[teb1013]

I'm glad that you re awakened this thread. I am currently involved in considering a tube amp. I'm old enough to remember, as a kid, wanting a transistor radio and looking at tube stuff as antique. But, now I'm fascinated with distant memories of those glowing tubes. The nice thing about head fi is that there is tube gear that is affordable for those of us with limited incomes. I' m currently considering what will sound best with HD558s. 

 

[Joong]

Quote:

This is a very good and interesting point.  We "weight" many of the important measurements of an amplifier - S/N, Dynamic Range, etc.  Yet, harmonic and intermodulation distortion are accepted as absolute measurements.  I suspect that this is the case because amplifier manufacturers with solid state designs (the predominate configuration for the mass markets) are able to very easily compete with low levels of THD and IM.  Yet, the deviations that they produce are very often odd-order results.  This may easily explain why tube amps with THD and IMD of several magnitudes higher than comparable SS are easy to listen to, whereas the SS with magnitudes lower THD and IMD may be objectionable.
 
I would love to see the THD and IMD results weighted.  They'll probably never do it, though, because in the mass markets, tubes don't exist. 

What is IMD or intermodulation distortion?
 
I am interesting in this audio parameters which define SQ.
We all speak about subtle quantity, which is usually not fundamental quality but 2nd order or higher order ones.
Consequently the words for expressing what they listen is quite complex one.
This makes me very interesting, because I do believe "what could not be measureable should not be felt."
However I already begun what I hated before and begun to buy an expensive cable for my phones.
What I am doing is disaligned with what I believed before.

 

[radgroza]

What I really wanted to experiment with is to find a good and precise tube emulation plugin for any of the Audio players available for PC or MAC.
Actually I found some for WinAMP, but these are not documented as of what kind of distortion they introduce.
What I was thinking of was a non-linear amplifiers (to simulate tube single ended amp behaviour) that would introduce 0.1-2% of THD (even harmonics mostly)
and a slight roll-off for lower and upper end of the bandwidth and see how it influences mucisality of the presentation.
Especially for long listenings not at the full power you can bear, but someting that won't make you tired after 1-2 hours of listening.
 
That would be an interesting experiment and if I am manage to do such one I will share results here.
Possibly I could even do some tests with a real tube amp (OTL - a real hi-end modded to the max)
 
Cheers, Rad

 

[radgroza]

Sorry for not replying to Joong's question (overlooked it).
IMD or intermodulation distorion is usually measured with the signal consisting of 2 sinewaves. It may be 13kHz and 14kHz or 18-19kHz. Others prefer 250kHz and 8kHz. It measures how two fequencys influence each other. It is quite an inportant factor to the amplifier, but as you said I am becoming also a bit disaligned with what I believed before that the perfect listening experience aligns with 0% THD, 0% IMD and so on.
I am not so sure anymore.  

 

[Joong]

Thanks for a good explanation.
It is actually similar to signal modulation between the two sinusoidals, which generates sidebands of the signal from original ones.
 
I think it is simply a measure of non-linearity action of the amplifier.
 
The other factor can be the load of the amplifier, which is headphones' inductive loading to the source that is the amp.
 
For my expectation, the othoplanar type phones like He-400 has almost resistive nature, whereas hi-Z phones like HD800, or T1 is regarded as high inductive load.
This nature of load makes the entire system to be more than 2nd order filters when we consider cable ( not twisted ones) has considerable inductance on top of that.
Consequently the dynamic phones have a strong colour which is no expected and hence not very good SQ.
 
To my experience, HD650 with Little dot mkvii+ (SS amp) is not good but  with HE-400 is excellent.
 
Therefore we need to understand SQ in terms of all system including human's higher order SQ interpretation capability of phones.
Here Headfier's forum is nothing but a series of explanation of subtlety of SQ with bizarre wordings like airy, color, or other weired words.
The definition of subtlety-expression of Headfiers forum can be defined as human's effort to measure and charting the order quantity that is beyond THD, IMD, or etc.

 

[germanium]

One thing that has not been touched on here is feedback. Feedback is often used to get amazing specs but often does very little for sound quality especially for tube amps. Of coarse the amp has to be designed from the outset to run with zero feedback but I have heard amps that could be run either way & the amps sounded better without the feedback. The feedback sound sounded squashed compared to without feedback. Almost all solid state have feedback & most require feedback to even operate where most tube amp designs can operate without feedback with minor modification & most often sound at least as good without as they do with feedback. This CANNOT be done with most solid state

 

[p a t r i c k]

Quote:

One thing that has not been touched on here is feedback. Feedback is often used to get amazing specs but often does very little for sound quality especially for tube amps. Of coarse the amp has to be designed from the outset to run with zero feedback but I have heard amps that could be run either way & the amps sounded better without the feedback. The feedback sound sounded squashed compared to without feedback. Almost all solid state have feedback & most require feedback to even operate where most tube amp designs can operate without feedback with minor modification & most often sound at least as good without as they do with feedback. This CANNOT be done with most solid state

 
I am a bit skeptical about the problems described by some with feedback in amplifier designs. However I would be very happy to have my skepticism proven to be in error.
 
