Why the hositily? I have no opinion about you or your clue about sound, just as you don't know about me. If it sounds great to you, great! There's nothing to discuss about that. I was just wondering if there was a technical explanation we could use and/or reproduce in our own systems.
jan didden
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We often read Stereophile Class A reasoning here. Here is another Stereophile class A list:
ftp://static-173-87-66-34.dsl1.plcd...EREOPHILE Recommended Components OCT 2010.pdf
These products definitely sound very different from no-global-feedback designs. Regardless this FACT they are also class A rated. Should there be a 'superclass A' rating established?
ftp://static-173-87-66-34.dsl1.plcd...EREOPHILE Recommended Components OCT 2010.pdf
These products definitely sound very different from no-global-feedback designs. Regardless this FACT they are also class A rated. Should there be a 'superclass A' rating established?
Back to work. In making the Vendetta Research SCP-1 pre-preamp and later, the SCP-2 phono stage, I took on the whole world of audio, in order to make the best possible solid state gain stage that I could possibly make. At first, it was the change of circuit topology and using complementary jfets exclusively, that got me going, but soon, other factors became almost as important, including, really good layout. I worked with several people on this, and ultimately was introduced to Carl Thompson (you know, the 'T' in CTC) by Jack Caldwell, I do believe, and we started a long relationship, that continues today. He is one of the best layout guys I have ever found, and I have tried more than a few.
However, there was more to learn. First what resistors to use? What capacitors?
Due to a series of good recommendations, I settled on Resista for the resistors, and Rel for the capacitors, mostly due to Peter Moncrieff. (sp) Peter also showed me that POWER SUPPLY BYPASS CAPS could be all important as well. This changed everything and made a larger enclosure necessary. Carl T handled that as well, thank goodness, as I am a real klutz when it comes to fine packaging.
We had many decisions still to make, what circuit board material, gold plating or not, what input and output wires, what RCA connectors? Each of these factors makes its own contribution to the overall sound quality of the product. Topology alone will NOT win listening comparisons. I have found that the overall physical integrity of the product, from materials, soldering, layout, etc will make or break a first class audio product. This it why I emphasize it so much, here, when given a chance.
However, there was more to learn. First what resistors to use? What capacitors?
Due to a series of good recommendations, I settled on Resista for the resistors, and Rel for the capacitors, mostly due to Peter Moncrieff. (sp) Peter also showed me that POWER SUPPLY BYPASS CAPS could be all important as well. This changed everything and made a larger enclosure necessary. Carl T handled that as well, thank goodness, as I am a real klutz when it comes to fine packaging.
We had many decisions still to make, what circuit board material, gold plating or not, what input and output wires, what RCA connectors? Each of these factors makes its own contribution to the overall sound quality of the product. Topology alone will NOT win listening comparisons. I have found that the overall physical integrity of the product, from materials, soldering, layout, etc will make or break a first class audio product. This it why I emphasize it so much, here, when given a chance.
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Well that pretty well sums it up. First, the topology and thru-path must be elegant and inherently distortion canceling. The circuits should be fast, and hopefully with a high open loop bandwidth.
Then, using the best known parts and materials the circuit should be properly laid out and assembled. Once again, in my experience, almost everything matters, and I would not quibble about 'overbuilding' a product, just one oversight can reduce the subjective character of the sound. If you get it right, others will know, as well, in time, and hopefully the design, properly implemented will be successful, at least to a select group of audiophiles. I will leave it there.
Then, using the best known parts and materials the circuit should be properly laid out and assembled. Once again, in my experience, almost everything matters, and I would not quibble about 'overbuilding' a product, just one oversight can reduce the subjective character of the sound. If you get it right, others will know, as well, in time, and hopefully the design, properly implemented will be successful, at least to a select group of audiophiles. I will leave it there.
Good point, John. I have long advocated 19+20kHz CCIF distortion testing because the IM products lie in-band. The CCIF plots are one of my favorite tests that John Atkinson does in his Stereophile reviews.
As you point out, with RIAA you REALLY get a big boost on the even-order terms.
