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    Senior Member uneumann's Avatar
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    understanding fet noise curves

    I'm looking at noise curves for the 2SK209 FET. (see below) They show lowest noise with ~10K "signal source resistance". I'm trying to understand what exactly that term means - and then how to achieve the lowest noise in a common-source circuit driven by a low-z source.

    My best interpretation is that the device would contribute it's lowest noise if I use a 10K gate resistance (gate stopper). However, that 10k resistor adds noise of it's own. So, does the best circuit noise performance boil down to finding the lowest combination of gate stopper and device noise?

    Also, does anyone know of a low noise FET (n-ch) in a through hole package like a TO92?
    It seems the 2SK117 was common in the past, but is no longer produced.

    capture.jpg
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    Quote Originally Posted by uneumann View Post
    I'm looking at noise curves for the 2SK209 FET. (see below) They show lowest noise with ~10K "signal source resistance". I'm trying to understand what exactly that term means - and then how to achieve the lowest noise in a common-source circuit driven by a low-z source.

    My best interpretation is that the device would contribute it's lowest noise if I use a 10K gate resistance (gate stopper). However, that 10k resistor adds noise of it's own. So, does the best circuit noise performance boil down to finding the lowest combination of gate stopper and device noise?

    Also, does anyone know of a low noise FET (n-ch) in a through hole package like a TO92?
    It seems the 2SK117 was common in the past, but is no longer produced.

    Click image for larger version. 

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    Try LSK170 New Ultra Low Noise JFETs Target Audio, Instrumentation, Medical and Sensors


    jfet300_lsk170familyspicemodels.zip

    LSK170A_LSK170B_LSK170C_LSK170D_Low_Noise,_Low_Capacitance,_High_Input_Impedance,_N-Channel_JFET.pdf

    Last edited by nickb; 08-12-2017 at 08:37 PM.
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    Beat me to it, Nick. I would just add that for audio, FETs work best for SNR when the source is transformed to a high impedance. If you must use one at low source impedance, you must go for the lowest nano volts per root Hz, and for ready availability, that is the LSK170. A truly remarkable device.

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    I was expecting high noise in an piezo-equipped acoustic guitar FET buffer with 11M Ohm input impedance. No such problem and it's whisper quiet with a 2N5457. Maybe the noise is below the noise floor of my Laney acoustic amp. Noise is relative - what looks noisy on paper depends on the specific application.

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    Senior Member uneumann's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Beat me to it, Nick. I would just add that for audio, FETs work best for SNR when the source is transformed to a high impedance. If you must use one at low source impedance, you must go for the lowest nano volts per root Hz, and for ready availability, that is the LSK170. A truly remarkable device.
    Thanks Mike + Nick. It seems supply does follow demand. I'll look around for the LSK170.

    On the first issue though. I don't really understand why a high source impedance should lead to lower device noise. I guess I just don't understand the principles and mechanisms involved. Anyone venture a layman's explanation or intuition about why source impedance matters?

    Connecting back to guitar amps - would you advise a 10K gate resistor or something lower for a LSK170 amp input stage?
    I've been using 470 - 1K ohm gate resistors on J113s and that seems to work OK - but I'm trying for even lower circuit noise.
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    Quote Originally Posted by uneumann View Post
    I'm looking at noise curves for the 2SK209 FET. (see below) They show lowest noise with ~10K "signal source resistance". I'm trying to understand what exactly that term means - and then how to achieve the lowest noise in a common-source circuit driven by a low-z source.

    My best interpretation is that the device would contribute it's lowest noise if I use a 10K gate resistance (gate stopper). However, that 10k resistor adds noise of it's own. So, does the best circuit noise performance boil down to finding the lowest combination of gate stopper and device noise?

    Also, does anyone know of a low noise FET (n-ch) in a through hole package like a TO92?
    It seems the 2SK117 was common in the past, but is no longer produced.

    Click image for larger version. 

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    Of the three curves the 10Hz one is clearly the top, so in that area it is the 1/f noise or flicker that dominates the noise figure. It is sometimes useful to think of 1/f noise as a 'drift'.

