might build a phaser...i know you need "matched" JFETS. Curious...what exactly is being "matched"?
Great question, it was only a few months ago I was in the same boat.
After determining what JFETS I needed, I bought 100 of them, although you can probably get away with about 25-50.
I then built a Greatly Improved JFet Matcher II (http://tagboardeffects.blogspot.co.uk/2012/07/greatly-improved-jfet-matcher.html).
I then got a piece of paper and then drew 100 boxes and numbered them. After that I measured each of my JFets using my matcher circuit, stuck them in a box on my paper and wrote down both the Vp reading and Idss reading in a spreadsheet.
I chose to match on Vp (I believe) because I'd read RG Keens article on JFet matching (http://www.geofex.com/article_folders/fetmatch/fetmatch.htm), and that's what he used. (I hope I got that right). That article will probably answer your question better than my response! I just thought I'd share my technique.
By using a spreadsheet, I was able to order them and find the closest matches.
Actually if J112's will do for you, I now have 94 spare! If you PM me your address I'll send a batch of matched ones out! Save you a job.
you bought 100!
I bought 4. I guess I got extremely lucky
On my list project there is a Phase 45, and I need JFET matched, I have some kind.
I'l try several combinations and I'll choose the better to my hear.
You could try, too, and get lucky, maybe. :P
(With Phase 45 somebody gets a less subtle effect with J201s ;)).
Waiting your news.
Quote from: MrBinns on June 15, 2015, 04:50:37 AM
you bought 100!
It was one of those things, when you bought one it was about £1 but when you got a 100 it was 9p each, so at £9 for a 100 I couldn't go wrong.
But I'm happy to give them away if anyone does need some I'll happily post.
Quote from: plexi12000 on June 15, 2015, 02:45:23 AM
might build a phaser...i know you need "matched" JFETS. Curious...what exactly is being "matched"?
Good question. "Matched" means that multiple devices have been selected to be alike in some way. That phrase "in some way" is not a casual toss-off to end the sentence. There are many characteristics of a semiconductor that can and are measured. Usually only one or a few characteristics are important to be similar. The more characteristics that have to be "matched", the more difficult it is to find matched devices.
It's worse for JFETs. JFETs are wildly variable, much more so than bipolar transistors.
In a phaser, you want all of the JFETs to be at about the same resistance from drain to source when their gates are at the same voltage compared to the source voltage. Matching JFETs for phasers refers to testing them for having very close drain-source resistance at a given voltage from gate to source. My JFET matching setup and the derivatives do specifically this. They do not match other characteristics. But Rds@Vgs is generally enough for a phaser.
... or a "dynamics" circuit (comp/gate etc.) ...
Just for added information, look up the Paul Nelson phaser which uses quad CMOS analog switches which are switched on and off above audio frequencies for varying duty cycles to achieve the phaser effect. The advantage of this is there is no need for matching since the devices are either open-circuited or shorted with a negligible resistance of about 100 ohms. It is a little more complicated, but you order only the parts you need and every one you build sounds like every other one you build.
Quote from: R.G. on June 15, 2015, 09:04:26 AM
Quote from: plexi12000 on June 15, 2015, 02:45:23 AM
might build a phaser...i know you need "matched" JFETS. Curious...what exactly is being "matched"?
Good question. "Matched" means that multiple devices have been selected to be alike in some way. That phrase "in some way" is not a casual toss-off to end the sentence. There are many characteristics of a semiconductor that can and are measured. Usually only one or a few characteristics are important to be similar. The more characteristics that have to be "matched", the more difficult it is to find matched devices.
It's worse for JFETs. JFETs are wildly variable, much more so than bipolar transistors.
In a phaser, you want all of the JFETs to be at about the same resistance from drain to source when their gates are at the same voltage compared to the source voltage. Matching JFETs for phasers refers to testing them for having very close drain-source resistance at a given voltage from gate to source. My JFET matching setup and the derivatives do specifically this. They do not match other characteristics. But Rds@Vgs is generally enough for a phaser.
are you the real RG ? I think its really cool that you post here :) I own some visual sound products and very happy with them ..the one spot is a great gadget
Quote from: LightSoundGeometry on June 15, 2015, 12:10:03 PM
are you the real RG ?
Do you think that a fine DIY site such as this would host a fake R.G.? :icon_lol:
In the flesh...well...virtual flesh!
And we are glad to have him!
Quote from: armdnrdy on June 15, 2015, 12:16:35 PM
Quote from: LightSoundGeometry on June 15, 2015, 12:10:03 PM
are you the real RG ?
Do you think that a fine DIY site such as this would host a fake R.G.? :icon_lol:
In the flesh...well...virtual flesh!
And we are glad to have him!
