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In Reply to: RE: Slew rate, TIM hogwwash posted by gusser on July 11, 2017 at 10:00:45
Gusser
As Otala has performed tests demonstrating that TIM in transistor amps is audible and that it can be detected down to a level of 0.003% by 6 out of the 68 listeners in his test group, then it would seem that there would need to be tests with a group of at least that size to seriously contradict Otala's findings. Neither you or I have been able to find any references to tests like this. Let's try it another way, can you supply the name of a transistor amp from the 80's with TIM specs lower than the 0.003% which was clearly audible to the 6 golden ears? Just what number would you buy as being below the level of audibility, and what amps have specs matching this? Feel free to add in any TIM specs for SET's too as we are on the Tube DIY Asylum
I quoted directly from sentences in both of the abstracts of the JAES papers I referenced, and I assumed that this would be obvious to anyone who has read the papers as you claim to have As to digging occult information out of these papers, well I would have to go down to the library, I would need help with the microfiche machine, and my knee is acting up causing me to drag my foot at times. Well you get the idea. If you were to write a rebuttal to the Otala papers to the JAES, then I would drop the Hogwash stuff, and you'll have to use your real name as you'll be subject to peer review. I've been to a number of JAES meetings and nobody wore a mask or used a pseudonym.
Paul
Follow Ups:
> As to evidence contra to this, Otala (and others) published two JAES
> papers ... In the first paper a group of six of the most sensitive
> subjects (from a previous test of 68 listeners) were tested and it
> was found that "The results show that in certain passages of music,
> 0.003% of distortion is clearly audible".> As Otala has performed tests demonstrating that TIM in transistor amps
> is audible and that it can be detected down to a level of 0.003% by 6
> out of the 68 listeners in his test group ...6 out of 68 is a really poor sampling, on the level of a statistical error. While TIM theory looks profound, real audibility is an open question. Its a common behavior in human society to raise a problem (often miniscule or even non-existent), in order to develop and sell "unique solution". BTW, TIM issue was widely used in advertising material back in 198x.
I own two amps engineered to minimize TIM (Sansui G Pure DC series). They sound great. I heard, had (and built myself) others, solid state and vacuum tube, which design don't take into account TIM at all. They also work great.
Thus, I assume, TIM made more noise by itself rather to contribute anything useful.
Edits: 07/12/17
LGIt would be nice if there was an easily accessible example of TIM distortion on the web or disc, demonstrated with a short musical performance repeated with more and more levels of TIMD until the effect became obvious. Unfortunately none of the Stereophile Test Discs have this, or anything else I have. To be honest I'm not really sure what it would sound like. The 6 golden ears of the 68 testees really couldn't detect it as distortion at 0.003% in Otalas test, they could only tell there was a difference. It would certainly be nice to have another test following Otala's with a larger group which would provide a better base rate, but how likely this is to happen I can't say. My original comments in the present discussion were directed at intermodulation distortion in loudspeakers, which is related to FIM in amps but really quite a bit off topic from the original post by Tre' up page.
As to how important FIM distortion in amps in the entire audio scheme of things is I really can't say. The subject is somewhat interesting to me but it's largely overshadowed by issues in speakers. I built a Hafler DH200 in the 80's but the specs show only IM distortion @ 0.005% and TIM is not mentioned. I though it sounded great when I built it, but it got replaced by what became a highly modified Dyna ST70 in my horn rig, and after that it served in our HT until a Yamaha 5.1 amp bumped it out several years ago. The Hafler sounded good in the HT with Pioneer CS80's until one time the Dyna was awaiting parts and I brought the Hafler back down into the horn rig in the basement and it sounded terrible! Very dull and slow, typical of what has been described as the "mosfet haze". Turning the volume up changed things, but you don't really want to listen to a string quartet for instance at heavy metal levels. This is likely due to the capacitance at the gate of the mosfets changing with the drive level as per the late Alan Wright's observations. So I really don't know what to make of the TIM thing and how it fits into the audio hierarchy, but until we get something better I'm going with Otala's test results. BTW my ST70 is currently triode wired with the ultra-linear tap disconnected, and I wouldn't expect it to do well in THD tests, but it's going to stay this way until I get something better.
Paul
Edits: 07/13/17
> It would be nice if there was an easily accessible example of TIM
> distortion on the web or disc, demonstrated with a short musical
> performance repeated with more and more levels of TIMD until the
> effect became obvious. ...
> To be honest I'm not really sure what it would sound like.
> The 6 golden ears of the 68 testees really couldn't detect
> it as distortion at 0.003% in Otalas test, they could only
> tell there was a difference.
It would require recording from test amplifier dummy load, and playing recorded data again on benchmark unit, thus, subjecting test data again to distortions of benchmark amp (no matter how miniscule they are), speakers, and room acoustics.
As Gusser stated before, TIM issue doesn't make practical sense, and IMHO, it is entirely correct. Someone wanted to make a party, they had a drink and show, now its over.
High stability of the amplifier is much more important factor, and it something that is time to time neglected with aftermath from HF spurious oscillation and loss of clarity to burned components and speaker coils.
LG
It's always nice to have a civil discussion here about audio concepts which don't get get looked at very often.
Paul
But you still don't acknowledge the engineering problems I raised that clearly contradict Otala's claim that this is audible with program material. Ahh, but did he even say that? I do agree it might be audible with test signals but then that's not real program material we listen to.
Just more "appeal to authority". Typical audiophile response when presented with real engineering problems that dispel their beliefs..
I think I see where you're going with this, you are intending to write a JAES paper and you are testing some of the ideas out here. Here's some suggestions to clean up the argument:
To say something is hogwash is to say it's useless and should be thrown away, so drop the hogwash and just say that. You would be expected to provide at least some rudimentary test results, so just have some buddies over for beers and test them. If you add some TIM to a musical selection you are adding energy to the signal and thus effectively raising gain, and that's an old Hi fi salesman trick to turn the volume up a bit to make the thing you are trying to sell sound better. With this you can probably get some people to actually prefer the TIM'd examples, and Otala did note that some of the test subjects did. It's also fairly easy to prove that people are guessing during audio tests if you make them guess, so do rapid switching among orchestral recordings with both loud and quiet passages. After awhile they won't even know what ocean they're in.
You have criticized Dennis and Jeff for promoting speculative and unsubstantiated theories, and you also leave yourself open for criticism in this regard if you don't provide something a bit more concrete. Lisa Randall once said to me when I asked her a question about Loop Quantum Gravity that "It's just a theory", though she did admit she respected the author of LQG. So far you just have an interesting theory, but the ear is the final arbiter in all things audio to paraphrase Olson. And it wouldn't hurt for you to make some appeals to authority too.
BTW Otals did say that the 0.003% TIM was clearly audible to the six golden ears on musical selections, but he noted that they could only hear a difference, but not identify which version had the TIM added. That alone would seem to open an area to study. Also Otala did use square waves as you stated, but he also used triangle waves in some of the tests too.
We're pull'n for ya!
Paul
"You have criticized Dennis and Jeff for promoting speculative and unsubstantiated theories, and you also leave yourself open for criticism in this regard if you don't provide something a bit more concrete. Lisa Randall once said to me when I asked her a question about Loop Quantum Gravity that "It's just a theory", though she did admit she respected the author of LQG. So far you just have an interesting theory, but the ear is the final arbiter in all things audio to paraphrase Olson. And it wouldn't hurt for you to make some appeals to authority too."Stupid analogy! I have provided evidence based on simple physics why TIM is not an issue with acoustically produced sound. You have yet to comment on the theory I have presented. All we hear is "Otala says, Otala says".
As for listening tests, first of all 6 out of 83 is hardly a convincing sampling. We also don't know the controls present or how these tests were conducted. ABT, DBT? "I'm going to play some TIM for you, tell me if you hear it?" Yes some classic audiophile tests were that unprofessional.
You also fail to separate the technical facts of TIM versus the final effects on the systems as a whole. Sure it can be simulated, but does it present an issue in real world audio reproduction? Basic atmospheric and mechanical physics says it can't.
Ok so you don't like TIM being called hogwash. Perhaps you are right in that TIM is real and can be demonstrated - with test equipment. But what is hogwash is that we can hear TIM with modern audio amplifiers designed with late 70s / early 80s technologies.
I'm tired of hearing your monotonous references to 40 year old research papers - OK, 39 years old in fact. Show me how an acoustically produced square wave rise time within the human hearing range can exceed the path delay of a typical DC coupled audio amplifier. Show me a microphone that can capture such an event even if out atmosphere would allow such a pressure wave to propagate in the fist place?
Again show me the math that supports TIM audibility! I know a test generator can do it. But what about real music, sound effects, and speech?
P.S. "If you add some TIM to a musical selection you are adding energy to the signal and thus effectively raising gain"
This is not totally correct either. You are thinking solely of harmonic distortion. IM distortion does not follow the same rules. Look up Bessel functions.
Edits: 07/12/17
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