    At low source resistance the JFET flicker will add a lot of noise relative to source resistance noise (thermal and uniform), thus you have higher noise figure.
    With source resistance going up - the JFET flicker gets relatively smaller noise source compared to source's inherent resistance white noise - making noise figure closer to 0dB.

    So it is not noise going down with source resistance, it is the noise figure go lower - remember the noise figure definition is a ratio.
    You still have the lowest total noise with low source resistance.

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    Quote Originally Posted by uneumann View Post

    On the first issue though. I don't really understand why a high source impedance should lead to lower device noise.
    Actually it does not. If you have a signal source with a certain impedance, you can, at least in principle, put a transformer between it and the amplifier and see which transformation ratio gives the best signal to noise ratio. Two things happen when you increase the transformation ratio: 1. the voltage into the amplifier increases, 2. the impedance looking back towards the signal source increases. If the total effective noise at the input of the amplifier due to the amplifier increases when you raise the source impedance, you might find that you should not make the transformation
    ratio too high. (The resistive part of the source impedance also contributes noise, so this can get complicated.) In general an amplifying device has two kinds of noise: noise voltage and noise current. As the source impedance increases, the noise current makes a bigger contribution to the total effective noise (E = IR), while the effect of the noise voltage remains the same. At audio, both tubes and FETs are dominated by noise voltage at typical impedances, while for bipolar junction transistors it is usually current.

    (OK, it is really a bit more complicated than that, but it has been along time since I have thought about these things!)

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    Senior Member uneumann's Avatar
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    Thanks to all of you ... this is new ground for me. I'm not sure I get it on first read, but you've given me some leads to follow.

    I'm going to breadboard some circuits for testing and hopefully I will see some cause/effect relationships that make sense of all of this.
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    Quote Originally Posted by uneumann View Post
    I don't really understand why a high source impedance should lead to lower device noise.
    We need to be careful exactly what we're talking about - noise from the source resistance itself, noise from the amplifying device, total noise, or noise figure.

    Noise from the source resistance itself will increase with higher source resistance. The familiar v = sqrt(4kBTR) formula.

    Noise from the amplifying device includes both a noise current component, and a noise voltage component. If you imagine the source feeding the device has zero source resistance, then that will actually short out the noise current from the amplifying device. You'll be left with the noise voltage from the source, and the noise voltage from the amplifying device (but no contribution from the noise current of the amplifying device). In such a situation, it would be good to have an amplifying device that has very little noise voltage of its own, but you can tolerate a bit of noise current, as it doesn't affect the total noise at the output much. BJTs usually do better in this sort of role.

    On the other hand, if the source has high resistance, then the noise current from your amplifying device, flowing through that high source resistance, generates lots of noise voltage (by Ohm's law). This noisy voltage caused by the flow of the device noise current, is likely to be much bigger than the actual noise voltage generated by the amplifying device itself. So when the source resistance is high, you want an amplifying device that has very little noise current of its own, but a bit of noise voltage may be fine. JFETs usually do well in this zone.

    Adding an amplifying device always adds some noise to the signal. Noise figure is, basically, how much more noise you have than before, in decibels. It's a ratio, and it tells you how much worse the amplifying device made things. This is very useful information to the circuit designer - arguably, more important than simply knowing how much noise voltage (or current) you have. If you have a good noise figure, your amplifying device has added a negligible amount of noise to what was already in the signal - which means, you can't really improve on what you already have.

    To elaborate, a noise figure of 0 dB would mean you had a magical, perfectly noiseless amplifying device (which, unfortunately, doesn't exist). A noise figure of 3 dB means your device doubled the total noise power at the input (twice the power, +3 dB). A noise figure of 10 dB would be problematic - it means most of the noise at the output comes from your amplifying device, and not from the source itself. (However, it is possible that, if the source was very quiet indeed, a noise figure of 10 dB will still be quiet enough to work just fine.)

    If you look at BJT noise curves, you will typically see that they have their lowest noise figures at fairly low source impedances - that's where they do the least amount of damage to the incoming signal, by adding relatively little noise to it.

