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DIY Stompboxes => Building your own stompbox => Topic started by: stm on February 03, 2005, 04:22:44 PM

Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 03, 2005, 04:22:44 PM
Greatly Improved JFET Matcher

So you wanna build a Phase 45 or Phase 90 or one of its derivatives?  Ahh, you need *matched* JFETs, so just read on... This was motivated by my latest experiments with a Phase 45 clone.

Current JFET matching techniques rely on matching at a single "point" along the response curve.  In practice, accurate JFET matching needs to be done on *two* different points, since the drain-to-source resistance (Rds) is a function of *two* parameters related to the manufacturing process: Vp (pinch-off or cutoff voltage) and Idss (drain-to-source saturation current), as indicated by the following equation:


Code: [Select]
          Vp^2           Valid IIF:
Rds = ---------------     (a) Vds aproximately 0
      2·Idss·(Vgs-Vp)     (b) 0 >= Vgs >= Vp



Example: Typical values for an MPF102 could be Vp=-2V and Idss=6mA (note the negative sign on Vp!). At zero Vgs, we get the ON resistance, which is given by:


Code: [Select]
          -Vp      -(-2)
Rds(on) = ------ = ------- = 166.7 ohms
          2·Idss   2·0.006



On the other hand, if we make Vgs = Vp, we get a division by zero, meaning infinite resistance (JFET fully OFF).

Where does this lead us?  Well, if we can measure the two *key* parameters for two or more JFETs, Vp and Idss, then we can do the following:

1) Any pair of JFETs that have close Vp *and* Idss are *matched*. Coincidence on just one parameter is not enough!

2) Knowing Vp and Idss allows me to effectively calculate the required Vgs for a given Rds, meaning I can really know a-priori where to adjust the control voltage for the JFETs. Manipulation of the first equation leads to:


Code: [Select]
        Vp^2
Vgs = ---------- + Vp
      2·Rds·Idss



Example: For the same MPF102, let's calculate the required Vgs for a 10k Rds.


Code: [Select]
       (-2)^2             4
Vgs = ------------- - 2 = --- - 2 = -1.96 V
      2·10000·0.006       120



Ok, now let's see the circuit that will let us read Vp and Idss:

(http://tinypic.com/1j5fz7)

As you see, the circuit is fairly simple. Only keep in mind the OpAmp MUST be MOSFET input to avoid altering the weak current through the 10Mohm resistor for the Vp reading, and must be capable of going down to GND (or the negative supply rail) for proper Idss reading.

After you characterize both Vp and Idss for each JFET you have, just find those with closest Vp AND Idss.

Have fun,

STM[/b]
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 03, 2005, 04:26:19 PM
Good work. I had not been through the math to realize that only two parameters needed measured; that's not true for bipolars, you need to match several.

My harangue on matching JFETs previously in the archives points out why I left it at only one parameter - it's simple, and it works well enough.

If two points are all that are needed, great, this will be a real step forward.
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 03, 2005, 06:16:29 PM
Further thoughts:

It think it needs a second battery. If you do that, you can use garden variety TL072's for the opamp, not a rail to rail CMOS opamp.

Yes, I know that rail to rail is easy; but beginners can't often get anything except the most common things.

I'd still be happier with a way to unload the Vp measurement. 10M isn't much of a load but it is some. The input of a FET opamp is much less.

I'll so some thinking.
Title: Greatly Improved JFET Matcher is here!
Post by: brett on February 03, 2005, 06:54:17 PM
Good work!!  It'll be interesting whether closer matching (and of which parameter) makes for more "musical" phasing.  I have no factual basis for suggesting this, but I wonder whether "close, but not too close" will make the notches a pleasant depth.???

thanks lads
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 03, 2005, 07:14:33 PM
Quote
I have no factual basis for suggesting this, but I wonder whether "close, but not too close" will make the notches a pleasant depth.???

Unlikely. Notch depth depends primarily on how well the magnitudes of the dry and phase delayed signals match (i.e. null out) when they are mixed at the end of the phase delays. Typically, 20db of notch depth is enough, more doesn't hurt.

