Does the JFET Doubler really work?

[daeg]

RG, do you have one boxed and working?

[R.G.]

I don't know. I did, long ago. I've moved house twice since then. You know the rule of thumb for moving, right? Three moves equals one major fire.   

[Eb7+9]

The descriptions are pretty accurate in the page.It's not a full wave rectifier, in spite of the visual similarity to the rectifying section of something like a Superfuzz.

hilarious ... replace the devices in a bipolar full-wave rectifier by jFET's (or any other type of transconductance device, incl. tubes) and suddenly it's no longer a full-wave rectifier

well, I guess that's true ... but only because now the circuit just won't work with anything less than precision matched FETs ...

case in point, the circuit goes for a dump with just 50mV offset between devices
(here I'm using the common 2n5457 which he endorses as a usable device for his invention)



add the low-gain transfer slope in the vicinity of the zero-crossing and you got yourself a circuit exhibiting poor S/N (ie., puts out lots of noise, as he states in fact) ...

---

another place where he did the same thing is with Bob Moog's Bipolar transistor matcher // but with the temperature referencing missing ... again, an original jFET replacement idea that somehow the engineering community missed

https://dragonflyalley.com/synth/images/TransistorMatching/MOOGmatch-01-pg9-3B.jpg

I've mentioned this several times already ...

go to my list of 2n5457 data and look at the following two data points:

Vgs(off) = -1.51v and Idss = 1.96mA
versus
Vgs(off) = -1.31v and Idss = 2.64mA

https://viva-analog.com/characterizing-and-matching-2n5457-jfet-transistors/

doesn't take an IQ expert to realize these will cross ... and if they happen to cross in the vicinity of the 400uA that the "improved jFET matcher" forces into the FET then guess what ... typical meter resolution will return a positive match // which is false ... that is, unless you consider 200mV Vgs(off) difference as some form of matching

if you want to try to make this thing work you'll need something like LSK489 matched pairs with |Vgs1-Vgs2|<20mV

http://www.linearsystems.com/lsdata/datasheets/LSK489_LOW_NOISE,_LOW_CAPACITANCE_MONOLITHIC_DUAL_N-CHANNEL_JFET.pdf

may luck be on your side

[R.G.]

Give it a rest, J.C. - it's been about 20 years since "A Rainbow of Sound" (I still have my copy, I just checked) and you just won't let go. You try to call me out about everything that hits key words involving me and JFETs, and anything else that looks easy.

I used to get irritated, but now it's just tiresome. You can try to "prove" that things work or not. I don't care. It works, it's not a full wave rectifier, it is set up to cancel fundamental, it's finicky to get it set up correctly, as stated. I can generate pictures and equations to support or refute most topics, as you seem to think is "proof".

Give it a rest, already. Or we can go at it hammer and tongs again. That's so much effort. Why?

[daeg]

Didn't realize you guys had a feud. As a long-time lurker I've appreciated both of your work over the years.

JC, I built your Super Full Wave Octaver years ago. It didn't work as well as I had hoped, but it was cool to hear how FWR sounded without gating. I understand it was more of a blog article about an experiment than an effect. I still have a 4-pin module of the SFWO I used for breadboard fun way back.

RG, I appreciate the novel approach but the doubler circuits seem like more of proof-of-concepts than something to build and stick on a pedalboard. The matching would be less of an issue than the noise.

Does it not amaze everyone that after almost 50 years, an army of highly intelligent hobbyists has not managed to improve on the simple Green Ringer? Myself, I run a search for 'Analog Octave' here every couple of years thinking that someone must have cracked the code.

[11-90-an]

Does it not amaze everyone that after almost 50 years, an army of highly intelligent hobbyists has not managed to improve on the simple Green Ringer? Myself, I run a search for 'Analog Octave' here every couple of years thinking that someone must have cracked the code.

What about it needs improving..?  everyone uses it with a fuzz/dist/overdrive, so it should still be fine...

[daeg]

It gates, it inter-modulates, it gives sine waves an ugly face-lift.

Most of us here have a soft spot for the FWR effect but there is reason you only ever hear it on Purple Haze covers or in Stoner Rock / Jam bands.

[11-90-an]

try looking at the theremin fuzz by parasit studio.... very interesting... 

[R.G.]

Sorry about the rant. As I mentioned, the origins are about 20 years ago. J.C. has never gotten over me calling him out on some early intellectual misappropriation, I think. I consider it closed. But J.C. periodically sees something here concerning JFETs and me, and feels called on to correct me, tell me I'm wrong and stupidly so, as he did again this time. Generally with equations showing what he thinks of as his deep insight into FETs and some graphs, as he did again this time.

