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#1
Posted to rec.audio.tubes
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Tube Amp Phase Response
I am hoping to be able to borrow the necessary equipment
to make some phase measurements of various bits of tube equipment. As far as RC coupled power amps are concerned, I would expect the LF output to lead the input, and the HF to lag it. Is this correct? I cannot recall having seen any graphs from commercial amp builders, but I am hoping that these figs, open and closed loop might tell me something about amplifier stability, which, for the homebrew builder of PP amps with feedback is perhaps the greatest challenge of all:-) Has anyone taken these kind of measurements? I would be interested to hear what kind of figs have been plotted, and what might be regarded as typical/good. Festive greetings to all fellow rodents Iain |
#2
Posted to rec.audio.tubes
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Tube Amp Phase Response
Iain Churches wrote: I am hoping to be able to borrow the necessary equipment to make some phase measurements of various bits of tube equipment. As far as RC coupled power amps are concerned, I would expect the LF output to lead the input, and the HF to lag it. Is this correct? What does very basic LCR theory predict? What were your observations with a dual trace CRO with various R & C and R & L arrangements? What is your knowledge of basic equivalent modelling of amplifiers? If you are searching for answers to the above, then you now know how to spend your time off over christmas. Out of the pub, and locked in your workshop I'd say. I cannot recall having seen any graphs from commercial amp builders, but I am hoping that these figs, open and closed loop might tell me something about amplifier stability, which, for the homebrew builder of PP amps with feedback is perhaps the greatest challenge of all:-) The phase response of many amplifiers both before and after NFB was applied used to be published but not now because it is of such little significance. Has anyone taken these kind of measurements? I would be interested to hear what kind of figs have been plotted, and what might be regarded as typical/good. Usually less than 20 degrees of phase shift maximum anywhere across the band is regarded as OK. Most amps with NFB easily achieve such criteria with a resistance load. The R loaded amp phase response is not an indicator of unconditional stability. Just rememeber that the crossover filters used in most speakers plus the drivers themselves add the majority of phase shift in a system. There is much phase shifting in the recording process unless kept all digital. A study done in Scientic American in about 1967 concluded that nobody could detect the relative phase of harmonics in a given tone, so that if for example you have a 200Hz fundemental musical tone, then if its 2H or 3H which could be 20% of the fundemental amplitude was phase shifted relative to the fundemental by +/- 90 degrees, nobody heard anything different; "the sound" didn't change. The article concluded it was just as well our ears do not react to phase too much, or else we would spend all day being aurally muddled. Evolution evolved ears which are very good at determining direction of sound, since phase does matter, but audiophile concerns about phase and its effect on "the sound" are largely twaddle. Just don't let me catch you doing anything to wreck the phase though when you record something. Oh, and speakers often have the tweeters reverse phase connected, ie, 180 degrees different to the midrange, and yet folks tell me that sounds better than having everything "phase coherent". By what amount do the realtive phase relationships of say 50 frequencies present in an arriving musical signal at our ears change when leaning forward say 400mm in a chair? Patrick Turner. Festive greetings to all fellow rodents Iain |
#3
Posted to rec.audio.tubes
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Tube Amp Phase Response
Iain Churches wrote:
I cannot recall having seen any graphs from commercial amp builders, but I am hoping that these figs, open and closed loop might tell me something about amplifier stability, which, for the homebrew builder of PP amps with feedback is perhaps the greatest challenge of all:-) Has anyone taken these kind of measurements? A guy named Nyquist published a paper on this about 75 years ago :-). "Regeneration Theory", Bell System Technical Journal 11.1 (Jan 1932) p 126. Bode and Black were doing important stuff too. Obviously prior to the Bode plot and the Nyquist diagram, negative feedback had been used advantageously, but the simple diagrams turn all the complex stability criteria into very beautiful pictures that just make sense. Even though in today's classrooms Nyquist's work is talked about rather abstractly in terms of information theory, this is a shame because he was solving the most important on-the-ground important problems in electronics and telegraphy and telephone technology. It certainly is impressive that people still talk in fancy language about all the stuff Nyquist was doing back then :-). The Radiotron Designers Handbook 4th edition has a chapter nearly a hundred pages long that walks you through every step for several amplifier topologies, including (very importantly) measurements necessary to draw the Nyquist diagram. Look in particular around page 338. Note when comparing old and modern references that the sign of the imaginary axis will randomly flip. The long tabulations of complex math from the middle of last century would probably be done on a spreadsheet today :-). The RDH is floating around the net, google for "RDH4", you want specifically Chapter 7 but the rest of the book is wonderful too. I have been lucky enough to locate several paper copies which I believe is far more convenient than tying myself to a computer to read PDF's. As to your original question of lead vs lag, for a typical RC coupled amplifier yes there is a lead at LF and a lag at HF. Chapter 17 has graphs for a "typical" Williamson. I put "typical" in quotes because I do not believe that a real Williamson was really that flat, but then again I never had his magical transformer. Tim. |