The problem some feel that feedback brings is time domain distortion. The reason for my skepticism is that the amount of time domain distortion introduced is really tiny, whereas the time domain distortion elsewhere, such as in speaker designs, must be vast in comparison.

 

[dsavitsk]

No, the problem with feedback is that it introduces high order distortion while it corrects low order. For instance, say you have a circuit that produces 2nd order distortion. If you feed back the signal from the output to the input, the fed back 2nd order cancels the generated 2nd order. However, because the amp produces 2nd order, the fed back signal will be distorted, generating a 2nd harmonic of the 2nd harmonic -- that's 4th harmonic. While this new distortion will be lower in amplitude, and indeed some will be canceled by feedback, the higher the order of the distortion the more impactful it is. In other words, 1% 2nd harmonic is no big deal, but .001% 13th harmonic is.

There is a second problem, too, which is that the back EMF generated by the speaker can interact with feedback to inject junk in to the circuit.

This does not mean that feedback is necessarily awful, but it is to suggest that it is not a panacea, and that an amp that measured at 0.001% THD into a resistor load may behave very differently into a real one.

There is a Nelson Pass article on this at http://firstwatt.com/articles.html called Audio Distortion and Feedback, but the link is not working right now.

 

[p a t r i c k]

Quote:

No, the problem with feedback is that it introduces high order distortion while it corrects low order. For instance, say you have a circuit that produces 2nd order distortion. If you feed back the signal from the output to the input, the fed back 2nd order cancels the generated 2nd order. However, because the amp produces 2nd order, the fed back signal will be distorted, generating a 2nd harmonic of the 2nd harmonic -- that's 4th harmonic. While this new distortion will be lower in amplitude, and indeed some will be canceled by feedback, the higher the order of the distortion the more impactful it is. In other words, 1% 2nd harmonic is no big deal, but .001% 13th harmonic is.

There is a second problem, too, which is that the back EMF generated by the speaker can interact with feedback to inject junk in to the circuit.

This does not mean that feedback is necessarily awful, but it is to suggest that it is not a panacea, and that an amp that measured at 0.001% THD into a resistor load may behave very differently into a real one.

There is a Nelson Pass article on this at http://firstwatt.com/articles.html called Audio Distortion and Feedback, but the link is not working right now.

 
Aha, it is more complicated than I thought
 
Thanks for the explanation of this issue.
 

 

[Steve Eddy]

Quote:

No, the problem with feedback is that it introduces high order distortion while it corrects low order. For instance, say you have a circuit that produces 2nd order distortion. If you feed back the signal from the output to the input, the fed back 2nd order cancels the generated 2nd order. However, because the amp produces 2nd order, the fed back signal will be distorted, generating a 2nd harmonic of the 2nd harmonic -- that's 4th harmonic. While this new distortion will be lower in amplitude, and indeed some will be canceled by feedback, the higher the order of the distortion the more impactful it is. In other words, 1% 2nd harmonic is no big deal, but .001% 13th harmonic is.

 
That's only true up to a point. Beyond that point, feedback reduces all orders, to the point that they're all lower than they were before applying feedback.
 
Did you read Bruno's article about this in Linear Audio? Basically it concludes that if you're going to use feedback, don't use a little, use a lot.
 
se

 

[dsavitsk]

if you're going to use feedback, don't use a little, use a lot.

NP refers to this as a pyramid scheme -- add more gain stages just so you can add more feedback to correct the errors introduced by the additional gain stages ...

 

[Steve Eddy]

Quote:

NP refers to this as a pyramid scheme -- add more gain stages just so you can add more feedback to correct the errors introduced by the additional gain stages ...

 
But you don't necessarily need more gain stages.
 
Ah well. You and I are getting gain by other means and the followers have enough gain on their own to get over the "hump."
 
se

 

[SneakyPelican]

Do any of you know whether a tube or a SS work better with a DT 770? I think I prefer the sound of tubes but I haven't ever been able to A/B them. Thanks
 

 

[Skylab]

The 250 ohm and 600 ohm DT770 are definitely better with OTL tube amps, generally speaking, IMHO.

 

[SneakyPelican]

Ah I forgot to mention I have the 80 ohm version... If I find the treble to be a bit sharp on my ears at times would a tube amp kinda take that feeling away? I'm a fan of textured, warm sound.