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#1
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Slew rate and slew rate limiting
I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful:
http://www.ethanwiner.com/slew_rate.htm |
#3
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Slew rate and slew rate limiting
wrote in message ... I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful: http://www.ethanwiner.com/slew_rate.htm Thank you Ethan! (btw, excellent article on "mastered For iTunes" in the February edition of Recording) Poly |
#4
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Slew rate and slew rate limiting
I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful: hmmmm.... it would be very interesting if someone could make a demo of what various amounts of slew rate limiting sounds like... Mark |
#5
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Slew rate and slew rate limiting
Mark wrote:
I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful: hmmmm.... it would be very interesting if someone could make a demo of what various amounts of slew rate limiting sounds like... Mark Download libsndfile and write some 'C' code. -- Les Cargill |
#6
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Slew rate and slew rate limiting
"Mark" wrote in message ... I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful: hmmmm.... it would be very interesting if someone could make a demo of what various amounts of slew rate limiting sounds like... You can find a circuit of a real-time adjustable slew rate limiter he http://home.provide.net/~djcarlst/abx_slew.htm This is part of a device we used to demonstrate ABX testing: http://home.provide.net/~djcarlst/abx_coh.htm Bottom line - slew rate has to be rediculously low to be audible with real-world music. The exagerrated reports about the audibility of slew rate limiting from the late 1970s were due reliance on sighted evaluations which we now know to be invalid for determining the audibility of subtle differences. |
#7
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Slew rate and slew rate limiting
Mark wrote:
hmmmm.... it would be very interesting if someone could make a demo of what various amounts of slew rate limiting sounds like... It sounds like the Seventies. It's interesting to note that slew-limiting (also going under the name transient intermodulation distortion) isn't really a serious problem with modern electronics any more. In great part that's the result of work by Marshall Leach and Matti Otala back in the seventies when it was recognized as a massive, massive problem. When it sounds fine at low levels but it starts to get mushy as you turn the gain up, slew limiting is apt to be the problem. --scott Oh, it _is_ still a huge problem with transducers, but what isn't? -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#8
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Slew rate and slew rate limiting
Arny Krueger wrote:
Bottom line - slew rate has to be rediculously low to be audible with real-world music. The exagerrated reports about the audibility of slew rate limiting from the late 1970s were due reliance on sighted evaluations which we now know to be invalid for determining the audibility of subtle differences. Strange, I thought they were due to power amplifiers from major manufacturers with slew rates only around 0.1V/microsecond. (At least two of these vendors were also noted for amplifiers that burst into flame, as well, so clearly stability concerns were not addressed well.) After folks learned to stop using TV horizontal sweep transistors for audio output stages, and working engineers learned that not all amplifier distortion issues can be solved by adding more feedback, things got much better. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#9
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Slew rate and slew rate limiting
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#10
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Slew rate and slew rate limiting
Don Pearce wrote:
On 12 Feb 2013 15:55:35 -0500, (Scott Dorsey) wrote: Arny Krueger wrote: Bottom line - slew rate has to be rediculously low to be audible with real-world music. The exagerrated reports about the audibility of slew rate limiting from the late 1970s were due reliance on sighted evaluations which we now know to be invalid for determining the audibility of subtle differences. Strange, I thought they were due to power amplifiers from major manufacturers with slew rates only around 0.1V/microsecond. (At least two of these vendors were also noted for amplifiers that burst into flame, as well, so clearly stability concerns were not addressed well.) After folks learned to stop using TV horizontal sweep transistors for audio output stages, and working engineers learned that not all amplifier distortion issues can be solved by adding more feedback, things got much better. Slew rate limiting doesn't happen in the power stage. It is due to the input stage's inability to source and sink enough current to charge and discharge the dominant pole cap around the voltage amplifier stage. It current-limits on large HF signals. You can generally fix it by increasing the emitter current in the long tail pair and decreasing the dominant pole cap. This is true. Sorry for my not being clear and piling a number of different amplifier problems of the seventies together in one category. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#11
