Actually that's called AC coupling or LF roll-off.
Any LF roll-off introduces phase shifts, some of which can be heard in some circumstances. You MUST do this is any velocity (cardioid type or fig-8) microphone unless you are into recording air-conditioning, underground trains, distant thunder or want extra use as a seismograph .. ie bad sound.
There's a huge thread on gearslutz purporting to compare ADC/DACs which pans some converters based on their funny waveform response when really all that is showing is different LF response.
Not true. I pontificate at length on this subject on the Yahoo MicBuilders group for those who are interested.
Just pointing out that what Mr. Marsh wants isn't CMRR but LF roll-off.
... though CMRR is important too.
I am not sure to understand.
Did-you say that the transmission "delay" on any signal change (may decrease) on those slow amps, according to the signal level ?
If so, you can measure the distortion, and, on square waves from -2 to +2v, see strange slopes where the top of the signal arrive before it starts (witch is impossible) ?
Or is-it something witch is not "time" correlated (delay change slowly according to the average level of the signal) ?
Pure curiosity, as i use exclusively current feed-back for audio since decades, with so high slew rates...
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Actually that's called AC coupling or LF roll-off.
Any LF roll-off introduces phase shifts, some of which can be heard in some circumstances. You MUST do this is any velocity (cardioid type or fig-8) microphone unless you are into recording air-conditioning, underground trains, distant thunder or want extra use as a seismograph .. ie bad sound.
There's a huge thread on gearslutz purporting to compare ADC/DACs which pans some converters based on their funny waveform response when really all that is showing is different LF response.
This is not even remotely close to what I am talking about.
Our ears are horns, and its membrane is not symmetrical at all. They produce their own distortion .(Mostly H2, while cones are H3)
I believe that, in a home system, distortion produced by a good horn/driver, is near the one of the ears, in regard to SPL. On a similar nature. I believe my brain interpret the driver's distortion as "level".
It don't bores-me at all.
May-be just the feeling that my system is slightly louder than measure would show... ?
The explanation about the fact i never notice those distortion i can measure, never feel-them as unnatural ?
I noticed, that, in PA, at big distance from the stage, the distortion is so high, and acoustic level so low that this distortion is disagreeable. If you go near the stage, where SPL is very high, you are not bored anymore by those.
Thinking about horns and cones, in our reptilian brains, we associate horns for PA, and cones for home (apart J.C..
May-be all wrong ?
I believe that, in a home system, distortion produced by a good horn/driver, is near the one of the ears, in regard to SPL. On a similar nature. I believe my brain interpret the driver's distortion as "level".
It don't bores-me at all.
May-be just the feeling that my system is slightly louder than measure would show... ?
The explanation about the fact i never notice those distortion i can measure, never feel-them as unnatural ?
I noticed, that, in PA, at big distance from the stage, the distortion is so high, and acoustic level so low that this distortion is disagreeable. If you go near the stage, where SPL is very high, you are not bored anymore by those.
Thinking about horns and cones, in our reptilian brains, we associate horns for PA, and cones for home (apart J.C.
.May-be all wrong ?
In regards to PIM etal --- While waiting to read R.Quan's paper --How would an amp get dc offset to show phase change.
I am suggesting it is thru the asymmetrical waveforms' CM signal component and how that CM signal affects the amp topology. [note that the CM component is zero in a sine wave...it is the average level in an asymmetrical waveform] I brought this up to JC decades ago.... after publishing a paper on Common-mode signals. However, I didnt relate it to PIM directly. Thx -RNM
I am suggesting it is thru the asymmetrical waveforms' CM signal component and how that CM signal affects the amp topology. [note that the CM component is zero in a sine wave...it is the average level in an asymmetrical waveform] I brought this up to JC decades ago.... after publishing a paper on Common-mode signals. However, I didnt relate it to PIM directly. Thx -RNM
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I'm very curious to see this paper too. As i know, phases turns are just a consequence of delays, in regard to frequency.
What the hell can we discover that a square wave signal don't show ?
Found it -- now if it is avail to download yet --- > Still on island off an island. Traveling tomorrow. Talk at ya later.
