Drive long lines and heavy loads
There are situations in which very long cable runs are necessary from the pedalboard to the amplifier, or a more powerful driving circuit is needed to the heavy load presented by devices in parallel. A good buffer box is the solution.
The best buffer board will present a high impedance to the input while having a low impedance output. This insures that the maximum clear signal will pass through unchanged to the output, and a low impedance drive can send the signal through long cables without losing much strength or response.
There are numerous special purpose chips that are designed as buffers for driving very long lines and/or capacitance loads. These ICs can be difficult to find and expensive. A simple solution for stompbox purposes is to parallel opamps and thereby boost the output drive. With attention to a few details, this is a simple project to build.
I have designed the AMZ Super Buffer to be flexible and easily modified to suit individual tastes. The schematic is shown below:
Resistor R5 is an pop prevention component and can be eliminated if desired. The value of R6 is essentially the input impedance if R5 is not used. When R5 is included, the input impedance is the parallel value of R5 and R6.
Two capacitors are used on the output - since we do not know what load this circuit may be driving and we want to get full response, a 10uF output capacitor (C3) is a good value to chose. A 0.1uF metal film (C2) is placed in parallel with C3 to insure that the response remains flat at high frequencies where the rising impedance of electrolytics could effect the sound. C2 could be left out without sacrificing much.
An additional tweak is to make the R6 resistor a metal film component for lower noise. If you are making this mod, you might as well make R5 and R7 metal film also since they are all the same value.
The circuit could be made with a quad opamp package but it is more interesting to use a pair of dual opamps. While the circuit is designed to use two identical dual opamps, there is no reason that one cannot make substitutions to check out the sound. For instance, put an NE5532 in the IC1 socket and an RC4558 in for IC2. Is there are difference in the sound? Feel free to experiment!
Also, the circuit works well with only one IC chip in place. It can be used in this manner to reduce power consumption requirements. To reduce current requirements even more, use FET opamps like the TL072 or similar.
If you want to disable all but one of the buffers, remove IC2 from its socket and also remove R2 (100 ohms). This will provide only a single opamp output drive via IC1a for use as a basic buffered output but it is more fun to be able to mix and match opamps for subtle variations of tone.
The best place for the Super Buffer is at the end of your chain of pedals. In this position the buffer will be a line driver for the long cord back to your amp or mixer. Alternately, it can be at the front of the pedal chain which is good if you have all true bypass pedals. No matter where you place it, the AMZ Super Buffer can clean up your sound and allow the maximum tone from your pickups to shine through!
If the 5532 or OPA2604 opamps are used, there is enough drive from this circuit to power a set of headphones! Of course, it is not optimized for that purpose and the flat frequency will not sound good, but the power is there.
©2005 Jack Orman This page last modified on Sunday, 14-May-2006 06:52:43 PDT
103774 hits since May 18, 2006
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©2005 Jack Orman
This page last modified on Sunday, 14-May-2006 06:52:43 PDT
103774 hits since May 18, 2006