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Old 24th November 2012, 04:16 PM   #29821
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Quote:
Originally Posted by mikelm View Post
OK

So do we think that this audible effect comes from the fact that when this resonance is excited by the music signal and the some oscillation begins the transistor has been transformed from a reasonably linear device to a non-linear device and this causes audible distortion ?

Is it that simple ?
You have to be precise about the term excite, an impulse excites activity at all frequencies. Dynamicly moving the operating points of an amplifier stage into a region of downright oscillation is not the same thing. The later is predominantly what is happening in many cases especially when you include amp, cable, and speaker together.

As for bypassing issues, you would probably do better looking at RF ingress. At least without eliminating that you can not make other speculations.
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Last edited by scott wurcer; 24th November 2012 at 04:23 PM.
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Old 24th November 2012, 04:19 PM   #29822
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Originally Posted by mikelm View Post
1) higher FB designs more prone to local resonances.
...
The circuit will be prone to resonance or oscillation lets assume the former - prone to resonance but not actually oscillating.
What the point ? A square wave at high frequency, fast transition time, is a perfect finger to hit the Cristal glass.
Just don't allow overshoot a any level.

[edit]Cross post with Scott, we said the same (as often)
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Last edited by Esperado; 24th November 2012 at 04:21 PM.
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Old 24th November 2012, 04:21 PM   #29823
DF96 is online now DF96  England
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Perhaps my use of the word 'trigger' was misleading. I meant the situation where the music/test signal acts as a bias change for an RF oscillator buried in the circuit. A bypassed capacitor could of course act as an unintended RF resonator for such an oscillator.
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Old 24th November 2012, 04:31 PM   #29824
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Originally Posted by DF96 View Post
Perhaps my use of the word 'trigger' was misleading. I meant the situation where the music/test signal acts as a bias change for an RF oscillator buried in the circuit. A bypassed capacitor could of course act as an unintended RF resonator for such an oscillator.
Have-you ever noticed such a behavior, playing with square waves at various frequencies/levels ?
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Old 24th November 2012, 04:39 PM   #29825
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Here, we have to cut in our thoughts preamps, where feedback look only to passive devices (impedances) from amps, where feedback look at loudspeakers acting like microphones.
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Old 24th November 2012, 04:41 PM   #29826
mikelm is offline mikelm  England
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sorry I thought "excite a resonance" was precise enough.

If one hits the soundboard of a guitar one impulse comes to the face the guitar and this excites a resonance in each of the strings. The frequency is different for each string according to it's own conditions but the same impulse has excited all of the strings.

In this way in an amplifier the musical impulse comes in and the devices in the amp react according the to signal but when this signal reaches a part of the circuit which has a natural resonance that part of the circuit will add it own characteristic to the signal.

I have spent years of my life working on preventing RF ingress to audio circuits - the effect I'm talking about is independent of RF ingress. More accurate to call it RF excitation due to insufficient damping of Capacitive & Inductive Reactance

Last edited by mikelm; 24th November 2012 at 04:51 PM.
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Old 24th November 2012, 04:51 PM   #29827
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Set the bandwidth of a preamp to be flat with no ringing whatever the capacitive charge with a correct serial resistance in output and no problem when short-circuit after the resistance or no charge. Set a low pass filter before the input for no overshoot at any level with the same charges... Set them for max signals and small ones... What do you expect ?

[edit] Milkem: "Soundboard of a guitar" or "Cristal glass".
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Last edited by Esperado; 24th November 2012 at 05:00 PM.
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Old 24th November 2012, 05:21 PM   #29828
RNMarsh is offline RNMarsh  United States
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Default Wild speculation or RFI ....??

It May be that RFI causes problems that are not seen under single unit testing... but would show up if the whole system was tested (here we go again with marsh and his system testing) --- check this out: See the attachments. Equipment on the ac line produce wide bands of rfi and most power supplies in audio gear are not equiped to keep it from getting into the circuitry. It has been shown that when this happens, significant increases in THD in the amp occures.... Attached are spectrum of 1.25Mhz bandwidth of some equipment (CD player and CPU) that I measured (powered ON) on the ac line. measured with a wideband current pickup probe on the DUT equipment's ac cord. All items on the ac line (TV, lamp dimmers, etc) put a lot of RF on the ac line with thier own signature. I used this type info to create a line of ac power conditioners with outlet to outlet isolation.

This rfi on the ac line may be the real reason why people hear improvements when they use small value caps in shunt on the DC power lines. Or, better, in general with wideband low Z power supplies. A well developed ac line filter with isolated outlets often makes an audible improvment according to listeners. BTW - almost no noise of wide BW and of continuous nature comes in on the ac line. Its coming from within the home, guys. We are our own worst enemy! Thx-RNM
Attached Images
File Type: jpg Line noise.jpg (721.7 KB, 108 views)

Last edited by RNMarsh; 24th November 2012 at 05:43 PM. Reason: Must have isolated outlets --
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Old 24th November 2012, 05:53 PM   #29829
bcarso is offline bcarso  United States
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Originally Posted by RNMarsh View Post
Its coming from within the home, guys. We are our own worst enemy! Thx-RNM
The power line in my apartment is filthy now. It (whatever it is) gets into everything and renders high frequency differential measurements essential below a few tens of millivolts on the line-powered oscilloscope.

And I had thought the mandated vacuum fluorescents were bad enough!
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Old 24th November 2012, 05:59 PM   #29830
mikelm is offline mikelm  England
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Quote:
Originally Posted by RNMarsh View Post
Its coming from within the home, guys. We are our own worst enemy! Thx-RNM
That's a very interesting study and I completely sympathize with anyone who suggests that RFI can be injected into the mains power from audio equipment, and be injected into audio equipment from the mains and mess up the sound big time - I think this is why systems at HiFi shows often sound so bad compared with home systems.

What Scott didn't realise when he suggested RFI as a culprit regarding my original question is that in my designs I am about as fanatical as it is possible to be about preventing RF getting into my circuits.

Because switching diodes ( even schottky diodes ) and transformer secondary resonance ( when diodes switch off ) generate their own RFI I usually concentrate my filtration efforts after the bridge.

I don't think there is a power rail or earth line anywhere in my system that does not have CLC filtration. For me noise in audio circuits is public enemy No1.

Having taken all these measures I then have to make sure these efforts have not been wasted by having elements inside my "RFI free zone" that are merrily producing their own RFI due to lack of damping.

I hope this clarifies why I am interested in local resonant elements within circuits and my question about how it is that we can hear the effects of a 15Mhz resonance remains.

thanks

mike

Last edited by mikelm; 24th November 2012 at 06:01 PM.
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