    In the same way, if you look at JFET noise curves, you will typically see that they have their lowest noise figures at fairly high source impedances. That means they do the least amount of damage with a high impedance source. (That doesn't mean a high impedance source has less noise voltage - it means the fraction of the total noise added by the JFET is lower if the source impedance is high. In other words, the source is much noisier than the JFET, which is what you want. The JFET isn't worsening things much.)

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    As an addendum to this post, I have my doubts about the accuracy of the SPICE model. I suspect the drain to gate capacitance is wrong as I'm seeing a surprisingly early HF rolloff. For a common source amplifier with a gain of 24.7dB it was showing -3dB down at about 1.2KHz with a 100K source resistance. That an equivalent input capacitance of 1.3nF. The data sheet specifies Cdg and Cgs as 5pF and 20pf respectively. That comes to about 110pf with the miller effect. Seems that something is arwy here.

    My plan was to give a plot of total noise vs freq for various source resistances. I'll not do that now as it won't be helpful.
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    More generally, noise figure fails as a convenient measure. This is mostly a result of the the need to refer to a particular reference noise temperature. Point a low loss antenna around the sky, and there is a huge variation in the noise temperature as a function of where you point and frequency; that is, the sky temperature varies, and the ground temperature can matter. It is simpler to work with a characteristic of the device alone, its noise temperature, still a function of frequency and source impedance.

    Even for audio, it can be important to work with noise figure as a function of frequency. An example relevant here is the guitar pickup. It might be 8K resistive at low frequencies, but at the resonance of the pickup coil and the coil plus cable capacitance, it could be 100K, leading to an unexpectedly large contribution from noise current. This of course favors tubes and FETs over BJTs even more than might be expected.
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    Quote Originally Posted by nickb View Post
    As an addendum to this post, I have my doubts about the accuracy of the SPICE model. I suspect the drain to gate capacitance is wrong as I'm seeing a surprisingly early HF rolloff. For a common source amplifier with a gain of 24.7dB it was showing -3dB down at about 1.2KHz with a 100K source resistance. That an equivalent input capacitance of 1.3nF. The data sheet specifies Cdg and Cgs as 5pF and 20pf respectively. That comes to about 110pf with the miller effect. Seems that something is arwy here.

    My plan was to give a plot of total noise vs freq for various source resistances. I'll not do that now as it won't be helpful.

    In [180]: 10.**2.47
    Out[180]: 295.1209226666387


    In [181]: 295*5
    Out[181]: 1475


    In [182]: 1./1.e5/(1475.e-12 + 20.e-12)/2./pi
    Out[182]: 1064.5815591431128

    Getting nano volt noise performance, good BW and good dynamic range is not so easy, but there are circuits around.

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    Quote Originally Posted by Mike Sulzer View Post
    ...the guitar pickup...might be 8K resistive at low frequencies, but at the resonance of the pickup coil and the coil plus cable capacitance, it could be 100K...
    And that's if the volume pot in the guitar is turned all the way up. It can be worse, a lot of humbucker-equipped guitars have 500k volume pots, which provide 125k of source resistance (!) if set to half-resistance with an imaginary zero-impedance pickup, and even more than that with a real guitar pickup.

    Interestingly, though, I have never noticed any of my guitar amps hissing more loudly when I turn down the guitar volume (though many times I've noticed the dreaded "tone suck", treble loss caused by the same increase in source resistance).

    Perhaps input noise is dominated by something else, perhaps I don't hear it because I tend to play using relatively low gain, and at modest volume.

    All the back-of-the-envelope calculations I've done invariably point to JFETs as being the quietest input devices at typical guitar source impedances. Not all JFETs, though, as we also want low flicker noise down to 80 Hz.

