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Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
In article , Patrick Turner
wrote: At what frequency and power did you do the distortion measurements? 1 kHz. It may be worth your while to also do measurements at 20Hz or use LF/HF intermod. The thermal effects of fuses on distortion are likely to be greater at LF than at 1kHz due to the time constants involved. But I know that gain / feedback quantities are high OK at 20 Hz, so there is little need for testing at 20 Hz. But what about the fuse (thermal) nonlinear effects at 20Hz? I am assuming your fuse is out-of-loop from what you have said previously. If so, the feedback and gain of the amp are not directly relevant. The paper by Greiner that Arny has pointed out in another thread shows fuses giving quite high levels of distortions under some quite plausible conditions. The thermal dynamic changes and resistance changes would be negligible with a soldered in peice of copper wire. I am not at all sure you are correct. The working method of a metal fuse is that it becomes heated until it ceases to hold together. This means that at or above the rated current the temperature of the metal has to rise sufficiently for this to occur. All normal metals have similar behaviour in that their resistivity rises with temperature. I don't know of any reason why a soldering in piece of copper wire would not follow the same physical laws as other types of metal link arranged to fail at a similar current level. Sure the fuse link may get warm, but not much during normal operation. Well, I can't vouch for Greiner's measurements as I have not repeated them. But he reports distortion levels of the order of 1 percent or more when using fuses rated at the order of just a few amps. If you are building an amp that might be expected to deliver currents for medium-term periods of more than a few amps this may be relevant. If the fuse gets warm, then its resistance will be changing. As before, I'd recommend that you actually measure this to be sure it is OK in your case. It may well be, but a few relevant measurements would establish this one way or the other. No idea what the relevant values would be for the fuses you are using, but unless you do some LF measurements at high currents I would not assume the effects are "negligable". I have never had cause to do much about it. Alas, this may mean you have never been *aware* of having any 'cause' to do much about it if you have not made appropriate tests. As I mentioned in an earlier posting, I was surprised by how large the effects were that were reported by Greiner. e.g. he quotes 1 percent THD for 5A fuses with a 4 Ohm load in one set of conditions. Higher values for other conditions/fuses... Often it isn't the fuse wire itself, its the connection that causes the bothers. That may be so in some cases. However... IIRC you mentioned in an earlier posting that you have some regard for the work of Doug Self. I have just noticed the following in my copy of the 3rd edition of his "Audio Power Aplifier Design Handbook": On page 385. "Fuses running within sight of their nominal rated current generate distortion at LF due to cyclic changes in their resistance caused by I2R heating. The THD would be expected to rise rapidly as the frequency falls and Greiner states that harmonic and intermodulation distortion near the burn-out point can reach 4 percent." Self then plots some measurements he made of the THD before and after a T1A fuse with 25W into 8Ohms. This shows levels around 0.002 percent at 1 kHz both before and after a fuse, but at 15 Hz he got 0.01 percent after the fuse and around 0.002 percent before it. As he says, his plot shows the distortion rising swiftly as the frequency is reduced. As I have said, for all I know, the amp/fuse arrangement you have chosen may well be OK. However it is not clear this *is* the case from measurements at 1 kHz as that may not show the same magnitude of thermal effects as might show up at lower frequencies, etc. It is obviously your choice, but I would have thought it easy enough to check. Slainte, Jim -- Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
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Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
Don Pearce wrote: On Tue, 07 Dec 2004 20:57:12 +1100, Patrick Turner wrote: It may be worth your while to also do measurements at 20Hz or use LF/HF intermod. The thermal effects of fuses on distortion are likely to be greater at LF than at 1kHz due to the time constants involved. But I know that gain / feedback quantities are high OK at 20 Hz, so there is little need for testing at 20 Hz. Is the fuse inside the feedback loop then? That would be an unusual topology since a blown fuse would immediately put the amplifier into open loop mode, and cause massive signals in the output stage. The fuse links are of course after the LR zobel, and most certainly not included in the NFB loop. Patrick Turner. d Pearce Consulting http://www.pearce.uk.com |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
On Sat, 11 Dec 2004 03:27:23 +1100, Patrick Turner
wrote: Don Pearce wrote: On Tue, 07 Dec 2004 20:57:12 +1100, Patrick Turner wrote: It may be worth your while to also do measurements at 20Hz or use LF/HF intermod. The thermal effects of fuses on distortion are likely to be greater at LF than at 1kHz due to the time constants involved. But I know that gain / feedback quantities are high OK at 20 Hz, so there is little need for testing at 20 Hz. Is the fuse inside the feedback loop then? That would be an unusual topology since a blown fuse would immediately put the amplifier into open loop mode, and cause massive signals in the output stage. The fuse links are of course after the LR zobel, and most certainly not included in the NFB loop. Patrick Turner. What made me ask was the comment about gain and feedback being high at 20Hz - so ne need for testing. This would only make sense if the fuse were within that feedback loop. As it isn't, what did you mean? d Pearce Consulting http://www.pearce.uk.com |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
