A few days, a few Jfets

[petemoore]

  Buy a Fetzer.
 So..all the calculations seem perfectly reasonable, and the 3rd Jfet, the one I held most hope for, a 2n5457 with Vp measured at 1.14..
 I punched in the numbers I calculated from the test rig for Vp/Idss measuring and started off with a 1k, more than twice the calculated Rs value.
 3k3 [calculator at ROG suggested 3.1k] on the drain, gate is 0.0v, the source insists on being .5v with 1k Rs, or .62v with 2k Rs, any bigger and the gain is nilch.
 Screwey Jfets !
  The MPF102 was gatey with BMP driving it [actually pretty cool], the NTE458 seems to sound pretty good, I wanted to get a stable bias on the 2n5457 so I put a 1meg to ground on the gate, a 1k on the source to Gnd. [then I added another to = 2k Rs], and put the suggested 3k3 on there to V+...

[stm]

 I punched in the numbers I calculated from the test rig for Vp/Idss measuring and started off with a 1k, more than twice the calculated Rs value.

Why didn't you stick to a 470 ohm resistor?

 3k3 [calculator at ROG suggested 3.1k] on the drain, gate is 0.0v, the source insists on being .5v with 1k Rs, or .62v with 2k Rs, any bigger and the gain is nilch.

The calculator gives you an Rd value that matches Rs.  If you altered Rs then Rd as given by the calculator is no longer valid.

Considering your JFET parameters (VP=1.14V and IDSS=1.95mA) you will get a voltage gain of 3x at the drain if you follow the recommended values.  If you prefer to go with a fixed RD resistor, choose the next lower value (i.e. 3k0 for your device).  Please correct me if I'm wrong Pete, but it seems it didn't work as you expected/required in terms of gain, but you didn't follow the recommended values either.

The online calculator is a tool that provides values for proper biasing for a PARTICULAR case.  It is not a general biasing tool, so you can't make changes in one of the component values without affecting the end result.

[petemoore]

  It took a few minutes to figure out an answer that makes sense to me.
  I used this time to parallel instead of series the 2x 1k source resistors.
  Why didn't you stick to a 470 ohm resistor?
  Here's what was on my mind, whether it's a viable response is of course not yet completely understood...
  I forsee a future of Jfet understanding that doesn't cause the logic of the texts [that source above gate by Vp is desirable] to clash with the DMM readings.
  AIUit, the source resistor is what causes the source voltage to be 'elevated', larger source resistor elevates it.
  ...elsewhere reads...I read that conservative...go slightly larger value Rs, so the Jfet isn't pinched off...are elsewhere reads.
  Back to the DMM to see what the readings are now that the values are as suggested on calculator.
  I started with 1k because I can then parallel to x value, and went to 2k as an attempt to get the source voltage above the gate by at least the Vp value.

[petemoore]

  Rs = 491R
 Rd = 3k3
 8.6v battery:
 S = .43v
 G = 0.0
 D = 5.76
 The S = .43 volts looks low considering 1.4v Vp ?
 I'm finding it a little easier to disregard my misunderstands of how a Jfet works since the boost and sound now, is sweet.
 I expected it wouldn't be [which now seems to be based on preconceptions based on misunderstandings] sweet, would be pinched-off sounding. Instead it sounds like a perfectly biased booster !
I'm finding it a little easier to disregard my misunderstands..still very difficult...

[stm]

If I understand correctly, what bothers you is that the source voltage is not exactly at VP when biased.  Well, it's not supposed to.  If the source voltage were VP volts above the gate then the JFET would be already in a full-off condition instead of being in the active region.

In practice, you want to aim for a "middle" operating point where you can support both positive and negative audio excursions at the input (gate).  A natural and intuitive starting point would be to adjust RS to establish the source voltage at VP/2.  In case of the Fetzer Valve, the source voltage is set lower, around 36% of VP only.  Why?  Because this is what you get when you use RS = 0.83*VP/IDSS.  Why this value for RS?  Because this value for RS produces a 3/2 square-law AROUND the operating point, as indicated in Dimitri Danyuk's paper referenced in the Fetzer Valve page.  Then, RD is calculated to have maximum symmetrical output voltage at the drain, which conveniently occurs when the gate of the JFET varies between -VP and +VP.  Don't forget that this analysis is applicable only to the particular case of the Fetzer valve.