It's tiresome, and not productive for the group, but when I've ignored it in the past, he rants on. I ignore his commentary until he gets personal. He can do some things well. If he'd stick with that and quit trying to show the world how very badly I'm wrong, wrong, wrong, his actual useful work could be more noticeable.

[11-90-an]

...concerning JFETs and me...

BJT Doubler? 

Already done before, i think...

[Marcos - Munky]

Does it not amaze everyone that after almost 50 years, an army of highly intelligent hobbyists has not managed to improve on the simple Green Ringer?

Well...
https://www.diystompboxes.com/smfforum/index.php?topic=32326.0

[daeg]

Well...
https://www.diystompboxes.com/smfforum/index.php?topic=32326.0

I've built it and yes it's quite good.

What I really mean is that in 50 years we haven't found another (analog) way to produce a prominent 2nd harmonic besides the Phase-Splitter / FWR approach. RG's approach is clever, novel and the exactly the kind of solution I want to see work.

The reason I asked for others experiences building it is because life is demanding at the moment and I can't afford late nights or weekends tinkering on breadboard anymore.

[11-90-an]

The reason I asked for others experiences building it is because life is demanding at the moment and I can't afford late nights or weekends tinkering on breadboard anymore.

Same... 

[R.G.]

Let me add another yea vote for being time restricted. I'm more tightly scheduled now than I ever was while nominally working full time. It's crazy!

[GFR]

Ok, found this amazing software that allows you to run spice simulations in real time using actual audio as input and actually hearing the results.
https://www.livespice.org/

And decided to try it with the JFet Doubler
http://www.geofex.com/article_folders/jfetdoub/jfetdoub.htm

As you can see, not FWR, the pair of FETS is biased in class A with some distortion. Then the fundamental is cancelled out while keeping the octave, that needs a reasonable amount of gain to be usable. You can see the output waveform is very sine looking

The amount of distortion depends on the input signal, so the octave decays quickly as the note dies. You can hear how it sounds, a very pronounced octave and perhaps "cleaner" than the usual FWR.

Obviously, in the simulation everything is perfectly matched, easier than real life

[Eb7+9]

JC, I built your Super Full Wave Octaver years ago. It didn't work as well as I had hoped, but it was cool to hear how FWR sounded without gating. I understand it was more of a blog article about an experiment than an effect. I still have a 4-pin module of the SFWO I used for breadboard fun way back.

um, you probably made a silly mistake // it's a standard time-tested circuit ... indeed, the "point" of the Super Full-Wave octaver was to show how much upper harmonics get produced when you've got a FW DC-transfer that goes down to zero (ie., with hardly any dead-band at all) ... an academic exercise, as you correctly point out

if you understand how the shape of a continuous-time waveform translates into harmonics - via Fourier - specifically speaking, if there's a (rapidly changing, ie., non-smooth) corner somewhere in the periodic cycle ... you get lots of upper harmonics

take a square wave for example, we know that an infinite sum of odd-harmonics will give you that square wave "in the limit", which means as you approach adding an infinite number of terms ... only add the first 20 or 40 terms and you still don't get very close to creating a corner ... so, it's the infinitude of higher terms that give you those corners ... ipso-facto, a corner can be seen as a mathematically equivalent sum of higher harmonics (said in simplistic terms) ... in an "ideal" FW rectifier circuit this fast change of direction (corner) would correspond to the sharp spike created where the two signal half-phases transition into each other ...

the whole point of using FETs (j's or MOS) is you get rid of that spike (or, rather, not create it in the first lace) due to the soft-cutoff of the devices ... this then does away with all those upper harmonics - ending up with a "cleaner" up-Octave response without having to use any post-FW filtering ...

I'm not saying the later FET approach is necessarily better, just something worth experimenting with as note of reference // probably lying at the opposite end of the fizz spectrum to the Super FW circuit ... in fact, I think experimenting with the jFET DOUBLER (pref. the corrected version) is a must for anybody who's checked out all the other approaches ... btw, I now have two new alternate versions that use jFET's in lieu of straight diodes - with prob. several more waiting to be uncovered ...

as I said before, I'm only talking in terms of producing a "clean" up-Octave - as clean as it can be in the context of instrument (non-pure sine) signals ... I understand that some of the really great up-Octave pedals combine clipping as part of the effect ... in all this I'm just trying to get a more complete understanding of the octaver part on its own ...

yO!