#4
Posted to rec.audio.tubes
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Tube Amp Phase Response
In article ,
Patrick Turner wrote: Iain Churches wrote: I am hoping to be able to borrow the necessary equipment to make some phase measurements of various bits of tube equipment. As far as RC coupled power amps are concerned, I would expect the LF output to lead the input, and the HF to lag it. Is this correct? What does very basic LCR theory predict? What were your observations with a dual trace CRO with various R & C and R & L arrangements? What is your knowledge of basic equivalent modelling of amplifiers? If you are searching for answers to the above, then you now know how to spend your time off over christmas. Out of the pub, and locked in your workshop I'd say. I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. A study done in Scientic American in about 1967 concluded that nobody could detect the relative phase of harmonics in a given tone, so that if for example you have a 200Hz fundemental musical tone, then if its 2H or 3H which could be 20% of the fundemental amplitude was phase shifted relative to the fundemental by +/- 90 degrees, nobody heard anything different; "the sound" didn't change. The article concluded it was just as well our ears do not react to phase too much, or else we would spend all day being aurally muddled. You are in trouble now, audiophiles pride themselves on being able to hear phase, and like to think their ears are special. By what amount do the realtive phase relationships of say 50 frequencies present in an arriving musical signal at our ears change when leaning forward say 400mm in a chair? Neglecting room effects, in other words if you are listening in an anechoic chamber, leaning forward say 400mm in a chair has no effect on the "relative phase relationships of say 50 frequencies". Leaning forward only affects the time delay, and the time the CD spends siting on your shelf has a much greater effect on the time delay. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#5
Posted to rec.audio.tubes
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Tube Amp Phase Response
John Byrns wrote: In article , Patrick Turner wrote: Iain Churches wrote: I am hoping to be able to borrow the necessary equipment to make some phase measurements of various bits of tube equipment. As far as RC coupled power amps are concerned, I would expect the LF output to lead the input, and the HF to lag it. Is this correct? What does very basic LCR theory predict? What were your observations with a dual trace CRO with various R & C and R & L arrangements? What is your knowledge of basic equivalent modelling of amplifiers? If you are searching for answers to the above, then you now know how to spend your time off over christmas. Out of the pub, and locked in your workshop I'd say. I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. A study done in Scientic American in about 1967 concluded that nobody could detect the relative phase of harmonics in a given tone, so that if for example you have a 200Hz fundemental musical tone, then if its 2H or 3H which could be 20% of the fundemental amplitude was phase shifted relative to the fundemental by +/- 90 degrees, nobody heard anything different; "the sound" didn't change. The article concluded it was just as well our ears do not react to phase too much, or else we would spend all day being aurally muddled. You are in trouble now, audiophiles pride themselves on being able to hear phase, and like to think their ears are special. I have a flame suit on. But many audiophiles have no better hearing than anyone average. They have a very close mental connection to sound, perhaps they are obsessive about it all.... Some are most definately more aurally perceptive than others, and their brains make a lot more from a given bit of music than average ppl, and some understand each word of an opera where the avereage needs sub-titles, and they hum along, since they have perfect pitch.... The significance of phase problems in amps may mot worry them too much. By what amount do the realtive phase relationships of say 50 frequencies present in an arriving musical signal at our ears change when leaning forward say 400mm in a chair? Neglecting room effects, in other words if you are listening in an anechoic chamber, leaning forward say 400mm in a chair has no effect on the "relative phase relationships of say 50 frequencies". Leaning forward only affects the time delay, and the time the CD spends siting on your shelf has a much greater effect on the time delay. But the relative phase is the myriad of phase angles relative to a reference phase angle which could ultimately be related to the time of the day, 0 degrees being midnight. The relative phase is all we are ever going to experience meaning fully. We listen to constantly changeing different relative phases of frequencies all day long. Even in an anechoic chamber the slightest head movement causes gross relative phase differences because every F has a different wave L. We just notice about nothing, although we are gifted with mamal brains which have evolved to sense direction based on relative phase using TWO microphones each side of the head. All the critters who didn't know where the lion was got eaten and their attempts at evolving hearing were curtailed. Then the musicians move around a lot....... It still sounds like an orchestra regardless of whether they sit still as possible or dance with their violins. Without constant relative phase changes continuously, music fast becomes boring, like an organ without real vibrato, ie, shifting the note F either side of the wanted note F. So the more phase variation, the better. Patrick Turner. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#6
