DIYstompboxes.com

Hi there,

so I have recently finally converted my old Status Stealth to passive. As I have said many times before, ripping out whatever active electronics your bass may have is, to me at least, hands down the best upgrade you can get. Even if you have a fancy expensive onboard preamp. Anyway, I now have a passive bass with a battery compartment so I thought, why not add a switchable buffer that (ideally) does nothing at all except lowering the output impedance. The question is, what design do I want? Important factors are:

9V (there is no space for a second battery and I'm not doing voltage conversion for battery fed circuits unless absolutely necessary)
very low noise
(fairly) high input impedance (>1M)
(moderately) low output impedance (<1k will suffice)
low current draw
must be fairly compact, there is not much space left in the electronics compartment

These will have to be balanced well, of course. There will be trade-offs, e.g. between noise and current draw.

Has anyone done something like that? My knee-jerk reflex tells me to use a JFET source follower. But how exactly? Simple with a source resistor or with a constant current source made with a second JFET? If the latter: Any suggestions for a dual-JFET-on-a-chip (with low Vgs and low noise, roughly equivalent to a 2SK170)? Or better go BJT? Opamps are pretty much out of the question because the impedance and gain benefits they bring are not worth the noise and/or current draw penalty compared to low-part-count discreet disigns for this particular purpose, in my opinion (feel free to tell me I'm wrong, though).

The main point here is to limit the influence of guitar cable capacity on the sound. So an alternative approach would be to use a cable with no capacity at all. More precisely: A cable with a dual shield (so: one core conductor surrounded by a shield conductor surrounded by another shield conductor), where the innermost conductor is signal, the outermost is ground and the middle one is the "ring" fed from a buffer which is first in line on the pedal board. That should also eliminate (almost) all capacity in the cable without the need for an onboard circuit. It would need a very specific cable though (any idea what such a cable would be called/sold as?). So one may as well just use a fairly short high quality cable, whose total capacity is ow enough to not matter to the signal. The "capacity killing cable" only makes sense if you absolutely need a 100ft guitar cable for your elaborate stage show on your stadium tour in 1970. Else you'd be using shorter cables or wireless packs. So the cable thing is more of a theoretical musing. But it may be fun to try if the appropriate cable is available in a sane thickness.

Cheers,
Andy

I've used JFET and JFET-input op amp buffers for this kind application and didn't notice a difference. But if you want to go discrete there's also this one from mr. Keen: http://www.geofex.com/FX_images/Onboard_Preamp.pdf
The cable you're looking for is triax (https://en.wikipedia.org/wiki/Triaxial_cable) and it's not the thickness i'd be worried about, it's the price. But doesn't the driven shield trick work with regular coax?
For what it's worth, I think a regular buffer is plenty for eliminating cable influence, but please experiment away.

I was also going to suggest RG's onboard buffer -- although, alas, it doesn't take 9v, just 6v in the form of a couple of lithium coin cells. You can't beat that for compact, at least not in DIY-land.

R.G.'s buffer can be adapted to 9V with minor changes to some resistor values, that is not the issue. I just think it is massive overkill in terms of performance and complexity. It has a higher input impedance, lower output impedance and better gain (i.e. closer to 1.00) than I need and pays for these properties with extra complexity. I had something much simpler in mind. 1M input, 1k output and gain = 0.9 are perfectly fine for my purpose and that should be possible with a lot less parts.

Triax, huh? Now that you spell it out like that, I feel like I should have been able to guess that that would be how it is called. Never heard of it before. OK, google: "Triax cable ten feet". These things cost HOW much? Holy cow. I have seen these in use a lot at work but never had to buy any. I guess I'll scratch that idea. It also seems to be difficult to find that stuff being sold by the foot in quantities less than 100ft. I may need to do some more digging. I don't think the driven shield works with a regular coax cable because then we are missing a ground lead. Would work with two parallel regular coax cables but that becomes awfully clunky and does not fare so well in electromagnetically noisy environments. I agree that an onboard buffer is plenty good enough for eliminating cable influence. The driven shield thing is just something that popped into my mind as a sort of default solution to killing pesky capacitance influences (although usually in much more demanding use cases than getting a guitar sound from A to B across a few meters).

Andy

So, if I'm going with a constant current JFET source follower, I might like a matched pair of JFET's for that, no? Seems like the (discontinued) Toshiba 2SK389 or Liner Systems clone thereof, the LSK389, are the best suited candidates. Any idea how to get those in Europe? They do pop up on ebay for cheap, but suspiciously cheap (like 5 LSK389 for 15€), so I'd bet a modest sum that those are fake.

Andy

You'll want to use an oscilloscope to find what the actual peak signal voltage excursions are from your bass before committing to a simple source follower. 
Source followers seem simple until you have to make them live in the real world. As a practical matter, you have to worry about the current capability of the JFET and the size of the signal compared to Vgsoff. Signals which are comparable to the size of Vgsoff can drive the FET close to cutoff. You might like this distortion, but be clear about what you're getting into before going with it. If you don't like the distortion, you may find yourself using a constant current source load and bootstrapping the gate bias voltage.