    -Gnobuddy

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    In [180]: 10.**2.47
    Out[180]: 295.1209226666387


    In [181]: 295*5
    Out[181]: 1475


    In [182]: 1./1.e5/(1475.e-12 + 20.e-12)/2./pi
    Out[182]: 1064.5815591431128

    Getting nano volt noise performance, good BW and good dynamic range is not so easy, but there are circuits around.
    The voltage gain Av was 24.7dB
    => Av= 10**(1.235) = 17.1 not 295
    So Ceff= (Gds*(Av+1) + Cgs) = (5 * 18.1 +20) =110pF
    and F-3db = 14468.6
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    Quote Originally Posted by Gnobuddy View Post
    Interestingly, though, I have never noticed any of my guitar amps hissing more loudly when I turn down the guitar volume (though many times I've noticed the dreaded "tone suck", treble loss caused by the same increase in source resistance).
    -Gnobuddy
    I do have one guitar that gets really noisy as the volume control approaches halfway - more hum and hiss that isn't there on full. I thought the hum was the ground-referencing, but I never could rationalize the hiss that crept in.

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    Quote Originally Posted by Gnobuddy View Post

    Interestingly, though, I have never noticed any of my guitar amps hissing more loudly when I turn down the guitar volume (though many times I've noticed the dreaded "tone suck", treble loss caused by the same increase in source resistance).

    Perhaps input noise is dominated by something else, perhaps I don't hear it because I tend to play using relatively low gain, and at modest volume.

    All the back-of-the-envelope calculations I've done invariably point to JFETs as being the quietest input devices at typical guitar source impedances. Not all JFETs, though, as we also want low flicker noise down to 80 Hz.

    -Gnobuddy
    You are right; you have to use a very quiet input stage and then you can hear it. The low pass effect of the cable capacitance removes lot of pot hiss. So that 33k grid resistor matters.

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    Quote Originally Posted by nickb View Post
    The voltage gain Av was 24.7dB
    => Av= 10**(1.235) = 17.1 not 295
    So Ceff= (Gds*(Av+1) + Cgs) = (5 * 18.1 +20) =110pF
    and F-3db = 14468.6
    I thought db was defined in such a way that you get the same number whether you are referring to power or voltage. So I would say that your gain is 12.35 db, and then agree with your math..

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    Quote Originally Posted by Mick Bailey View Post
    I do have one guitar that gets really noisy as the volume control approaches halfway - more hum and hiss that isn't there on full. I thought the hum was the ground-referencing, but I never could rationalize the hiss that crept in.
    Or you might have a bad pot.

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    Quote Originally Posted by Mike Sulzer View Post
    I thought db was defined in such a way that you get the same number whether you are referring to power or voltage. So I would say that your gain is 12.35 db, and then agree with your math..
    Voltages are what we are interested in here. It comes from the definition https://en.wikipedia.org/wiki/Decibe...wer_quantities

    24.7dB is the correct figure.
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    OK, you are right 24.7 is the gain, corresponding to a voltage gain of 17.

    Sorry, Nick,
    Last edited by Mike Sulzer; 08-14-2017 at 06:34 PM. Reason: got it all wrong
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    Great info here. I'm much more comfortable with all of this now. Thanks to all.
    I've built the attached test circuit. The J113 is in cascode with the LND150. I've used the LND150 by itself just to confirm how horribly noisy it is. It's horrible. In cascode however, it's fine and the overall circuit is very quiet. I've fed this into a SS PA and I can get really (uncomfortable) loud levels with barely a trace of hiss or hum.

    The guitar PU noise (even humbuckers) far exceeds the circuit noise. Hiss at zero guitar vol is almost inaudible. Hum+noise at max volume is MUCH louder.

    capture.jpg

    The test configuration gives about the same gain as a standard 12ax7 stage (response curves below). I don't have an LSK170 yet, but the spice model shows similar gains to that of the J113. When I get a LSK170 I can compare noise levels.

    capture2.jpg

    I've also verified some relationships experimentally and they now agree with my understanding of all of this (thanks again).

    - R11 and R8 both produce increased hiss as their values increase. This is no surprise, but now that fits with my new understanding of noise figure curves.

    - Noise in the cascode device (Q3) has minimal impact. I've read this before and the arguments make sense now. The test circuit is really quiet, so the evidence support that too.