Cheers,
Bob
I just noticed that I can use the AD825 to show what I meant. Circled in red are the points where some base current is injected back into the input stage (just like an offset trim) to cancel some OLG loss due to OS loading. Now if you tweak it just right you can make Aol 120dB rather than 76dB or even overcompensate it to be negative. You get any OL corner you want 1Hz, 1kHz, etc. The crossover frequency stays the same at 41MHz. Yes, NADA.
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Hi John,
Barrie Gilbert is right, and his papers are always good reading. However, he does not state that open-loop bandwidth is important for fidelity. He's right, but what he says does not support your position.
Cheers,
Bob
Actually, I think that *may* be PM or FM to AM conversion, not the other way around.
The two questions then are:
How would get FM or PM in a signal in the first place?
How does feedback affect this?
(Note: No opinions on the latter two are being offered from me!)
It is important to read Barrie Gilbert's article to make good sense of it. It is called: 'Are Op Amps Really Linear?
It appears that when you modulate the open loop gain with normal [AM] distortion, you move the effective bandwidth back and forth, creating FM or PM distortion, due to the difference between the working bandwidth and the open loop bandwidth, and the 90 degree phase shift necessarily due to the compensation capacitor. This creates a distortion term that is 90 degrees out of phase with the input signal, and the vectorial summation of the sine (initial waveform) and the cosine (distortion waveform caused by the 90 degree slope) add vectorially and cause a dynamic phase rotation that is directly related to the square of the input signal, and [signal frequency/gain bandwidth]squared.
In other words the phase of the output signal varies with different dynamic conditions, or all the time. Perhaps it is a small amount, like jitter appears to be, or it can be a fairly large amount when you use cheap IC's as most manufacturers do. Either way, it appears to be real, at the moment, difficult to measure, and probably subjectively an important key to quality audio design.
For example, open loop vacuum tube amps would NOT have this distortion. Solid state CAN have some, even operating open loop, BUT it should just get worse with yet another phase shifting mechanism added.
Once again, I am only the messenger.
It appears that when you modulate the open loop gain with normal [AM] distortion, you move the effective bandwidth back and forth, creating FM or PM distortion, due to the difference between the working bandwidth and the open loop bandwidth, and the 90 degree phase shift necessarily due to the compensation capacitor. This creates a distortion term that is 90 degrees out of phase with the input signal, and the vectorial summation of the sine (initial waveform) and the cosine (distortion waveform caused by the 90 degree slope) add vectorially and cause a dynamic phase rotation that is directly related to the square of the input signal, and [signal frequency/gain bandwidth]squared.
In other words the phase of the output signal varies with different dynamic conditions, or all the time. Perhaps it is a small amount, like jitter appears to be, or it can be a fairly large amount when you use cheap IC's as most manufacturers do. Either way, it appears to be real, at the moment, difficult to measure, and probably subjectively an important key to quality audio design.
For example, open loop vacuum tube amps would NOT have this distortion. Solid state CAN have some, even operating open loop, BUT it should just get worse with yet another phase shifting mechanism added.
Once again, I am only the messenger.
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Oskar Heil (inventor of lots of things, including a very early Fet) was very
big on the sensitivity of our ears to this sort of thing - subtle phase
modulation. He was occasionally wrong in his observations, but I suspect
this was not one of them.
John, I have had this article for many years. I am not sure that it is on the track of audible differences that we speak about, in modern circuits. Not uA741, not TL071, not 4558. We should forget once for ever reasoning supported by results on such devices.
I affirm that OLG corner is not important, in case we have similar circuits with same slew rate and same GBW and they differ only in OLG corner (for the reason of VAS load). The only result of such 'improvement' would be higher distortion - maybe this is the real reason why VAS load resulting in lower Ao is appreciated by some listeners.
Yes, but it's only half of the story. It does NOT improve by just making the OL BW higher. It ONLY improves if you increase the available OL gain at the frequencies where the modulation would appear. That decreases the AM distortion and decreases the AM-FM conversion.
That usually means higher OL gain overall and not necessarily a higher bandwidth. Squashing the LF OL gain will give you higher OL BW but doesn't do one iota to alleviate this issue.
Saying that a higher OL BW is required to decrease PIM is incorrect and misleading.
jan didden
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