Posted to rec.audio.tubes
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Tube Amp Phase Response
Patrick Turner wrote:
The phase response of many amplifiers both before and after NFB was applied used to be published but not now because it is of such little significance. Not much for listening, but if you are like the Iain and me and are just learning the ropes of NFB and stability criteria I think there is a lot to learn. I learned a lot from your schematics on the web, for example. But I learned much much more when I actually built some amps, tested and measured and graphed phase and gain and Nyquist diagrams, and then actually understood why you stuck those shelving networks in :-). Tim. |
#7
Posted to rec.audio.tubes
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Tube Amp Phase Response
I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. IN addition to the above, I'd take a look at a square wave. See how well it gets reconstructed. If the various harmonics have the same time delay (aka "group delay"). A study done in Scientic American in about 1967 concluded that nobody could detect the relative phase of harmonics in a given tone, so that if for example you have a 200Hz fundemental musical tone, then if its 2H or 3H which could be 20% of the fundemental amplitude was phase shifted relative to the fundemental by +/- 90 degrees, nobody heard anything different; "the sound" didn't change. The article concluded it was just as well our ears do not react to phase too much, or else we would spend all day being aurally muddled. Yes for continious tones, but what about transient spikes? Music has lots of those. -- Posted via a free Usenet account from http://www.teranews.com |
#8
Posted to rec.audio.tubes
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Tube Amp Phase Response
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#9
Posted to rec.audio.tubes
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Tube Amp Phase Response
Robert Casey wrote: I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. IN addition to the above, I'd take a look at a square wave. See how well it gets reconstructed. If the various harmonics have the same time delay (aka "group delay"). A study done in Scientic American in about 1967 concluded that nobody could detect the relative phase of harmonics in a given tone, so that if for example you have a 200Hz fundemental musical tone, then if its 2H or 3H which could be 20% of the fundemental amplitude was phase shifted relative to the fundemental by +/- 90 degrees, nobody heard anything different; "the sound" didn't change. The article concluded it was just as well our ears do not react to phase too much, or else we would spend all day being aurally muddled. Yes for continious tones, but what about transient spikes? Music has lots of those. But there is nothing that happens faster in music than a what happens in tone burst from nothing to full power of a sine wave at 20kHz, mainly due to the bandwidth limiting of the cd format, and certainly hi-fi FM transmisions don't have anything above about 16kHz. Try as hard as you like, but there will be no signal in any audio amp which rises from zero volts to clipping voltage in less than 1/100,000 of a second. A 20kHz sine wave takes 1/20,000 seconds to happen. The rise from zero V to the positive or negative peak takes 1/80,000 seconds, so the slope at zero crossings is such that zero volts to clipping should be doable in about 1/100,000 seconds, which = 10uS. If an amp can make 32 watts into 8 ohms, that's 16Vrms, or about 22.5 peak volts. A rise of 22.5 peak volts in 10uS = 2.25V/uS To ensure such performance can be attained, we try to build our amps to be able to achieve a sine wave at 65kHz into the rated R load at up to 0.7 x 1 khz sine wave clipping voltage, and without having any active device going into grid current/overloaded/saturated condition. I will leave you guys to figure the V/us the HF pole of 65kHz allows, and to consider that an amp producing 128 watts into 8 ohms needs to have a "rise time" = 2 x that of the amp making 32 watts. If these conditions are met then the amp will withstand any "transient" produced in music with ease. Transients in music are simply a complex array of HF sine waves of various F and with various decay rates, but nothing faster than can happen does happen. Solid state amps ensure the output is bandwidth limited due to the Zobel LR and CR networks, and these ensure the amp with a lot of NFB is never embarrassed by being forced to achieve absurdly fast rise times which are simply not required. Patrick Turner. -- Posted via a free Usenet account from http://www.teranews.com |
#10
Posted to rec.audio.tubes
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Tube Amp Phase Response