The onboard preamp at geofex started out as a simple follower, and got to where it is by running my nose into various walls.

Some experienced musician once said "ripping out whatever active electronics your bass may have is, to me at least, hands down the best upgrade you can get." Maybe you should listen to that thought?

I mean, what are you missing with your "high" (but unspecified) impedance? You can't smell or taste impedance.

The number of parts *may* be less than 2^n, where n is the number of items on the Features List. Your list has 6 items, so the notional number of parts is 64. Do not laugh-- that's about a TL071 and support bits. When I tried to "simplify/improve" R.G.'s buffer, I realized he HAD bounced into every wall, and any change I'd make would be a matter of style, not substance or cost. Certainly a well-picked opamp can give hiss in guitar circuit as low as any single active device. And can drive tougher loads with less idle current than any single-device stage.

Put your weed in there.

Just throwing the late Albert Kreuzer's onboard JFET preamp (https://web.archive.org/web/20160910041328fw_/http://www.albertkreuzer.com/preamp_onboard.htm) into the pot.  :)

Quote from: Fancy Lime on February 03, 2020, 02:45:27 PM
Has anyone done something like that? My knee-jerk reflex tells me to use a JFET source follower. But how exactly?

That would be my choice too. A simple source follower, either the simplest possible, or maybe with the gate pulled up to half-rail. No CCS nonsense, simple is beautiful, just like the triode input your grandfather used to use. If not that then a TL071, which doesn't draw much more current.

Quote from: merlinb on February 04, 2020, 04:31:00 AM
just like the triode input your grandfather used to use.

Or a bootstrapped BJT Emitter follower, like his father used to use..  :icon_wink:

I've done the opposite and thrown Baja's  low current bass preamp into both of my passive basses and i couldn't be happier. should be noted that the majority of my playing is into a di box to the foh mixer so for me having more than just a passive treble cut at my fingertips it's super useful
sounds good into my tube and solid state amps too though

Quote from: R.G. on February 03, 2020, 06:59:41 PM
...
Source followers seem simple until you have to make them live in the real world. As a practical matter, you have to worry about the current capability of the JFET and the size of the signal compared to Vgsoff. Signals which are comparable to the size of Vgsoff can drive the FET close to cutoff. ...

Correct me if I'm wrong with the following, FET biasing always makes my head spin until I can no longer tell myself if I understand it:
The Vgsoff problem is obvious to me when running the source follower self biased with the gate tied to ground because the source cannot swing below ground (assuming an n-channel JFET and negative ground here). But  the linear region should extend from Vgsoff to almost Vdd, no? So biasing the gate right in the middle between those two (say, -1V and +9V) should give my a much bigger clean input swing (almost +/- 5V when biasing at +4V). Again, this is how I always thought it worked based on my mental synthesis of many poorly labeled graphs and half baked hints at explanation rather than an actual proper explanation about how to bias a JFET source follower and why. So any correction of my misconceptions is greatly appreciated. And if anyone knows a book called "understanding JFETs for dummies" or anything of the sort please point me to it.

Quote from: PRR on February 03, 2020, 08:31:17 PM
Some experienced musician once said "ripping out whatever active electronics your bass may have is, to me at least, hands down the best upgrade you can get." Maybe you should listen to that thought?

I mean, what are you missing with your "high" (but unspecified) impedance? You can't smell or taste impedance.

The number of parts *may* be less than 2^n, where n is the number of items on the Features List. Your list has 6 items, so the notional number of parts is 64. Do not laugh-- that's about a TL071 and support bits. When I tried to "simplify/improve" R.G.'s buffer, I realized he HAD bounced into every wall, and any change I'd make would be a matter of style, not substance or cost. Certainly a well-picked opamp can give hiss in guitar circuit as low as any single active device. And can drive tougher loads with less idle current than any single-device stage.
...

Well, in a way I am trying to solve a non-existing problem here. I can use a shortish (3m) low impedance cable with a total impedance of around 150pF and I perceive no obvious lack of shiny treble frequencies in passive mode. In fact, I have installed a 6-way switch that lets me choose the tone capacitor between 2n2 and 68n because the small caps (<10n) actually increase the perceived treble content because it forms a resonant low pass with the impedance of the pickups (obviously depending on the pickup selector, which allows series as well as parallel mode) and the resonant peak sits quite nicely near the top end of the "useful frequencies" of a bass guitar. So converting the impedance to avoid the additional cable influence is not *necessary* for my practical needs. I just thought that It'd be fun to try and make the simplest reasonable buffer and add that since I have an empty battery compartment and an empty whole for a switch anyway. Who knows, maybe I'll learn something along the way and can pass that along to people for whom such a buffer is more useful. BTW, if people can "hear" the influence of a oxygen free silver core mains cable on their 50,000$ home stereo system, I can definitely "smell" impedance.
Also, I'm not sure the 2^n equation fully applies here. A simple JFET source follower with 6 parts ticks all the boxes with current draw and noise being a matter of balancing resistor values right but not adding more components. Also the "low complexity" or "small size" requirement surly should not count towards a higher part count.

