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AMZ Jfet Splitter
Multiple Buffered Outputs

This is the schematic of a simple jfet buffer from the Basic Buffers article in the AMZ Lab Notebook, with the addition of pulldown resistors at the input and output.

The circuit has a high input impedance and a low output impedance suitable for driving long cable runs or low impedance pedal inputs. Current draw is low and it is quiet and simple to construct. A stripboard (veroboard) layout and a pcb design are included in the original article about this circuit.

The jfet transistor Q1 can be any common small signal device such as the 2N5457, MPF102, or J201.


I was looking at the article for the buffer and realized how easy it would be to extend the circuit so that it is a signal splitter... a high impedance input buffer but with multiple independent outputs. All that is required is to parallel extra source follower units with the original.

As you can see here, two more fets have been added to the original circuit. Each has separate low impedance drives from their source connections. Since the gate junction of a jfet is extremely high impedance, it does not change the input characteristics of the buffer circuit when additional transistors are put in parallel.

With three totally independent buffered outputs, one can be dedicated to a tuner while the remaining two may be used to drive stereo pedal inputs or even two separate amplifiers.

This splitter would work great at the input of your pedal chain, immediately after the guitar so that the pickups are loaded by the high impedance of the buffer/splitter. If you want to change the input impedance, the values of R2 and R3 can be altered to suit your taste. If the signal seems too crisp and bright, lower the values of those two resistors to 470k each. Always keep R2 equal to R3.

For an even more neutral sound with less loading of pickups (especially good for dual humbuckers), set the values of R2 and R3 at 10M, or even 22M.

It is not necessary to use all three outputs. If you have a need for only two, leaving the output jack of the third buffer without a plug in it will not cause a problem.

A fourth section could be similarly paralleled with the transistors shown here if a four-output splitter is required.

One more option would be to add a mute switch to any output that you need to be able to silence. As an example, if you want to be able to turn off Out3, you could connect an SPST in parallel with the R9 resistor. When open, the output performs as normal. When the switch is closed, output 3 is pulled to ground and no signal passes. Similar switches could be added across the Out1 and Out2 pull-down resistors if desired.


It is easy to modify the stripboard layout of the original buffer to accomodate the additional jfet sections. The gate and drain leads of each of the jfets are in parallel so only the source has to be isolated.

The four red boxes with red dots in the middle are places where the copper stripboard traces have to be cut to make the circuit function properly. These cuts will isolate the source connections of the transistors. Just cut away the copper while leaving the epoxy substrate intact.

If you want to make a pc board for the splitter, I have these images available for download: PC Board and PCB parts placement. The pcb board is the same basic layout as the stripboard.

Source follower buffers are not too demanding about the choice of transistor so any jfet that you have in your parts bin would work in this circuit. Typical devices that I would suggest here are the J201, 2N5457, 2N5486, 2N3819, MPF102, BF245, 2SK117, 2SK130 and similar parts.

To put a fourth output on the stripboard, merely duplicate columns 14 through 18 at the right side starting with the new column 19.

Easy to make, inexpensive and excellent performance. What more do you need from a pedal design? Make one and expand your pedalboard options!

 


©2007 Jack Orman
All Rights Reserved

This page last modified on Friday, 26-Sep-2008 09:18:10 PDT

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