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.
In the last year or so, I have noticed a number of new pedals being introduced that are being called preamps. So what is a preamp, and can a pedal serve as one?
In audio applications, a preamp is a circuit that amplifies very weak audio signals to a level that will drive a power amplifier to its rated capacity.
The concept of a pedal that sounds like an overdriven amplifier has been around a long time, and in the late 1990s, I contributed to the field with several designs. The most influential of them has to be the Booster 2.5 pedal, which was a derivative of my earlier Mini-Tubes Preamp.
Fake LM308 in a Mooer pedal… well, not an original National Semiconductor chip anyway.
So, is it a fake if you mark a plain cheap op-amp that meets or exceeds the specs of the original LM308 with that part number but the internal circuit is not the same?
It will work in circuits designed for the LM308 and perform well. It does the job. But at the fringes and limits of the electrical characteristics where many fx pedals operate, it will not be the same as an IC with the internal circuit design of a real original NatSemi LM308. Is that a fake or just a poor substitute?
A great resource about building stuff, including electronic projects, HackSpace magazine is packed with projects for fixers and tinkerers of all abilities.
Why are US-made pedals significantly more expensive than Chinese clones?
TL;DR: Chinese companies receive subsidies from China’s central and provincial governments. They also have an artificially low currency-exchange-rate, smaller profit margins, few worker safety programs, little environmental control, low taxes, low wages, long hours and few worker benefits, so the price of Chinese products will always be lower than similar products made in the USA.
