JFET Buffer and buffer cable

[JohnH]

My first thought would be just to add treble bleed, and use a 10' cable.

But with a buffer in the guitar, the next thought is to not put a volume after it, but use the standard guitar volume before it. That way you get the full benefit of low impedance output. To add it some 'virtual cable' capacitance, you can wire a small cap just before the volume pot, ie across the outer lugs of the volume pot. In that position, its effect should stay constant as you roll down volume. You can try that on GuitarFreak, either by using a very small tone cap and tone at 0, or increasing the pickup capacitance to suit. If you are using the current version 4, keep bass-cut at zero to test that way. I've tried adding a few small caps in that way, and tended to find that I want to add a bit more than the 500pF, maybe 1000pF or even up to 2.2nF to explore the effects.

But you can have a volume control after the buffer. The lower value that you use, the lower should be the source resistor so the buffer has enough grunt to drive it. This version of the buffer would be a good basis for it:



The 3.3M would be replaced by the volume pot, and I would suggest that 100k would be a good value for your purpose. The highest impedance out of that will be 1/4 of the pot value, at 25k which is not going to have any problem sending signal to your interface

In your first post you asked about power. You can run these buffers from any type of supply suitable for guitar pedals, using a cap and resistor to smooth things a bit more. But not just any wall wart is good, many of them hum. Im this stomp-box circuit, you can see the buffer forming the last output stage, with a the cap and resistor providing supply. Also, there is a switched LED, taken across the power supply before the cap/resistor:

BlueJuice overdrive

One thing though, its best not to try to switch the power on and off at any time that you might be playing, because it makes a thump or surge that is very hard to suppress. So the power to the circuit is kept engaged, and just switch the signal if you want to.

[petre]

Hi John,
Thanks again for your help.  You are absolutely right.  Having the "virtual cable" cap doesn't interfere with the volume pot as long as it is BEFORE it.  I just did the testing that you suggested, set the output to Buffer, increased the capacitance of the pickup, and sure enough, the curve stays constant when moving the volume pot.  I thought the pot needs to be moved after the buffer in order not to taper off the treble from the added cap.  That was also my reasoning then for having the buffer inside the guitar.  I agree with you that having 1nf or even 2.2nf could be beneficial to explore more tonal variations.  Maybe two push/pull pots with two 1nf caps in parallel.  It is very revealing how the capacitance of the cable can have such an effect on the tone.  It acts in a different way than what the tone pot does, having very low capacitance but "no resistance".  In my first post I was thinking of different ways to add external power supply so that I don't have to worry about the battery, but in reality there is no practical need for it with such a low power draw.
I'm thinking now whether it is possible to have the circuit turn on/off from the plug that goes into the guitar, rather than the stomp box.  Could we have a 3-core cable, run the negative lead directly from the battery into the middle post of a stereo plug that goes into a stereo jack of the guitar that has the ground and middle posts soldered together?

[JohnH]

If the battery is in the guitar, with + connected to the circuit and - to the ring terminal of a stereo jack socket, then pushing in a standard mono plug connects battery - to ground and engages power. A standard cord is all thats needed.

[petre]

I was thinking of having the Buffer cable, plus the stomp box with the battery inside it, not in the guitar.  Basically your buffer cable diagram but having the "switch" for the circuit happen at the jack of the guitar.

Edit: I just reread my previous post, and realized that my wording was a bit confusing.  The only reason why I thought I needed the buffer inside the guitar in my second post was because I thought I need the volume pot after the buffer in order to have it not roll of the treble, which I was wrong about.  I am planning on building the Buffer cable.

Edit 2: An edited image has been uploaded in my next post.

[JohnH]

Your power idea is interesting. A good point is that by having ground and battery negative as the two exposed connections that engage power, nothing bad happens at any interim moment when various things get shorted inserting the plug. This contrasts with other alternatives where + is being connected, with risk of shorting it to ground.

The stereo jack on the guitar, wired as you describe, would not prevent its use with a standard cable. But would your new buffer cable then end up requiring only this guitar to make it work?

If you are putting the transistor in the plug, then it already needs two cores plus ground, so battery -ve would mean a 3rd inner core. Thrtes not much room to get it all connected. Its also good to usr a plug which had a very positive grip on the cable, since the main wires are only connected to fine transistor leads.