Jim Lesurf wrote: In article , Patrick Turner wrote: At what frequency and power did you do the distortion measurements? 1 kHz. It may be worth your while to also do measurements at 20Hz or use LF/HF intermod. The thermal effects of fuses on distortion are likely to be greater at LF than at 1kHz due to the time constants involved. But I know that gain / feedback quantities are high OK at 20 Hz, so there is little need for testing at 20 Hz. But what about the fuse (thermal) nonlinear effects at 20Hz? I am assuming your fuse is out-of-loop from what you have said previously. If so, the feedback and gain of the amp are not directly relevant. I believe the thd caused by the soldered in fuse links to be negligible. The paper by Greiner that Arny has pointed out in another thread shows fuses giving quite high levels of distortions under some quite plausible conditions. The thermal dynamic changes and resistance changes would be negligible with a soldered in peice of copper wire. I am not at all sure you are correct. The working method of a metal fuse is that it becomes heated until it ceases to hold together. This means that at or above the rated current the temperature of the metal has to rise sufficiently for this to occur. All normal metals have similar behaviour in that their resistivity rises with temperature. I don't know of any reason why a soldering in piece of copper wire would not follow the same physical laws as other types of metal link arranged to fail at a similar current level. Sure the fuse link may get warm, but not much during normal operation. Well, I can't vouch for Greiner's measurements as I have not repeated them. But he reports distortion levels of the order of 1 percent or more when using fuses rated at the order of just a few amps. If you are building an amp that might be expected to deliver currents for medium-term periods of more than a few amps this may be relevant. If the fuse gets warm, then its resistance will be changing. The change of temperature is quite slight. When I tested the amp, the thd didn't seem to be the worse for the presence of the fuse link. I got less than 0.01% at just under clipping into 180 watts, and at 5 watts, less than 0.005%. Most listening will be at 2 watts..... As before, I'd recommend that you actually measure this to be sure it is OK in your case. It may well be, but a few relevant measurements would establish this one way or the other. There are no more measurements I need to make, imho. Often it isn't the fuse wire itself, its the connection that causes the bothers. That may be so in some cases. However... IIRC you mentioned in an earlier posting that you have some regard for the work of Doug Self. I have just noticed the following in my copy of the 3rd edition of his "Audio Power Aplifier Design Handbook": On page 385. "Fuses running within sight of their nominal rated current generate distortion at LF due to cyclic changes in their resistance caused by I2R heating. The THD would be expected to rise rapidly as the frequency falls and Greiner states that harmonic and intermodulation distortion near the burn-out point can reach 4 percent." The fuse links will never get near their burn out temperature during their lives unless there is a short circuit. I made the thd measurements at 180 watts and gave the fuse links adequate time to heat up more than they would with music. No severe thd/imd was noticed due to the fuse heating. Self then plots some measurements he made of the THD before and after a T1A fuse with 25W into 8Ohms. This shows levels around 0.002 percent at 1 kHz both before and after a fuse, but at 15 Hz he got 0.01 percent after the fuse and around 0.002 percent before it. As he says, his plot shows the distortion rising swiftly as the frequency is reduced. I need to have a fuse. I have to wear whatever imd introduces. I have no reason to think there is a lot. As I have said, for all I know, the amp/fuse arrangement you have chosen may well be OK. However it is not clear this *is* the case from measurements at 1 kHz as that may not show the same magnitude of thermal effects as might show up at lower frequencies, etc. It is obviously your choice, but I would have thought it easy enough to check. But the thermal variations of an inch of 8 amp fuse wire are neglible, even at 20 Hz. There is only 4.7 amps rms at 180 watts, 8 ohms. The fuse is there for a short circuit protection or a too low value load. If the rail voltage of 56v appears at the output, 7 amps will flow in 8 ohms. The fuse will stay intact. The DC protect circuit will turn off the amp if 1v DC appears for longer than 1 second at the output. Patrick Turner. Slainte, Jim -- Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
In article , Patrick Turner