Quote from: bluebunny on February 04, 2020, 03:02:47 AM
Just throwing the late Albert Kreuzer's onboard JFET preamp (https://web.archive.org/web/20160910041328fw_/http://www.albertkreuzer.com/preamp_onboard.htm) into the pot.  :)

Thanks but no thanks. That is exactly the sort of thing that has no business in any of my basses. It is a great circuit snippet for a bass preamp, though.

Quote from: thetragichero on February 04, 2020, 10:44:11 AM
I've done the opposite and thrown Baja's  low current bass preamp into both of my passive basses and i couldn't be happier. should be noted that the majority of my playing is into a di box to the foh mixer so for me having more than just a passive treble cut at my fingertips it's super useful
sounds good into my tube and solid state amps too though

You don't happen to have a schematic, do you? I usually find that a pickup selector switch with series and parallel options provides all the tonal flexibility I need. Fitting that into the band sound or a venues acoustics is the FOH engineers job.


Sooo, long story short: Looks like I'll be experimenting with some JFET and a TL071 circuit and try to figure out what works best for me and my DMM.

Thanks for the input and cheers,
Andy

Nobody has mentioned the Tillman preamp so far.  Very simple, has gain, output impedance is the drain resistance of about 6800 ohms but you can always lower the gain and get lower output impedance.

(http://www.till.com/articles/GuitarPreamp/images/preamp.gif)

A source follower would give almost unity gain:

(http://1.bp.blogspot.com/-Z9GmvfMBlUs/T-_KtwLsckI/AAAAAAAABds/7-WZ4v-sr4Y/s400/FET%2BBoost.gif)

I prefer to think in terms of op amp buffers for this type of thing.  The noise level is acceptably low, the input impedance can be high and the output impedance is low and can definitely meet the < 1000 ohm level you specified.  The gain is unity but can be adjusted up or down.  For battery operation, there is no need for the power supply rejection the op amp affords but it is nice to know it is there.  You may want to increase the coupling capacitors for bass operation.

> bootstrapped BJT Emitter follower

(https://i.postimg.cc/N5DYv07r/Y960-42.gif) (https://postimg.cc/N5DYv07r)

Quote from: PRR on February 04, 2020, 09:04:33 PM
> bootstrapped BJT Emitter follower

(https://i.postimg.cc/N5DYv07r/Y960-42.gif) (https://postimg.cc/N5DYv07r)

Mind giving a quick rundown of how it works?  Best I could come up with was "increased negative feedback at higher frequencies".  My transistor brain is getting atrophied.

I'm this terrible combination of cheap and lazy (but *dang* I'm good looking!). Why invent the wheel?  I'd go with one of these, if there's room:  http://www.muzique.com/pcb.htm

Quote from: Digital Larry on February 04, 2020, 09:12:59 PM

Quote from: PRR on February 04, 2020, 09:04:33 PM
> bootstrapped BJT Emitter follower

(https://i.postimg.cc/N5DYv07r/Y960-42.gif) (https://postimg.cc/N5DYv07r)

Mind giving a quick rundown of how it works?  Best I could come up with was "increased negative feedback at higher frequencies".  My transistor brain is getting atrophied.

Probably due to "digital" operation of your transistor brain..  :icon_redface:
(just kidding, of course..) :icon_lol:

It utilizes the almost identical voltage swing between Base & Emitter, so for sake of simplicity, raise C7 positive leg from second BJT Emitter and connect it to first BJT one..
Now, input signal "sees" no voltage difference across R86 as it appears as an almost(*) infinite resistance item, hence no AC current flowing through it..
(*) actually multiplied with inverse of V_B-V_E attenuation - for an arbitrary gain value value of 0.990, R86 apparent value should be 100 times its DC (ohmic) value..
R87/R88 could have any reasonably low value (compromise between robost voltage divider biasing & "light" Emitter loading) without any signal loss, 'cause R86 "blocks" signal leakage due to its apparent very high value..
(for DC purpose (bias), its actuall value is takern into account..)
Second BJT is used for raising first BJT Base input impendance (h_FE X R89) by beta multiplication (h_FE1 X h_FE2), permiting for using low enough R89 value for better linear response..
(although here isn't utilized the above..)

P.S.
My sole argument should be the use of higher value cap (47μF, say) for higher gain on low frequencies..
(C7 capacitive reactance forms an extra voltage dividing effect between Emitter & parallel combination of R87/R88..)
edit: Just realized C7 value is 2000μF (2mF)..