Matching JFETs has to do with where the notches move in step with the LFO sweep. Ideally, they should all move in synchrony. If not, the notches move in odd ways, and it's the movement that we hear as phasing. If they're not well matched - and well matched to the LFO - then the sweep is not pleasant. Some JFETs won't sweep at the same times as others, and some will be completely off or on when the others are sweeping.
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 03, 2005, 07:22:39 PM
Quote from: R.G.
Further thoughts:

It think it needs a second battery. If you do that, you can use garden variety TL072's for the opamp, not a rail to rail CMOS opamp.

Yes, I know that rail to rail is easy; but beginners can't often get anything except the most common things.

I'd still be happier with a way to unload the Vp measurement. 10M isn't much of a load but it is some. The input of a FET opamp is much less.

I'll so some thinking.


R.G.,  yes, a second battery would alleviate the special characteristics for the OpAmp.  I think there could still be some chance to adapt it for a TL071 at the expense of adding additional components.

You always need some loading on the Vp measurement.  The datasheets usually specify Vp at a given drain current.  In the case of the J201 datasheet I have, it says at 10 nA (really low!).   So, for a 2V cutoff voltage and a 10nA current you would need a 200 Meg resistor.  Not very practical, especially considering the input bias current of the opamp will be in the neighbourhood and interfere with the measurement.

Nevertheless, as the cutoff voltage is approached, the Vgs varies very little, and in practice the important fact here is that every JFET will be measured under the same conditions.

Let's assume an MPF102 with -2V Vp.  With a 9V battery, the current through the 10Mohm resistor will be 2V/10Meg = 200nA.  Then Vds=7V, which gives a JFET Rds of 7V/200nA = 35 Mohm, so the JFET is pretty shut down anyway!!!  The measurement will be pretty accurate anyway.

Regards,

STM.

P.D.  A simple way to avoid the additional 9V battery would be to use one of these voltage inverters. A cheap one would work well here, since there are no problems with the oscillator getting into some audio path (like the LMC7660 et. al.)
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 03, 2005, 07:34:30 PM
Quote
yes, a second battery would alleviate the special characteristics for the OpAmp. I think there could still be some chance to adapt it for a TL071 at the expense of adding additional components.

I don't think that would be worth it. TL072's are more common than 071's and usually cost the same or insignificantly more. I'd just use the dual. In fact, the price is so close, I'd always use a dual instead of a single unless i was matching a pinout.

Quote
You always need some loading on the Vp measurement.

Yeah, I know. Quoting Jerald Graeme in "Applications of Operational Amplifiers",
Quote
In order to easily measure the pinchoff voltage of an FET, the current in the FET must be nearly pinced off. The low current level is sensitive to voltmeter loading, but can be bufffered with an operational amplfiier which also biases the FET. With the FET source voltage driven to zero at the amplifier input, the output voltage is Vgd. This voltage approximates Vp for a specified, small source current supplied through the resistor R.


The trick is to get that current down. Hey... use a reverse biased signal diode, the Millenium Bypass trick. Plop in a reverse biased 1N914 or 1N4148 and you get 10-25 nA of current flow. Bingo, you can measure almost to the datasheet spec.

Yeah, yeah it won't matter to matching. OK, use the 10M.

Quote
P.D. A simple way to avoid the additional 9V battery would be to use one of these voltage inverters. A cheap one would work well here, since there are no problems with the oscillator getting into some audio path.

Yes, I was saving that one for the version with the switching automated by the CMOS switch and indictors on whether it's showing Vp or Idss.
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 03, 2005, 08:00:43 PM
You know, the very next thing we need to get done is have someone run 50  or a hundred JFETs through this thing and see what the yield turns out.

I did some thinking about matching, based on another post in the page.

I think Vgs matching is more important than Rds matching if you have to choose for reasons of low yield.

Different Rds values at any given Vgs value are not tragic, it just means that a given notch is not where it is expected to be in the frequency spectrum. Mismatched caps do the same thing, and we not only don't match caps, we "univibe" phasers by distributing the values in steps of about 10x.

So if the yield is low for two-parameter matches, the thing to do is to grab the best Vgs matches and give a nod to Rds matching.


Course, we won't know til we see some distributions.

Hence this note.
Title: Greatly Improved JFET Matcher is here!
Post by: Prive on February 04, 2005, 12:08:44 AM
I don't think MXR was so deep looking in the matching stage of construction, maybe that was the reason why some P90 sounds different to others.
BUT... if we can, why not to do a hard search for us?