I read two of his articles this morning ! very good stuff for a newbie like me. The article are so in depth I have to book mark them and reread a few times lol
I keep three folders containing R.G.s material on my PC.
The first is titled: Writings by R.G. that I understand.
The second is titled: Writings by R.G. that I do not understand.
and the third folder is titled: Writings by R.G. that I will never understand. :icon_lol:
Quote from: armdnrdy on June 15, 2015, 01:26:44 PM
I keep three folders containing R.G.s material on my PC.
The first is titled: Writings by R.G. that I understand.
The second is titled: Writings by R.G. that I do not understand.
and the third folder is titled: Writings by R.G. that I will never understand. :icon_lol:
HAHAHAHAHA!!! -- aint that the truth, armdnrdy!!!
Thank you guys for answering- i appreciate it! Maybe its my beer talking but.....RG, I love you ,man!!! hahahaha!!!!
So i guess, i'm not able to use my meter to match, huh? (i can check HFE's on it though.)
oh- wait a sec....i guess i'll start with RGs article linked above--- thanks for that too!!
Quote from: LightSoundGeometry on June 15, 2015, 12:10:03 PM
are you the real RG ?
There is no real R.G. He didn't exist, so we created him.
Yes. RG is a spambot. LOL ;D 8)
Actually, a very helpful and very real dude.
A JFET tester will become a "tool of the trade" you will probably need to build. Along with an "Audio Probe" and "Signal Injector" eventually, if you don't already have them.
Here is my JFET matcher--
(http://i800.photobucket.com/albums/yy286/italianguy63/ROG_JFET_zpseb91de1b.jpg)
EDIT-- note the "free" harbor freight DMM! It is a great use for one.
MC
And-- off subject. But realated to my last post...
If you are nice to him, Henry Rollins will hold your probe for you:
(http://i800.photobucket.com/albums/yy286/italianguy63/Rollins_zps06d9b7d2.jpg)
If you are not nice.. he will pound you into oblivion.
Carry on...........
(EDIT NOTE: The red probe was made to discharge caps in amps.. so they don't shock the crap out of you, the black one is the audio proble.)
MC
WHY do they need to be reasonably matched?
Because a phase-shifter relies on shifting the phase. That is, the effect comes from notches being moved around. What that implies is that the JFETs - or whatever kind of control element the design uses...because there are several kinds - have to always respond to the LFO controlling them. Any given JFET will have a range of applied gate-voltage/current, outside of which it will not change drain-source resistance. This is why the P90, and any other JFET-based phase shifter always has a bias trimpot. The bias moves the JFETs into the range where they can be made to keep "moving"/responding to the LFO.
When the JFETs are not matched, then finding a bias range where all of them are responding to the LFO, at all points in the sweep cycle, becomes much much harder. The result is that, for example, all 4 JFETs are changing drain-source resistance for part of the sweep, but at a certain point one or more just stops changing for the remainder of that direction of the sweep, until the sweep again returns to that point where those JFETs are susceptible to change.
All the JFETs have to have their oars in the water for the phaser to move in the desired direction. When they are not matched, some may have one or both oars "out" some of the time. I will point out that their respective drain-source resistance does NOT have to be matched, only their ability to change in response to the LFO.
Because the risk of inability to respond occurs as one moves to the extremes of the sweep, mismatched JFETs run the risk of having less musical-sounding "turnarounds", whether at the top or the bottom of the sweep. Naturally, this will be most likely to occur when using a wider sweep. You will note that some posted schematics use a 3M3 resistor on the LFO output, while some will use a 3M9. The larger value yields a more limited sweep, which is a little better suited to faster speeds. I don't know if mismatchng can be entirely offset by adjusting speed width, but it might help a little, at the potential forfeiture of wider slow sweeps that you might like to have.
One of the reasons why some folks can have such an affection for photocell-based, PWM-based, or OTA-based, phase shifters is that matching is a non-issue. The photocells will always respond - even if non-identically - to the light shining on them. The CMOS switches (used for PWM) will all respond identically, and the OTAs (e.g., in Small Stone, Moog, or similar) will also respond similarly. It is only in the case of JFETs that there can be a point where there is a risk of them performing non-identically.
QuoteCurious...what exactly is being "matched"
What needs to be matched is the Vgs_off, the gate source voltage where the current is zero.
In reality this converts to matching the Vgs voltage when the drain current is low, say 10uA.
Most matching circuits match Vgs at low current.
When this is done, and assuming the all the JFETs are the same part, the JFET resistance will tend to match well enough over the whole range of Vgs voltages.
If they don't match the phasing effect will sound weak.
But the resistances actually don't have to match. Consider photocell-based phasers. VERY little chance the resistances match. But they sound great, just the same. Why? Because the LDRs all respond to light variation at all points in the sweep cycle.