[petemoore]

If I understand correctly, what bothers you is that the source voltage is not exactly at VP when biased.  Well, it's not supposed to.  If the source voltage were VP volts above the gate then the JFET would be already in a full-off condition instead of being in the active region.
  I'm pretty sure you do...and my understanding just clicks with the inevitable logic it seems.
  In practice, you want to aim for a "middle" operating point where you can support both positive and negative audio excursions at the input (gate).  A natural and intuitive starting point would be to adjust RS to establish the source voltage at VP/2.
  I noticed this works, started re-re-reading the Fetzer article.,,I'm sure it's all in there, I'm the type that'll run into every brick wall at least a few times before ''the path'' becomes clearer. Sorry to have to sweat out answers like this...I'm working on the math, it's getting better.
  In practice, you want to aim for a "middle" operating point where you can support both positive and negative audio excursions at the input (gate).  A natural and intuitive starting point would be to adjust RS to establish the source voltage at VP/2.
  I had to build the rest of my understand of course also, but this is what caused the 'click' that rang the bell when it sounded out how the logic works...duhh...of course the pinch-off point is going to sound pinched off...
  @@rate bonehead thinks he's got it now < !
 

[stm]

I'm glad to see it is starting to make sense for you.  Personally I hate to do things without understanding the underlying principles.

[stm]

By the way, an interesting corollary can be derived from the following formula in the Fetzer Valve article:

Av = 0.54 * ( Vcc/|Vp| - 2)

Gain is higher for higher supply voltages, and inverse to the Vp of the JFET.  In particular, if you set Av=1 and isolate Vp, you will get that Vp=2.34 will produce a voltage gain of unity at the drain.  This means that JFETs with Vp higher than 2.34V will produce less than unity gain, and in particular, MPF102 JFETs are not suitable for a 9V supply if you expect them to produce voltage gain when configured as per the Fetzer Valve principles.  You can still have greater than unity gain with an MPF102 and a 9V supply, but this is outside the scope of the Fetzer Valve.

Another thing to notice is that since the input dynamic range is given by:

Vi = +/- |Vp|

this means that voltages outside this range will produce clipping.  This is not necessarily bad, but you must take this into account with respect to your goals.  If you want a clean booster, you shouldn't choose a J201 to be used with a guitar equipped with hot humbuckers.  This is because a J201 will have Vp around 0.75V, and hot humbuckers can easily produce 3V peaks, thus you will get noticeable clipping.  On the other hand, if you want as much gain as possible, then go with the JFET with a small Vp.  In this sense, it is possible to provide a basic guideline if you want a Fetzer Valve just for coloring you sound without much overdrive:

a) Single coils: use J201 (Vp around 0.75V) and 9V supply
b) Normal humbuckers: use 2N5457 (Vp around 1.5V) and 9V supply
c) Active pickups or hot humbuckers: use MPF102 (Vp around 2.5V) and 18V supply

In general, a 9V supply is not the best you can get for JFET preamplifiers, just as a 12AX7 will not sound as good when starved as when it is fed from a 300V supply.

[petemoore]

  I'm not the first to hit brick walls with Jfets @9v, not the last I'm sure.
  Sometimes I just have to be hardheaded and ''learn on my terms''...get a real clear understanding of where the hurdles I keep tripping over are, how high they are. Learning the hard way has it's advantages [it seems that persistance is the only way for me...just keep hitting it...study...try some other approach].
  @@Rate, the studies of Fetzer have paid off, it was working perfectly and I didn't even know it [following the instructions worked].

[Eb7+9]

finally ...

looking at the parameter pair that matters to a jFET circuit, maybe the phasor guys will jump on board with these numbers ...
knowing these might also help make circuits like Keen's jFET octaver stage run more optimally, or at least help cut to the chase during assembly - the octave effect the is maximized when the circuit is balanced

In practice, you want to aim for a "middle" operating point where you can support both positive and negative audio excursions at the input (gate).  A natural and intuitive starting point would be to adjust RS to establish the source voltage at VP/2.  In case of the Fetzer Valve, the source voltage is set lower, around 36% of VP only.  Why?  Because this is what you get when you use RS = 0.83*VP/IDSS.  Why this value for RS?  Because this value for RS produces a 3/2 square-law AROUND the operating point, as indicated in Dimitri Danyuk's paper referenced in the Fetzer Valve page.  Then, RD is calculated to have maximum symmetrical output voltage at the drain, which conveniently occurs when the gate of the JFET varies between -VP and +VP.  Don't forget that this analysis is applicable only to the particular case of the Fetzer valve.

