Thanks again. Once last question (hopefully!) Can I use a two 10M in parallel (=5M) for R2 (I have in "inventory"), or am I better to pick up a 4M and/or 6M?
You can try a 5M pot to replace both R2 and R3 ...
Just connect the center wiper to the Gate of your JFET....
This way you can quickly trim in the bias to where you like best... some times you can do it with a scope where slowly increase input signal and trim for equal onset of clipping on either side of sine wave..
If you go too big on the R2 and R3 resistors you can kill some of your high frequencies as well as introduce additional noise....
Just connect the center wiper to the Gate of your JFET....
This way you can quickly trim in the bias to where you like best... some times you can do it with a scope where slowly increase input signal and trim for equal onset of clipping on either side of sine wave..
If you go too big on the R2 and R3 resistors you can kill some of your high frequencies as well as introduce additional noise....
Thanks. That's an interesting idea. Would that also allow me to adjust the pot to handle single coil or hummer pups?
The pot would allow you to bias the buffer stage for maximum signal excursion prior to clipping... This is irrelevant as for SC or HB coils...
Keep in mind that this is just a simple buffer stage and is not perfect but should get the job done ... I hope 🙂
A good source in Canada for caps and resistors is justradios (Dave Cantelon) in Toronto/Scarborough, ON.
I've ordered from him quite a few times.
High Voltage Resistors and Resistor Kits for Tube Radios and Electronics
Mouser and Digikey are also OK to Canada (IME) since they fixed the shipping costs/system a few years ago.
Thanks. I think I'll try to pick up a 5M pcb trim pot. Thanks for the parts link. I've had pretty good success finding most parts from Sayal in Canada.
Hi Guys
The original splitter circuit uses voltage divider bias to set the output idle voltage point.The voltage at the jfet source will be slightly higher than on the divider (gate), which is nonproblematic provided only typical guitar-sized signals are passing through. If you want to tweak the output voltage exactly to half the supply, just make the upper R larger or the lower R small.
The input impedance is set by the parallel combination of both divider resistors and the leakage resistor for the input cap. For 2M2 x2 and 10M this works out to about 1M, which is okay for standard guitar pickups but not for a raw peizo.
The 2M2 resistors are noncritical in value and can easily be 10M each.
A large filter cap should be added to assure that the the supply is at a good AC ground. Relying on the battery or some wall-wart supply for this is not recommended.
The current through each jfet is set by its individual source resistor and the half-voltage bias. Assuming 9V input, the voltage across the source resistor is about 4.5V and current is then 450uA using 10k. This does not make a very good driver for long cables. A lower R should be used, but you can judge performance in your own system.
Removing the upper resistor of the divider is not a good idea. This will drastically reduce signal head room and the possibility of distorting the signal is greatly increased. To bias the jfets in this situation requires changing the source resistors to a much lower value, say 1k or so. Output signal swing and actual idle conditions now depend much more on the individual jfet characteristics than with the divider bias where the circuit set the performance.
As with all single-element voltage followers, there will be a slight signal loss at the output.
Have fun
The original splitter circuit uses voltage divider bias to set the output idle voltage point.The voltage at the jfet source will be slightly higher than on the divider (gate), which is nonproblematic provided only typical guitar-sized signals are passing through. If you want to tweak the output voltage exactly to half the supply, just make the upper R larger or the lower R small.
The input impedance is set by the parallel combination of both divider resistors and the leakage resistor for the input cap. For 2M2 x2 and 10M this works out to about 1M, which is okay for standard guitar pickups but not for a raw peizo.
The 2M2 resistors are noncritical in value and can easily be 10M each.
A large filter cap should be added to assure that the the supply is at a good AC ground. Relying on the battery or some wall-wart supply for this is not recommended.
The current through each jfet is set by its individual source resistor and the half-voltage bias. Assuming 9V input, the voltage across the source resistor is about 4.5V and current is then 450uA using 10k. This does not make a very good driver for long cables. A lower R should be used, but you can judge performance in your own system.
Removing the upper resistor of the divider is not a good idea. This will drastically reduce signal head room and the possibility of distorting the signal is greatly increased. To bias the jfets in this situation requires changing the source resistors to a much lower value, say 1k or so. Output signal swing and actual idle conditions now depend much more on the individual jfet characteristics than with the divider bias where the circuit set the performance.
As with all single-element voltage followers, there will be a slight signal loss at the output.
Have fun
I like to bias these so the source is just above halfway. the reason is that on the upswings, the Jfet switched on has a reasonably low resistance, and can pull the output voltage up against a load (due to cable capacitance or sometimes input impedance, say if you use it to drive a line input). But on the downswings, the most the circuit a=can do is to pull the output down via the 10k source resistor, with jfet off. If say, you bias the source for about 5V out of a 9V supply, you could drive say a 10k input impedance, with a voltage swing of up to 2.5V. Greater than that, the resistor cant pull the level down enough and it clips. But 2.5V swing is more than plenty for a passive guitar, even a hot one, and that's only for such a 10k input impedance. If driving a high impedance load, there is even more headroom. Actually, my standard design for these is with a 15k source resistor.
The output impedance of this circuit is determined by the Jfet rather than the resistor, and in my experience with this circuit (analysis and testing), it does a good job with long cables. There is a form or negative feedback inherent in this type of follower circuit, such that if the load tries to distort the linearity of the output, by say, dragging it down, the gate-source voltage difference increases to let the Jfet switch on more, to keep the voltage where it should be.
Its not as linear as more sophisticated circuits, but the very small non linarites that it has are very musical, so it works great for instruments. .
The output impedance of this circuit is determined by the Jfet rather than the resistor, and in my experience with this circuit (analysis and testing), it does a good job with long cables. There is a form or negative feedback inherent in this type of follower circuit, such that if the load tries to distort the linearity of the output, by say, dragging it down, the gate-source voltage difference increases to let the Jfet switch on more, to keep the voltage where it should be.
Its not as linear as more sophisticated circuits, but the very small non linarites that it has are very musical, so it works great for instruments. .
I'm very grateful for all the assistance I've received this post. My problem is that I don't know very much electronics and just build a few projects from a la "cookbook".
As luck would have I haven't had an opportunity to change R2 to 5M and R3 to 2M.
Is that still my best option?
Thanks.
As luck would have I haven't had an opportunity to change R2 to 5M and R3 to 2M.
Is that still my best option?
Thanks.
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