Usually the user throws it away by improperly posing the problem. We are back here I see. The FFT "throws away" nothing.
For people who have access to the AES Convention Paper 8722:
Any similarities with the content of this patent?
METHOD AND APPARATUS TO MEASURE DIFFERENTIAL PHASE AND FREQUENCY MODULATION DISTORTIONS FOR AUDIO EQUIPMENT
George
Any similarities with the content of this patent?
METHOD AND APPARATUS TO MEASURE DIFFERENTIAL PHASE AND FREQUENCY MODULATION DISTORTIONS FOR AUDIO EQUIPMENT
George
Jan, I'm probably completely missing the point, but isn't the phase shift very high for 15KHz and thus likely configured with some very high closed loop gain.
Even for a simple single pole at 22 phase I think it would take just over 0.5% gain change per degree phase shift, consistent with cmrr of ~40db at 15KHz.
Thanks
-Antonio
I dont mean in theory... but often in practice. Thus the word - ignore was included. did you ignore that? Anyway - beside the point of my main thrust... re cmr and PIM.
[the FFT algor usually drops the dc term as it isnt needed to determine harmonics etc. But yes, it is there in the theory books]
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It looks to be the testing set up for the AES paper is here. No data on test results but that is the test concept, I'm sure. Thx-RNM
Hi Jan,
Although I haven't seen Ron's paper, I'm really looking forward to seeing it.
I think the phase deviations you show here are probably fairly easy to explain. As the operating points change, the forward gain of the amplifier changes and the closed-loop bandwidth changes. A far-out change in the closed-loop bandwidth, often acting roughly like a single pole at the gain crossover frequency, will cause a change in phase shift at frequencies well below that crossover frequency. That is what causes PIM.
These operating point forward gain changes can occur due to numerous things happening, but at high frequencies one of them could be the voltage-dependency of junction capacitances.
One thing of interest in the numbers above is that they seem to suggest that the closed-loop 3 dB frequency is fairly close-in, unless there is some other source of phase lag in the setup. Bear in mind that the phase lag of a single pole one octave below that pole is about 22 degrees. If the numbers above were attributable to a single pole, one would be inclined to speculate that the closed-loop pole was only about an octave above the test frequency of 15 kHz, implying a closed loop bandwidth on the order of only about 30 kHz. This would be very, very low.
It would be interesting to know what the closed-loop amplitude response of the amplifier under test is.
It would also be interesting to know how much the 20-kHz THD of that amplifier was under those same conditions.
The idea of looking at phase lag as a function of a DC offset is largely the same as Matti Otala's original proposal for measuring PIM. That is, mix a 60Hz signal with a 6 kHz sigbal and look at the phase modulation on the 6kHz "carrier" after the test signal is passed through the amplifier under test. This is sort of like the SMPTE IM test, except that we look for phase modulation on the carrier instead of amplitude modulation (AIM). The coherent IM analyzer I built to measure this is described in my JAES PIM paper published in 1983 (a copy of which is on my web site at CordellAudio.com - Home). As long as we can argue that the 60Hz stimulous is low enough in frequency to look like a DC offset for purposes of measurement, the two approaches are essentially the same.
In that paper, a TL071 operating at an inverting gain of 10 and with an output of 6V rms or the p-p equivalent of that value was found to have THD-20 of 0.013%, SMPTE IM (AIM) of 0.0017% and PIM of 1.6 nanoseconds rms when measured in accordance with Otala's scheme.
It is virtually impossible to have any significant amount of PIM without also having an easily measureable amount of THD-20, since the same nonlinearity that causes PIM will also cause HF THD.
Cheers,
Bob
I wonder how we can be concerned in 2012 with phase's problems with amps able to have a flat bandwidth up to 1Mhz, witch is one of the requirements, on my point of view, for amps pretending to be very "high-end" for audio.
The others being level linearity and low noise
What else ?
[edit] In accordance to the last Bob Cordell's post, and its old and accurate papers on TIM, open loop bandwidths, and closed loop amps.
The others being level linearity and low noise
What else ?
[edit] In accordance to the last Bob Cordell's post, and its old and accurate papers on TIM, open loop bandwidths, and closed loop amps.
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