Quote from: amptramp on February 04, 2020, 03:16:48 PM
A source follower would give almost unity gain:

(http://1.bp.blogspot.com/-Z9GmvfMBlUs/T-_KtwLsckI/AAAAAAAABds/7-WZ4v-sr4Y/s400/FET%2BBoost.gif)

You can build that simple buffer on a piece of stripboard:

http://www.muzique.com/news/jfet-buffer-on-stripboard/

regards, Jack

Great thread.

I use a Klon buffer strapped to my belt but I've been thinking about going onboard to. I thought opamp designs in general were lower noise, less current draw and better in- out- impedance.

Here's another discreete design I found floating around the webb.
(https://i.postimg.cc/3d8CY2cc/JFETBJTBUFFERSCH.png) (https://postimg.cc/3d8CY2cc)

Opamps have superior input and output impedance characteristics. But with noise and current draw, things get complicated. There is an inherent tradeoff between those two both in discreet designs as well as in opamps. A TL071, for example has around 1.5mA of current draw and decent-ish noise characteristics. You can get opamps with only a few hundred µA draw but much more noise (e.g. TL061) or you can buy really quiet ones with several tens of mA of draw. With a discreet design, you can decide the source current yourself but a given transistor will have an optimum current for least noise, which s usually not super low either. My favorite cheap-but-good JFET, the 2SK117, is pretty quiet at source currents above 1mA. Below that, it gets increasingly noisy. That means in practical terms I would expect it to perform not too differently from a TL072 in terms of noise/current balance. Of course impedances are still better with the opamp, and so is consistency between devices. So a really good 2SK117 may perform better, a bad one worse than the TL072. Silicon lottery.

I still need to do more research before deciding...

Andy

Quote from: Fancy Lime on February 05, 2020, 08:01:52 AM
I still need to do more research before deciding...

Decide about what exactly, Andy..??  :icon_cool:

Are you making a state-of-art research or you just wish to design an input buffer "quieter" than your guitar pickups..??
(for the last case, almost all the above suggestions should be considered quite satisfactory.. if not, a shielded breadborad & a noise analyzer could save you a lot of mind trouble..)

Quote from: Digital Larry on February 04, 2020, 09:12:59 PM

Quote from: PRR on February 04, 2020, 09:04:33 PM
> bootstrapped BJT Emitter follower

Mind giving a quick rundown of how it works?  Best I could come up with was "increased negative feedback at higher frequencies".  My transistor brain is getting atrophied.

It's an emitter follower using a Darlington pair. The input is biased with a potential divider (R87/R88) and then bootstrapped with a feedback cap C7. Dead simple, but it uses PNPs and is drawn 1969 style so everything is upside-down and inside out. Here's it is after being converted into non-Australian mode:

(https://i.postimg.cc/c6VXNk3N/Y960-42.jpg) (https://postimg.cc/c6VXNk3N)

A bunch of stuff has been posted at this forum over the years.

A link to an over designed for guitar/bass buffer posted more for people to get ideas from

https://www.diystompboxes.com/smfforum/index.php?topic=105522.msg954811#msg954811 (https://www.diystompboxes.com/smfforum/index.php?topic=105522.msg954811#msg954811)

I would search for FET input rail to rail opamps that operate at 9VDC

Or use the sony textbook buffer from the above link.

> Just realized C7 value is 2000μF (2mF)..

No, just micro. There were not greek so didn't have the symbol. 2uFd is ample for pushing 75k around.

Quote from: antonis on February 05, 2020, 09:08:02 AM

Quote from: Fancy Lime on February 05, 2020, 08:01:52 AM
I still need to do more research before deciding...

Decide about what exactly, Andy..??  :icon_cool:

Are you making a state-of-art research or you just wish to design an input buffer "quieter" than your guitar pickups..??
(for the last case, almost all the above suggestions should be considered quite satisfactory.. if not, a shielded breadborad & a noise analyzer could save you a lot of mind trouble..)

Decide if the whole idea of a buffer is a good one to begin with (considering the unavoidable trade offs) or if I want to include a passive treble boost instead. The latter would mean that I need to tune the cable capacity such that the RLC lowpass filter of the pickup resistance and inductance with the cable capacity has its resonant  peak somewhere useful. That also means shelling out biggly for a decent LCR meter. Which shifts the decision to: What LCR meter?

Andy

I use an AideTek DM4070 which is a direct reading LCR meter with ranges from 200 pF to 2000 µF, 200 µH to 20 H and 20 ohm to 20 megohms.  I also have a number of LCR bridges but they are not direct reading - I collect test equipment and certain pieces are better for different purposes.

Quote from: Fancy Lime on February 05, 2020, 03:10:23 PM

Quote from: antonis on February 05, 2020, 09:08:02 AM

Quote from: Fancy Lime on February 05, 2020, 08:01:52 AM
I still need to do more research before deciding...

