Boosters Are Not Buffers

  
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Boosters are not buffers! While it is possible to make a booster that also provides a buffering action, the vast majority of the boosters commercially available ready-made or kits, do not have the proper design to be buffers. A buffer should have a high input impedance (Z) and a low output impedance. The LPB1, shown at the top of this post, has a
Boosters Are Not Buffers - schematic

fairly low input impedance, which automatically disqualifies it from the buffer category. The mini-booster has a high input impedance but a moderately high output Z, and is not a good buffer either. Another part of these designs that prevents their use as buffers is that they have a volume control potentiometer on the output. The volume pot impacts the impedance of the circuit. If the volume pot is at the 50% position, represented above by R5 and R6, the output pot is acting as a voltage divider. Here we have the 100k pot in parallel with the output impedance of the LPB1 (10k) that is driving the cable to the amp or next pedal. The cable capacitance (Cc) even on a good cable can be 800pF, as I measured in a previous test, and is shown on the schematic in light blue. The output impedance of the booster and the resistance of the pot are driving the cable and form a low pass filter with the cable capacitance. In this example, the high frequencies are rolled off starting at around 7. 2kHz. This is obviously not what we want from a buffer! This is a typical opamp distortion pedal schematic fragment, and the high frequency limit is similar to the transistor booster example (7. 5kHz corner frequency). You should note that as the volume is turned up, the frequency limit rises as well, letting more high frequencies through. The shunt resistance of the pot, shown as R6 on the examples, is in parallel with the input impedance of the...



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