[petre]

I made this schematic of the complete circuit plus a CAD drawing showing how the parts can fit in a Hammond 1590B box.  Inside the guitar there is an extra 500pf cap installed before the volume pot to emulate 10' of cable engaged with the Push/Pull Tone pot.

[JohnH]

Petre, that is an awesome amount of detailed drawing. It should be fine, but make sure you rig it up in some rough temporary way before you build all that permanently. Need to make sure it all works with your parts, check voltages across the 33k etc. (it depends on the specific JFET, but expect about 2.5 V to 3.5V).

I have built that buffer inside a guitar, run on two 3v lithium button cells, for a 6V supply, and that seemed OK, so that might be a good lower limit at which to get a new battery, in about 10 years time!

Which pots are you asking about? I don't know about that battery indicator circuit.

Also, with the 0.5nF cap, will you be making that switchable? - if you do then the guitar will also work as normal with a standard cable to amp, not that that small cap makes more than a very slight difference though, so maybe its not a big deal.

[petre]

10 years!  It seems that this indicator circuit is a bit silly, especially with that stomp button on top.  I considered a really tiny SPST switch with a little hole that you have to press with a pencil or something, but then I changed my mind.  Maybe that's what I'll do.  Having 2 button batteries inside the guitar sounds interesting.  No added weight, and any guitar will have enough space for such a small circuit with 2 button batteries.  Can it really go 10 years on 6V with those button batteries?  This seems to be the simplest and most elegant solution.  Less things to carry around and can plug into any cable having the added benefit of a buffer.  What kind of batteries do you have?

Edit: Also I'm curious where did you put the batteries?  Maybe this is what we need, Fender S-1 9v knobs!  www.premierguitar.com/ext/resources/archives/f9b5ffce-f1b7-4615-96d2-adc7bb79b32a.JPG?1371672717

[JohnH]

This is what I did with the button-cell buffer:



The cells are two CR2025, mounted in a holder, which was taped to the back of a pot. Hot glue might have been better.  The buffer was assembled point to point around the jack. I left off he 2.2M input resistor for this application, and added 1M (could be higher)to tie the output of the cap down to zero. Looks like I used a 39k source resistor, and the Jfet was MPF102.

As to how long these things run for, it's gonna depend....

This circuit, as described above, if you used a 33k resistor, and if the JFET ended up with 2.5V at the source, will draw 0.076mA.

Here is a data sheet for CR2025:

CR2025 data

It seems to be good for at least 100mAhours, at low current, still at nearly full voltage, which gives about 1300 hours run time. But it will be also limited by the battery shelf life, and 10 years, maybe not! It worked fine for me for about a year, then I took it out to build in something more complicated. The buffer and batteries have been in my spares box for the last 6 years, and are now reading about 5.5V total.

If you could find space for a 9v lithium pp3, it would have 10x that capacity, and a larger headroom for discharge, so could last several further years.

[petre]

John thanks for posting all this information.  I just looked a bit more into those button batteries. It seems like there is a CR2450 that has 620mAh down to 2V. If I put 3 together I'll get 9V at about half the weight and size of a pp3.  Mouser has a Lithium Panasonic with tabs, so I can just solder them together and attach to the back of the pot or use a little bigger holder for CR2477.  Those should give close to 10,000 hours of playing for 10 years, which is like 3 hours every day during that time.  Even if it's soldered onto the pot, changing them together every 10 years is a good idea.  Making sense of that number just showed me how absurd it is to have a "battery indicator", especially since a 9V battery should give 2x that number.  Now I just have to decide which one to build.  I have edited the previous post with a new schematic without the extra unneeded circuit.  Just looking at all the parts that go into it, building the cable with the box could end up costing quite a bit more, but maybe it might still help somebody not technically knowledgeable like me to feel more at ease making it.

[petre]

Mar 21, 2014 19:57:21 GMT -5 JohnH said:

Need to make sure it all works with your parts, check voltages across the 33k etc. (it depends on the specific JFET, but expect about 2.5 V to 3.5V).