I have both PCB's P90 and P45 waiting to populate, and now i have more things to think.

Guys STOP please!!!!!!!!

 :lol:  :lol:  :lol:

Just kidding.

Saludos, Marcelo.
Title: ..
Post by: petemoore on February 04, 2005, 01:18:04 AM
Wwell I have 4 + 6 [10] Jfet phaser stages and lots of dual OA's.
  No CMOS...and 90 Jfet transistors for phaser use, plus the 10 that are in the 90 and the SS...hint hint...
Title: Greatly Improved JFET Matcher is here!
Post by: Johnny G on February 04, 2005, 05:25:52 AM
hey now this isa pretty nifty lol. i havnt actually got round to making any phasers yet since the idea of having a large build on perfboard doesnt really sound to me like the fun of DIY should do lol

however all of a sudden im just getting these thoughts about the ultimate rack mounted analogue phaser with switchable phase stages and a whole host of other things.

actually, apologies for hijacking this but im on a roll and can repost this in its own thread later lol. if you could match JFets this closelly then you could get a pretty good idea of where it is that the phases would be shifted. so effectivlly you could have a (rather large) phaser with say 20 phase stages but you could switch in the ones that you wanted, so being able to get very particular sounds. maybe having different LFOs for different frequency ranges. oh dear, im running away with myself so ill leave it there :D

nice work again lol
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 04, 2005, 06:03:35 AM
Well, after posting this circuit yesterday in the afternoon, I continued at night with the experimentation on the Phase 45.  I had 15 MPF102's, which I had previously matched for resistance (Rds) at two points:  1 kohm and 20 kohms.  I found several units that matched at one of these values but not at the other, but finally got two pairs that were reasonably close.  The problem is I had to use an analog voltmeter, so I didn't have much precision on the readings.  Today I'm gonna buy a DVM and try the matching with the proposed technique.

Anyway, I finally redesigned the Phase 45 LFO to have an hypertriangular waveshape that matches the Rds v/s Vgs relation I posted above.  Great improvement, now the sweep is much more even and behaves nicely at high speed settings, instead of making me feel seasick. The con is that it needs a Quad OpAmp + 78L05 + 2N5088 + 4 diodes + resistors and caps instead of the single Opamp, but absolutely worth it in my opinion, so I'll make a PCB for it.  I'm not following the KISS philosopy for my boxes anymore. I just want the best possible sound.

Will let you now when I do the next matching round with the Vp and Idss parameters.

STM
Title: Greatly Improved JFET Matcher is here!
Post by: Eric H on February 04, 2005, 09:29:35 AM
Quote from: stm
I'm not following the KISS philosopy for my boxes anymore. I just want the best possible sound.



STM
I believe Einstein said something like:  "As simple as possible --but not too simple"

I see a LOT of over-analysis over circuit path simplicity, and component-type,
with no consideration of the circuit itself.

Kinda like life, I guess ;-)

-Eric
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 04, 2005, 09:47:12 AM
Eric, you really nailed it with that Einstein's quote.  It reflects perfectly my philosophy.  I am adopting it for my signature, if you don't mind.

STM
Title: ..
Post by: petemoore on February 04, 2005, 10:35:35 AM
THe ohter side of that one is 'if it works don't fix it".
  It can be more complicated to make something more simplified..
  If it's working....what could possibly be simpler than letting it continue work ???
Title: Greatly Improved JFET Matcher is here!
Post by: Vsat on February 04, 2005, 02:04:23 PM
Did some work on an op amp circuit that automatically adjusts Vgs to whatever voltage is required to achieve a specified channel resistance. This idea may be applicable to FET matching.