The effect depends on the cumulative phase shift across the stages. It doesn't matter that much *where* in the spectrum any single stage produces phase shift (within reason), so long as where the shift occurs is moving. What one tries to avoid is any single stage choking in one spot for part of the sweep, while the others change.
QuoteBut the resistances actually don't have to match. Consider photocell-based phasers. VERY little chance the resistances match. But they sound great, just the same. Why? Because the LDRs all respond to light variation at all points in the sweep cycle.
They don't need to match in terms of fine tolerance but they have at at least be in the ball park. Despite LDR's poor tolerance they do seem to fall in that practical ballpark.
The "off point" issue you get with JFETs is largely eliminated with LDR's. One advantage LDRs have over JFETs is a very wide range of resistance adjustment. In theory JFETs can act as a high resistance but the Vgs_off mismatch puts a practical limit on the upper resistance.
For example, suppose you have two JFETS matched to Vgs_off within 50mV. Both are supplied from a common gate supply, as is done in a phaser,
Q1: Vgs_off = 1V, and
Q2: Vgs_off = 1.05V
Assume rsd_on is perfectly matched at rds_on = 250ohm (yfs0 = 4000 uS)
Suppose we bias both JFETs at 50mV above the lowest Vgs_off of the two JFETs
ie. Vgs = 1V-0.05V = 0.95V.
The resistances each JFETs are,
rds1 = 250 / (1-0.95/1) = 20 * 250 = 5000 ohms
rds2 = 250 / (1-0.95/1.05) = 10.5 * 250 = 2625 ohms
So despite what at first seemed like a good match, the effect of even a small Vgs_off mismatch produces a very large tolerance at the high resistance end.
As the Vgs bias point approaches zero the resistances will both approach the same value of 250ohms.
As you can see the practical range of resistance adjustment is going to be in the 10:1 to 20:1 range. For a compressor the practical range is higher because we don't have to match two JFETs.
[Links for JFET resistances:
http://educypedia.karadimov.info/library/322Lecture31.pdf
page 12 of,
http://www-personal.engin.umd.umich.edu/~jwvm/ece414/PowerPoint/JFET_Overview.ppt
]
Quote from: Mark Hammer on June 27, 2015, 10:23:47 PM
But the resistances actually don't have to match. Consider photocell-based phasers. VERY little chance the resistances match. But they sound great, just the same.
I hate to disagree with you Mark but, to me...it seems logical that the resistance of the vactrols/JFETs be in a similar range rather than have one move from 2K-100K and the next from 30K-300K.
It seems that matched resistances would have a smoother, more linear sound.
If the resistances were are grossly unmatched, that would be similar to the VGS off being unmatched.
I have performed testing with R.G.s matcher, and the ROG matcher to check the min/max resistances of matched JFETs while connected to a phaser LFO.
I have found that the resistances are similar as a general rule but, in the same "batch" of matched JFETs there will be a few that are "way out" from the group.
Here is one of the factory component pages for the MXR Phase 100. Under "Special Note" it shows that MXR did match the vactrols for this phaser.
(https://dl.dropboxusercontent.com/u/53299166/DIYstompboxes/Phase%20100%20parts%20list%20pg.2%20-%20Copy.jpg)
I think our point of disagreement lies in what constitutes "matching". My own original purpose was to allay any fears some folks might have that their JFETs weren't matched enough (i.e., on multiple parameters). It matters that they all sweep up and down together (which vgs-matching and proper biasing will take care of), and are - as Rob said - "in the ball park". But more than that is gilding the lily.
I urge people to keep in mind that it is always the sum total of phase-shift at any given frequency, across all stages, that is the objective. If, at the lowest point of sweep, one stage provides 87 degrees of phase shift at 150hz, another provides 86, a third provides 90, and a 4th also provides 90, then we have 353 degrees of phase shift at 150hz, and don't hit the full 360 until a little higher up in the spectrum.
I might point out that the range of sweep is also a function of the fixed resistor placed in parallel with the JFET, and the cap value. Getting a reasonably good match on JFETs simply means one doesn't have to monkey around with either of those other components. But, in principle, as an example, if the zone where a given stage is applying maximum phase shift (as a function of the JFET parameters) swings between 1.3khz and 9khz, then that range could be dropped to 325hz-2.25khz by quadrupling the value of the cap, and even a little further by increasing the 22k resistor to 24k or even 27k.
The MXR note is interesting. I just wish we knew more about what was being used as a benchmark for "matched", and what the tolerances were for that part at that time in production history. Unfortunately, it's hard to peek inside sealed optoisolators! :icon_lol:
Just want to say a thank you to armdnrdy, Rob Strand and Mark Hammer for your detailed explanation. I happen to be reading through my new copy of Horowitz & Hill and I'm in the JFET chapter lately. Between the three of you and Messrs. H&H, I feel like I am understanding what is going on here at a much better level than ever before.