    - I have not done a precise measure of tube vs FET noise (I'll work on that) but I've found plenty of arguments for FETs producing less noise. I found some numbers to back that up. Seems like the data is plausible. Maybe someone here can vouch for it.
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    Quote Originally Posted by uneumann View Post
    I've found plenty of arguments for FETs producing less noise. I found some numbers to back that up. Seems like the data is plausible. Maybe someone here can vouch for it.
    Broad rule of thumb is 1uV EIN for a garden variety triode, 0.3uV EIN for a garden variety JFET, both assuming negligible source resistance.
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    Quote Originally Posted by Merlinb View Post
    Broad rule of thumb is 1uV EIN for a garden variety triode, 0.3uV EIN for a garden variety JFET, both assuming negligible source resistance.
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    Thanks! The pieces are all falling into place and I'm left wondering if I'll ever have reason to use a triode input stage.
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    Quote Originally Posted by uneumann View Post
    Great info here. I'm much more comfortable with all of this now. Thanks to all.
    I've built the attached test circuit. The J113 is in cascode with the LND150. I've used the LND150 by itself just to confirm how horribly noisy it is. It's horrible. In cascode however, it's fine and the overall circuit is very quiet. I've fed this into a SS PA and I can get really (uncomfortable) loud levels with barely a trace of hiss or hum.

    The guitar PU noise (even humbuckers) far exceeds the circuit noise. Hiss at zero guitar vol is almost inaudible. Hum+noise at max volume is MUCH louder.

    This is an interesting circuit. The idea of using two FETs with different transconductances in order to suppress the noise of the second (upper) one in the cascode is described here: http://nvlpubs.nist.gov/nistpubs/jre...n6p383_a1b.pdf. I do not know if this is the first place it was described, but it is the first I have seen. The LND150 has a transconductance as much as 10 times lower than the J113, and so the suppression can be quite good. The cost is a bit more input capacitance from a small Miller effect C, but it is not a problem. And the LND150 can take the high voltage, which you must have in order to be able to utilize the same gain as a triode without overload given the high output of some guitar pickups. I think this is a very good way to replace a triode input stage.

    Of course tube purists could use a tube-tube cascode where the first triode would have high transconductance and the second would have low. I suppose that if you look around, you would find that someone did this in 1939.

    Edit: One more thing. I do not think that it would help to replace the J113 with a lower noise FET. The 2K source resistor would have more much more noise than the FET. It might be the limit with the J113 as well. With that 2K resistor, the noise is no lower than a 12AX7, but a good 12AX7 stage without the usual 33K resistor in series with the grid. (It is possible that you could use a small resistor and still achieve proper biasing with some FETs.)
    Last edited by Mike Sulzer; 08-15-2017 at 06:48 PM.
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    Another possibility (which I assume that someone has tried?) is to modify the FET/FET cascode back in the other direction: use a low noise, high transconductance FET in the bottom part of the circuit, and a low transconductance triode in the top, say the usual 12AX7, although you might need a bit more plate current than it can provide for some FETs. Then you keep the output characteristics (including some degree of nonlinearity), of the usual tube, but get reduced noise (although in this case probably not as good as a 170 is capable of, but still very good). Of course you also get a much reduced Miller effect, and thus a reasonably small Miller effect input C, although not as small as a cascode circuit using devices with equal transcendence.

    (You would have to fool around with the source resistor to get the right biasing, and perhaps use two resistors in series, one bypassed wit a C in order to get the best compromise between noise and effective transconductance in order to not overload the circuit with a high output guitar pickup.)

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    Quote Originally Posted by uneumann View Post
    Thanks! The pieces are all falling into place and I'm left wondering if I'll ever have reason to use a triode input stage.
    I still like the sound of a triode input stage. Contrary to some published opinions, I found that, with almost all my electric guitars, there was a slight, but clearly audible, difference between a triode input stage, and a truly clean (lots of NFB) solid-state one. The triode produced a more attractive, more "valvey" or "tubey" clean, i.e., there were audible amounts of low-order harmonic distortion.