The basic idea is to servo the Vgs to hold the drain voltage constant at a small value, while injecting a small constant current  into the drain (the source is grounded).  I used about 30 mV on the drain, corresponding to the low peak-peak signal swings used in phaser circuits. The idea is to maintain the FET in the triode (resistive) region.  A Howland current pump was used as an adjustable current source. By varying the current, the channel resistance also varies, since the drain voltage is held constant. To match FET's, you could  measure the Vgs for each FET in a batch with this circuit, using several current settings, corresponding to the range of channel resistances the phaser circuit requires. The original application was a sweep circuit that provides a V/oct sweep in response to a linear control voltage, but using FET's instead of OTA's.
Regards, Mike
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 04, 2005, 10:11:24 PM
I tested 15 MPF102 JFETs with the proposed method and got the following values (ordered according to Idss):

....Idss(mA).Vp(V).....
01) 4.64 -2.10
02) 4.78 -2.24
03) 4.98 -2.26
04) 5.61 -2.35
05) 5.84 -2.54 *
06) 5.87 -2.54 *
07) 6.28 -2.61
08) 6.30 -2.70
09) 6.68 -2.69
10) 7.02 -2.92
11) 7.06 -2.83
12) 7.37 -2.91
13) 7.65 -2.99 #
14) 7.67 -3.00 #
15) 8.00 -3.16

As you can see, there are two good candidates for pairing, marked with * and #.
Notice devices 08 and 09 have almost identical Vp (-2.70 & -2.69), however their Idss are quite different (6.30 & 6.68). This validates that matching in terms of Vp or Idss only is not enough.
I chose pair 13 & 14, using a representative Idss of 7.66 mA and -3.00 V for Vp.

With this I calculated that I needed:

Vgs = -2,988 for Rds = 50kohm
Vgs = -1.825 for Rds = 500 ohms

I adjusted my LFO to move within this range with a DVM at very low speed, and wow! Very intense phasing effect!

So far this experiment gives me peace of mind on this improved matching method. Initially it appears that the Vp grows monotonically with Idss, however, if you look at device 11 you'll notice its Vp went down.  I double checked this reading and the ones around devices 10 and 12 to make sure this wasn't a procedure mistake. Also, on some deices Vp remains nearly identical within two consecutive devices while Idss increases, as is the case for devices 08 & 09.

Regards,

STM
Title: Greatly Improved JFET Matcher is here!
Post by: R.G. on February 04, 2005, 11:25:01 PM
Interesting results.

I would be quite interested in seeing the differences in phasing between the devices you selected and the pairs you'd otherwise select by Vp only, for instance 8/9 and 10/12. It would be interesting to see scope traces for the differences in phase notches.

As I read the equations, Rds varies as the square of Vp and inversely with Idss. So I'd expect that Vp matching is critical, and Idss matching, while desirable, only adds the remaining ratio between Idss-s. For instance:

I would expect 8/9 to vary only 6% in Rds, 1.06 being the ratio of the two Idss-s.

So we need to see some traces.
Title: Greatly Improved JFET Matcher is here!
Post by: Eric H on February 04, 2005, 11:41:24 PM
Quote from: stm
Eric, you really nailed it with that Einstein's quote.  It reflects perfectly my philosophy.  I am adopting it for my signature, if you don't mind.

STM

The actual quote is more like this (I simplified it ;-)

" Everything should be made as simple as possible - but no simpler"

Don't mind a bit --they aren't my words :-)

-Eric
Title: Re: ..
Post by: Eric H on February 04, 2005, 11:52:34 PM
Quote from: petemoore
THe ohter side of that one is 'if it works don't fix it".
  It can be more complicated to make something more simplified..
  If it's working....what could possibly be simpler than letting it continue work ???

That always sounded good to me --on paper. ;-)
Hand me an object and I'll mod it ; doesn't matter what it is. It's a hopeless addiction --one that I enjoy.
 I'm a perfectionist living in a world that isn't interested. Sound familiar?

-Eric
Title: Greatly Improved JFET Matcher is here!
Post by: Peter Snowberg on February 05, 2005, 12:03:42 AM
Quote from: Eric H
The actual quote is more like this (I simplified it ;-)

" Everything should be made as simple as possible - but no simpler"

-Eric

I thought that's what that was. It's my favorite Einstein quote. 2nd favorite is "If I had known about the end result of my work, I would have sold shoes."
Title: Greatly Improved JFET Matcher is here!
Post by: zachary vex on February 05, 2005, 01:29:13 AM
my favorite Einstein quote (of late) is this one:

Things should be made as simple as possible, but no simpler.

works well for designing things.

on edit: oops, i didn't notice someone else had posted this above.
Title: Greatly Improved JFET Matcher is here!
Post by: Rob Strand on February 05, 2005, 03:09:08 AM
OK while we are getting all scientific an everything I'll show you why two point matching is of no use.  There's been too much misinformation regarding JFET matching and phasers on this group.  A few years back I did a whole heap of analysis and concluded that VP matching was the most important parameter.  The analysis involved writing software to analyse the matching of the all-pass networks with JFETs.