Decide about what exactly, Andy..??  :icon_cool:

Decide if the whole idea of a buffer is a good one to begin with (considering the unavoidable trade offs) or if I want to include a passive treble boost instead. The latter would mean that I need to tune the cable capacity such that the RLC lowpass filter of the pickup resistance and inductance with the cable capacity has its resonant  peak somewhere useful. That also means shelling out biggly for a decent LCR meter. Which shifts the decision to: What LCR meter?

Unless you just enjoy the dithering, I think you're seriously overthinking (or at least over-typing) the issue. Your forum reading and typing time on this issue alone would probably have been enough to breadboard and try a couple of approaches.  :icon_lol:
Not that there's anything wrong with overthinking, of course.  :icon_lol:

Quote from: antonis on February 05, 2020, 06:19:05 AM
Probably due to "digital" operation of your transistor brain..  :icon_redface:
(just kidding, of course..) :icon_lol:

No worries, thanks for trying!  I now recall my senior year at university when I began to study DSP.  I thought, "this seems much easier, you just tell the thing what you want it to do!"  Realizing that one still needs a well behaved high impedance preamp before one shreds the signal into screaming helpless ones and zeros.

https://xkcd.com/1952/
(https://imgs.xkcd.com/comics/backpack_decisions_2x.png)

Quote from: Digital Larry on February 05, 2020, 10:43:21 PM
Realizing that one still needs a well behaved high impedance preamp before one shreds the signal into screaming helpless ones and zeros.

:icon_lol: :icon_lol: :icon_lol:
(couldn't figure out better description..)

Quote from: PRR on February 05, 2020, 02:39:29 PM
> Just realized C7 value is 2000μF (2mF)..
No, just micro. There were not greek so didn't have the symbol. 2uFd is ample for pushing 75k around.

<off-topic on>

In such a case, apparent R86 value should be raised hardly X50 for open low E string and even lower (x10) for 20Hz signal..(supposing it's incorporated into an audio console..)
Can't see any reason, other than cost, for using such a low value capacitor..

<off-topic off>

https://www.talkbass.com/threads/onboard-tillman-inspired-jfet-preamp.1389669/ if you have previously done the variable cap and parallel series sw, i would try like the one at this post, maybe an active/inactive sw with the tillman is all you need

Quote from: R.G. on February 05, 2020, 07:10:55 PM

Quote from: Fancy Lime on February 05, 2020, 03:10:23 PM

Quote from: antonis on February 05, 2020, 09:08:02 AM

Quote from: Fancy Lime on February 05, 2020, 08:01:52 AM
I still need to do more research before deciding...

Decide about what exactly, Andy..??  :icon_cool:

Decide if the whole idea of a buffer is a good one to begin with (considering the unavoidable trade offs) or if I want to include a passive treble boost instead. The latter would mean that I need to tune the cable capacity such that the RLC lowpass filter of the pickup resistance and inductance with the cable capacity has its resonant  peak somewhere useful. That also means shelling out biggly for a decent LCR meter. Which shifts the decision to: What LCR meter?

Unless you just enjoy the dithering, I think you're seriously overthinking (or at least over-typing) the issue. Your forum reading and typing time on this issue alone would probably have been enough to breadboard and try a couple of approaches.  :icon_lol:
Not that there's anything wrong with overthinking, of course.  :icon_lol:

Me, overthinking something? How dare y... uhmm ya that checks out, actually. As a kid I used to start lists of potential Christmas wishes in October to compare things I *might* want to want for Christmas. With tables comparing pros and cons. Other kids just wanted everything, I wanted to know what I really wanted much rather than actually wanting to have it. All absolutely moot because I knew full well that I never got what I wanted anyway. That realization did not stop me then, I'm afraid it still doesn't today. This forum seems to have become my year-round Santa but with a much better delivery rate. You are of course right, I should just build the obvious candidates and see what sticks. And as soon as I have my DIY bench back (having visitors at the moment), I will. But until the weekend, theorizing will have to do.

Quote from: PRR on February 06, 2020, 01:34:10 AM
https://xkcd.com/1952/
(https://imgs.xkcd.com/comics/backpack_decisions_2x.png)

I want royalties for being in that comic. Like I said, Christmas lists.