btw, you mean a "3/2 law" (ie., 1.5 power) and not a "3/2 square" law ... jFETs without source degeneration commit to a roughly "square" law (ie., 2.0 power) ... also, just to be clear, triode circuits which operate with "un-bypassed" cathode degeneration are not 3/2 law (an exception to this are the early grid-biased circuits) ... the "capacitor-bypassed" case is a little more complicated, but does exhibit 3/2 law behavior ...

sometimes I wonder if we're just getting hung up on this 3/2 power target thing ... I think it's likely Danyuk followed this route also/mainly to prove the validity of his method - IMO that's not so much the point as far a producing "considerable" amounts of coloration - so why stop at 1.5 when we can have 2.0 ?! ... to me the goal is to enrich sound in a similar manner, not so much "emulating" an entire set of characteristics - that's where I agree with the nay-sayers ...

there's another important dynamic aspect missing that I won't go into here, again due to the partial emulation of characteristics

it seems we principally want curvature in the transfer - that's what makes things interesting to the ear in terms of generating output harmonics, at least in a sound production setting ... and also, conversely, why jFET circuits operating at considerably higher supply volts (eg., Roland Bolt preamp) sound bland in comparison ... may as well go op-amp based at that point (linear transfer = absence of dynamic borne harmonics)

so moral of the story: you're working with low supply voltages, you need low Vp devices ...
we all know this from experimenting

now ... I'm not trying be negative or anything,
but here's an interesting aspect of the game which applies directly to the Fetzer approach

as you did find, though unlike Danyuk's approach setting the transfer curvature to yield a 3/2 power (imitating a pure "undegenerated" triode transfer) with a trans-impedance amplifier, your source-degeneration-only approach leaves your idling point lying close to the gate turn-on limit (Spice models don't typically show the change of curvature near that point) ... drop the resistance to get more curvature, the wall moves in closer to the y-axis, increase it move the wall away and you loose curvature ... you mention this I think

this is ok if you want easy clipping as part of an overdrive package - I'm not saying it's bad or anything of course
just pointing out a fact ...
but from my observations it's in this "other" area where jFET/ triode circuit behavior differs greatly
call it what you want, it's sharp diode loading to me (as opposed to the soft "input" diode load curve seen in tubes)

next, question - as you point out below - is why these circuits will sound "clearer" at higher supply voltages
same with triodes, parasitic capacitance is voltage dependent - ie., they get worse (bigger) at lower terminal voltage differences

lastly, you may want to look at how choosing such low source degeneration resistance impacts bias current levels, and therefore sets overall circuit noise ... this is the launching point for the direct-source-voltage biasing a method I devised for maintaining full transfer curvature (ie., max harmonics) while idling at lower current ... something you can't do with a source resistor - there's a compromise you can't get out of there as stated above ...

of course, now it's not a hard clipping circuit anymore but you can operate the principle with less noise ... in this direct-voltage approach you get two birds with one stone, curvature is maintained independently of where you set the idling point inside the transfer range

http://isp.lynx.net/~jc/transferCurvature-TubeSimulation.html

[stm]

JC, you mention several interesting aspects that I'd like to further discuss for the sake of further informing people who might be interested, and also because something interesting might surface from the interchange of ideas.

I'll be adding comments on your previous post as time permits.

btw, you mean a "3/2 law" (ie., 1.5 power) and not a "3/2 square" law ... jFETs without source degeneration commit to a roughly "square" law (ie., 2.0 power)

Yes, I meant "3/2 power law" as it is written in section 4 of the Fetzer Valve page, instead of "3/2 square law" (it was a mental typo)

[stm]

...also, just to be clear, triode circuits which operate with "un-bypassed" cathode degeneration are not 3/2 law (an exception to this are the early grid-biased circuits) ... the "capacitor-bypassed" case is a little more complicated, but does exhibit 3/2 law behavior ...

Agreed in both statements.  I assume the complication you mean comes from the fact that the 3/2 exponent
includes both the gate-to-cathode (Vgk) and the plate-to-cathode voltage (Vpk), the latter being also
dependent on (Vgk), as given by Child-Langmuir equation for the plate current:

Ip = K*( u*Vgk + Vpk  ) ^ 3/2    (valid for positive values of the term inside parentheses)

I am aware that this equation is just an approximation for the actual tube behaviour, but I understand it is quite valid if you stay in the central portion of the operating region (let's say a triode with a large cathode bypass capacitor whose Vpk ranges between 150V and 250V in a 350V supply system).

