A popular article on my web site is the one about pickup simulation, and questions come up several times every year about the transformers recommended for this project. One technique that works well is to get a cheap guitar pickup and use it as the input inductor.
The black wire of the pickup goes to the input jack while the red wire connects to the input of the electronic circuit. If there is also a braided shield, it can be connected to ground. The 10k trimmer and the 330pF capacitor are simulating the cable connecting the guitar to the pedal.
Cheap pickups are readily available for $5 or $6 on Ebay, or from US suppliers of parts for cigar box guitars (cbg). The 3-pole pickups for the cbg projects will be physically smaller and there are even versions with a metal shield over the coil.
The pickup has the electrical characteristics of a guitar pickup (because it is one), and in this circuit it works quite well to modify the tone and response of a fuzz circuit or other project.
This is a small test board that I made for my own use. It goes between a power source and a pedal being tested. The objective is to provide access to the voltage source terminals. A battery or unregulated power supply will often have voltage sag when loaded, so this lets me measure the actual voltage with the pedal connected without having to remove the back of the box.
The power jack was mounted on the back of the pcb to make sure that it is out of the way of the probes, but now that I have assembled it, that placement seems unnecessary.
There is not only easy access to the actual voltage being used, but also the yellow jumper block can be removed and a meter placed across the two pins so that a measurement of the current draw can be easily made.
This graph illustrates why the Mini-Booster has a tube-like sound. The second harmonic (2HL) is much larger than the third (3HL). In fact, the total harmonic distortion (THD) is almost all second with only small amounts of third at lower levels. Only when the pedal begins to clip at higher input signals does the third harmonic begin to rise.
Mini-Booster pc boards and pre-assembled modules are available on the AMZ order page.
The forward voltage of a diode is not fixed, and this is easily seen on the datasheet for the 1N4148, for example. At 200ma, the 1N4148 diode Vf is over 1v but if the current is only 1ma then the Vf is less than 0.625v. As the current goes lower, so does the diode’s Vf.
This circuit will vary the peak clipping of the diodes, but only over a modest range. Also, the opamp driving it must be capable of sourcing enough current to make a difference, and you have to be careful that the power rating of the 47 ohm resistor is not exceeded.
The circuit is of limited practical use, but there it is for your experimentation anyway!
While getting acquainted with my new audio distortion analyzer, I decided to do a quick test to compare a true bypass pedal with another that used jfet bypass switching.
Since there was a new Boss SD-1 in a box next to my workbench, that is the pedal that I chose for the fet bypass test. The true bypass test was actually performed twice using a prototype of the Mosfet Boost module pedal and a similar mosfet boost pedal that I made using through-hole parts, but there was no difference in the results when the pedals were bypassed.
The bottom line is that jfet switching with buffers, as used in the Boss pedals, results in 135 times more distortion! It also has a signal loss of 0.81dBV while true bypass has no loss.
True bypass did not introduce any distortion and the THD shown in the listing is the residual distortion from the sine wave generator. It was the same with the true bypass switch as with a straight loopback without the pedal.
I will post a more complete article when I have had time to run further tests and optimize the settings on the test equipment.
Update: Further tests on bypass switching were conducted and the article is now in the Lab Notebook. Enjoy!
All resistors have a certain amount of noise when current flows through them. Other factors then add more noise on top of that baseline, such as size, composition and quality of construction. No-name cheap smt resistors will have 200x more distortion than common 1/4w metal film through-hole resistors! Smaller smt resistors have more noise than larger sizes; example 0603 vs. 0805 smt sizes.
Good quality Panasonic or Susumu smt resistors will be better than cheap no-name resistors, and can be close to through-hole devices in performance. Dave Hill of Crane Song shared some real world measurements of resistors which clearly shows that there is a difference in performance between smt types vs through-hole. The diff is small but measurable.