Andy

So I finally just did what I intended to do before starting to overthink it. Simple JFET source follower using a 2SK117GR with gate bias at V/2 and the source resistor tuned for the optimum balance between noise performance and current consumption as per the datasheet. The lowest quiescent source current that we want to use for that is about 750µA. With 9.2V from the battery, I measured 4.7V at the source of Q1, giving me 840µA across a 5k6 resistor. Close enough to not shed tears about 90µA wasted. Since the bias network has 2M across the rails, it does not contribute much more to the current consumption, so 840µA is also what I measured as the total current cunsumption. Should give me a few hundred hours of operation from an average 9V block; good enough for me. I used a "noiseless biasing" scheme because my ocd told me to and you guys did not stop me in time. Muhuhahaha. May not make a world of difference, though. The 10k series resistor on the input keeps the input resistance in the optimum range for best noise performance even when this thing is fed with a low impedance source. So this little thing does all I want it to as good or better than I expected it to. At only eleven parts, that was way easier than I (over)thought. And it does not audibly clip even at the loudest peaks from two Status Hyperactive pickups (which are passive despite the name but pretty hot) in series, so it should work fine with most pickup setups. Anyway, here's the layout (I did not bother drawing a schematic but that should be obvious to most readers and for the others it will be a good exercise to trace the schematic from the layout):

(https://i.postimg.cc/0KkNdRdr/onboard-JFETbuffer.png) (https://postimg.cc/0KkNdRdr)

The layout is not verified at this point in time. [[[EDIT: I just found an error: the input resistor R1 is shorted by a trace. This trace should of course not be there. I'll fix it in the schematic as soon as I can.]]] I still have to build it and see if there are errors (only on the breadboard so far). I am not unhappy with the layout, there is only little wasted room for improving the compactness unless you use smaller footprint caps. One row could probably be scraped of by a more talented layout artist. An SMD version could probably be made to fit into most 1/4" plugs, so you could phantom power it through a stereo cable and thus make any passive bass or guitar active. I have seen this concept somewhere but I don't remember where. If someone knows, please add the original reference, I don't want credit for ideas that were not mine. If you want to power this from a wall wart instead of a battery, you can add a 100µF-470µF cap across the rails on the right edge of the schematic by making the board one row wider. Or make it two rows wider and use a 100µF-470µF electrolytic in parallel with a 100nF (or similar) film cap for really troublesome power supplies. In that case I would also change the diode to something with a bit more internal resistance, like a 1N5819 or even a 1N400x.

Hope that helps people who tend to run circles inside their own brains, like I do.
Cheers,
Andy

Quote from: Fancy Lime on February 10, 2020, 06:19:57 AMAn SMD version could probably be made to fit into most 1/4" plugs, so you could phantom power it through a stereo cable and thus make any passive bass or guitar active. I have seen this concept somewhere but I don't remember where. If someone knows, please add the original reference, I don't want credit for ideas that were not mine.

http://www.till.com/articles/PreampCable/ (http://www.till.com/articles/PreampCable/)

> The 10k series resistor on the input keeps the input resistance in the optimum range for best noise performance even when this thing is fed with a low impedance source.

Never add series resistance "to suit OSI". The resistor is not The Source.

Yes, in stage work a 10k is good protection and probably a non-issue for hiss.

Quote from: Ben N on February 10, 2020, 08:24:40 AM

Quote from: Fancy Lime on February 10, 2020, 06:19:57 AMAn SMD version could probably be made to fit into most 1/4" plugs, so you could phantom power it through a stereo cable and thus make any passive bass or guitar active. I have seen this concept somewhere but I don't remember where. If someone knows, please add the original reference, I don't want credit for ideas that were not mine.

http://www.till.com/articles/PreampCable/ (http://www.till.com/articles/PreampCable/)

Yes, that's the one. I remembered incorrectly that it was a buffer, though, instead of a booster. The concept can be adapted for a buffer, I think.

Quote from: PRR on February 10, 2020, 02:03:23 PM
> The 10k series resistor on the input keeps the input resistance in the optimum range for best noise performance even when this thing is fed with a low impedance source.

Never add series resistance "to suit OSI". The resistor is not The Source.

Yes, in stage work a 10k is good protection and probably a non-issue for hiss.

Paul, you are one of those people here whose word I'm willing to take without an explanation if I have to. Nevertheless, I would greatly appreciate if you could expand a little on that topic, since it still baffles me. Why is a series resistor bad in terms of noise/hiss? There is no DC voltage across it, so it does not give of thermal noise, right? Or does it? Any noise contribution that is only present when there is an AC signal would certainly matter if we were to build an input for a super accurate scientific measuring probe but for audio, especially a guitar signal, I don't think it matters because human ears would not be able to tell the difference (as long as we're not going completely outside of reasonable design territory).
And what is the "source resistance" anyway? The signal comes from a guitar pickup, so the reactance of the inductor swamps any ohmic resistance for audio band AC signals. What does that mean in relation to the (usually) terse info given in datasheets? Some tell me the optimum source resistance at 1kHz (https://www.onsemi.com/pub/Collateral/2N5457-D.PDF), but most datasheets do not specify the influence of source resistance at all, let alone the relationship that has with frequency. And when this sort of information is given it is mostly "typical" curves, when "best and worst" curves would be so much more useful. Very frustrating not to be give this information which I assume the manufacturers must at least partially have.
Also: what do I protect the gate from with the series resistor? Gate-Source breakdown voltage being exceeded by electrostatic discharge? If so, how does the resistor help?