[stm]

sometimes I wonder if we're just getting hung up on this 3/2 power target thing ... I think it's likely Danyuk followed this route also/mainly to prove the validity of his method - IMO that's not so much the point as far a producing "considerable" amounts of coloration - so why stop at 1.5 when we can have 2.0 ?! ... to me the goal is to enrich sound in a similar manner, not so much "emulating" an entire set of characteristics - that's where I agree with the nay-sayers ...

Hung up on this 3/2 power thing?  Absolutely!
Practical experience has convinced me that, as you mention above, adding some coloration can enhance the sound, regardless of the exponent.  There are some further aspects I'd like to expand here:

1) Is the 3/2 exponent is THE BEST exponent you can have for coloring a guitar sound?  I don't think so.  Is the square the best exponent? I don't think so either.  If we had a "continuously variable exponent circuit" and let lots of people tune it to their preference, I'll dare to guess that you would have values ranging from 1 (no coloration for the purists) to perhaps 3 (which in controlled amounts can produce a nice signal compression like tape saturation).  So, 3/2 and square are just in-between values of a wider range of valid and useful possibilities.

2) To my ears, the Fetzer Valve adjusted as per Danyuk's criteria sounds great on a clean guitar, especially when the guitar peaks are close to +/- VP, i.e. the JFET stage is used in its full dynamic range with little or no clipping. This is of course my personal opinion on the subject, but many will agree.  To further expand on a little noticed fact: the Fetzer Valve exhibits the 3/2 exponent only AROUND the operating point.  As the gate voltage goes positive the exponent is reduced and gets closer to unity, and as the gate voltage goes negative the exponent increases and gets closer to 2.  Anyway, the harmonic content you get from a sinusoid is akin to a pure 3/2 exponent, so "on average" the Fetzer Valve behaves in a similar way to a pure 3/2 exponent device.  (Clipping characteristics are of course different to a valve, but we are not analyzing them yet.)

3) I've built a SLFV (square-law Fetzer Valve), which is a variation of the Fetzer Valve arranged for a pure cuadratic exponent by using a large source bypass capacitor and choosing resistors as:

Rs = 2*VP/IDSS  (VP is positive, Rs should be bypassed by a large capacitor)
Rd = (Vcc-2*VP)/IDSS  (VP is positive)

This biases the JFET at a drain voltage where the input can swing between -VP/2 and +VP/2 without clipping, so it has about twice the gain of a conventional fetzer valve or THFV (three-halves Fetzer Valve).

I performed repeated listening tests on both circuits.  The input to the SLFV was attenuated in half so as to maintain a comparable clipping and output level in both circuits.  I have to admit that despite of my preconceived preference towards pure 2nd harmonic as the SLFV does, in the end the THFV was the  winner according to my personal taste. In spite of the similar volume and matched frequency response, the THFV sounded more open, warmer and more natural. On the other hand, the SLFV sounded a bit harsh, perhaps a little fatiguing, and somewhat darker. The difference in coloration was clear and evident.  Anyway I'm certain both circuits have the potential to be someone's favourite.  I admit that in order to make a comparison under similar conditions I had both the SLFV and the THFV set for their full dynamic range usage, which produces different amounts of THD in each circuit.  Next time I do this I'll try to have similar THD levels as the reference point for comparison.  Bottom line: this is an are open for experimentation for sure.

4) In favor of Danuyk's work, I can add that even though his circuit tried to produce a 3/2 power which is a limit condition and probably unlikely to be found in a hi-fi audio amp, his circuit had a mixer knob that allows mixing clean and processed signals so as to have any amount of coloration in between, leaving the choice to the user.  The same concept can be applied in circuits like the THFV or SLFV or any non-linear transfer invention.

[petemoore]

  Wow...learning curve spike, thanks everyone for helping out, STM said something that is probably in the article, @@rate I went bingo right after reading it, understanding of Jfets...whew..once I got put back on track..isn't that bad.
  Dunno what to say, here goes.
  Thoughts of just putting an LM317 as source voltage adjuster were superceded by 'I need to figure This circuit out first'.
  The renewed interest now has me looking for opamp and ready to order Jfets...now that I know what I know which includes:
  I can't get the distortion I expected from tubes, they just don't distort like Jfets. I don't even know where I got such expectations since I've never played a tube amp that really does that anyway, must be preconcieved notions...talk about a brick wall and repeatedly running into it !
  I think I tried everything:
  Fender, marshall, vox, vintage HiFi, Si, Mosfet, amplifiers.
  Fender Marshall Vox Boogie, Vintage Hifi, Bipolar, Mosftet, Ge, Jfet, Opamp, tubes at various voltages, CMOS, 6ls7 and 12a-7 "preamps".
  I mean preamp in the broadest possible sense: everything from tube/solid state device[s between 9v and 30v, preamp tubes up to >300v.
  There's a certain smoothness involved with the Jfet and the CMOS which put them into a category IME. I found both devices a little harder to screw around with, Jfets as stated above and beyond, CMOS I just had circuit failures, probably not chip-failure related [just seemed that way at the time...me and others noted the chip seeme to stop].