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Slew rate and slew rate limiting
On 12 Feb 2013 16:21:36 -0500, (Scott Dorsey) wrote:
Don Pearce wrote: On 12 Feb 2013 15:55:35 -0500, (Scott Dorsey) wrote: Arny Krueger wrote: Bottom line - slew rate has to be rediculously low to be audible with real-world music. The exagerrated reports about the audibility of slew rate limiting from the late 1970s were due reliance on sighted evaluations which we now know to be invalid for determining the audibility of subtle differences. Strange, I thought they were due to power amplifiers from major manufacturers with slew rates only around 0.1V/microsecond. (At least two of these vendors were also noted for amplifiers that burst into flame, as well, so clearly stability concerns were not addressed well.) After folks learned to stop using TV horizontal sweep transistors for audio output stages, and working engineers learned that not all amplifier distortion issues can be solved by adding more feedback, things got much better. Slew rate limiting doesn't happen in the power stage. It is due to the input stage's inability to source and sink enough current to charge and discharge the dominant pole cap around the voltage amplifier stage. It current-limits on large HF signals. You can generally fix it by increasing the emitter current in the long tail pair and decreasing the dominant pole cap. This is true. Sorry for my not being clear and piling a number of different amplifier problems of the seventies together in one category. --scott And there ere plenty of problems. It wasn't until we had the serious circuit analyses courtesy of folks like Doug Self that all of the problems found solutions. d |
#12
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Slew rate and slew rate limiting
And there ere plenty of problems. It wasn't until we had the serious circuit analyses courtesy of folks like Doug Self that all of the problems found solutions. d my point for bringing up a demo of what it sounds like is because slew rate issues are a good example of something that is now easily MEASURED at levels well below where it impacts the sound.... but there was a time when slew was not clearly identified as an issue to be measured..... to make progress (as was made with slew) the "golden ears" and the "propeller head engineers" (thats me) have to work together to solve these problems... if you think you hear something, the name of the game is to identify something measurable associated with what you (think you) hear so that the engineers can go fix it. Mark |
#13
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Slew rate and slew rate limiting
"Mark" wrote in message ... to make progress (as was made with slew) the "golden ears" and the "propeller head engineers" (thats me) have to work together to solve these problems... if you think you hear something, the name of the game is to identify something measurable associated with what you (think you) hear so that the engineers can go fix it. Nah the "golden ears" will never believe you can ever measure what they *think* they can hear, and for the rest of us amplifier tech has already surpassed the human auditory system capabilities (unlike speaker tech). Which is not to say a bad amp can't still be made (and are) by those who try hard enough :-( Trevor. |
#14
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Slew rate and slew rate limiting
"Scott Dorsey" wrote in message ... Arny Krueger wrote: Bottom line - slew rate has to be rediculously low to be audible with real-world music. The exagerrated reports about the audibility of slew rate limiting from the late 1970s were due reliance on sighted evaluations which we now know to be invalid for determining the audibility of subtle differences. Strange, I thought they were due to power amplifiers from major manufacturers with slew rates only around 0.1V/microsecond. I don't think that there ever were any serious audio amplifiers any where near that bad, at least not by the early 1970s. My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. |
#15
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Slew rate and slew rate limiting
On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave.. Samuel Groner's measurements of opamps suggest otherwise. Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion. And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too. Peace, Paul |
#16
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Slew rate and slew rate limiting
Mark wrote:
to make progress (as was made with slew) the "golden ears" and the "propeller head engineers" (thats me) have to work together to solve these problems... if you think you hear something, the name of the game is to identify something measurable associated with what you (think you) hear so that the engineers can go fix it. Amen! --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#17
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Slew rate and slew rate limiting
Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. There were lots of not-halfways competent amps out there at the time. Lots of people out there developed a great distaste for transistors, believing that transistors all sounded bad, when it fact it was just the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad. If you ever get a chance to measure slew rate on those SWTPCs or on some of the bigger Phase Linears, you'll be horrified. I think you forget just how bad things used to be, and in terms of amplifier sound quality things went _down_ in the seventies as we made the transition from tubes to solid state while trying to design circuits with the same capacitively-coupled techniques. We're still dealing with the fallout today. There is a lot of misinformation floating around in the audiophile world about "solid state sound" which was in fact nearly true at one time but is effectively bunkum today. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#18
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Slew rate and slew rate limiting
PStamler wrote:
And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh= ich had abysmal slew rates under usual operating conditions. And those cons= oles sounded like crap, too.=20 Although... surprisingly enough Studer made some consoles using LM301s that actually sounded very good. I boggled when I discovered those things were full of 301s. Then I looked at the schematics and noticed that almost all of them were working at unity gain, or maybe a gain of five or ten at most... --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#19
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Slew rate and slew rate limiting
On 2/12/2013 10:19 PM, Arny Krueger wrote:
I don't think that there ever were any serious audio amplifiers any where near that bad, at least not by the early 1970s. That was the golden age of golen ears. Their high end amplifiers were indeed pretty good, but they were the ones who pointed out the problem in the "popular" amplifiers of the day. The San Francisco Lunatic Audio Fringe was instrumental in defining the problem. -- For a good time, call http://mikeriversaudio.wordpress.com |
#20
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Slew rate and slew rate limiting
We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which was in fact nearly true at one time but is effectively bunkum today. Perhaps the biggest piece of misinformation is the belief that because modern solid-state amplifiers are good, solid-state amplifiers have always been good. The following is an blatant plug for a friend's products... I find it interesting that both Stereophile and Abso!ute Sound both recommend the Parasound A21 (which I own) as one of the best amplifiers available, despite its modest price (for an amp that can dump 300 W/ch into an 8-ohm load). |
#21
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Slew rate and slew rate limiting
Scott Dorsey wrote:
PStamler wrote: And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh= ich had abysmal slew rates under usual operating conditions. And those cons= oles sounded like crap, too.=20 Although... surprisingly enough Studer made some consoles using LM301s that actually sounded very good. I boggled when I discovered those things were full of 301s. Then I looked at the schematics and noticed that almost all of them were working at unity gain, or maybe a gain of five or ten at most... --scott The studio board we built at onion audio used LM301AN's, the military version, which could run at a higher supply voltage, for buffer amps. We ran them just under their max. v. spec. The board sounded great. -- shut up and play your guitar * http://hankalrich.com/ http://hankandshaidrimusic.com/ http://www.youtube.com/walkinaymusic |
#22
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Slew rate and slew rate limiting
On Tue, 12 Feb 2013 22:16:14 -0800 (PST), PStamler
wrote: On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote: My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave. Samuel Groner's measurements of opamps suggest otherwise. Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion. And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too. Peace, Paul It is certainly fair to say that if it isn't slew-rate limiting, then slew rate isn't a problem. Slew rate limiting is the same as any kind of limiting. It's doing it, or it isn't. There's plenty of scope for other stuff to be wrong of course. d |
#23
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Slew rate and slew rate limiting
"PStamler" wrote in message ... On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote: My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave. My spread sheet says 1.381737, so check! Samuel Groner's measurements of opamps suggest otherwise. I have his 2009 book right here and I don't see that. Page number? For lurkers, it is a freebie download that you can probably search for and find. Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion. Your conclusion seems to be several miles west of what I actually said. ;-) At this point we know that "real and measurable distortion" can be fantastically low compared to what it takes to have flawless SQ. And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too. The performance of a 301 can be improved to about 0.88 v/uSec which gives you a 5 volt RMS signal at 20 KHz. Not +20 but a pretty useful signal level, especially if this is a consumer device and reference level is -10. These days there is a fair amount of pro stuff that only goes up that high. It's only a few dB shy of +20. I think that those consoles may have sounded like carp but SR wasn't the reason why if the designers knew what they were doing. http://www.ti.com/lit/ds/snosbs0c/snosbs0c.pdf page 7 |