Of course that's a mistaken impression, but I'm enjoying it so let me have my moment, OK?
Quote from: tubegeek on June 28, 2015, 08:06:41 PM
Just want to say a thank you to armdnrdy, Rob Strand and Mark Hammer for your detailed explanation. I happen to reading through my new copy of Horowitz & Hill and I'm in the JFET chapter lately. Between the three of you and Messrs. H&H, I feel like I am understanding what is going on here at a much better level than ever before.
Of course that's a mistaken impression, but I'm enjoying it so let me have my moment, OK?
Hahahahhaa.. I am just hearing Charlie Brown's teacher... whaaaa whaaaa whaa wa wa... ;D
Way over my comprehension.
An interesting perspective on how far things can be mismatched is to look at the large capacitor ratio used on the univibe phaser.
....and that is intended to result in broad and shallow dips, rather than narrow and focussed notches, by staggering where the maximal phase-shift occurs.
I'd be curious as hell to find out how in the deuce they stumbled onto that one. For me, it's right up there with the Worcestershire sauce recipe. I mean, it tastes/sounds good and all that, but just what possessed you to think of combining those ingredients?
There was a paper presented to the 1971/2/3/4? JAES on the spacing of RC time constants in phasers and their audible effects. I can only recall the conclusion that identical RC sections produced the more pleasing effect, better than staggered sections. But I can't recall the metric they used for "better". I keep intending to find that article and read it again, but I keep refusing to pay for it or join the AES to do it.
Another thing comes to mind. Staggered phase shift stages in broadband-quadrature setups. Phasers don't do aligned phase shift differences, of course, but I suspect that there's some good work there for understanding.
So, we've seen the combining of fixed and swept stages, and we've seen the combining of staggered/non-identical swept stages.
Have we ever seen any cases where identical stages are used, but not swept in identical manner (e.g., some swept only half as wide)? And would there be any musical value in doing so?
So, as an example, let's feed the gates of two Phase 90 stages from the LFO with one current-limiting resistance, and feed the other two stages with a different resistance.
There are fixed four-stage allpass filters of the sort seen in a Phase 90 or Univibe used for voice broadcasting. The idea there is that different frequencies will show amplitude peaks at different times, permitting more signal amplitude without overmodulating the carrier. These filters usually have dissimilar sections like the Univibe. This may be an interesting effect - will the music sound louder at a live gig with the phaser connected but not necessarily sweeping?
Quote from: Mark Hammer on June 29, 2015, 09:54:03 AM
Have we ever seen any cases where identical stages are used, but not swept in identical manner (e.g., some swept only half as wide)? And would there be any musical value in doing so?
You may remember some years ago you and I exchanged some notes on non-synchronous moving notches. It was in the context of emulating the varying resonances/anti-resonances of a sitar bridge, but we talked about phasers as one means to that end.
Psychoacoustically, the ear detects the moving notches of flanging as a detection of motion, and phasers seem to hitch onto a bit of that. Fixed notches are hard to detect, but fixed resonances impart a woody, resonant (well, duh! :) ) note, while moving resonances get vocal in some instances. I would guess that musically, non-synchronous notches and/or resonances would hitch onto one or more of these as they moved. It's worth looking into.
Notice that the phase change produced may not even be audible on its own. A moving phase change imitates motion as Doppler effect, but many of the effects of phase change are most easily heard by their notches and peaks. A phaser, with dry mix, is mostly a peaks/notches device, not a vibrato.
QuoteSo, as an example, let's feed the gates of two Phase 90 stages from the LFO with one current-limiting resistance, and feed the other two stages with a different resistance.
IIRC, that was one of the possibilities I suggested, feeding different pairs of stages from different modulation sources. It's also possible to do this directly, with something like a twin-T per notch and just wobble the notches around directly. Probably about the same number of components, but one might be easier to mess with than the other. The Twin T cell lends itself notches in the forward path, peaks in the feedback path, so there may be some elegance there.
I would think that the synth guys would have plumbed this well long ago, but I don't remember reading anything about it. Anyone else?
Let's distinguish between differential modulation of stages , in which each cluster is combined with dry signal to produce one or more notches, and that scenario where the summed amount of phase shift prior to mixing is jerked around by feeding some JFET gates a different amount of a common LFO than other gates.
The first case is essentially individually-controlled notches (and any accompanying resonances), in a way that would be similar to a couple of people playing with the individual centre-frequency controls on a multi-band parametric equalizer. The second case is....hell, I have no idea whatsoever! :icon_lol:
Quote from: R.G. on June 29, 2015, 01:39:57 PM
You may remember some years ago you and I exchanged some notes....
I saw a picture of that meeting:
(http://i680.photobucket.com/albums/vv166/tubegeek-original/yalta-conference.jpg)