[stm]

Pete, if you are experimenting with an opamp to set the source voltage, I recommend you use an LM324 (quad) or LM358 (dual) fed from 9V and GND.  Their outputs can go down to a few mV of GND, and input range also accepts GND.  You should also consider a capacitor to bypass the opamp output to GND (provided it doesn't generate instability), like a 220nF film capacitor or higher, just to make sure high frequencies actually see a low impedance path.

If you use an industry standard opamp like RC4558, TL072 or NE5532 you will need to feed the opamp from a +/- supply, which is less convenient.

P.D.  What does "@@rate" stand for?  Is it by chance "at any rate"?

[petemoore]

  Yupp...sometimes this stuff jumps off the page and makes sense right away.
  Other times a thousand things takes a long time to soak in, at any rate, I'm learning.
  The process of learning occasionally reaches a saturation point at which I either "get it or give up on it" until the next 'wind up'.
  It's cool when the 39'th time reading another explanation makes all the other 38 reads all suddenly become worthwhile.

[petemoore]

  DSG / MPF 102
  Now the Vp's are 1.2, but the raw Idss voltage is 3.99.
  Must be a pinout, but the measuring seemed simple and like it was working.
  Whakki Jfets up to here, these two MPF's read about the same, D gets 9v, Sgets the 1megleg, G gets the 100R/1meg pole, 100R and batteryblack/Gnd.
  A wire jumpers either the 1 meg or the 100ohm. Who knows...

[stm]

The values you mention don't make sense for an MPF102.  If you have verified the tester with other JFETs like J201 and 2N5457, then I can only say your pinout is not right.  If I remember correctly, the MPF102 comes in two different pinouts depending on the manufacturer, so I'd also try SGD to see if it works.

[petemoore]

  If I remember correctly, the MPF102 comes in two different pinouts depending on the manufacturer, so I'd also try SGD to see if it works.
 I tried DSG and DGS for kicks, RS shows a some unnecessary codes on their packages, when 1's 2's and 3's are cyphered out it boils down to DSG with MPF102 facing me, legs down.
 I appreciate the verification about the non-ballpark numbers for MPF102.
 The IDss particularly looked way high, but the Vp of 3.99 seemed within parameters, so I twisted up a quick Fetzer, got 2v on the source, ~1/2v on the Drain...at 18v supply sounds fine...close enough for that one for now.
 The other new 102 went in a Fetzer circuit and the gate insists on being 2.some volts [source is about the same] until I can get these values 'ballparked' I'll figure there's something wrong with the transistor..the only other things involved in bias are the resistors, all measured etc.
 Leg broke off the last MPF102 for todays messing around, the NTE458 has much much more robust leads.

[Eb7+9]

Agreed in both statements.  I assume the complication you mean comes from the fact that the 3/2 exponent
includes both the gate-to-cathode (Vgk) and the plate-to-cathode voltage (Vpk), the latter being also
dependent on (Vgk), as given by Child-Langmuir equation for the plate current:

Ip = K*( u*Vgk + Vpk  ) ^ 3/2    (valid for positive values of the term inside parentheses)

I am aware that this equation is just an approximation for the actual tube behavior, but I understand it is quite valid if you stay in the central portion of the operating region (let's say a triode with a large cathode bypass capacitor whose Vpk ranges between 150V and 250V in a 350V supply system).

you're right, that equation constitutes very coarse boundary-value style modeling and is almost totally useless for some work
it's a sexy looking equation fer sure, but unfortunately it's not wieldy at all - even for hand analysis

finite range / hand analysis is not an important goal in my thermionic work anyway because the small-signal parameters
that everyone has expertize in, things like "gain" and "bandwidth" ain't where it's at ... in reality they're quite secondary past basic requirements ... rather, it's the dynamic response (large-signal stuff) more than anything that paints a real picture to the musician

that's one reason why I decided it was ok to take a numerical-analysis/empirical modeling approach (instead of a more common algebraic one) when modeling Triode two-port characteristics - the other being a likeness to polynomial functions in the transfer curves ... more importantly, for accurate full dynamic-range distortion analysis we need full-range accuracy throughout - except maybe around the origin where the tube never ventures in most if not all working audio circuits

getting a way-more accurate view of things was the basis for doing my triode modeling work in the winter of '95 (btw, this makes up the first 80 pages of IFMTA - to be released on it's own shortly) ... I realized that modern engineering techniques could be applied to this near obsolete technology with more accuracy once a better modeling approach was found ...

