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Author Topic: How an AKG 451 Discussion Went Global...  (Read 21412 times)

klaus

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Re: How an AKG 451 Discussion Went Global...
« Reply #45 on: September 08, 2013, 05:22:20 PM »

To be honest, unlike the phase smear of capacitors, which I hear, I have not been able to hear that of resistors, but made a (false?) assumption that ANY component in the path of a straight wire line will display audible artifacts.
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boz6906

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Re: How an AKG 451 Discussion Went Global...
« Reply #46 on: September 08, 2013, 08:53:48 PM »

Some resisters do exhibit inductive or capacitive reactance, wirewound resisters can be the worst.

I think old-style carbom comps may be the best for audio but they exhibit thermal/vibration noise.

I wonder about metal film resisters as I think the metal film may be etched in such a manner to cause inductance, though I've never measured them.  I do like how quiet they are.

This whole idea of voltage/current phase is a real problem in AC power sistribution.

Most loads are inductive (because of power xfmrs and motor windings), it's known as 'power factor' (PF) and power companies go to great lengths to correct the imbalance.

You may notice banks of very large oil-filled capacitors high on the power poles near large consumers of electricity; it's the only available way to correct this E/I phase problem because it can cause tremendous currents to be reflected back down the line and destroy xfmrs and even the generators.

I theorize that in audio circuits this also happens, causing a sort of 'comb filtering'.

Old tube amps used coupling caps, balanced by interstage xfmrs and output xfmrs, leading to a coherent signal path that just sounds better and less cluttered.
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Kai

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Re: How an AKG 451 Discussion Went Global...
« Reply #47 on: September 09, 2013, 09:00:14 AM »

...it can cause tremendous currents to be reflected back down the line ... I theorize that in audio circuits this also happens, causing a sort of 'comb filtering'.
No, reflection only appears if the line is long compared to the wavelenght, then you have a "transmisson line". In these, BTW, signal speed is much lesser then speed of light.
This happens, e.g. on a long analog telephone line (cross country) not properly terminated - you get an echo.

For shorter cables this cannot happen, as the signal is progressing close to the speed of light, so it's the same (in phase) all over the whole cable or element.
You cannot find a "wave" riding on the cable.


Commenting on delay in analog circuitry (a myth):
Analog circuits (or elements) that have almost linear frequency response do not have any delay besides minor phase shifts at both ends of the transmission range.
In fact it's hard to build a delayline even for some microseconds of delay using analog circuitry.
It's done for special purposes, e.g for color tv sets: http://en.wikipedia.org/wiki/Analog_delay_line
http://de.wikipedia.org/wiki/Verz%C3%B6gerungsleitung

Regards
Kai
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soapfoot

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Re: How an AKG 451 Discussion Went Global...
« Reply #48 on: September 09, 2013, 09:12:51 AM »

Speaking of inductance in resistors-- It does exist in film types, but in minuscule amounts only. Some carbon film and metal film resistors have the resistive element spiral-wound against the substrate, which causes some inductance. I was always told that this is almost always too small to have great significance at audio frequencies, but can have some impact in other applications-- for this reason, manufacturers like Caddock and Vishay make metal film resistors with almost zero inductance.

But I'm sure someone with better, more first-hand information can set us straight.
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boz6906

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Re: How an AKG 451 Discussion Went Global...
« Reply #49 on: September 09, 2013, 09:40:26 AM »

Kai, we're not talking about phase shift of one freq vs. another, it's the voltage vs. currentphase shift that causes distorted waveforms.

The AC power distribution circuits operate at 60 Hz, well within audio bandwidth.

"Capacitance has the property of delaying changes in voltage as described in Module 4.3. That is, the applied voltage reaches steady state only after a time dictated by the time constant. In AC circuits voltage and current are changing continuously, and in a purely capacitive AC circuit the peak value of the voltage waveform occurs a quarter of a cycle after the peak value of the current. Therefore a phase shift is occurring in the capacitor, the amount of phase shift between voltage and current is +90° for a purely capacitive circuit, with the current LEADING the voltage. The opposite phase shift to an inductive circuit."

It's this phase shift that I theorize causes audible 'smearing'.

And yes, massive currents can reflect back into the source, causing the proverbial 'standing wave' in the audio bandwidth.

In AC power circuits the current can be large enough to burn down a skyscraper, as in "The Towering Inferno", a real occurrence caused by this phase imbalance.