John are there any critical values that I need to check on the circuit, and where exactly do I need to check them? It was discussed previously in the thread, but I assumed it was relating only to the gain buffer.

So far I have two options, but I previously asked if it is possible to build a buffer cable phantom powered.  Tillman claims that his cable can work connected directly to a phantom 48V source, and shows a diagram with 1 transistor and 3 resistors inside the plug.  Will his buffer cable work OK with phantom power, and if not do you think your circuit can be modified to work in this configuration?

[JohnH]

The source follower circuit used in the buffers that I build is very tolerant of component variations. But JFETS are also very inconsistent, which is why they are not used in as many commercial applications as they might be.

The key voltage that we would like in this circuit is the source voltage to be at or slightly above half of the supply voltage. That way, there s the maximum range for the output to swing up and down, being pulled up when the jfet switches on more, and pulled down by the source resistor when the jfet is more 'off'

Jfets work by the conductivity between source and drain being controlled by the voltage between gate and source. Each one has a particular characteristic, voltage known as Vgs or Vgs-off. the balance between on and off is achieved when the source is at about this voltage above the gate. In steady state, with the buffer cable, the gate is based to zero. and at the source we get about 2.5 to 3.5V for that, usually, but different jfets will vary. Thats not half of 9v, but its ok and works fine.

When a + signal voltage swing comes, the gate voltage rises, switching on the jfet a bit more making it more conductive, allowing more current to flow, which then raises the source voltage until equilibrium is obtained again. So gate voltage has gone up, and source voltage has also gone up and they stay locked together, if one varies from the other, the gate to source voltage changes, changing the conductivity drain to source, which brings source and gate back together. If gate voltage changes, the source follows and its a form of neagative feedback that makes the source voltage a very close match to the gate voltage. Hence there is very low distortion, but much more current flowing at the source and plenty available for a low impedance output, which is what this circuit achieves. That's how it works.

So when you set it up, and power it up, you measure the source voltage, and you might actually try a few different jfets to get one with a Vgs that gives you a nice nearest to mid-supply voltage. Really it always seems to work, but the more central the source voltage is, the better the available voltage swing before clipping. Its unlikely to clip though. Most guitars even on a string-breakingly hard strum will only put out about 1.5V to 2V peak, though I've measured 3V with crazy amounts of series wiring and hot pickups. Once the initial transients have passed, the signal is much smaller. So we would like to have at least say +/- 2V swing available, preferably some more

With the non-cable buffers, I show a different jfet and two bias resistors at the front. This lets you pick resistor values to get that mid voltage which optimises the head room, and you can shoot for about 5V on a 9V supply. But then the gate is not at zero, so it also needs an input cap. The buffer cable is ending up maybe lower than half supply, but still enough voltage above ground to give adequate output swing.

Tillman is biasing his gate to ground, but using a J201 which has a typical Vgs of only around 0.75V, much lower than the MPF102 or 2N5457 that I use. That ends up as a limiting factor on his clean headroom, which is why it can clip on a loud input. A different jfet or more biasing components could help.

The Tillman circuit I think can work with phantom power, but it depends on the phantom power impedance being consistent at 6.8k, which is replacing his drain resistor. I don't know if that is an absolute standard across all equipment.

The source follower buffers, I cant see how to run that way without more wires and parts, and maybe not at all. Not sure there.

[petre]

After reading your first post and this last couple of times while looking at the diagrams, and after I watched 4 videos on youtube explaining transistors I think I now understand something which must look too simple to you.  I should have really done my homework before making you explain here everything.  Again thank you John for your detailed explanation and for your patience.  I have decided to build the in-guitar buffer.  Basically the closer Vgs of that J201 is to its specs (-1.5V), the closer the voltage will be to the optimum 4.5V and the more headroom available.

If anybody is unfortunate enough to have read all 6 pages of this thread, and is still scratching their head like I was:
1. have couple of J201 or MPF102 (5?)
2. close the circuit
3. measure the voltage across R1
4. pick out of the bunch the one that gives closest value to 4.5V

Mentioned youtube videos, including this exact basic amp configuration:
"What is a transistor? How does a transistor work?" (2 parts)
"Field Effect Transistors" (2 parts)

[JohnH]

One last thing, for now:

The buffer cable is contarined by very little space to have very few components. But given a square inch of circuit board, this one is a better option:



The two resistors biasing the gate can be picked so you get 4.5V to 5V at get source, its not quite so sensitive to components. Best way is to rig up the jfet and resistors, measure the source voltage then adjust to optimise it. It still not critical but you might as well have it working the best it can. The circuit above is fine for either a 2n5457 as shown, or a J201. It will also work with MPF102, but probably the upper resistor R12 will get higher.