 I posted the results on this group and the conclusions were:

- two point matching is a waste of time
- matching JFET using (hard to do) AC resistance matching is a wast of time
- VP should be used to match JFETs in phasers
- The easiest way to match JFET is to do a VP measurement when the JFET is operated at low currents
- RG's matcher matches the JFETS at much too high a current and doesn't produce optimal results.  The 10M mode of stm's circuit solves this but I think 10M is asking for trouble.  I recommended testing at 10uA or so.

Only the other day we had the thread about using separate trimmers on phasers and the point still hasn't got through.   Am I wasting my breath on this issue?

Here we go (for the last time):

Lets start with stm's measurements:

     Idss Vp      rd0 = Vp/(2*Idss)
01) 4.64 -2.10      226
02) 4.78 -2.24      234      
03) 4.98 -2.26      227
04) 5.61 -2.35      209
05) 5.84 -2.54 *   217
06) 5.87 -2.54 *   216
07) 6.28 -2.61      207
08 ) 6.30 -2.70      214
09) 6.68 -2.69      201
10) 7.02 -2.92      208
11) 7.06 -2.83      200
12) 7.37 -2.91      197
13) 7.65 -2.99 #   195
14) 7.67 -3.00 #   196
15) 8.00 -3.16       198

Average rd0 = 210
min rd0 =195  (-8%)
max rd0 = 234 (11%)
(min rd0 + max rd0)/2 = 215


Incidently rd0 is 1/Yfs  where Yfs is the parameter you see in JFET data sheets.

From this it is immediately clear that despite the very wide variations in Vp and Idss the tolerance of rdss is small ie. the Vp and Idss values are correllated.  There is a physical reason for this as Jfet parameters Idss, Vp and rd0 are all related to the geometry of the Jfet's channel.

The most important point is the tolerance of the (intrinsic) channel resisance rd0 is in the order of capacitance tolerances.

The ac resistance of a JFET is:

   rd(Vgs) =  rd0 / (1 - Vgs/Vp);  Vgs and Vp both signed

With a phaser *all the Jfets* operate off a common VCO.  The
VCO voltage plus the JFET bias network provides a range of gate-source voltages, Vgs.  When the gate-source voltage is close to Vp the ac resistance is high.  When the gate-source voltage moves away from Vp and approaches 0V the ac resistance is low.

When Vgs = 0 the tolerance of the ac resistance is determined
entirely by rd0.  And from the above reasoning the tolerance
on this is fairly low.  Note that the variation in Vp
has no effect.

When vgs is near Vp, the real problem of JFET matching becomes evident.  Consider the case of two JFETs where rds0 = 210ohm for each JFET, VP1 = -4.7V for the first JFET and VP2 = -4.75V for the second JFET.  The two JFETs would be considered fairly well matched. Now suppose the VCO + bias produces a gate source voltage of -4.65V:

For the first JFET:

   rd(-4.65)  = 210 / (1- (-4.65)/(-4.7)) = 19740 ohm

Whereas for the second JFET,

       rds(-4.65) = 210 / (1- (-4.65)/(-4.75))  = 9975 ohm

As is blatently obvious the effect of small mismatches in
Vp produce massive tolerances when the VCO is sweeping the
JFET through the high resistance region, 2 to 1 variation on a good day.
This totally swamps the tiny variations in rds0.

To put this in context two point matching is quite useless.
Vp matching should be the most important thing to match!

The last point is the resistance depends on rd0 and VP and that
Idss is largely an irrelevant parameter. We know the tolerance on rd0 is good so it's measurement is not warranted.  We know Vp is imporant
to match.  To match Vp all you need to do is a measurement at
low Id currents - it doesn't get simpler than taking one measurement.
[edited]
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 05, 2005, 09:44:00 AM
Rob, I agree with your conclusions.  The sensitivity analisys around Idss and Vp for Rds is conclusive. Thanks for taking the time to point this.

