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Valve amp (preferably DIY) to drive apair of Wharfedale DiamondII's
Patrick Turner wrote:
SS amps are usually all direct coupled, which means their response *should* extend down to 0.0 Hz, but in fact this is impossible, unless we have infinitely low impedance of the PS, which is almost possible if we used good active regulation, or about 50 truck batteries.. Homer nodding again? The LF response of an amp is generally determined by an input coupling capacitor and has nothing to do with the PSU. Most amps only use the earth as a voltage reference and are insensitive to psu ripple. -- Eiron. |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
On Fri, 26 Nov 2004 07:44:52 +0000, Eiron wrote:
Patrick Turner wrote: SS amps are usually all direct coupled, which means their response *should* extend down to 0.0 Hz, but in fact this is impossible, unless we have infinitely low impedance of the PS, which is almost possible if we used good active regulation, or about 50 truck batteries.. Homer nodding again? The LF response of an amp is generally determined by an input coupling capacitor and has nothing to do with the PSU. Some amps are truly d.c. coupled, although this is *very* foolhardy for any commercial amp being let loose on the public! Most amps only use the earth as a voltage reference and are insensitive to psu ripple. Indeed. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
Mike Gilmour wrote: "Jim Lesurf" wrote in message ... In article , Mike Gilmour wrote: "Jim Lesurf" wrote in message Unfortunately, you may find that conventional distortion measurements may not show up the effect of the fuse in series with the output. The effect will depend upon how the thermal time constant of the fuse compares with the time of a cycle of the test waveform. The effect shows up more clearly if you either carefully measure the apparent gain (into say 8 Ohms) or the output impedance, each as a function of signal level. Jim, do you have details of any research on the temperature coefficient of low amperage cartridge fuses in audio use i.e. both quick acting (F) & anti surge (T) ? Alas, no. :-/ I did do some measurements on this 20+ years ago, but at that point decided to avoid them simply to avoid extra o/p resistance, etc. I then did some quick measurements on the fuses in my friend's amp (this was about 5-10 years ago). But in each case I didn't keep results, and only did measurement on a few types and examples. so don't know how general the results would be. I was thinking about this again this morning when I went in to 'work' and asked another colleague who had been involved for the amp tests. But he could not recall details, either. I could re-do some measurements sometime, but these would then just be on a few 'example' fuses, so the results might not apply to all types. In principle, you just use a bench supply to run in constant-current mode, and a couple of meters to plot out the quasi-static IV curve. However this does not tell you the dynamics of the response time if it is quick. For that pulsed measurements are needed. I assume someone *has* done this, but I don't know where the results may be. Can't recall seeing a paper on it anywhere. I suspect that many engineers had the reaction, "Oh, that might be a problem, so I'll avoid it rather than study the details!" :-) Fuses do have some frustrating/surprising properties. FWIW I use 5A fuses in the power rails for the 732 and this cheerfully delivers peak currents well over 30A on music[1] with no signs of fuse failure. Yet when I checked some fuses, some blew in given conditions when others did not, despite being the same rating, and out of the same packet... [1] The music means the current is in brief peak spikes, and each fuse only sees the current for that 'half' of the waveform. This seems to make a lot of difference and the fuses I've used seem to survive this despite a d.c. current much less than 30A probably causing the fuse to blow in less than a second. IIRC I used 'Fast' fuses as the Slow ones never seemed to be any use as they seemed to take far too long to be of value in protecting anything. However since this is mostly recalling 20+ years ago, I can't recall details, and for all I know modern fuses differ in some ways. The snag is that the behaviour is highly non-linear. As you increase the current, the teperature rises, and the resistance rises with it. This, in turn, tends to increase the heating effect for a given current. As the fuse heats up the heat loss due to radiation varies as T^4 (-ish). So you end up with a non-linear and time-varying behaviour. Below a given level the situation stabilises. Above some level the system may go into thermal runaway, thus producing a fuse-blow time that varies in a complex way with the current level. My reaction was along the lines given above - to simply avoid them in the signal path. I have assumed that other people do the same. However if they do not, it is a problem we may have to re-visit. Does anyone know of data on this? If not, I, or someone else, could perhaps do some measurements of some standard example types and report the results. 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 In effect a postive temperature coefficient resistor in series with the signal path. I assume L can be generally ignored as straight piece of wire. It's interesting to observe the 'element' of a clear glass cartridge fuse of the fast blow variety near to its I rating, you can see the wire 'element' bow slightly only to return when the current is reduced (the element seems too short for this to be a result of expansion). I understand this to be the reason why some older fuses just fail below their rating because they've been through this heat-cool cycle once too often. But is there an ongoing change to the fuse's resistance due to these heating-cooling cycles? Does a new fuse have the same resistance as an old fuse when heated? Too many unanswered questions.... Experimenting with a fuse in the signal path bypassed using a relay contacts controlled by the operator/listener may be a way to highlight changes. Mike During this thread I mentioned I was using 3AG fuses in an SS amp held by very tighly sprung fuse holders. During the testing of the latest second channel of the amp I witnessed the thd I was trying to measure drifting up and down, making nonsense of the measurements, and by a factor of 20 dB. Wriggling the fuse cover altered the thd, so the intermittent culprit was found. I have since removed the fuses, and will never ever use clipped or pressure clamped fuses again. I have since rewired the output to include a board with soldered links of fuse wire. The metalic interaction of the ends of the pressure clamped fuse causes quite some thd, as would any connection when not perfect. Once the fuse links were in place, I started to get stable sensible results from thd testing, with about 0.01% at a dB below 150 watts into 8 ohms, 0.005% at 50 watts, 0.004% at 12.5 watts, and 0.002% at 1 watt. Patrick Turner. |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
In article , Stewart
Pinkerton wrote: On Fri, 26 Nov 2004 07:44:52 +0000, Eiron wrote: Patrick Turner wrote: SS amps are usually all direct coupled, which means their response *should* extend down to 0.0 Hz, but in fact this is impossible, unless we have infinitely low impedance of the PS, which is almost possible if we used good active regulation, or about 50 truck batteries.. Homer nodding again? The LF response of an amp is generally determined by an input coupling capacitor and has nothing to do with the PSU. I've not yet had a chance to read Patrick's posting in detail and consider it. However from the context I assumed he meant the internal coupling was usually all d.c. these days. Not sure about his comment re requiring "infinitely low impedance" in the PSU. Having a high gain open loop at DC does not automatically mean that the amp has to deliver a very high output. Nor does having a non-zero closed loop gain at DC even if no input DC break cap is used. Depends on the level of any dc error or input. But I may have misunderstood what he was trying to say here... 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: During this thread I mentioned I was using 3AG fuses in an SS amp held by very tighly sprung fuse holders. During the testing of the latest second channel of the amp I witnessed the thd I was trying to measure drifting up and down, making nonsense of the measurements, and by a factor of 20 dB. Wriggling the fuse cover altered the thd, so the intermittent culprit was found. This was the kind of reason I took to putting fuses in the power lines and avoided them in the o/p. In the power lines any such problems tend to be dealt with by the rail rejection of the amp if it is designed with this in mind. I have since removed the fuses, and will never ever use clipped or pressure clamped fuses again. I have since rewired the output to include a board with soldered links of fuse wire. The metalic interaction of the ends of the pressure clamped fuse causes quite some thd, as would any connection when not perfect. That solves the distortion problem, but may not deal with any medium-term dynamic variations in fuse resistance produced by the music. It also is a bit awkward for the user if they have to change a fuse. :-) Mind you, I was happy enough to put the fuses on the boards so the user had to undo the lid to replace them. Did this to give the PSU caps time to discharge before the user could touch the fuses. c100,000microF caps up at over 80V can make the user jump. :-) 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 , RdM
wrote: I wrote this several days ago, but held back ... it being early am, and I wasn't sure of what I'd written, especially the paragraph beginning "Perhaps again" ... and I'm still not ... but I want to release it anyway [snip] Perhaps at least you could list your website ... If Patrick has already given the relevant evidence I was asking for on websites I would be happy to look there. My problem was that when I asked he simply seemed to dismiss the idea of bothering to give me the evidence. [snip] I think Jim is genuinely searching to understand your POV, and his gentle questions for clarification, and honest feedbacks, might be considered edifying, rather than disputatious. It may be useful to revisit them ... My intent is clarification and education. I am not trying to argue for the sake of argument or trying to 'prove' I am clever. I am fully aware that I make mistakes, just like everyone else. However I'm also aware that other people make mistakes as well. So my interest is in investigating such issues with the aim of leading to improved understanding. [snip] Smooth pebbles, after all, are formed from collisions of rough edges ;-) You mean that I must have rocks in my head? ;- 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 ,
Kurt Hamster wrote: Pinky's helpfulness is eclipsed by the sheer quantity of hot air. Perhaps you'd point at a recent post of yours where *you* were helpful? Otherwise, pot, kettle. HTH. -- *Upon the advice of my attorney, my shirt bears no message at this time Dave Plowman London SW To e-mail, change noise into sound. |
Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
In article , Patrick Turner
wrote: [big snip] I did make some remarks here earlier about the non flat and peaked response the open loop response of a typical SS amplifier, one which I am completing as we type. When measuring an SS amp for open loop response, it is necessary to shunt the AC signal NFB with a large value cap from the NFB port of the amp to 0V, so the DC voltage at the output can still be fed back to maintain CD stability at least, necessary because passive trimming of the DC gain is impossible because in fact the gain is around 250,000 times, or over 100 dBv. One point I'd like to raise here is that we need to take care with quoting values for voltage gain as the actual value may well depend upon the output load. This is particularly the case when considing open loop values as the system then lacks the feddback network that would control the output level and tend to stop the voltage gain varying with o/p load. I'm not clear what value of o/p load you are assuming or using for the values you quote in your posting. A side-point here is to be wary of quoting gains in "dB's" in this context as they can be misleading or confusing unless we take the impedances through the system (and the load) into account. So when a cap is used to shunt the DC, the cap does have some impedance, even if it is 300 uF, and so the response does not measure flat below the open loop HF pole, which is around 1 kHz. I assume that a large cap will also exhibit resistance and inductance as well, so this would need to be taken into account if you are trying to asses the 'true' open loop gain via measurements with a cap. Curious as to why your amp starts its open loop rolloff as low as 1 kHz. SS amps are usually all direct coupled, which means their response *should* extend down to 0.0 Hz, but in fact this is impossible, unless we have infinitely low impedance of the PS, which is almost possible if we used good active regulation, or about 50 truck batteries.. Afraid I still am puzzled by the above comment. Having a high gain at dc should not mean the amp must have rails with zero impedance so far as I can see. Can you explain what you mean here? FWIW I have used instrumentation power amps that go down to dc and these didn't need what you say, nor can I see a theoretical reason for it in the practical cases I have in mind. Hence I am wondering if I've understood what you are saying here. [snip] The output stage is emitter follower, and has about 40 dB of NFB applied there. At 400 Hz, and when global NFB is applied, the open loop gain is reduced from 262,000 to 33, a reduction of nearly 8,000 times. Since thd at 34vrms of output is around 0.005%, we could have expected thd to be 8,000 times worse with no NFB, and that would mean it would be 40%. When i did test the amp with no NFB, thd sure wasn't 40%, more like 5%. so the use of NFB in SS amps does not always seem to follow the laws of gain/feedback equations like it generally does in tube amps using far less NFB. There are various possible reasons for that, and they may not have much to do with choosing SS devices. Two possibilities. 1) That some of the 'open loop' and test gains you quoted are voltage gains and hence may be subject to the comments I made above about being loading dependent and not taking current gains or impedances fully into account. 2) Some distortion mechanisms may be 'outwith the loop' so aren't affected. An obvious example here is that in some AB designs you might find that some of the rail current variations induce error voltages at the ground reference point used for feedback. This then can inject distortion by a route that the feeback loop can't really deal with. I suspect you will have noticed that when looking at low-levelTHD figures, particularly at higher frequencies, you can sometimes adjust the reading by moving around wires or components and inducing various effects similar to (2). On some cases this gives almost a 'null' in some circumstances, but the low value this gives can be highly misleading. Perhaps the fuse at the output really is contributing all that thd; my oscillator which makes about 0.002% at the 1v level. And the wiring of this re-engineered amp isn't state of the art, and so star earthing not entirely optimal, Ah. Perhaps (2) above may be relevant here. but then 0.005% at 144 watts isn't too bad a result. The snag is that to check for things like 'nulled' distortion effects we really need to try things like intermod and asymmetric signals. Unfortunately, these can be harder to do well than thd... That said, it becomes open to question if this matters when the distortion levels are genuinely low. Maybe I can wheel out the 1 kHz LC filter with 30 dB attenuation at 3 kHz, and that may improove the thd measurememnts, along with placing a shunt across the fuse, or using a 40 amp auto fuse, which should have less effect than the 10 amp one I have their now. The fuse is outside the NFB loop. What current is required with these fuses to make them blow in, say, less than 100 milliseconds? I don't know about currently pun! available fuses, but I found in the past that you had to use surprisingly low values to get them to blow quickly enough to protect bipolars. [snip] I may appear to not be willing to discuss every widget and twadget in the thread; I have to earn a living, and I am time poor. I appreciate that. I am now retired, but still find I don't have enough time to do everything I intend/plan. :-) But I hope I have been of use to those actually building something, rather than spending time all night picking holes in people's arguments and building nothing, observing nothing, measuring nothing, and understanding less. No idea who you are referring to here. :-) Can't speak for anyone else. But I hope I have been asking questions and making comments with the aim of improving *all* our understanding and aiding better design. FWIW I have built, measured, etc, quite a few things in my time, so I assume you are not referring to me above. ;- And BTW, I do have a website, and one due for re-vegetation after all the browsers have passed over the last 4 years, and I will get around to it, along with my tax return, and all the other work. There is little at my website regarding solid state design and build methodology. The master at that appears to be Douglas Self, from whom I have learnt a lot. Ben Duncan is another who inspired me to consider the considerables. Fritz Langford-Smith taught me more about tube craft than anyone else. he wrote the Radiotron Designer's Handbook, along with his merry team of helpers. FWIW I also have a high regard for Doug Self and Langford-Smith. Alas, I can't say the same for Ben Duncan as I have found too many misleading, ambiguous, out-of-context, or inaccurate things in his published work to be comfortable with a lot of what he has written.[1] I regret to say that many textbooks contain things which should be treated with some caution. Slainte, Jim [1] If interested: For a specific example, go to the 'Analog and Audio' section of the "Scot's Guide" and have a loop at the linked page on "Current dependent phase shifts in cable?" -- 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
I have since removed the fuses, and will never ever use clipped or pressure clamped fuses again. I have since rewired the output to include a board with soldered links of fuse wire. The metalic interaction of the ends of the pressure clamped fuse causes quite some thd, as would any connection when not perfect. That solves the distortion problem, but may not deal with any medium-term dynamic variations in fuse resistance produced by the music. It also is a bit awkward for the user if they have to change a fuse. :-) The soldered in fuse link seems to be quite blameless, since it is but a piece of plated copper wire. The distortion measurements indicate it has no large effects worth a worry. Mind you, I was happy enough to put the fuses on the boards so the user had to undo the lid to replace them. Did this to give the PSU caps time to discharge before the user could touch the fuses. c100,000microF caps up at over 80V can make the user jump. :-) All you need with 100,000 uF at 80v is 1k5 at 10 watts as a bleed resistor across the cap. The time constant is 150 seconds, no? so rails would be down by the time you opened the case. Patrick Turner. |
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