Thanks and Cheers,
Andy

> how does the resistor help?

Limits current flow.

Quote from: Fancy Lime on February 10, 2020, 03:58:58 PM
There is no DC voltage across it, so it does not give of thermal noise, right? Or does it?

Yes it does, it's called Johnson or thermal noise.

Quote
And what is the "source resistance" anyway?

The intrinsic internal resistance of the signal source (e.g. the pickup coil).

Quotethe reactance of the inductor swamps any ohmic resistance for audio band AC signals.

Reactance does not generate noise, resistance does. (However, impedance does provide a way for noise current coming out of the amplifying device to produce a noise voltage which is then amplified.)

QuoteAnd when this sort of information is given it is mostly "typical" curves, when "best and worst" curves would be so much more useful. Very frustrating not to be give this information which I assume the manufacturers must at least partially have.

That information applies to the impedance-matched world of RF (power signals), not AF (voltage signals). The optimum source impedance for audio is "the internal impedance you're stuck with after removing all unnecessary source impedance!"

Thanks Merlin, that clears things up, to a degree. And as it is the habit of any good answer, it generates more questions :)

Am I right in assuming that the "impedance noise" is coupled to the current that flows into the gate, then? In other words, probably negligible for a JFET but potentially troublesome inf form of a base current when using a BJT instead?

QuoteThat information applies to the impedance-matched world of RF (power signals), not AF (voltage signals). The optimum source impedance for audio is "the internal impedance you're stuck with after removing all unnecessary source impedance

"Don't worry  bout it", gotcha.

Quote from: PRR on February 10, 2020, 04:51:00 PM
> how does the resistor help?

Limits current flow.

What current, though? Should the current into a JFET gate not be very limited already by the fact that is a JFET gate?


Thanks guys, I would be even more lost without y'all,
Andy

> the fact that is a JFET gate?

Which conducts HARD ("infinite") if taken past +0.7V or -35V. Both of which can happen on a stage. Look-up JFET data, and consider what crap they may survive with zero or 10k in series.

Quote from: PRR on February 11, 2020, 05:21:46 PM
> the fact that is a JFET gate?

Which conducts HARD ("infinite") if taken past +0.7V or -35V. Both of which can happen on a stage. Look-up JFET data, and consider what crap they may survive with zero or 10k in series.

Ah, I see. Thanks!



BTW, here is the corrected layout:

(https://i.postimg.cc/sBbnBhbP/onboard-JFETbuffer.png) (https://postimg.cc/sBbnBhbP)

Andy

So are you gonna reduce R1?

Quote from: merlinb on February 12, 2020, 10:08:35 AM
So are you gonna reduce R1?

Well... I don't know, really. I mean, unless I know what voltage and total charge I want to protect the input against, R1 is pretty much a stab in the dark, is it not? The 2SK117 can tolerate 10mA at the gate, so with a 10k resistor I can protect it from up to 100V. Not a lot for ESD. Then again, there is also C1, which will eat some charge before we even need to start worrying about voltage. And ESD pulses usually have relatively small charges. How small? No idea. I feel like the R1-C1 combo should somehow take care of the the charge variable but I am way to tired right now to figure out how. But the smallest voltage deviation from the bias point at the gate that we need to avoid is 4.5V + 0.7V on the plus side. To get 5.2V across the 100nF cap, we need 520nC of charge. Is that realistic for the type of ESD we want to protect against? I don't have the slightest clue.

TL;DR: I stuck with 10k for R1 because that is what Boss do in all(?) their JFET input buffered pedals. Is that a great reason? No, but it is the only one I have at this point. Better reasons to change it will be greatly appreciated. Basically, I would need to know what discharged (V and Q) I am up against. I won't be sticking parts of the instrument in a mains socket, so maybe 100V protection against large-charge sources is overkill. Maybe I'll get hit by lightning while playing and should install a kilofarad supercapacitor for C1. But those can only take a few volts at the moment, so maybe lightning protection will have to wait until capacitor technology catches up.

Andy

I'd use 1k at most, otherwise all that talk about wanting low noise is moot. If you need ESD protection, use a small series inductor or break R1 into two 470R resistors in series, with diodes to each power rail from the junction of the two resistors.

ESD protection often takes the form of a pair of reverse-biased diodes from the input to ground and the power rails.  If the input goes below ground or above the power supply, the diodes conduct and short out the ESD, limiting it to just beyond the rails.  You may still need a series resistor to avoid blowing the diodes.

I know the "reverse diodes to both rails" protection scheme from MOSFETS but have never seen in on JFETs and started to wonder why. I tried to find useful info on ESD protection for JFETs and did not find much very useful info. So, reluctantly, I had to use my own brain:

A positive ESD pulse on the gate means that the G-S junction starts to conduct. So voltage in itself is not a problem here but may become one if the current into the gate exceeds 10mA (for the 2SK117). So uncle Ohm says that if I place a 1k resistor right in front of the gate, anything below +10V over forward breakdown is OK. Forward breakdown should occur at Vgs=+0.7V, which happens when the gate is a positive suppl plus 0.7V. 