This one has very good drive capability and can run into a mixer line in, though it uses a bit more current. If you don't want that facility, you can raise R4 to say 39k, and drop the output cap size. 

[petre]

Thank you again John for the input.  That's an extra resistor and a capacitor.  I just did a quick search on mouser, and turns out there's plenty of ceramic capacitors that are very very small.  0402YD393KAT2A this one is only .25x.5x1mm in size, a MLCC at 16V!  Tillman was able to fit 2 transistors and 4 or 6 resistors in his stereo jack.  So maybe having 1 transistor, 2 resistors and a tiny cap in the plug is still a viable option even with this circuit.  It is always a good thing to have more headroom or more gain.  What do you think?

Edit:  0402YD393KAT2A was just theoretical, there are bigger ones that would be easier to solder.

[JohnH]

Mar 23, 2014 12:06:25 GMT -5 petre said:

Thank you again John for the input.  That's an extra resistor and a capacitor.  I just did a quick search on mouser, and turns out there's plenty of ceramic capacitors that are very very small.  0402YD393KAT2A this one is only .25x.5x1mm in size, a MLCC at 16V!  Tillman was able to fit 2 transistors and 4 or 6 resistors in his stereo jack.  So maybe having 1 transistor, 2 resistors and a tiny cap in the plug is still a viable option even with this circuit.  It is always a good thing to have more headroom or more gain.  What do you think?

Edit:  0402YD393KAT2A was just theoretical, there are bigger ones that would be easier to solder.

There's an in-between version where c4 and R8 are inside the guitar, then its still just one jfet and one resistor in the plug. The guitar will still work passively with a normal cable. Maybe make C4 a bit bigger, say 100nF in that case

[petre]

I did some testing and I was able to fit 3 resistors, and a transistor in a a right angle switchcraft jack without much difficulty.  It should be the same with those tiny caps.  John please correct me if I'm wrong, but from what I understand in your design having two bias resistors allows higher voltage at the source, idealy half of the supply, so more available headroom.  In other words the original buffer cable design that has only one bias resistor inherently has less headroom, but that's ok because 2-3V is enough in most circumstances (medium gain pickups in normal wiring, medium length cable, normal high impedance input)?  I'm asking because you showed the picture with the button batteries, and from what you said it looks like you built it according to your buffer cable schematic.  If I can fit those 4 components inside the plug no problem I can build either one for a cable or inside the guitar.  Besides having a little less tolerance with the MPF102 to try to get above 3V at the source, are there benefits of one of the circuits over the other?  I'm talking here about the 1x buffers.

Another thing that I read from an old thread is you mentioned how you tested the batteries using a stereo plug.  I want to mention it here because that is not mentioned in this thread, but I think it is a very useful thing to be able to do.  Looking at the diagrams, am I right to assume that the output voltage at both buffers should be about the same as what they have at the source?

Edit: Also if what I said above is correct, that would make say a 6V battery in the circuit with the J201 give about the same headroom and battery life as the circuit with MPF102 and 9V?  Assuming I use two or three of the same button batteries.

[JohnH]

I guess all that is true. Having the second bias resistor is better, particularly if you pick its value to suit the actual jfet.

Testing the batteries with stereo jack measures battery voltage through the circuit unpowered. You get a low reading depending how conductive it is. So best to try wifh a good battery to get a benchmark.

[petre]

Hi John,
Thank you once again for your informative input.  I have a question which might go beyond the scope of this thread if it has any relevance at all.  Does the capacitor on the input of the buffer have any reasonable influence on the sound of the guitar?  This cap isn't just dumping the high frequencies like what the tone cap does, but it stands directly on the signal path.  If this was a regular amp, if I'm not mistaken this cap would be of importance.  I understand this is just a guitar sound, but I was thinking maybe those tiny caps that I mentioned in a previous post are not good enough.  If anything of what I'm saying here has any relevance, what do you think would be a reasonably "good" cap to use here.

