If you are a pedal builder that likes to experiment with different op amps in your Rat pedal, then you should try the NJMOP07D chip that is still available from Mouser.
The slew rate is about 40% slower than the coveted LM308 but it is a single op amp with a similar pin configuration that should be a direct drop-in, and you can quickly test the sound of it for slightly more than a dollar!
Blind testing a 1 db difference in volume: https://www.audiocheck.net/blindtests_level.php?lvl=1
I was surprised that when I repeated the test 10 times, I could identify the 1db change with 99.65% confidence. That was on the first pass, and with a little practice, I am certain that I could do it with 100% results. However, when testing audio devices, 1db is a large change!
There are smaller decibel differences in the testing that you can try. It is an interesting test site.
This is the basic schematic of a PNP germanium fuzz face. Some players find that the output volume is too low, but there is a simple mod to fix it.
Just got a new package from OSHpark with some prototype pc boards. Besides the Squeezer compressor, there is a new op amp fuzz and 3 sets of mini-boards, one of which is a tiny mini-booster. More info soon.
While looking through some boxes of old material, I came across this completed pcb for the ADV-Snare project that I designed with Thomas Henry. The article on this project was published in Nuts-and-Volts magazine over 20 years ago!
Low input impedance is not really the issue making a fuzz incompatible with a buffer.
A fuzz face has a lot of gain…. VERY high. It is basically a crude inverting op amp with a 100k resistor in the negative feedback loop. There is no resistor in series with the audio input that would otherwise reduce the gain. The simplified gain equation is Rf/Ri where Rf is the feedback resistance (100k) and Ri is the input resistance. When connected directly to a guitar, the combined resistance and impedance of the pickups are in series with the input (acting as Ri) and will reduce the gain, thereby preventing the fuzz from going bonkers.
If you drive the fuzz with a buffer, the output resistance/impedance of the buffer is very low so the result is that the fuzz circuit runs at max gain with oscillations and other mayhem often as the result. Buffered input = max gain = bonkers, and guitar pickup = lower gain = fuzz heaven 🙂
The Rangemaster does not have this problem because the available gain from the Ge transistor is much lower than that available from a FF.
The Big Muff does not have this problem because each gain stage has a series resistor to reduce their gain… 39k on the input stage, for example.
Charge pumps are fairly simple devices. They involve charging a capacitor to the voltage of the supply, and then stacking it on top of that power supply so that the voltage potentials are added together, effectively doubling the output.
It looks like the pcb on this Daphon E20CM compressor was supposed to have a square cutout in the top left corner to match the one on the right side, in order to fit around the mounting post. When cutting the pcb, the board fractured and chipped off a small piece including a portion of a copper trace. Instead of trashing the damaged board, they added a bodge wire to replace the missing trace.
I ordered a selection of prototype mini-boards from Oshpark, including tiny boosters, fuzz, and more.
I asked the manufacturer of my audio analyzer instrument to modify one of their plugins to be able to measure compression. They did so, and it works really great, so I am planning on doing some tests on various guitar compressors.
In my first trial run of the compressor test setup, I learned 3 things:
I’ll have an article about the compression results in the near future.
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Jack Orman has been involved in FX design and construction since the mid-1970s.