turns out there would be many advantages to pulling this off aside from finally seeing exactly how two cascaded Triode voltage gain stages drive and load each other dynamically - also showing how "all that" gives rise to dynamic compression effects - again, the thing about tubes that talks to musicians ... now, that's something we can't do in the lab or on the bench without high-sensitivity/low-noise and hopefully very accurate "floating" current sensors

not too surprisingly, one of these opportunities would present itself as backdrop for establishing a basis of comparison between Triode circuit response and emulators - that's partly how I got the idea to use jFET's this particular way (compare the transfer curves in the paper linked to below and the image shown next) ... in particular these suggest a link between the transfer curves of an un-degenerated jFET, an un-bypassed common-Cathode Triode and the overall Dynamic Transfer Response of a Single-Ended tube amp (Champ) - all exhibiting un-canny similarity ... it was a long and slow ride for me until a few years ago when I read Danyuk's paper and figured out how to generalize his approach and at the same time do it with simpler class-A circuitry ...

what you say above about a voltage-gain limitation in the Fetzer is very interesting to me ... just as a title of comparison, one of the things I learn to do in school was work more with currents rather than move to voltage form right away using resistance - this is by no means unique, anybody schooled in VLSI design is accustomed to the idea ... I fit this in my discrete circuit amusements whenever I can, so a lot of my analysis (and design) approach tries to focus a bit more on that intermediary step ...

I demonstrate the view it affords in some of my published simulated transfer curves,
seen here for typical/RCA 12ax7 (Fender/Marshall/etc. 100k/1k5) voltage gain circuits :



notice how the 3/2 power thing seems quite vague when looking at a more realistic profile of un-bypassed Triode gain stage transfer curves ... if anything it suggests the modeling shouldn't be necessarily aiming at a fixed-curvature target ...but these plots do not give a complete picture of all Triode gain stage transfer behavior either - like I said above, Capacitive bypassing complicates things even more ...

one of the advantages of designing with currents is one doesn't run into voltage/gain restrictions as invariably as one does in the usual stacked-voltages approach ... that's the idea behind Danyuk's scheme as well incidentally ... I use a different trick (that I can't divulge) for yielding almost full rail-to-ground headroom when converting to voltage, and irrespective of gain - this way I'm basically making the most of the non-linear response or using only a portion of it ...

I also do the voltage conversion with the right signal polarity so that when I place that circuit directly in front of a tube amp a synergistic situation creates a very natural sounding compression effect by dynamically shutting off the front tube, and in a more controllable and potentially exaggerated way relative to the usual amounts seen on tube amps (a novelty) ... without trying to sound too immodest about it I'd say it's quite unlike a side-chain based compressor/limiter circuit - I built two versions, the other being a bias-shifter circuit, to compare the two ideas ... I refer to them as my "Mojo Booster" circuits ...

the main thing is that if I look at the output of my emulator circuit on a scope, I see pretty much the same thing that I see at the output of a cranked Champ, only scaled down (which needs be anyway) ... that's the main thing that counts for me ... the other being the fact that the output noise is very low - making it useful as a Boogie preamp like Toy Caldwell had - that's mainly where I'm coming from with all this... I've heard from one person so far that the idea was used in building a "tubey" sounding/feeling amp using their version of the idea and a SS power stage ... something I have yet to try myself

for an idea of what these magical "transfer shapes" look like this paper gives a good perspective:

-> DAFX02 Moeller Gromowski Zoelzer Non-Linear Circuit Measurement (pdf)

of course, I'm personally interested in the SE version of the non-linearity ... well, because from what I read in Jim Ferguson's GP many years ago about Toy Caldwell and Charlie Daniels being originators of sorts with the whole "Boogie" idea, where they used Champs to do it with ... I've done it with Champs and it's one of the best sounds ever (beats any transistor booster/OD pedal - no question), but it can be a chore to set up and invariably involves use of an isolation transformer in order to avoid hum and get a Pro response

not sure if this helps place the whole emulation thing into perspective or not

thanks for bearing with me - I knew it was a typo btw ... (and a good opportunity to get into it with you again)