    The exception was one Squier guitar with very low-output single coil pickups (read that as: Fender cheaped out on this model), which evidently didn't drive the triode hard enough to create audible amounts of harmonic distortion.

    Of course your JFET also inherently produces 2nd harmonic distortion, and you might very well find it sounds just as good as a triode input stage. I don't know, I've never tried a JFET front-end, though it's been on my list of things to do for some years now. If you want, you could bypass that JFET source resistor with a capacitor to remove local negative feedback and maximise JFET nonlinearity.

    -Gnobuddy

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    Quote Originally Posted by Mike Sulzer View Post
    This is an interesting circuit.
    <snip>
    I think this is a very good way to replace a triode input stage.
    Evidently, you're not the only one with that opinion. On a different forum, someone posted a commercial guitar amp schematic that had exactly the same input configuration (a small signal N-channel JFET cascoded with an LND150 MOSFET). I forget exactly what amp that was.

    I'm still curious about sonic differences, if any, between a JFET input stage and a triode input stage. Aside from the audible triode nonlinearity I mentioned in my last post, a "classic" 12AX7 input stage operates at Vgk only about (-1.5 V). We know grid current starts to flow in a 12AX7 if the grid reaches maybe (-0.9 V), and we know plenty of guitar pickups can put out peak voltages of 1 V or bigger.

    This suggests to me that some signal clipping may actually occur right there at the input grid of the first 12AX7 triode that the guitar sees, because the high source impedance of a guitar is not going to supply the grid current drawn by the triode without having the peaks flattened. I don't know if this is audible or not, but I wouldn't be surprised if it turns out to be a factor in the sound of, say, heavily strummed chords played through a triode vs a typical solid-state guitar amp. To me, almost every solid-state guitar amp sounds much harsher (than a valve amp), even when played entirely clean, i.e., not audibly overdriven.

    It is possible that choosing a JFET with the right Vgs (and carefully chosen gate resistor) might allow one to rougly replicate the grid current effect of an input triode; the gate-source diode will also start to conduct once Vgs gets to around (+ 0.5 V) or so.

    Quote Originally Posted by Mike Sulzer View Post
    One more thing. I do not think that it would help to replace the J113 with a lower noise FET. The 2K source resistor would have more much more noise than the FET. It might be the limit with the J113 as well.
    A few months ago, I decided to buy a few JFETs before they went entirely extinct. There were only a pitiful few part numbers to choose from at Mouser, and after looking at a few datasheets, the J111, J112 and J113 were ones I thought were good candidates, because all of them looked to have very good noise figures, and good flicker noise down through the entire guitar bandwidth.

    If the input resistor is eliminated entirely, it might be necessary to take some other steps to keep RF and high-frequency hash out of the input. Maybe a ferrite bead with a few turns of wire wrapped around it.

    -Gnobuddy

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    Quote Originally Posted by uneumann View Post
    Thanks! The pieces are all falling into place and I'm left wondering if I'll ever have reason to use a triode input stage.
    Because even a triode is quiet enough for guitar, and it likes high voltage, which is what you conveniently have in the amp already. Noise in guitar amps normally comes not from the valves but from the resistors, especially the input grid stopper which is commonly rather large. Reduce it to 10k or even less (with suitable extra shunt capacitance if necessary) and you can approach the irriducible noise of your pickups.

    Mike Sulzer
    Another possibility is to modify the FET/FET cascode back in the other direction: use a low noise, high transconductance FET in the bottom part of the circuit, and a low transconductance triode in the top
    Yep. But you don't need to fret about the relative transconductances in a cascode (well, maybe if you're designing RF antenna amplifiers, but not guitar amps). Even if you use two identical devices, EIN will always be doninated by the lower device.
    wes.jpg

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    Quote Originally Posted by Merlinb View Post
    Even if you use two identical devices, EIN will always be doninated by the lower device.
    I do not see how you arrive at that conclusion. The analysis in the paper (http://nvlpubs.nist.gov/nistpubs/jre...n6p383_a1b.pdf.) shows that the two contribute equally (with identical gms) unless I am grossly misreading something. In any case, the obvious approximate analysis, without writing anything down, is that the lower device has unity gain (gm/gm) and that therefore its noise contributes the same as the top device. If you have a better analysis, I would be happy to listen.