Considering what you mention, a simpler, efficient and effective way to go would be to use a 1Mohm resistor instead of the 10 Mohm, and omit the OpAmp entirely and measure directly with a decent DVM.  This greatly simplifies the matching fixture to: one resistor, one 9V battery, and a DVM.

As additional information, last night I ended up using a 6 Mohm resistor on my matching fixture (5 x 1.2 Mohm, since I didn't have anything higher available at home).  The voltage difference in Vp when measuring directly across the resistor string or the OpAmp output was consistently 20 mV (like an offset).  So in practice this won't affect matching since it is identical for all devices tested.  This value should be even lower when using a 1 Mohm resistor.  Vor Vp=-3V, the current through the JFET will be 3V/1Mohm = 3 uA.

On another matter, it is really hard to set the LFO offset, since tiny variations on the lower (most negative) voltage greatly affect the sound. The resistance relation of the JFET is highly nonlinear, so I'm moving to study LDR's for now.

Another important hing to notice is that according to this new light, there are two additional matched pairs in my batch: 8 & 9, and 10 & 12.  It is sad to see there are no good candidates for a triplet, not to say a quad.  o building a Phase 90 would require actually 50 or more JFETS to choose from to get a good matched set.

Regards,

STM
Title: Greatly Improved JFET Matcher is here!
Post by: Rob Strand on February 05, 2005, 10:28:40 AM
Quote
and omit the OpAmp entirely


Good idea!  Since the same meter is used it's effect is constant.  It might be wise to add say 220om resistors in series with the drain and gate to prevent damage if the JFET is put in wrong.  A cap might be useful to prevent noise affecting the results. (I've used this method to do quick JFETS measurements for SPICE models.)

Quote
it is really hard to set the LFO offset, since tiny variations on the lower (most negative)


That's right on the mark.  While the bias can be made stable (with a regulator), unless you regulate the LFO supply as well the LFO voltage is subject to variations.  The commonly used zeners, < 4.7V, don't actually regulate too well anyway.  All in all when the battery goes flat, or you run of a plug-pack and battery, the phasing quality moves around.   The stability issue is something I looked at when I did my analysis a while back.  The common designs are just acceptable.  One of the EH phasers used a more elaborate scheme with a MOSFET as a reference and MOSFET resisstances (from a 4049 cmos gate) - this also tracks with temperature.  I seemed to remember this design was fairly stable.

Quote
It is sad to see there are no good candidates


That's where two bias pots would be useful.
Title: Greatly Improved JFET Matcher is here!
Post by: puretube on February 05, 2005, 11:50:35 AM
take care to test LDRs under constant surrounding (leaking) light circumstances;
take time - they`re slow (esp. from light to dark, esp. at high R);
don`t forget: LEDs vary too, and are little thermometers...
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 07, 2005, 12:51:53 PM
Well, I used an LM78L05 instead of the 4.7V zener, and I powered the LFO IC from this stable 5 volts.  I also connected the JFET sources to these 5V, so everything should be as stable as the LM78L05 allows.

With the original LFO, the sweep seems concentrated on one of the sides, especially at high rates between 2 to 5 Hz, which I dislike.  Is this normal for the original Phase 45?

I made a more complex hypertriangular LFO and I got a more even sweep at high rates, but still too pulsating on one side to my taste.

Any comments/ideas?
Title: Greatly Improved JFET Matcher is here!
Post by: Vsat on February 07, 2005, 05:44:40 PM
Hi stm,
The FET channel resistance changes most quickly near pinchoff - which also happens to be where the resistance is largest (and corresponds to the sweep "bottom") - so the sweep seems to "decelerate" as the phaser sweeps upward. This may be perceived as a problem -lopsided sweep-  but it is also a part of the "FET phaser sound". If you use linearization techniques the sweep can be warped into something different, and possibly better sounding. But at that point it would be simpler to use OTA's.
Cheers, Mike
Title: Greatly Improved JFET Matcher is here!
Post by: Rob Strand on February 08, 2005, 03:07:54 AM
Quote
Any comments/ideas?