#24
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Slew rate and slew rate limiting
hank alrich wrote:
Scott Dorsey wrote: PStamler wrote: And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh= ich had abysmal slew rates under usual operating conditions. And those cons= oles sounded like crap, too.=20 Although... surprisingly enough Studer made some consoles using LM301s that actually sounded very good. I boggled when I discovered those things were full of 301s. Then I looked at the schematics and noticed that almost all of them were working at unity gain, or maybe a gain of five or ten at most... --scott The studio board we built at onion audio used LM301AN's, the military version, which could run at a higher supply voltage, for buffer amps. That sounds like the critical issue then - it's an impedance lower-er, not a gain-add-er-er. We ran them just under their max. v. spec. The board sounded great. Clipping* parts are clipping. *realizing of course that they not have actually been clipping, but there was distortion based in abuse of the parts on the basis of expected Vout.... -- Les Cargill |
#25
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Slew rate and slew rate limiting
Scott Dorsey wrote:
Arny Krueger wrote: My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. There were lots of not-halfways competent amps out there at the time. Lots of people out there developed a great distaste for transistors, believing that transistors all sounded bad, when it fact it was just the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad. If you ever get a chance to measure slew rate on those SWTPCs or on some of the bigger Phase Linears, you'll be horrified. I think you forget just how bad things used to be, and in terms of amplifier sound quality things went _down_ in the seventies as we made the transition from tubes to solid state while trying to design circuits with the same capacitively-coupled techniques. So it's the designs that were bad, not the parts? Something I had forgotten is that the entry of solid state into consumer electronics was more driven by market-seeking from Fairchild and folks like NatSemi ( teh Fairchildren ) after the space thing cooled off. SFAIK, NASAians are and always have been obsessively spec-driven, so I am wondering how part quality could have dropped when the transition happened ( becoming price driven perhaps?) IOW, it was bad slew rate because the poor three-legger was charging a cap? Hoo boy... We're still dealing with the fallout today. There is a lot of misinformation floating around in the audiophile world about "solid state sound" which was in fact nearly true at one time but is effectively bunkum today. --scott -- Les Cargill |
#26
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Slew rate and slew rate limiting
Les Cargill wrote:
hank alrich wrote: Scott Dorsey wrote: PStamler wrote: And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh= ich had abysmal slew rates under usual operating conditions. And those cons= oles sounded like crap, too.=20 Although... surprisingly enough Studer made some consoles using LM301s that actually sounded very good. I boggled when I discovered those things were full of 301s. Then I looked at the schematics and noticed that almost all of them were working at unity gain, or maybe a gain of five or ten at most... --scott The studio board we built at onion audio used LM301AN's, the military version, which could run at a higher supply voltage, for buffer amps. That sounds like the critical issue then - it's an impedance lower-er, not a gain-add-er-er. We ran them just under their max. v. spec. The board sounded great. Clipping* parts are clipping. *realizing of course that they not have actually been clipping, but there was distortion based in abuse of the parts on the basis of expected Vout.... Board was clean and very quiet. It worked, very well. API pres, EQ's, summing amps, and some of the limiters. -- shut up and play your guitar * http://hankalrich.com/ http://hankandshaidrimusic.com/ http://www.youtube.com/walkinaymusic |
#27
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Slew rate and slew rate limiting
Les Cargill wrote:
So it's the designs that were bad, not the parts? Something I had forgotten is that the entry of solid state into consumer electronics was more driven by market-seeking from Fairchild and folks like NatSemi ( teh Fairchildren ) after the space thing cooled off. There was some of each going on. Also, the transition to solid state made it possible to have truly enormous amounts of power, and people went berserk over that. The market became very focussed on high power operation, often leading to amplifiers with lots of crossover distortion because the designs were optimized for getting great numbers at high power. SFAIK, NASAians are and always have been obsessively spec-driven, so I am wondering how part quality could have dropped when the transition happened ( becoming price driven perhaps?) The transition from tubes to solid state was a very dramatic change in philosophy. A lot of the simple and reasonable techniques that worked well in the tube era fell apart in the solid state world. You build a tube amp, you have an input transformer, a tube stage, a coupling capacitor, another tube stage, an output transformer. The impedances and voltages are high, so you can use paper or film caps for coupling. In the solid state world to do the same thing the impedances and voltages are much lower, which drives you to electrolytics for coupling. A lot of the simple straightforward designs wind up having a lot of side-effects. Now, careful engineering managed to work around those side-effects, in part because transistors are tiny and cheap so it suddenly becomes easy to just add a constant current source with another transistor rather than using a resistor to fake a current source (a tube era technique that works well when you have lots of voltage to throw away, but not so well with a 12V rail). It was worse for the audio guys than the RF guys by a long shot, since the RF guys always keep a tight handle on distortion products (and usually filter them out if they can) so they at least know where they are. Lots of audio designers just didn't know what their distortion spectrum was until the age of FFT analyzers.... --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#28
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Slew rate and slew rate limiting
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#29
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Slew rate and slew rate limiting
Chuck wrote:
The SWTPCs were also inherently unstable and went into hf oscillation even when just driving resistor loads. I used to modify them by adding caps to the driver stage so they would even be functional. Does anyone know of a worse designed commercial amps than these? The Dynaco ST120 had a flammable circut board that was stocked with carbon resistors. One amp of a customer of ours burst into flames and was sitting under some curtains. His house burned down. On the bright side, with amps failing right and left back then, being an audio technician was far more lucrative occupation than today. Chuck The phase linears were the best, though. I remember going to a concert at the Little Five Points Pub in Atlanta when the monitor amp spewed a three-foot jet of flame out the top. The audience cheered! --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#30
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Slew rate and slew rate limiting
Chuck wrote:
On 13 Feb 2013 07:38:00 -0500, (Scott Dorsey) wrote: Arny Krueger wrote: My calculations show that it takes about 3.55 v/microsecond slew rate to produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to 50 watts into 8 ohms). Just about any even halfways competent amp in those days could do that. There were lots of not-halfways competent amps out there at the time. Lots of people out there developed a great distaste for transistors, believing that transistors all sounded bad, when it fact it was just the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad. If you ever get a chance to measure slew rate on those SWTPCs or on some of the bigger Phase Linears, you'll be horrified. I think you forget just how bad things used to be, and in terms of amplifier sound quality things went _down_ in the seventies as we made the transition from tubes to solid state while trying to design circuits with the same capacitively-coupled techniques. We're still dealing with the fallout today. There is a lot of misinformation floating around in the audiophile world about "solid state sound" which was in fact nearly true at one time but is effectively bunkum today. --scott The SWTPCs were also inherently unstable and went into hf oscillation even when just driving resistor loads. I used to modify them by adding caps to the driver stage so they would even be functional. Does anyone know of a worse designed commercial amps than these? The Dynaco ST120 had a flammable circut board that was stocked with carbon resistors. One amp of a customer of ours burst into flames and was sitting under some curtains. His house burned down. On the bright side, with amps failing right and left back then, being an audio technician was far more lucrative occupation than today. Chuck My tigersaurises, or however you spell them, had those big bias resistors that ran hot. Eventually had to redo them, and added fan. With all the paranoia about SRL, one way to deal with it was to put a low pass on the signal before amplification. After redoing an overly complex Crown amplifier, I watched as it lit up. I hope I never go back to fixing that beast. Greg |
#31
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Slew rate and slew rate limiting
Worst case signal for slew rate tests is to jangle keys in front of a high-bandwidth microphone. Of course once it hits an a/d converter it gets band-limited to 20k so after that there is no issue.
