking out the RF even at idle. The unit would run hot as well. A simple way is remove the mains fuse and connect the ammeter in its place.

The AM radio methiod sometimes works, but some 50kHz might not be enough to swamp the radio front end.

You can make a simple RF detector for your meter. The meter will read AC OK but not up to 50kHz, 100kHz, or even in to the MHz range. But a simple diode detector can. Even if there is 20v of signal on the output, it averages zero on DC, and at high freqs your AC meter can't read it. But if we sample the signal and rectifiy it, now it becomes a DC signal we CAN measure.

Basically from the test point, run a small diode to your meter. Then a small cap from the meter hot lead to its ground lead will smooth this DC. COnsider the voltages involved. If teh amplifier has 40 volt rails, the signal can be as high as 80v p-p, so the diode and cap should reflect this. Use a 1N4004 or something and a 100v cap. Doesn't much matter on the cap, a 0.1uf ought to do it. I suppose you could also add a series cap at the test probe tip to block actual DC, again value not critical, 0.1uf comes to mind. But at these freqs the cap offers little resistance.

You can also make them with the diode in shunt, doesn't matter.

50 years ago when I was in amateur radio, "RF probes" were a common accessory for one's meter. I haven't seen one in a long time, and my scope makes it unecessary here, but they probably still make them commercially.

In fact, Google "RF probe" and the first few hits there show exactly how to make this simple tool.
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Old 10th February 2006, 07:03 AM   #9
lineup is offline lineup  Sweden
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Quote:
Originally posted by Enzo
My experience is that you will NOT see any DC on the outputs from oscillation. When a power amp goes into HF oscillation - going RF as we say - it will usually still pass audio, but it does have a certain weak, washed out sound to it, and it is noisy in a hummy sort of way.
.....

You can make a simple RF detector for your meter.

The meter will read AC OK but not up to 50kHz, 100kHz, or even in to the MHz range.
Even if there is 20V of signal on the output, it averages zero on DC, and at high freqs your AC meter can't read it.

But a simple diode detector can.
If we sample the signal and rectifiy it,
now it becomes a DC signal we CAN measure.
Enzo

Thanks for info, that there is often no DC-offset at output
so this is no good method to detect oscillation.

My guess was not so good.
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Old 10th February 2006, 07:26 AM   #10
Enzo is offline Enzo  United States
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To be clear, there may or may not be DC offset, just as there may or may not be oscillation. But they are not usually linked.
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