It depends what the actual problem is.  There's two possibilities.  The first is the on and off times for the LFO are not the same.  That can be caused by the LFO opamp not swinging symmetrically around the "centre" point - whis is more probable at low supply voltages.  You can see this by looking at the "square LFO output point on a CRO.  The second possibility is the one Mike mentioned:   tinkering with the hypertriangle shape  is the solution and that could take some experimenting before it sounds right to you.
Title: Greatly Improved JFET Matcher is here!
Post by: d@vide on February 08, 2005, 07:21:38 AM
...mmm
hey stm
how the new improved tester  will be ?
only a res, battery and dvm how above told or will post a new schem :-)
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 08, 2005, 07:50:03 AM
Mike, Rob, thank you for the response.

My LFO is symmetrical and perfectly triangular.  I am using a rail-to-rail opamp fed from the 5V, and the output swing is set to go exactly from 1 to 4 volts, and the reference is 2.5V.  I am using the typical two op-amp triangle generator circuit (one integrator and one comparator with hysteresis).  Then, I am using two additional opamps as precision diodes to introduce gradual clipping on the lower part of the triangle.

This is the schematic:

(http://tinypic.com/1okkrr)

And these are the curves:

(http://tinypic.com/1okksj)

Remember the JFET source are connected to the +5V reference supply, so the hypertriangular output is effectively negative with respect to the JFETs.  Finally, some buffering and offset control is added (not shown in the schematic) to adjust for the exact Vp of the JFETs in use.

Anyway the sweep is much more even than a pure triangular (or the nearly triangular wave you obtain using the single op-amp LFO in the original Phase 45), but still not "right" at high speed settings.

By the way, I adjusted the knee voltages and the limiting resistors according to the theoretical voltage that would produce an exponential resistance variation (i.e. linear when seen on a log resistance graph), and also took into account the effect of the two 10K (i.e. 20k) resistance in parallel with the JFETs.

Mhhh... Maybe I'm too perfectionist and this is just what it is.

Regards,

STM
Title: Greatly Improved JFET Matcher is here!
Post by: Paul Perry (Frostwave) on February 08, 2005, 08:57:41 AM
My 2 cents:

(1st cent) this fet matching is a great argument for using PWM switched resistors instead.

(2nd cent) the easy way to make a hypertriangle, is to run the triangle thru an overdriven 3080 to make a sine, then full-wave rectify the sine.

I've never built anything with a matched or selected fet, and I hope I never do :wink:
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 08, 2005, 09:30:22 AM
Paul, yes, I forgot the full-wave rectified sinusoid.  Anyway making a sinusoidal LFO is not so easy anyway.

Regarding the analog switches, I built a phaser many years ago with them, but the switching introduced some sort of white noise due to PWM leakage in the audio path (in fact, control signal leakage is in the order of 100 mV according to datasheet). I don't know if my circuit was poorly designed or if this is normal in phasers with analog switches.  Also, making the PWM generator has its own complexities as do other methods, so for a Phase 45 I would try to stick with matched JFETs.  But for Phase 90 and higher number of FETs I agree matching is quite a hassle and PWM analog switches would certainly rule.

Mike, any comment on noise when using analog switches with PWM?

Regards,

STM
Title: Greatly Improved JFET Matcher is here!
Post by: Paul Perry (Frostwave) on February 08, 2005, 09:56:28 AM
I must say, I havn't had much luck with PWM myself, but that was probably due to noise getting into the sawtooth going to the comparator & causing jitter. PWM requires care in construction. MXR seem to have got it to work OK, though. ANd nowadays most low end audio is PWM, believe it or not! (yeah, all those SMT audio amps in phones & computer gear & cheapass boom boxes).
Title: Greatly Improved JFET Matcher is here!
Post by: Vsat on February 08, 2005, 12:35:15 PM
stm,
For some reason (probably javascript ) aren't able to load any of the schematics/graphs you posted for this thread  :(  so comments will be general.

I assume you have built it so that the triangle LFO waveshape and the transfer function of the "Vgs-waveshaper" are independent of LFO rate - ie. no filtering involved that would change the overall modulation shape  as the rate is varied. In that case, you are probably hearing the normal behavior of a FET phaser. My guess is that the "hypertriangle" "make the  sweep accelerate as it approaches the top" needs some more work.