"The level of interference created when a three-phase wye system is split up and used as three single-phase circuits is truly something to behold. For example, as much as 20% (or more) of the power used by fluorescent ballasts is reflected back onto the power grid in the form of reactive or harmonic currents -- now that’s a lot of distortion. In the late 80’s, a 40-plus-story office building in Los Angeles actually burst into flames because of these reactive currents. Incredibly, the origin of the fire was determined to be from excessive harmonic distortion in fluorescent lighting circuits which created a high-frequency current overload and literally a meltdown of the electrical wiring system. The First Interstate Bank fire in Los Angeles in May of 1988 was the event dubbed by the media as "The Towering Inferno" a la the Hollywood movie. Codes were adapted to remedy the fire danger, but the noise problem itself was never completely resolved."

http://www.equitech.com/articles/enigma.html
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Jim Williams

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Re: How an AKG 451 Discussion Went Global...
« Reply #50 on: September 09, 2013, 11:30:53 AM »

My point.  Besides: anytime you press a signal (electricity) through a component like a resistor or capacitor, you will get time delay, compared to the same signal passed through a straight piece of wire. Is there any argument about that?

If you now press that same signal through two paralleled devices of vastly different storage capacity, you inevitably incur phase shift, i.e. different time delays generated by the two capacitors. Any argument about that?

Even if you can hear to 100 mhz, you will not find those errors in the audio band. If you ever get a hold of a nework analyzer like Agilent or HP you will find no measurable errors up to 20k hz. Capacitor dialectric absorbtion and dissapation factors will eat up your audio far before those timing errors will.

The time delays through a metal film resistor are approaching the propagation delays of electrons through solid materials. That is usually limited to about 70% of the speed of light. That's about 103,000 miles per second of operational speed though your passive components. If you can hear that the NSA has a cushy job waiting for you.
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Kai

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Re: How an AKG 451 Discussion Went Global...
« Reply #51 on: September 10, 2013, 01:53:30 AM »

Kai, we're not talking about phase shift of one freq vs. another, it's the voltage vs. currentphase shift that causes distorted waveforms.
It depends on the way the capacitor is used.
If a coupling cap (series capacitor) is made big enough, it behaves almost like a piece of wire in the audio band.
This means there is almost no phase shift in the waveform, neither relative between different frequencies nor absolute nor as "voltage vs. current" shift.

Here the voltage at the output of the cap follows the input without any delay or phase shift.

The mentioned phase shift between current and voltage isn't happening in this case.

Why is that?
Because we have an almost purely resistive circuit, the capacitic influence is almost neglectable. The bigger the coupling cap, the lesser is it's influence.

Phase shift only occurs if you use a cap in a parallel circuit, something that is not done this way in audio circuitry except if you want to filter out high frequencies.


Practically phase shift appears only at very low frequencies, e.g. the coupling cap loading the output x-former of a tube mic.
Usually this pairing is intentionally tuned to about 30 Hz x-over as low cut filter.
Here the (voltage) phase shift would be 90° for 30 Hz, approaching 0° for higher f's.

Remember: what our mic amp well see is only the voltage (voltage coupling), any currents flowing around do not affect what ends up in the recording.

Don't mix switched DC voltages, power lines and audio transmission, these are different animals and different laws of physics are relevant for those.

I hope this clearifies some things.

Regards
Kai
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Jim Williams

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Re: How an AKG 451 Discussion Went Global...
« Reply #52 on: September 10, 2013, 11:46:39 AM »

Phase shift also occurs in the audio band at the higher end of the audio bandwidth if there is a roll-off applied below 200k hz. This is because phase shift developes a decade above/below the roll-off point. Set the bandwidth from 2 hz to 200k hz and you will see a flat phase vs frequency plot on the Audio Precision analyzer.

The shift isn't audible but the roll-offs are if allowed to enter the audio band. Once I tested a Mitsubishi 32 track digital recorder with an all passive 10th order 48k hz low pass filter to avoid Nyquist frequencies. That rig had not phase shift but group delay of about 2500 degrees. Of course, no one complained about Madonna's hits cut with it. If you had a chance to sum that output with the direct source, it created quite a comb filter from all that group delay. Once all the tracks cut were played together, they did play well together. The original "black face" ADAT also had 2500 degrees of phase shift, that didn't stop millions from buying "Jagged Little Pill".

Compare to an analog recorder with about 90~100 degrees of phase shift at 20k hz due to the record/play equalization and roll-offs.

Anyone with a bit of curiosity can play with phase shift by building/using a tunable all pass filter with a couple of opamps and a dual pot. Commerial units are also available from Little Labs. All pass filters are the basis of the proverbial phase shifter pedals used by 1970's guitarists. They sum the phase shifted signal with the dry signal to create the comb filtering. If you get one of those and disconnect the dry signal, you won't hear anything even though the phase is shifting from zero to 720 degrees or so. That is the nature of phase, you can't hear it alone.
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Kai

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Re: How an AKG 451 Discussion Went Global...
« Reply #53 on: September 10, 2013, 02:31:41 PM »

...Once I tested a Mitsubishi 32 track digital recorder with an all passive 10th order 48k hz low pass filter to avoid Nyquist frequencies.
I used this machine several years here and I really liked (and still do like) it's sound and reliability.
I still have it in working condition, although it's stocked and not in use of course.

Regards
Kai
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