[JohnH]

Petre,

The cap is needed for the buffer design where the jfet gate is biased above 0 volts. It separates dc bias for the jfet from the guitar, which is all at ground. Any normal type of cap is fine, nothing special needed. It has negligible effect on tone unless it is a much smaller value and starts to block bass.

I drew it as 39nF. If you work out he impedance of that at say 80hz (just below low E on a guitar), it's impedance is 51k. The input impedance of the buffer is about 1M. The net impedance of input plus cap is worked out like the long side of a right angled triangle, sqrt(1000000^2 + 51000^2) = 1001300.

Net reduction in bass due to the cap is 1000000/1001300 = 0.9987 = -0.01db = nothing.

[matbard]

Mar 23, 2014 2:40:16 GMT -5 JohnH said:

One last thing, for now:

The buffer cable is contarined by very little space to have very few components. But given a square inch of circuit board, this one is a better option:



The two resistors biasing the gate can be picked so you get 4.5V to 5V at get source, its not quite so sensitive to components. Best way is to rig up the jfet and resistors, measure the source voltage then adjust to optimise it. It still not critical but you might as well have it working the best it can. The circuit above is fine for either a 2n5457 as shown, or a J201. It will also work with MPF102, but probably the upper resistor R12 will get higher.

This one has very good drive capability and can run into a mixer line in, though it uses a bit more current. If you don't want that facility, you can raise R4 to say 39k, and drop the output cap size. 

Sorry for resurrecting this thread...

What happens if I use a 2N5485 instead of a 2N5457 and change the power supply to 24V? My simulation software is not so accurate(or perhaps I'm unable to do a correct simulation...)

[JohnH]

Hi matbard, welcome to GN2

I think that Jfet would work in this circuit since it is quite tolerant. I don't expect a problem with 24V supply. What do you have in mind to use it for? It would be best to test it to measure the voltage at the source, then maybe change one of the input resistors to keep the source at around mid supply voltage. But I also think that with 24V available, it will work fine as drawn.

[matbard]

Hi JohnH,

Thank you for your precious help and the warm welcome.

I have one of those "24V mod" for EMG hanging around home, so I'm intrigued to use it to supply power for the preamp. I usually play a beautiful Electric XII replica that I built some times ago with a friend of mine, and 99% I use it plugging directly into a mixing console desk. Having an "universal" output that can drive both a  guitar amp input or a line level input would be a great benefit. Maybe I'm guessing if I can squeeze a preset compressor circuit directly inside the control cavity...

The preamp cable solution was the one I considered first, but your last schematic version intrigued me more...  

[johan]

Maybe this is an idea, but still trying it out, have built it.

• Seems to bias just fine with the very first jfet I took out of the box
  
• Should have about 15dB gain
• Trimpots allows different transistors/supply voltages to be used.
  

Tantalum cap maybe the best way to go inside the jack... you could do with lower value there, maybe omit... I just had the 22µF handy

[JohnH]

That looks like a very credible circuit. Congrats, especially if you can get that into a jack plug without shorting.

If it is only to go into a passive guitar, you can lose the input cap, since its already all at ground potential. In this thread, you will have seen what I did, with much less in the plug, but no gain. My approach to adding gain would have been to put all of those parts in the box at the other end (basically per a circuit like yours), just because I doubt I could successfully build so much into the jack.

[johan]

Mar 5, 2016 15:18:39 GMT -5 JohnH said:

That looks like a very credible circuit. Congrats, especially if you can get that into a jack plug without shorting.

If it is only to go into a passive guitar, you can lose the input cap, since its already all at ground potential. In this thread, you will have seen what I did, with much less in the plug, but no gain. My approach to adding gain would have been to put all of those parts in the box at the other end (basically per a circuit like yours), just because I doubt I could successfully build so much into the jack.

Well, I certainly didn't mean to step on your toes like that. Did I mention: soldering skills & patience required?
What you say about the input capacitor would apply to your design, but in the design I proposed, it is better to put it in.
The outset was not to put as few components in the jack as possible, but rather to solve the JFET inconsistency issue.