wrote: Jim Lesurf wrote: But I know that gain / feedback quantities are high OK at 20 Hz, so there is little need for testing at 20 Hz. But what about the fuse (thermal) nonlinear effects at 20Hz? I am assuming your fuse is out-of-loop from what you have said previously. If so, the feedback and gain of the amp are not directly relevant. I believe the thd caused by the soldered in fuse links to be negligible. By "the" here are you referring *only* to the specific fuses in your circuit? Or is you belief one for any fuses that are soldered in place? My understanding is that the fuse works because of the thermal/resistive properties of metal wire, and in the short term I can't see any reason to believe this is changed significantly by being soldered. Hence I'd recommend you check this and base your conclusions on an appropriate measurement rather than a belief. Sure the fuse link may get warm, but not much during normal operation. Well, I can't vouch for Greiner's measurements as I have not repeated them. But he reports distortion levels of the order of 1 percent or more when using fuses rated at the order of just a few amps. If you are building an amp that might be expected to deliver currents for medium-term periods of more than a few amps this may be relevant. If the fuse gets warm, then its resistance will be changing. The change of temperature is quite slight. Measured in what way, under what signal/load conditions, and with what time resolution? When I tested the amp, the thd didn't seem to be the worse for the presence of the fuse link. I got less than 0.01% at just under clipping into 180 watts, and at 5 watts, less than 0.005%. But are you still only quoting values for frequencies well about 20Hz? Most listening will be at 2 watts..... This makes me wonder. If you are certain of that, why are you building an amp that can provide higher powers or currents? Also, if 2 watts is the mean level, what about the musical peaks that might be 15 - 20 dB higher. - i.e. perhaps up to 200W? Does the distortion on these not matter? Also, IIRC you were developing an amp that a customer wanted to deliver high currents. For all I know they are using a nightmare load as speakers, or will do so in future once they have the amp. I assume you are not telling them that you are assuming limits like the above. :-) Alas, 8 Ohm loads (or even 4 Ohm ones) and sinewaves aren't always a reliable guide. As before, I'd recommend that you actually measure this to be sure it is OK in your case. It may well be, but a few relevant measurements would establish this one way or the other. There are no more measurements I need to make, imho. Well, you may be correct. However given the results of others that I report, then I think it possible that if you did make further suitable measurements you might then change your mind. However if you refuse to make the measurements you won't know this (unless someone else makes the measurements on your amp once the customer has it). Often it isn't the fuse wire itself, its the connection that causes the bothers. That may be so in some cases. However... IIRC you mentioned in an earlier posting that you have some regard for the work of Doug Self. I have just noticed the following in my copy of the 3rd edition of his "Audio Power Aplifier Design Handbook": On page 385. "Fuses running within sight of their nominal rated current generate distortion at LF due to cyclic changes in their resistance caused by I2R heating. The THD would be expected to rise rapidly as the frequency falls and Greiner states that harmonic and intermodulation distortion near the burn-out point can reach 4 percent." The fuse links will never get near their burn out temperature during their lives unless there is a short circuit. I made the thd measurements at 180 watts and gave the fuse links adequate time to heat up more than they would with music. No severe thd/imd was noticed due to the fuse heating. At what frequency, etc. (See above). Sorry, Patrick, but I am puzzled by the way you seem not to be understanding the points I am making. Have you a copy of Self's book and/or the JAES paper? If so, can you see the results that show rising distortion at LF, etc, as I have described? These show quite clearly why a 1kHz sinewave test may simply fail to show this problem. The point here is not how much the medium-term duration temperature of the fuses changes - i.e. on timescales of the order of a second, say. The point is what changes may occur on timescales short compared with, for example, the period of a 20Hz waveform on a cyclic basis. Self then plots some measurements he made of the THD before and after a T1A fuse with 25W into 8Ohms. This shows levels around 0.002 percent at 1 kHz both before and after a fuse, but at 15 Hz he got 0.01 percent after the fuse and around 0.002 percent before it. As he says, his plot shows the distortion rising swiftly as the frequency is reduced. I need to have a fuse. I have to wear whatever imd introduces. I have no reason to think there is a lot. But you might *find* a reason if you did the appropriate measurements. Then your view could be based upon *appropriate* measurements as opposed to a belief. OTOH the measurements may show there is no real worry. As I have said, for all I know, the amp/fuse arrangement you have chosen may well be OK. However it is not clear this *is* the case from measurements at 1 kHz as that may not show the same magnitude of thermal effects as might show up at lower frequencies, etc. It is obviously your choice, but I would have thought it easy enough to check. But the thermal variations of an inch of 8 amp fuse wire are neglible, even at 20 Hz. There is only 4.7 amps rms at 180 watts, 8 ohms. The fuse is there for a short circuit protection or a too low value load. If the rail voltage of 56v appears at the output, 7 amps will flow in 8 ohms. The fuse will stay intact. The DC protect circuit will turn off the amp if 1v DC appears for longer than 1 second at the output. The problem is that you are basing your conclusions on your beliefs and assumptions as opposed to a relevant measurement. Given that the references I have quoted include clear measurements that show that a problem may exist that you measurements so far might miss, I am less confident than yourself that your beliefs are well-founded when it comes to the amp being used with real music and real loudspeakers. It is your choice, though. If it were me, I'd make the measurements to establish that I knew what the fuse did, rather than simple operate on the basis of a belief. Slainte, Jim -- Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html |
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