An alternate approach to "hypertriangle" is to make a voltage-controlled triangle LFO and feed some of the output back to the input... you could try linear but a V/oct rate response would probably give better results. If you have well-matched FET's you could try linearizing the Vgs vs. resistance curve by servoing one FET with an op amp and applying the processed Vgs to the other gates. (BTW what is the proper definition of hypertriangle?)
Also the Eventide Instant Flanger has a CD4066-based plug-in board to convert it into an "Instant Phaser".
Regards, Mike
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 08, 2005, 01:25:55 PM
Mike, the links to the images are the following. They should work if you copy-paste them into the address bar of the browser.

http://tinypic.com/1okkrr

http://tinypic.com/1okksj

Yes, there are no frequency dependent elements in the waveshaping circuitry.

So far I think the behaviour I am witnessing is "normal" for phasers (I never really liked phasers, but the Phase 45 samples I've heard are quite sweet, and I really want that "shine on your crazy diamond" sound).  In fact, slow sweeps are definitely more even now than with the original LFO. High sweep rates (those suitable for vibrato) are still better that with the original LFO also, but not "good enough".

So what I'll try next is place a switch for reducing the amplitude of the triangular at high sweep rates before it goes into the shaping section, just as you would do on a Chorus with the Depth control, and see if this helps.

Will keep you informed.

P.D. as for a definition of the hypertriangular, I have not seen a mathematical one yet.  I think the best conceptual representation is the full-wave rectified sine, however you can also have parabolas and exponential functions as well to define the actual curve.
Title: Greatly Improved JFET Matcher is here!
Post by: puretube on February 08, 2005, 01:33:00 PM
Quote from: stm
P.D. as for a definition of the hypertriangular, I have not seen a mathematical one yet.  I think the best conceptual representation is the full-wave rectified sine, however you can also have parabolas and exponential functions as well to define the actual curve.


...maybe I`ll find tomorrow - but it is called "parabolic" or "hyperbolic" sometimes...
Title: Greatly Improved JFET Matcher is here!
Post by: Vsat on February 08, 2005, 05:04:07 PM
stm,
I'll see if the download works later on. Something else that might be the cause of the asymmetry - remember that the modulated allpass stages also provide phase modulation (in the true sense)... this will only be noticeable at the faster LFO rates... so you will get a vibrato component with the faster rates (that is not normally noticed at slower rates). Plus, the "shape" of the vibrato is a function of the LFO waveshape, the Vgs vs resistance curve of the FET, and the waveshaping you are using. Like a BBD, the vibrato is produced only when the phase delay is changing - so a derivative will be needed to determine the actual vibrato "shape" (sinusoidal or not...)
Regards, Mike
Title: Greatly Improved JFET Matcher is here!
Post by: Rob Strand on February 09, 2005, 03:11:45 AM
I think you might be better off getting rid of the opamp based ideal diodes by using just diodes and changing the reference points to suit.  In this case there may be a problem with the finite reference impedance applying this directly, so you may have to tweak your resistors.  An alternative is to use the opamps to buffer the reference.  The transistions will be smoother and it's highly like this will sound better.
Title: Greatly Improved JFET Matcher is here!
Post by: stm on February 09, 2005, 03:41:25 PM
Hi, I used one 1N4148 diode to clip the half bottom of the triangular waveform. Not very impressive.  Then I changed the diode for a 1N60 germanium device, readjusted the biasing, and wow! Now I do like the sound within all the speed range (several seconds to fast vibrato). I would describe the overall sound as "organic".  Thanks for the idea Rob. At this point I was able to play for over half an hour without even thinking that it might need some tweaking.

I tried also the "univibing" mod, switching the two 47n caps to 150n and 15n (i.e. 47n*3=150n and 47n/3=15n, which keeps the overal notch frequency identical to the original).  I would describe the sound as being "softer", but don't know for sure it is worth to add a switch, since the difference is rather subtle.

What I am really keeping is a pot that connects the dry and the phased output, which allows gradual transition from DRY, to MILD PHASER, to STOCK PHASER, to PHASER+VIBRATO, to PURE VIBRATO. This is certainly a keeper.

Regards,

STM
Title: Greatly Improved JFET Matcher is here!
Post by: puretube on March 28, 2005, 02:19:30 AM
FYI:
an interesting component tester article in latest "Elektor" mag:
http://www.elektor-electronics.co.uk/Default.aspx?tabid=27&year=2005&month=4&art=52735
for testing all kind of discretes.
looks complicated/expensive, though...