Sound Quality Vs. Measurements

[nigel pearson]

Many years ago I think I remember one UK reviewer saying slew rates were like an arms race . Curious to know more a figure of about 6 V/uS was said to be enough for 100 W if real music . Anything above that a bonus or silly ? Having so many times read of greater slew rates I feel certain there is more to this story . A home cinema guy said he measured 1.3 W for dialogue ( 93 dB efficiency speaker ) , 250 W for the spaceship flying in . We covered symmetry the other day which I suspect was approved of ? Any ideas on minimum slew rates for 100 W ?

This link says about slew rates near the bottom of the page . By coincidence I have built nearly all these at some time having asked myself the same questions . Glad he thought it worth documenting .

Elliott Sound Products - Audio Power Amplifier Design Guidelines

One guy showed a 1 kHz square-wave and how to use it for evolution .He then said do not use a speaker for the test as the tweeter will fry . Too obvious ? Well it does say even that simple test has no musical reality .

[fas42]

A simple way of looking at it is to consider a full power 20KHz sine wave. This has a period of 50uS, so to go from max -ve to max +ve rails takes 25uS. 25 x 6V/uS = 150, volts peak to peak, 75V rails. An amp with such could probably do near 400W into 8ohms; so I reckon we're safe ...

Frank

[bcarso]

Also, see Putzeys' article in Linear Audio for his comments about the effects of slew rate when the signals approach, but don't exceed, the slew rate limits of the equipment. Jan posted a link.

[john curl]

You know, both Walt Jung and Matti Otala beat this topic to death with REAL measurements more than 1/3 century ago. It was found that 50V/us was necessary for a 100W amplifier, to be relatively SURE that TIM (SID) would not be generated.
What many here STILL seem to forget is 20KHz may be the highest frequency considered important in audio listening, BUT the working bandwidth goes on and on in many situations.
For example, for phono cartridge mistracking, 200KHz is pretty normal with MC cartridges, 500KHz has been measured. Where is your 6V/us THEN?

[nigel pearson]

Thanks John . I had a hunch that would be the answer . One could almost say in audio a factor of ten more if you can . If overload margin of a phono stage that is probably a bit low . Not sorry I asked .

[john curl]

Of course, it is also to be considered that future digital will have more effective bandwidth, with 24/192, 24/96, and SACD already here. You would be surprised how well these sources can fit the TIM profile, making 50V/us for a power amp, but only 5V/us for a preamp, pretty much a necessary spec for highest quality audio.

[john curl]

A set of examples from 35 years ago. By the way, the newer Shure's are faster and wider bandwidth, even with normal loading, at least the best ones.

[jan.didden]

Originally Posted by john curl

For example, for phono cartridge mistracking, 200KHz is pretty normal with MC cartridges, 500KHz has been measured. Where is your 6V/us THEN?

John do you have any references for that, papers or measurement reports where we can read up on it?

Edit: we xposted, you posted just above me, thanks

jan

[fas42]

And of course the real answer, as John alluded to, for why really fast slew rate is necessary, is that it's required for the FB mechanism to function correctly. Funny concept, the amp has to work well enough to make sure the error correcting mechanism works well enough, hmmm ...

Frank

[magnoman]

My related question is, assuming the amplifier slew rate is fast enough to track the input, then is any additional slew rate margin a direct indicator of reduced SID.
It just seems to me this distortion is from larger nonlinear current swings in the input stage, so it could be reduced by reducing the current swing or making it more linear. If in the process this typically results in an increased measured slew rate, then maybe one can use this as in indicator but it still seems indirect to me.
Direct correlation to slew rate would imply that CFA type amplifiers would be automatically superior in this area (which may be true I don't know) but I dont even see a fundamental between a CFA and VFA if both are operating within the same i-v portion of the input transistors for the audio input.

Thanks (not sure I should have posted this, though crowd around here)
-Antonio