Bob |
#32
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Slew rate and slew rate limiting
wrote in message ... Worst case signal for slew rate tests is to jangle keys in front of a high-bandwidth microphone. Of course once it hits an a/d converter it gets band-limited to 20k so after that there is no issue. Not with a 96 or 192k sample rate it doesn't, but you'd have to look hard to find an amp that can't handle the frequency response of a microphone without slew problems these days anyway. Trevor. |
#33
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Slew rate and slew rate limiting
On Thursday, February 14, 2013 12:23:37 AM UTC-6, Trevor wrote:
wrote in message ... Worst case signal for slew rate tests is to jangle keys in front of a high-bandwidth microphone. Of course once it hits an a/d converter it gets band-limited to 20k so after that there is no issue. Not with a 96 or 192k sample rate it doesn't, but you'd have to look hard to find an amp that can't handle the frequency response of a microphone without slew problems these days anyway. Which is a result of people paying attention to the issue back in the 70s and 80s. Part of the problem back in the way-back was that this was the beginning of the transformerless era; people were champing at the bit to eliminate the transformers at the outputs of mics and the inputs of preamps which colored the low frequencies (and took up space and added weight and cost more money). What they didn't necessarily realize was that the ultrasonic bandlimiting the transformers added helped prevent nasty slewing issues and IM distortion. By the way, my apologies for an error: the tests that showed high-frequency distortion in opamps that were not into actual slewing came from Walt Jung, not Sam Groner. They're in "Audio IC Op Amp Applications"; unfortunately, my copy is buried at the moment, so I can't get the page numbers. Oh, and anyone who wants the Groner opamp test results can download them (the file is about 35 megs) from: http://tinyurl.com/opamptests Peace, Paul |
#34
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Slew rate and slew rate limiting
On 14/02/2013 7:54 AM, Scott Dorsey wrote:
Chuck wrote: The SWTPCs were also inherently unstable and went into hf oscillation even when just driving resistor loads. I used to modify them by adding caps to the driver stage so they would even be functional. Does anyone know of a worse designed commercial amps than these? The Dynaco ST120 had a flammable circut board that was stocked with carbon resistors. One amp of a customer of ours burst into flames and was sitting under some curtains. His house burned down. On the bright side, with amps failing right and left back then, being an audio technician was far more lucrative occupation than today. Chuck The phase linears were the best, though. I remember going to a concert at the Little Five Points Pub in Atlanta when the monitor amp spewed a three-foot jet of flame out the top. The audience cheered! --scott I remember mixing a gig in the 70's where two of those things shot flames, and the W bins caught fire. Real pyrotechnics! |
#35
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Slew rate and slew rate limiting
Agreed that there are plenty of good choices out there with slew-rate to spare. It's a good idea to over-design the slew-rate, because the performance of most op-amps begins to degrade as you approach the hard slew-rate limit.. This occurs because the gain of the input stage shows some soft clipping behavior and therefore the loop gain is dynamically lowered during a high-slope input event that almost but not quite hits the limit. This shows up in the the+n versus frequency plot on the data sheet. I wouldn't call this a major factor and its audibility is questionable, but its common practice in the design world to over-specify slew-rate compared with the simple 2*PI*A*F calculation.
Bob |
#36
Posted to rec.audio.pro
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Slew rate and slew rate limiting
On Thursday, February 14, 2013 5:39:26 AM UTC-6, wrote:
Agreed that there are plenty of good choices out there with slew-rate to spare. It's a good idea to over-design the slew-rate, because the performance of most op-amps begins to degrade as you approach the hard slew-rate limit. This occurs because the gain of the input stage shows some soft clipping behavior and therefore the loop gain is dynamically lowered during a high-slope input event that almost but not quite hits the limit. This shows up in the the+n versus frequency plot on the data sheet. I wouldn't call this a major factor and its audibility is questionable, but its common practice in the design world to over-specify slew-rate compared with the simple 2*PI*A*F calculation. And with many high-quality opamps available, it has become quite easy to over-spec the slew rate. Peace, Paul |
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