Switchable onboard buffer for otherwise passive bass

Started by Fancy Lime, February 03, 2020, 02:45:27 PM

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Fancy Lime

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
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

FiveseveN

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 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.
Quote from: R.G. on July 31, 2018, 10:34:30 PMDoes the circuit sound better when oriented to magnetic north under a pyramid?

Ben N

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.
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Fancy Lime

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
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

Fancy Lime

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
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

R.G.

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.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

PRR

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.
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bluebunny

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Ohm's Law - much like Coles Law, but with less cabbage...

merlinb

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.

antonis

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'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

thetragichero

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

Fancy Lime

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 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
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

amptramp

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.



A source follower would give almost unity gain:



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.

PRR

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Digital Larry

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.
Digital Larry
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antonis

#16
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.

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 VB-VE 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 (hFE X R89) by beta multiplication (hFE1 X hFE2), 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)..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

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ljudsystem

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.


Fancy Lime

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
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!