FET phaser Distortion and Harmonics

[markphaser]

Some phaser pedals use FET's in the interfacing section hook up to the LFO

What kind of FET's are used in phaser pedals?

I heard FET's distort and produce harmonics why is that ? what causes them to distort if the LFO circuit is hooked up to them

If the FET's distort and produce harmonics then the harmonics and distort frequencys will blend in the phase network?
giving the phase shift more richer sweeps and richer audio

[markphaser]

Matching FETs gives more notch depth? how more notch depth?

FETs have slow slew rates because the LFO changes faster than the FET's can change the resistance
the rate of change/slew rate how fast it can change voltage or resistnace VS time

FET's clip the LFO voltages?


 

[Mark Hammer]

No.

FETs are fast.  It is LDRs that often cannot change as fast as the LFO.  This can have a positive side effect in that as LFO rate goes up, sweep width is reduced a bit because the LDRs are too slow to go through the complete range of resistance change.  because the human ear is sensitive to amount of change over time, we usually reduce the sweep width/depth as the rate is set higher.  With LDRs, at least part of that compensation is already done for you.

FETs are selected so that they can provide the most change in resistance possible in response to the LFO output voltage.  The reason for matching them is to make sure that all the FETs are able to respond to the LFO with their "peers".  If one or more of the FETs is unable to change its resistance any more even though the other can still go up or down a little more in resistance, then the sweep will seem to have less richness at the top or bottom end of the sweep cycle (depending on whether the FET/s are limited in going higher or lower in resistance).  Think of it like 4 people pushing a car.  As long as all four push the same amount, the car moves at the same velocity.  If one or more of the people stops to catch their breath, the remaining ones can't push the car as fast or easily.  If one or more of them does not join in the pushing right away, it will take a while for the car movement to really "get going".  You want all of them to be pushing together all the time.  That's what the matching accomplishes.  Even when "matched" in the typical manner, their actual resistance may well be quite different.  What is matched is their capacity to respond to gate voltage change with a resistance change.

The notch depth will be a function of the matching between clean and phase-shifted levels.  If a person were aiming for max notch depth, and the clean and phase-shifted signals were mixed via a pair of, say, 47k resistors, then the normal component tolerances - both in those resistors, and all other relevant components along each of the two major paths -  might result in something other than a perfect 50/50 mix/balance.  The smart thing to do would be to stick a 25k trimpot in there, with 36k fixed resistors at each of the outside lugs of the trimpot.  This way, the clean and phased signals would "see" a resistance equal to the 36k plus their side of the trimpot (the wiper being the point where they are combined), and the total (36k + 36k + 25k) would be pretty close to the original combination of 47k + 47k, with the added apability of making one side a little higher or lower than the other.

Ideally, the way you set something like this up is to hold the sweep, feed the phaser with a fixed steady signal that falls where you know a notch exists, and adjust the trimpot until the greatest notch depth is attained.  This would be indexed on the scope by the lowest output amplitude at that signal frequency.

[stm]

The article AN-129 "A linear multiple gain-controlled amplifier" that appears in National Semiconductor's  Linear Applications Handbook explains the issues of distortion with FETs when used a variable resitors, and proposes several circuits and refinements to use them in gain controlled amplifiers, which is directly applicable to phasers as well.

Unfortunately I haven't been able to find a PDF or electronic version of this article.

[Mark Hammer]

Here's the chart you want: http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF

[stm]

Here's the chart you want: http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF

Great info!  To everybody, this info is valid both for MOSFETs and JFETs.

[MR COFFEE]

Hi Markphaser, STM and all,

Well, old NS took it down for some reason.

Here's a digital camera copy of AN-129 laying on my kitchen table in gif format. Yeah, I edited it a bit so it's not so dern big. A few notes in the margins FWIW. The scope shots are a bit dark, but most are visible.

http://home.bellsouth.net/p/s/community.dll?ep=331&folderid=226667&folderview=list&groupid=207065&ck=

You'll need to download the images and view them in a decent graphics program to get them sharp enough to read. Best I could do. Don't know how they'd do printed out, but readable on the monitor anyway.