In the other direction, I need to worry about G-D breakdown, which is at a minimum of -50V for the 2SK117.

So for a JFET like this, I think I should be in the clear with a single 9V1 Zener diode and a little rearranging of the components like so:
Input > C1 > 9V1 Zener reverse to ground > R1 (1k) > gate
This means that the gate can only swing from 0.7V below to 9.1V above ground (which is slightly outside the maximum range intended for normal usage)and will never see it's maximum gate current exceeded.

The only thing I was worried about is that the leakage of the diode may upset the gate bias. So I put it in and checked and got some very unexpected readings.
In the circuit as it is in the last schematic I get:
Power supply: 9.21V
Bias network point (junction R2, R3, R4): 4.29V (so far so good)
Gate: 2.95V
Source 4.70V
So I'm thinking R2 is too big at 4M7 and the gate current is pulling the bias low. But how, when the drain sits at 9V and the source at 4.7V? How can anything in the JFET drag the gate lower than any of the other terminals are? Anyway, I tried to stick in a reverse 9V1 Zener straight from gate to ground to see if that would pull the gate too low. But it did the opposite, it pulled the gate up to 3.26V. What the FET? If the only thing I changed was to add another connection to ground, how can the point that is now "more" connected to ground go further away from ground? I checked all connections and readings five times to make sure I'm not just being stupid. Any idea what's up with that?

I changed R2 to 2M2 to see if I get a more reasonable bias voltage at the gate and indeed it went up to 3.55V. Now sticking in that Zener does not change the voltage at the gate anymore. The source voltage also stays nailed to 4.70V.

Here I was, naively thinking I finally had a bit of a handle on JFETs...
Andy

QuoteHow can anything in the JFET drag the gate lower than any of the other terminals are?

Your multimeter is pulling the gate voltage down when you try to measure it.

Quote from: merlinb on February 13, 2020, 12:18:20 PM

QuoteHow can anything in the JFET drag the gate lower than any of the other terminals are?

Your multimeter is pulling the gate voltage down when you try to measure it.

Ah yes, that checks out. So I was being stupid after all. Disregarded the old rule to never trust measuring equipment blindly. I checked the apparent voltage between the gate and the supply rail (with the 4M7 for R2 back in) and got -3.58V. So yes, my DMM is pulling the gate around.

What I still don't understand, is what that diode does to the voltage reading. With the diode in (9V1 Zener reverse to ground straight from the gate), the apparent voltage at the gate is pulled up from 2.95V to 3.26V (compared to without the diode) when measuring with reference to ground. When measuring from the power rail, on the other hand, the apparent voltage goes down from -3.58V to -3.62V when inserting the diode. I don't understand that at all.

I tried to figure out the "equivalent resistance" of the reverse biased zener at 4.5V, to see if it would noticeably mess with my bias for real or if that is again some artifact. I put two 9V1 zeners in series between the rails (reverse biased of course) and measured the center point vs ground. That read 2.25V, which we have established is bull$#!+ but we can use it anyway. I checked the same point against the supply rail and lo and behold, -2.25V. The rails are, just for a sanity check, 9.21V apart. So I swapped the upper one of the diodes out for resistors to get the same 2.25V against ground reading, figuring that that would give me the "equivalent resistance". I went up to 32M, where I got 2.9V and had no more large resistors lying around on my bench. So that tells me, unless I'm being stupid again, the equivalent resistance of the 9V1 zener, reverse biased with 4.6V, is >32M. So It should not mess with the bias too much. But possibly enough to justify going down to 3.3M or 2.2M on R2. Should not make an audible difference anyway. I'll test it.

Thanks and cheers,
Andy

Quote from: Fancy Lime on February 13, 2020, 02:09:07 PM
But possibly enough to justify going down to 3.3M or 2.2M on R2. Should not make an audible difference anyway. I'll test it.

Stop measuring the gate voltage, measure the source voltage. Does it change when you add the Zener?

Quote from: merlinb on February 14, 2020, 03:32:08 AM

Quote from: Fancy Lime on February 13, 2020, 02:09:07 PM
But possibly enough to justify going down to 3.3M or 2.2M on R2. Should not make an audible difference anyway. I'll test it.

Stop measuring the gate voltage, measure the source voltage. Does it change when you add the Zener?

Good point. Source voltage changes reproducibly but very slightly from 4.70V without to 4.67V with the diode. So I'll put that down as a non-issue, shall I?

Thanks for all the mental hand holding,
Andy

Quote from: Fancy Lime on February 14, 2020, 05:12:43 AM
So I'll put that down as a non-issue, shall I?

We should be grateful, Andy.. :icon_wink: