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In Reply to: RE: Slew rate, TIM hogwwash posted by LinuxGuru on July 12, 2017 at 14:57:36
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. 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.
Edits: 07/13/17Follow Ups:
> 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.
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