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#161
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On 9 Oct 2004 15:26:05 GMT, B&D wrote:
On 10/8/04 11:20 AM, in article , "Chung" wrote: The listener listens to the amp, *NOT* the amp with the feedback loop open. You can have amps with vastly different open loop responses, but if the closed-loop response differences are beneath audible thresholds, then the amps will sound the same. (For example, there are dozens of op amps that can be used in the signal path, all with different open-loop responses. But many of them will sound identical if used properly.) That isn't true at all. Yes it is, and proveably so. Do you have any evidence beyond mere assertion, for your opinion? The open loop response of an amplifier is critical to the performance of the entire amp - and its response does influence the sound of transients. As someone who designs amplifiers in general, I have seen it and I am currently working on a upgrade to an existing design where the performance is to change the feedback to allow faster rise times and load changes in a reliable manner. As someone else who has been designing amplifiers - for about thirty years now - I agree that it is *preferable* to start with a reasonably linear open loop response. However, it is *not* essential, for sonic transparency. BTW, how do you propose to change the *feedback* to improve transient performance? One "listens" to am amplifier in its entirely - both the open and closed loop response. A well-designed amplifier will not allow the feeback loop to lose control. Hence, you will *only* hear the closed loop response. A previous poster mentioned a source slew rate - and that is important to keeping the amp under control - and I did mention "sufficiently slow loop" will be audible - and I suspect most amps do this. It is sufficient to place a filter at the input, which will not allow slew rate limiting at full output. Crude, but effective. Your 'suspicion' is irrelevant, unless you can *demonstrate* the existence of this effect in a real-world amplifier. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
#162
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On 9 Oct 2004 15:26:51 GMT, B&D wrote:
On 10/8/04 11:20 AM, in article , "Chung" wrote: Here's where you are wrong. The listener listens to the amp, *NOT* the amp with the feedback loop open. You can have amps with vastly different open loop responses, but if the closed-loop response differences are beneath audible thresholds, then the amps will sound the same. (For example, there are dozens of op amps that can be used in the signal path, all with different open-loop responses. But many of them will sound identical if used properly.) Put a capacitor as the feedback on the op amp - and put in a rising edge of a square wave and note the transient and long term response. Do it with a rising edge of various input signals. Why? What has this to do with the closed loop response of a linear audio amplifier? Do it with a variety of capacitors and rising times and you will see that the initial response and final response are different. THIS is a very simplified example of what I am talking about (though not 100% accurate - just to illustrate my point). Sure it's not 100% accurate, because you are describing an integrator, not an amplifier. Have you ever used op amps? Lots - and no one in history has ever attempted to use an integrator as a linear amplifier. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
#163
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On 9 Oct 2004 15:31:52 GMT, B&D wrote:
On 10/8/04 1:08 AM, in article , " wrote: The listener will hear both the open loop and closed loop response. In the steady state condition you will hear the closed loop response. In transients the sound will be influenced by the open loop response. Sure, if you're referring to obsolete dreck designs like a 741 op amp and the like. If one can hear this going on then that amplifier cannot be said to be 'nominally competent'. What news! Why worry about it these days? I am not particularly worried about it myself, but when you design an amplifier using global feedback, you do need to take this into account - not to mention that a lot of feedback can make an amplifier less stable as well. *Any* amount of negative feedback makes the amplifier less stable - but it's not usually a problem. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
#164
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On 9 Oct 2004 15:31:04 GMT, B&D wrote:
On 10/8/04 1:11 AM, in article , "Stewart Pinkerton" wrote: On 7 Oct 2004 03:13:59 GMT, B&D wrote: The listener will hear both the open loop and closed loop response. In the steady state condition you will hear the closed loop response. In transients the sound will be influenced by the open loop response. That is not necessarily true, and is certainly *not* true of a well-designed amplifier which absorbs transients at its input. Transients in this case would be something like a plucked string of a guitar, etc. In which case they will be accurately amplified by any competent audio amplifier, since the bandwidth of the highest frequency component of such 'transients' does not exceed 5kHz, let alone 20kHz. But your point is true - to use feedback properly, you MUST take slew rate into account and make sure the input cannot exceed the speed of the feedback and delay - probably by a factor of 10 or more to make the sound less or inaudible. A factor of three is more than adequate. Absorbing the leading edge of the transients through filtration or other means, you have to be careful as well since you are removing and altering the signal in a benign manner. No, you are simply limiting the bandwidth, in the case of an audio amplifier, to something like 30-40kHz. Any competently designed modern SS amplifier will have no problem exceeding this bandwidth without encountering slew rate limiting, and a well-designed one will have an *open loop* bandwidth greater than this. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
#165
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"Put a capacitor as the feedback on the op amp - and put in a rising edge
of a square wave and note the transient and long term response. Do it with a rising edge of various input signals. Do it with a variety of capacitors and rising times and you will see that the initial response and final response are different. THIS is a very simplified example of what I am talking about (though not 100% accurate - just to illustrate my point)." This is testable as to the threshold that is audible, do the tests. If a threshold can be exceed then it can be audible. As is also other factors in amps,ex. distortion, freq response differences, etc.; all of which illustrate the same thing. The core point in all of this is to demonstrate that in amps today these can be heard as difference, including your's and other proposed design differences that make the $800 and the $8000 amp clearly different using listening alone testing. |
#166
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#168
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#169
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"I will start:
1. Absolutely stable into all loads, 0 Ohms to open circuit, from +90 to -90 phase angles. 2. Rated power is at full power, full bandwidth (band limited white noise) into 8, 6 and 4 ohms" All you are doing above is tinkering with the usual "within current limits" guidelines for comparing amps,ie. if given an untypical load then all amps have such limitations as above and might distort so as to rise above the threshold of audibility. In addition, any amp can be modified to do as above with changes in power supply and number of output devices. This still does the amps sound differently folk no good, if the $800 amp can be changed to have the same current limiting as the $8000 amp any differences as above will disappear, regardless the majic claimed for the latter amp. Some in the latter category will sound different because each has a different current limit if pushed far enough, by which we have demonstrated the obvious. When the marketing/publishing folk claim night and day differences they are not making appeals to differences to current limiting but rather some majic the designer put into them. |
#171
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B&D wrote:
On 10/8/04 11:20 AM, in article , "Chung" wrote: Here's where you are wrong. The listener listens to the amp, *NOT* the amp with the feedback loop open. You can have amps with vastly different open loop responses, but if the closed-loop response differences are beneath audible thresholds, then the amps will sound the same. (For example, there are dozens of op amps that can be used in the signal path, all with different open-loop responses. But many of them will sound identical if used properly.) Put a capacitor as the feedback on the op amp - and put in a rising edge of a square wave and note the transient and long term response. Do it with a rising edge of various input signals. "Put in a rising edge of a square wave"? What exactly are you talking about? Are you trying to ac couple in just the edge? Putting a capacitor in the feedback loop would appear that you want to build an integrator. What is the relevance of that in this discussion? That one could design a circuit with a non-flat freqeuncy response? Do it with a variety of capacitors and rising times and you will see that the initial response and final response are different. Now, are you talking about a differentiator or an integrator? An integrator will turn a square wave into a rtiangular wave. As usual, I find it hard to understand what you are saying... THIS is a very simplified example of what I am talking about (though not 100% accurate - just to illustrate my point). How does that have anything to do with what I said, which is that different op amps with vastly different open-loop responses can sound identical in audio applications? Do you agree with that I said? What was your point that you tried to illustrate? Have you ever used op amps? Since the early '70's. Not to mention designing them in IC's. Of course, what you wrote so far raised the question of whether you really understand how to use op amps or not... |
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