[2607] Low Input Impedance Fuzz

Date: August 16th, 2019 | Comments : none | Categories: DIY.

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.

 

[2604] How Charge Pumps Work

Date: August 9th, 2019 | Comments : [2] | Categories: DIY.

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.

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[2599] Bodge wire

Date: August 1st, 2019 | Comments : none | Categories: DIY.

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.

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[2596] Mini Boards

Date: July 28th, 2019 | Comments : none | Categories: DIY.

I ordered a selection of prototype mini-boards from Oshpark, including tiny boosters, fuzz, and more.

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[2594] Compressor Testing

Date: July 26th, 2019 | Comments : none | Categories: DIY.

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:

  1. My Empress Compressor seems to be broken;
  2. The LM3080 based compressor is not as noisy as I anticipated;
  3. Opto-coupler compressors do not react linearly.

I’ll have an article about the compression results in the near future.

 

[2583] Power Jack Ratings

Date: June 21st, 2019 | Comments : [4] | Categories: DIY.

Did you know that DC power jacks have a voltage and current rating, and they may not be as high as you would think?

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[2579] Pickup Simulation

Date: May 16th, 2019 | Comments : none | Categories: DIY.

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.

 

[2566] Inside Ge Transistors

Date: May 10th, 2019 | Comments : [1] | Categories: DIY.

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[2562] Power Testing

Date: May 8th, 2019 | Comments : none | Categories: DIY.

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.

 

[2551] Closeup of LEDs

Date: April 29th, 2019 | Comments : [1] | Categories: DIY.

This is a rectangular LED shot with a closeup lens. The plastic housing is so clear that the internal parts of the diode are easily visible.

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