    And do not be concerned with my fretting. That is just how we "internet experts" are.

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    Quote Originally Posted by Merlinb View Post
    Because even a triode is quiet enough for guitar, and it likes high voltage, which is what you conveniently have in the amp already. Noise in guitar amps normally comes not from the valves but from the resistors, especially the input grid stopper which is commonly rather large. Reduce it to 10k or even less (with suitable extra shunt capacitance if necessary) and you can approach the irriducible noise of your pickups.
    But it is not irreducible.

    There are two ways that I use to reduce it:

    1. Medium impedance pickups: Just use fewer turns of larger wire and use Cs and Rs to get a switched flexible tone. For example, you can lower the impedance a factor of ten while losing a factor of about three in signal voltage. You do not need a transformer or a preamp if you improve the noise performance of the amp (assuming that you are playing without pedals).

    2. Pickups with a better magnetic return path. For example, you can make a single coil size pickup with the normal inductance, but a factor of three lower resistance. You can use lower value pots, or you can set up the circuit so that the tone control is normally nearer the middle like nearly all other audio devices.

    Even with 250K pots, if you play with the volume control of one pickup very close to zero in order to get a nearly clean sound while the other pickup is at full volume (you need a volume pedal at the amp master volume to make this work right) you need lower than "normal" noise.

    Sure, most guitarists just do the standard thing, but in this forum we discuss ways of pushing that.

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    Quote Originally Posted by Mike Sulzer View Post
    But it is not irreducible.
    When I say 'irriducible noise' I'm talking about the intrinsic noise of the pickups (and other guitar electronics) that you actually have; it's just the accepted jargon, sorry for confusion. Sure, you can design lower noise pickups and guitars, but few of us have that luxury! Your new super designed pickups will still have their intrinsic noise referred to as the 'irriducible (source) noise'.

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    Quote Originally Posted by Merlinb View Post
    When I say 'irriducible noise' I'm talking about the intrinsic noise of the pickups (and other guitar electronics) that you actually have; it's just the accepted jargon, sorry for confusion. Sure, you can design lower noise pickups and guitars, but few of us have that luxury! Your new super designed pickups will still have their intrinsic noise referred to as the 'irriducible (source) noise'.
    But when you say "Reduce it to 10k or even less (with suitable extra shunt capacitance if necessary) and you can approach the irriducible noise of your pickups.", I think that is not quite good enough if you make the "irriducible noise" lower as I described.

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    Quote Originally Posted by Mike Sulzer View Post
    the lower device has unity gain (gm/gm) and that therefore its noise contributes the same as the top device.
    That's true if they're operating in the saturation region as the analysis assumes. But wouldn't our practical circuit be more likely to operate in the triode region? i.e. the impedance seen looking into the top device will be larger than 1/gm2, so the lower device gain will be higher than unity.

    EDIT: I was wrong, the top device is not likely to be operating in triode mode unless your supply voltage is unusually low. So yeah, you need more gm in the lower device.
    Last edited by Merlinb; 08-17-2017 at 08:55 AM.

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    Quote Originally Posted by Mike Sulzer View Post
    But when you say "Reduce it to 10k or even less (with suitable extra shunt capacitance if necessary) and you can approach the irriducible noise of your pickups.", I think that is not quite good enough if you make the "irriducible noise" lower as I described.
    Well yeah... but "if you make the source even quieter (non standard) then eventually the amp needs to be made even quieter too" seems a little unhelpful and redundant, IMO.
    Last edited by Merlinb; 08-16-2017 at 01:46 PM.

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    ^^^ what publication is that excerpted from? Designing Hi-Fi Tube Preamps? I don't have that one.

    Fwiw It looks like Toshiba has obsoleted that FET.
    "Stand back, I'm holding a calculator." - chinrest

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