[JohnH]

Mar 12, 2016 6:25:02 GMT -5 johan said:

Well, I certainly didn't mean to step on your toes like that. Did I mention: soldering skills & patience required?

What you say about the input capacitor would apply to your design, but in the design I proposed, it is better to put it in.
The outset was not to put as few components in the jack as possible, but rather to solve the JFET inconsistency issue.

Hi Johan, no toes were stepped on! I'm happy to see new designs here, and your work is cool.

Those 2N5457's are mostly perfect for guitar drive and buffer circuits. They have a cut-off voltage that puts them in the right range for the voltages out of pickups. They seem to be getting rarer though. But luckily for me I have most of a x100 pack left, so I'm OK for any further experiments.   

[reTrEaD]

Mar 4, 2016 12:35:46 GMT -5 johan said:

Tantalum cap maybe the best way to go inside the jack... you could do with lower value there, maybe omit... I just had the 22µF handy

Hello, Johan!

I agree with John. Your work is cool.

Omitting the bypass capacitor on the source resistor will reduce the gain. The signal on the source will follow the gate fairly closely. So the gain will be a function of the ratio of the drain resistor divided by the source resistor. It will still be well above unity, so this is still a viable option. It all depends on how important gain is to you at this point. In my humble opinion, it's just a minor bonus. The more important feature of an active cable is removing the cable capacitance.

I'm not sure how to do the math but my gut tells me you're safely below the lowest frequency of a bass guitar for your LF rolloff caused by the 22uF capacitor. You can *probably* lower the value but it would make sense to do a spice simulation or test with a signal generator and scope.

I also agree with John about the input capacitor. It's completely unnecessary. Your gate is referenced to ground.

I very much like the fact that the cable only needs to have a single conductor. That's slick.

Voltage swing possible at the emitter of Q2 should be roughly (Vsourceresistor - 0.7V) on the bottom end and supply voltage - 0.7V at the top end. Setting the idle current to get 1/2 supply voltage on the emitter of Q2 seems to be just right if the idle voltage across the source resistor is about 1.5V. If the voltage across the source resistor is higher than that, you should adjust the voltage at the emitter of Q2 to more than 1/2 supply voltage.

But if you remove the bypass capacitor on the source resistor, your drain will never be able to approach idle voltage on the source resistor. The voltage across the source resistor will tend to follow in the same direction as the input signal.

.

The wonderful thing about these active cables is the freedom from the treble loss that occurs when the guitar volume is turned down. We no longer have a high-cut filter caused by the series resistance of the volume pot and cable capacitance. But in a guitar that's equipped with a treble bleed network, we should expect added brightness when the volume is turned down (when feeding an active cable). Not sure if that will be a nuisance. Maybe John could shed some light on that, due to his experience?

[JohnH]

Yes i think that 22uF cap, related to the 3.3k next to it, rolls off the gain below 2.2hZ. So you could use a smaller one!

Do you find with this circuit, you need to reduce guitar volume to get a clean tone? I found so with similar circuits on my guitars. No prob with that though, since the cable buffer means you can use the whole guitar volume control range with consistent tone.

[reTrEaD]

Mar 17, 2016 16:30:05 GMT -5 JohnH said:

Yes i think that 22uF cap, related to the 3.3k next to it, rolls off the gain below 2.2hZ.

I think it's more complicated than just using the value of the source resistor and its bypass capacitor to determine the rolloff frequency. You need to use the "source resistance" in parallel with the source resistor. I don't know how to calculate source resistance but I reckon the value of the resistor in series with the drain and some device characteristic of the FET will come into play. Maybe someone reading this can weigh in.

Barring that, it shouldn't be too hard to determine the appropriate bypass capacitor experimentally. Use a smaller bypass capacitor (maybe about 1uF) and feed the circuit with an audio generator, measuring output voltage with an oscilloscope. At higher frequencies we expect the output should remain constant. Decrease the frequency until the output drops to about 70%. This tells you the rolloff frequency with this capacitor.

Divide the frequency measured during the test by the desired rolloff frequency. Then multiply that times capacitor used in the test. This will give you the value appropriate capacitor.

Or something like that...