I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. Hi John, Yes I have previously used the method you described. I want to plot 10Hz to 100kHz so the above method takes some time:-) The system I am using is built by Feedback Electronics UK. It consists of an oscillator with two outputs and two independent gain controls, and a metering unit with two inputs. (Yes, you have guessed it) You plug one of the oscillator outputs straight into meter input A and the other into the amp. You bring back the output from the amp to the meter input B which has a digital read out to show you the phase difference. A LED shows if the return signal is lagging or leading. I have also used another method with the scope in XY mode, but the dedicated 2-channel oscillator and meter is far quicker and more accurate too. Regards to all Iain |
#11
Posted to rec.audio.tubes
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Tube Amp Phase Response
wrote in message ups.com... Patrick Turner wrote: The phase response of many amplifiers both before and after NFB was applied used to be published but not now because it is of such little significance. Not much for listening, but if you are like the Iain and me and are just learning the ropes of NFB and stability criteria I think there is a lot to learn. Hi Tim, Building a tube amp with quite an impressive performance is not beyond the capability of most dedicated constructors, but learning about NFB and critical damping is not so easy, but nevertheless necessary if one wants to progress as a tube amp builder. Iain |
#12
Posted to rec.audio.tubes
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Tube Amp Phase Response
Tim wrote:
I learned a lot from your schematics on the web, for example. But I learned much much more when I actually built some amps, tested and measured and graphed phase and gain and Nyquist diagrams, and then actually understood why you stuck those shelving networks in :-). Did this give you any insight into how you might design a decent amp, without shelving networks? cheers, Ian |
#13
Posted to rec.audio.tubes
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Tube Amp Phase Response
Tim wrote:
Even though in today's classrooms Nyquist's work is talked about rather abstractly in terms of information theory, this is a shame because he was solving the most important on-the-ground important problems in electronics and telegraphy and telephone technology. It certainly is impressive that people still talk in fancy language about all the stuff Nyquist was doing back then :-). Not in my classrooms. Nyquist is general, foundation level engineering. As ever, I would recommend a decent student textbook on control system theory. Such would place Nyquist and others (Bode, Laplace, Fourier, etc) in a broad engineering context. It is very useful to be able to see how all control systems have the same problems, and can be designed with the same analytical tools. cheers, Ian |
#14
Posted to rec.audio.tubes
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Tube Amp Phase Response
Ian Iveson wrote: Tim wrote: I learned a lot from your schematics on the web, for example. But I learned much much more when I actually built some amps, tested and measured and graphed phase and gain and Nyquist diagrams, and then actually understood why you stuck those shelving networks in :-). Did this give you any insight into how you might design a decent amp, without shelving networks? cheers, Ian Yes of course Ian, it did, and some amps with NFB need very little shelving. In amps with no global NFB can be the case with using triodes with a high OPT Z ratio to get a low Rout, one needs no shelving networks at all. Also in SS amps with emitter or source follower outputs, there is no need for such networks when there is no NFB, for example see the amps built by Sue Parker. But nearly all SS amps have a C or R+C between the collector output and base input of the VAS to give a "dominant pole" for roll off of HF at maybe only 500Hz and reducing at 6 dB/octave so that for all of the AF band above about 5kHz the open loop phase shift has a phase lag of -90 degrees. The applied global NFB corrects virtually all of this delay. In nearly all amplifiers unless there is a gain shelving network at HF at least, then the NFB will become PFB at some F, and the poorer the quality of the OPT, then the lower the F at which this occurs, and it is shown up by testing with pure capacitor loads which react with the inductive output resistance character of tube amps and their OPTs. One may spend years modelling and drawing equivalent models of amplifiers, or try to simulate them, but this is only an exploration into principles; real amp makers know the principles and concepts and just apply the right values of R&C to eliminate any chance of oscillation at any F under any load condition whatsoever. Observation with a CRO and a volt meter leads them to the one and only critical damping solution which is the best. They will achieve this without allowing the amp to become saturated and the HF performance is thus never sonically compromised. Many haters of NFB don't fully understand the technical issues which actually improve the sound rather than degrade it. And even in UL amps or amps with local CFB in the OPT there will usually be a need for critical damping to control HF behaviour. But you would already know all about it I presume. Patrick Turner. |
#15
Posted to rec.audio.tubes
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Ping: John Burns
"John Byrns" wrote in message ... I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. Hi John, Here is a pic of the Phase Oscillator made by Feedback Electronics,.UK http://www.kolumbus.fi/iain.churches...Oscillator.jpg And here is a pic of the Phase Meter which goes with it: http://www.kolumbus.fi/iain.churches...PhaseMeter.jpg And here is a screen shot chart of the phase response I plotted for a 30W monobloc (Press F11 to see the graph full screen) http://www.kolumbus.fi/iain.churches...seResponse.png The system can be used in several ways, and one can take measurements without the meter unit, but feeding the ref and return signals into the X and Y inputs respectively of a CRO and then adjusting the phase of the signal feeding the unit on test to give a straight line on the scope. You can then read off the phase angle from the variable oscillator dial. If you have both units, you can read the phase difference straight from the meter. Regards to all Iain |
#16
Posted to rec.audio.tubes
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Ping: John Burns
In article ,
"Iain Churches" wrote: "John Byrns" wrote in message ... I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. Hi John, Here is a pic of the Phase Oscillator made by Feedback Electronics,.UK http://www.kolumbus.fi/iain.churches...Oscillator.jpg Why is a special oscillator required, is it computer controlled or something? And here is a pic of the Phase Meter which goes with it: http://www.kolumbus.fi/iain.churches...PhaseMeter.jpg After you first mentioned the phase meter it occurred to me how simple it would be to build a phase meter for sine waves. Simply feed the reference signal and the signal to be measured through limiters. To drive an analog meter just feed the outputs of two limiters through an exclusive or logic gate, and use a D type flip-flop to drive a lead-lag led. For a digital readout use a microprocessor with on chip counter-timers to measure the period of the reference signal and the time offset, compute the phase from that data and display it. Or if you are more ambitious and like a challenge you could build some sort of complex counter circuit from discrete SSI type IC's. And here is a screen shot chart of the phase response I plotted for a 30W monobloc (Press F11 to see the graph full screen) http://www.kolumbus.fi/iain.churches...seResponse.png Was the data for this graph hand collected, or was an automated computer program used? It would be useful if the graph also included the amplitude response. I'm not sure why I would want "to see the graph full screen", which is probably a good thing because "F11" does something completely different than show "the graph full screen". The system can be used in several ways, and one can take measurements without the meter unit, but feeding the ref and return signals into the X and Y inputs respectively of a CRO and then adjusting the phase of the signal feeding the unit on test to give a straight line on the scope. You can then read off the phase angle from the variable oscillator dial. If you have both units, you can read the phase difference straight from the meter. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#17
Posted to rec.audio.tubes
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Ping: John Burns
"John Byrns" wrote in message ... In article , "Iain Churches" wrote: "John Byrns" wrote in message ... I wonder what "equipment" Iain is hoping to borrow to make these measurements? I always thought an audio oscillator and a dual channel CRO were all you needed to make measurements more than good enough for tube amplifier work? All one needs to do is simultaneously display the input and output waveforms, read the relative time delay using the graticule on the screen, and finally convert the relative time delay to phase based on the frequency being measured. Here is a pic of the Phase Oscillator made by Feedback Electronics,.UK http://www.kolumbus.fi/iain.churches...Oscillator.jpg Why is a special oscillator required, is it computer controlled or something? No. But it is intended as a stand-alone unit, and needs two outputs, one of which needs to be variable phase if you measuring with the scope and without the meter, And here is a pic of the Phase Meter which goes with it: http://www.kolumbus.fi/iain.churches...PhaseMeter.jpg After you first mentioned the phase meter it occurred to me how simple it would be to build a phase meter for sine waves. Simply feed the reference signal and the signal to be measured through limiters. To drive an analog meter just feed the outputs of two limiters through an exclusive or logic gate, and use a D type flip-flop to drive a lead-lag led. For a digital readout use a microprocessor with on chip counter-timers to measure the period of the reference signal and the time offset, compute the phase from that data and display it. Or if you are more ambitious and like a challenge you could build some sort of complex counter circuit from discrete SSI type IC's. The earlier version of this system had an analogue meter. The one I am using, with a digital display, is later. And here is a screen shot chart of the phase response I plotted for a 30W monobloc (Press F11 to see the graph full screen) http://www.kolumbus.fi/iain.churches...seResponse.png Was the data for this graph hand collected, or was an automated computer program used? It would be useful if the graph also included the amplitude response. I just noted the readings against frequency, and entered them into Excel which plotted the chart. I do most of my charts that way. I'm not sure why I would want "to see the graph full screen", which is probably a good thing because "F11" does something completely different than show "the graph full screen". Hmm. On most computers .png screen captures seem to display at reduced size, and some of the text may be illegible. On my two PCs (both Dell) F11 gives a full screen pic. Regards Iain Churches |
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