[Freo-1]

Cordell debunked the NFB>TIM myth. That's a bit more than a "disagreement".
Interesting. Please provide the links so we can see the specifics, controls used, statistical analysis of the positives, etc. TIA.

Disagree.  None other than Nelson Pass has cited the work regarding TIM and it's importance, as has John Curl.  Not exactly debunked, as you claim.


http://www.stereophile.com/content/boulder-500ae-power-amplifier-transient-intermodulation-distortion

[Freo-1]

More on T.I.M.

https://community.klipsch.com/forums/storage/3/1107724/01162904.pdf

[AJinFLA]

http://www.stereophile.com/content/boulder-500ae-power-amplifier-transient-intermodulation-distortion

Sidebar 2: Transient Intermodulation Distortion

TID is due to the active devices being unable to slew the voltages they are asked to handle sufficiently fast to keep up with the demands of the signal.
—J. Gordon Holt

The article discusses the origins of TIM and the transient nature of music that can bring it on. It points out that amplifiers with inadequate slew rate margin are the ones prone to TIM. Moreover, the article demonstrates that large amounts of negative feedback, when properly applied, do not in any way contribute to TIM. The bad rap that negative feedback gets to this day is largely a result of that misconception.
- Bob Cordell

It's helpful to actually read the articles being quoted.

None other than Nelson Pass has cited the work regarding TIM and it's importance, as has John Curl.

Appeal to Authority in no way addresses the facts Cordell presented. But the myth of NFB is "audiophile" by nature, thus immune to facts.
If you have some evidence of TIM/slew issues in modern amps, or NFB causing drama, including those biased Class A (as the thread topic implies) please do present it. Thanks.

cheers,

AJ

[Freo-1]

Appeal to Authority in no way addresses the facts Cordell presented. But the myth of NFB is "audiophile" by nature, thus immune to facts.
If you have some evidence of TIM/slew issues in modern amps, or NFB causing drama, including those biased Class A (as the thread topic implies) please do present it. Thanks.

cheers,

AJ

You are once again reading too much into this.   I found another paper Cordell penned:

http://www.cordellaudio.com/papers/multitone_test.pdf

Reads like he is proposing a lower cost method to test for TIM (not exactly debunking TIM).

Denying TIM is like denying men landed on the moon!   

[AJinFLA]

Denying TIM is like denying men landed on the moon!   

Fixation on it and incorrectly correlating it to NFB, is an audiophile thing. Myths persist. If it has any relevance, then you should have no problem citing some modern day examples. Feel free to go back to the 90s if needed. TIA.
Otherwise, yet another audiophile tempest in a teapot.

http://sound.westhost.com/articles/distortion+fb.htm

cheers,

AJ

[Freo-1]

Fixation on it and incorrectly correlating it to NFB, is an audiophile thing. Myths persist. If it has any relevance, then you should have no problem citing some modern day examples. Feel free to go back to the 90s if needed. TIA.
Otherwise, yet another audiophile tempest in a teapot.

http://sound.westhost.com/articles/distortion+fb.htm

cheers,

AJ

You are STILL reading too much (or the wrong point) into this.  I'm not arguing about being for or against global NFB as many audiophiles do.  I'm supporting the principals of feedback that provides lowering T.I.M, which IS an issue with all SS amps.  Use of local (and parallel) feedback used by designers such as Electrocompaniet does improve the audible performance, make the amp more stable, and lowers distortion of complex musical waveforms, as cited in the above references.


This subject is discussed at length over on DIY audio.  I'm sure John Curl can set us all straight on the subject over there is you care to know more.   

[sonicboom]

Commodity parts of amplifiers are just that. The only thing that matters is the electrical output at the terminals. Mind showing that lower distortion of triodes, full bandwidth into loads? TIA.

cheers,

AJ

It has nothing to do with 'commodity parts of amplifiers'. I referred to the "inherent" distortion of the amplifying devices in and of themselves, while I was careful not to touch on any specifics as to how a completed circuit using any type of device will behave. With NFB at our disposal we can make even the most non-linear device exhibit very low distortion on the bench.

As for me pointing out the lower distortion of the DHT's, one has only to look at the characteristic curves of the amplifying device of choice. The DHT's I mentioned in my previous post, are the closest to ideal as we've been able to get to, despite them having been around for as long as they have.

Additionally, it makes no sense to talk about full bandwidth into loads when we're talking about amplifying devices alone. Transistors or tubes alone, don't make circuits. In this instance, I'll assume you're talking about tube amps in general and SE tube amps in particular, where the bench performance of the entire circuit (of which these devices are a part of), is less than ideal. Well yes, I'll agree, but I will turn your question around and ask this: what will the performance of a BJT or MOSFET be, if arranged in a SE circuit and without the use of feedback? The answer of course is: ...horrendous! The fact that we can use a single device with out any tricks and obtain this kind of sound out of them, is pretty neat to say the least.

[AJinFLA]

Transistors are still inherently the highest distortion devices out there, with the possible exception of Nelson Pass's custom made SIT's. The old direct heated triodes (300B, 2A3, 845 etc), are still the lowest distortion amplifying devices extant.

Transistors or tubes alone, don't make circuits.

Bingo.

cheers,

AJ

[G Georgopoulos]

The physics underlying all amplifying devices has not changed. Transistors are still inherently the highest distortion devices out there, with the possible exception of Nelson Pass's custom made SIT's. The old direct heated triodes (300B, 2A3, 845 etc), are still the lowest distortion amplifying devices extant.

(transistors) highest distortion devices???are you for real!!!

[G Georgopoulos]

No, that is not a problem with "Class A". That would be component or system architecture/design.
No, that isn't a problem either, unless the speakers are chosen unwisely. The "20 watt" amplifier I linked would drive a HE (>98db/watt) speaker system, in room, to very high levels, where the ears own distortion would most likely start to mask moderate clipping (>113db peaks).

cheers,

AJ

well,depends on how you look at it..
you could still have good dynamic range but not as much as class-b...

have you ever built a class-a amp??

[Roger A. Modjeski]

(transistors) highest distortion devices???are you for real!!!

Perhaps some technical information would help clear the air.

First I should say that till we get to the output stage of our power amp all the amplification is voltage amplification. Starting from a moving coil cartridge to the speaker we have over 100 dB of available amplification. What is the best device for that job?

If viewed as voltage to voltage amplifiers triodes are the most linear device. It's apparent from the curves. The gain of pentodes is not quite as linear but still better than transistors. Selected vacuum tubes are very low noise. I have used them for moving coil head amps. (RM-4)

Bipolar Junction Transistors (BJT) are inherently very bad at voltage to voltage amplification although very good for current to current or current to voltage.

JFets are decent voltage to voltage amplifiers and behave a lot like pentode tubes. Still not as good as triodes.

MOSFETS are great switches. They come on at around 3.5 volts on the gate and by 4 they are conducting  many amperes of current. It is rather abrupt so they are a bit hard to bias. They make the greatest voltage followers in the world. They are used in Class D amplifiers.

The other advantage of tubes is that we can make very simple circuits with them. Generally a tube amp or preamp has fewer stages and fewer parts in the signal path than anything solid state. I hate to use the term fast, but tubes are faster than fast; they are immediate. The OTL amp I am building has such a short path that any feedback is immediate. The amp goes out beyond 200 KHZ.

[Pneumonic]

And it will still sond like crap unless you design it correctly:

I would submit that any amp which, when connected to a specific speaker load via specific speaker cables and positioned in your desired room and asked to pump out volume to certain SPL’s, is measured to perform with inaudible errors in frequency response and THD+N then it won’t sound like crap because it won’t have a sound per se. Of course, not every amp performs with such linearity, under what are so many different conditions, which is why we see so many different opinions on how amps sound and, in some cases, measure.

[sonicboom]

(transistors) highest distortion devices???are you for real!!!

You seem to not be able to understand that transistors are inherently very non-linear devices. Non linearity means distortion, plain and simple. All the hand wringing, sarcasm etc. on your part will not change this fact. like I said above, you don't have but to look at the transfer characteristic of the device in question, to figure out to what degree it distorts its input signal. The only reason we're able to design and built SS amps that measure low in distortion is because the very high amplification factor of transistors, allows us to use a lot of NFB usually in multiple local/nested loops. As I said before, try using any transistor without any sort of feedback like it's done with DHT's and then get back to us. I am sure you'll be singing a different tune then and a highly distorted one at that!

[Pneumonic]

As for TIM/Slew issues ...I have only seen real issues with low power/voltage output and limited high frequency response situations. Such instances invariably involved either tube amps or poorly designed, low power, SS gear.

My opinion on NFB. Implementation of NFB should be done only after getting the basic design proper. And then, my preference is to use only small amounts, around every stage instead of globally and to assure that the phase is compensated well so that it doesn’t shift and induce positive feedback at very high f’s.

One other thing.  I see tons of discussion, indeed much marketing effort, emphasizing the importance of the types of spectra of the involved distortions while ignoring completely that if said distortions are below the threshold of hearing and, thus inaudible, then so what ….. you can’t hear them anyhow.

[AJinFLA]

well,depends on how you look at it...

It's the only way I look at it. Systems create soundfields. I don't obsess over commodity parts and classes, etc.

you could still have good dynamic range but not as much as class-b...

Horses for courses.

have you ever built a class-a amp??

Nope. I'll stick with transduction, thanks. Voltage and current multipliers are dime a dozen these days.
 

cheers,

AJ

[Freo-1]

(transistors) highest distortion devices???are you for real!!!

Yep.  Check out this link:

http://spectrum.ieee.org/consumer-electronics/audiovideo/the-cool-sound-of-tubes


Read the sidebar links about jfets vs. 6AU6.  Interesting.   

[Roger A. Modjeski]

Yep.  Check out this link:

http://spectrum.ieee.org/consumer-electronics/audiovideo/the-cool-sound-of-tubes


Read the sidebar links about jfets vs. 6AU6.  Interesting.   

The sidebar was a bit  hard to find so here it is directly  http://spectrum.ieee.org/consumer-electronics/audiovideo/the-cool-sound-of-tubes/distortion

Interesting, thanks.

[Freo-1]

Thanks, Roger!

[Freo-1]

As for TIM/Slew issues ...I have only seen real issues with low power/voltage output and limited high frequency response situations. Such instances invariably involved either tube amps or poorly designed, low power, SS gear.

My opinion on NFB. Implementation of NFB should be done only after getting the basic design proper. And then, my preference is to use only small amounts, around every stage instead of globally and to assure that the phase is compensated well so that it doesn’t shift and induce positive feedback at very high f’s.

One other thing.  I see tons of discussion, indeed much marketing effort, emphasizing the importance of the types of spectra of the involved distortions while ignoring completely that if said distortions are below the threshold of hearing and, thus inaudible, then so what ….. you can’t hear them anyhow.

I am in general agreement.  One item I would like to expand on is the TIM/Slew issue.  The best way I can explain it is when a given amp is designed properly, the low level detail is CLEARLY reproduced with no audible distortion.  Many SS amps miss the boat on this issue.  When an amp does it correctly, one will often suddenly hear detail on well known to the listener recordings that more modest amps mask.  The masking of those details itself is a form of TIM.

[Roger A. Modjeski]

I assumed that after Otala most designers got the message. Are there some amps out there with terrible slew rates? It should be less of a problem with low voltage amps as they don't have as far to go voltage wise.

For the other readers here, making an amp Class A bias does not necessarily improve slew rate as that is mostly a problem in the driver.

Does anyone here want to take a shot at explaining slew rate and TIM in simple terms? The papers cited so far are written for engineers and not the layman. I have enjoyed reading them, but they are pretty heavy stuff for the general public.

It would also be nice to quote the slew rate of some good amps like the Electrocompaniet.