This won't be up long, so get it while it's here.

Best article on it I've read. Nowdays the author is chief of Applications at Maxim -

and that's NOT a coffee joke 

[Mark Hammer]

If that's the old orange softcover book they used to sell at Radio Shack, I may be able to provide a better quality scan.

[MR COFFEE]

Hi Mark,

Looks familar, eh? 

[markphaser]

Thanks Mark Hammer for the information and time

"It is LDRs that often cannot change as fast as the LFO"

Why do LDR's have a timing delay?
Why can't they change as fast as the LFO?

Do FET's have a resistance taper or sweep?

FET's get overloaded by the audio signal which causes them to Distort and the Distortion bleeds in the phase network filters adding in the notches and peaks with the FET distortion?

  What is slow tracking and Fast tracking a FET do ?

[MR COFFEE]

Hi Markphaser,

Do FET's have a resistance taper or sweep?

FET's get overloaded by the audio signal which causes them to Distort and the Distortion bleeds in the phase network filters adding in the notches and peaks with the FET distortion?

  What is slow tracking and Fast tracking a FET do ?

Did you read AN-129? It's only 8 pages, most of which is diagrams and graphs. It answers your questions; all you have to do is click on the link to download and read them.

JFET Distortion products vary in frequency with the remainder of the VCO-varied phase-shifted signal. They are not part of the notch filter cancellation except by infrequent accident

All LDRs are slower in changing resistance in response to varying light. Attack time is in the range of milliseconds for "fast" cells, 100's of milliseconds for "slow" cells. Maximum signal pass frequency is roughly 25 hz. If you want to know the physics of why, go check out a photocell manufacturers site like Vactec. They have tutorials. The remainder of your other questions answered in you other thread.

[markphaser]

Thanks MR. Coffee

JFET Distortion products vary in frequency with the remainder of the VCO-varied phase-shifted signal

So what distorts the JFETS the Audio or the LFO? and why do they overload or distort?

Fast cells are in Microseconds or milliseconds? Slow cells are in microseconds or milliseconds?

[R.G.]

Mark, MR - I could be wrong, but my sense of text style tells me that markphaser is the new name for our friend walters. If not, the resemblance is remarkable.

Why do LDR's have a timing delay?  Why can't they change as fast as the LFO?

LDRs simply can't change resistance value very fast as a result of the way that their materials respond to light at a molecular level. The real answer on the details of the materials and how they work is too complicated and involved to type in here. It's sufficient for this forum to know that they can't respond very fast. I suggest that if you need a detailed explanation of the actual response of the materials to light, you use a web search engine and look it up.

Do FET's have a resistance taper or sweep?

Yes. The resistance of a FET channel varies with the square of the applied Vgs.

FET's get overloaded by the audio signal which causes them to Distort and the Distortion bleeds in the phase network filters adding in the notches and peaks with the FET distortion?

No. The resistance of a FET channel also varies with the applied Vds. This variation of channel resistance with Vds is smaller than with the Vgs, but it does exist. This variation of channel resistance with both Vgs and Vds leads to intermodulation of the signal with the LFO in phasers. The distortion is unrelated to the notches and peaks. The signal through each phase stage gets distorted by the intermodulation. Small enough signals are not distorted by objectionable amounts. The exact details of this intermodulation varies from FET to FET.

What is slow tracking and Fast tracking a FET do ?

"Slow tracking" and "fast tracking" are just an attempt at characterizing what the FET does. They are not technical descriptions of the operation. A FET can respond to any signal very quickly - fast enough to follow signals of many megaHertz. The FET can then track the LFO signal and the audio signal very well indeed. This is opposed to LDRs which cannot respond quickly as a result of the nature of their inherent materials' response to light.

So what distorts the JFETS the Audio or the LFO? and why do they overload or distort?

Explained above. The JFET resistance is modulated by both the Vgs and the Vds - that is, both the audio and the LFO.

As to why they distort, this reminds me very much of Bill Cosby's humor recording of "Why is there air?" Distortion of any signal is a result of the signal running through some process that is non-linear. There are ...no... completely linear processes. There are processes that are linear enough that we can think of them as linear to a good approximation within a limited range. Resistors are nonlinear over a large enough range, for instance. We just use them over a small enough range of signal that the difference between truly linear and not linear is negligible.

They distort because the signal level and modulation voltages get into ranges for which the deviation from linearity is no longer something we can ignore. They distort because getting a big enough change in channel resistance for phaser use and applied to common guitar level signal is only barely within the usable range of the device. The device has that range of acceptable distortion levels because that's how the math of the universe works out. The physicists are still working on why the math of the universe is as it is.

Fast cells are in Microseconds or milliseconds? Slow cells are in microseconds or milliseconds?

Fast LDR cells are in tenths of a second. Slow LDR cells are in seconds. Fast LDR cells are only barely within the range of response times that they can be used for LFO stuff.
Why? Because that's how the LDR materials work. Because that's how the atoms of those materials work. Because that's how the subatomic particles of those atoms work. Because that's how the math of the universe is. The physicists are still working on why that last one is.

[petemoore]

  The physicists are still working on why that last one is.
  And probably as we speak, they are gettin' better at it...again.
 
http://en.wikipedia.org/wiki/Quark
  When they found the atom, it was called the 'smallest known matter', not 'long' after though, they found smaller nuclei, electrons, protons...
  Our perception of how small and how large 'things' can get seems to have 'improved'...and yup, they're working on this one.

[gez]

I could be wrong, but my sense of text style tells me that markphaser is the new name for our friend walters. If not, the resemblance is remarkable.

My thoughts exactly, which is why I haven't been replying.

Apologies if this isn't the case...

[MR COFFEE]

Hi Gez and RG,
Gez wrote

Quote from: R.G. on December 16, 2005, 03:23:25 PM
I could be wrong, but my sense of text style tells me that markphaser is the new name for our friend walters. If not, the resemblance is remarkable.

My thoughts exactly, which is why I haven't been replying.

Is there something I should know here? 

I don't know about "walters"; is he a troll or something?

From your response, Gez, I get the vibe "walters" is someone who poses questions because he gets a charge out of people trying to help him by personally answering his questions, rather than the sort of person who poses questions here out of a real interest in learning ? Correct ?

I noticed that after I went to the trouble to make and post a digital copy of AN-129 (which is no longer available from National Semi or anywhere else on the net that I could find), that markphaser asked questions, i.e.,

So what distorts the JFETS the Audio or the LFO? and why do they overload or distort?

that suggest he didn't download and read the information despite stm and I telling him it answers his questions about JFET behavior used as a voltage-controlled resistor with audio - actually quite specifically. 

Please clue me in on "walters"; I don't want to feed the trolls, either, if there is reason to believe that that is the case here.

Apologies to markphaser if that is *not* the reason he\you didn't take advantage of the resources made available to him\you.

[Mark Hammer]

Earlier this year, the gentleman being referred to joined this forum.  His question-asking style needed, how shall I put it, some "refinement".  I had encountered such a style many times before in students, but some folks here did not take to it very well.  It wasn't an aggressive style.  Rather, it just flung too many questions at once, many of which needed a bit more time to be fully formed.  In short, it was a style born of way too much enthusiasm, and too little self-restraint.  It was a style that was exasperating to respond to because of the unconnected nature of the questions.  However, there was nothing malicious in it. 

Some people, however, thought it was, and responded in kind.  The person in question reacted as many would to such taunting, and the episode was not quite the rabid meltdown I've seen in other places, but it was uncomfortable and regrettable, and people said unkind things that were not in keeping with the usual spirit here.  I spoke with the gentleman off-line and made some concrete suggestions to him regarding how to hem in his style and bring out better behaviour in others which would, in turn, benefit him and get him answers to his questions.  To his credit, he worked at it.  But of course, these are not the sorts of habits you change overnight, and I understand that he was not about to show a 180-degree turnaround and be a forum's dream come true.

What I guess is not so well known is that I also suggested to him that he should hang back for a little while, and possibly use a different handle when he returned so that he would not be saddled with an "Oh, THAT guy" reputation that might not acknowledge any change in his behaviour and improvement in his style.  I was hoping this sort of identity change would allow someone who was sincere but perhaps a bit disorganized to be accepted at face value,  We are generally a VERY tolerant group here, but this was at the very edge of what some people would put up with and I wanted us ALL to get past that.

The gentleman took my words to heart and we had a number of forum-like exchanges about pedals after that, part of which was, I guess, "training" in a way, so that he could pitch questions that got answers.  I do not know at all if the person being discussed right now is the same person.  My gut sense is no, simply because I know that "style" is a lot more common than many might think, and there are other cues that suggest a different person.

Either way, I would just say the following:

1) If it IS "you", my apologies for not getting back to you about your last questions.  I was lazy and when stuff gets bumped off the first screen of e-mails I tend to forget about it.

2) People here vary widely in their ability to ask clear, easy-to-answer questions.  Sometimes it is a question of English being their second language.  Sometimes it is a question of them just starting out and not having enough knowledge to even ask the questions.  Sometimes it is a question of their enthusiasm getting the better of them, or some other set of reasons.  Whatever the case, if they ask sincerely, no atter how much one's instinct is to mumble "What a stupid frigging question", they deserve a sincere answer.  That might be a referral to an FAQ, but it should be a sincere referral to an FAQ, not a "screw off and go away" referral.  It is exceedingly rare that people come here to cause trouble.  That doesn't mean they are socially graceful, but they don't come looking for an argument.  "Troll" is a word I have, thankfully, never really found a reason to use here.  I understand that they are encountered so frequently in other places that people who visit other sites are sensitive and apprehensive about running into one here, but I must emphasize that I have come across irritable types here but never trolls.  And I've been here since pretty much the beginning.

3) You don't have to, but personally I believe in the capacity of people to improve.  When they make efforts in that direction, you tip your hat to them and provide encouragement to keep it up.  When they fall down, you give them a hand up, and make some constructive suggestions.  To deny others the possibility of redemption is to deny oneself that very same capability.  Yeah, I guess that's a little pompous and heavy-handed, but dammit I believe in people.

4) markphaser - try to work on the way that you ask questions.  People seem to be willing to TRY and answer them, but some of the comments by others here are an indication that they are finding your posts as frustrating as the episode referred to earlier in the year.

There.  I hope that addresses everyone's concerns.  Now plug in your irons and get to work.

[MR COFFEE]

Hi Mark,

Thanks for bringing me up to speed on the "walters" business. I am both a frequent flyer here and absent for long periods of time and I missed that one.

[MR COFFEE]

Hi all,
Folks who are actually interested in reading and learning about the distortion behavior of JFETs used in audio circuits for either variable gain control or variable phase-shift networks can download a picture perfect pdf of National Semiconductor AN-129 from this link - much nicer than mine if you don't mind the 5 meg file-size.

My tiny gifs are still probably better for those of you who are still on dial-up.

http://headfonz.rutgers.edu/FET-gain.pdf

Enjoy

[markphaser]

VC FET Attenuator

1.) The FET behaves as a Linear resistance only for small values
     of source-drain voltages Vds in either polarity

2.) Non-Linearity (of resistance) increases as the control voltages
    Vgs approaches cut-off voltage Vp when the resistance is max.

3.) The relationship of resistance rd to Vgs is reciprocal rather than direct linear

4.) Vp requires separate bias set and gain set on each circuit