Category Archives: notes

Milling PCBs with Linux…

This is a quick outline of my LinuxCNC setup. Basically, I have an old HP Compaq Pentium IV that’s running Xubuntu 14.04. I had to install the Xenomai real-time kernel in order to make it work with LinuxCNC 2.5.2 which controls my Chinese CNC3020T.

I’ve been using 0.1mm 30° engraving bits for etching which have worked nicely. For drilling, I’ve got some carbide 0.7mm drills. I haven’t broken any of them thus far. I’ve setup a Dremel 3000 with a 220-01 rotary tool work station for additional drilling by hand.

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For sanding, I’ve been starting off a freshly etched board with a 180 grit sanding block and then moving down to a 320 grit for smoothing. So far, I’ve been impressed with the results. The following picture shows a single pass isolation trace after sanding at ~75x magnification.

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#NoDAPL

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I believe it’s time to put an end to this kind of insanity. There can be no economic growth if human beings continue to be reckless with the environment that supports them. Clean air, water, and soil are necessary for all life to flourish. That ought to be a priority.

Differential Amplifier w/ Cathode Follower

Differential Amplifier with Cathode Follower

This is a circuit designed for a classroom project whereby the instructions were to “improve” a differential amplifier circuit. The differential amplifier design is essentially an exercise in understanding the inner workings of an opamp, and it effectively works in the same way. The figure below is a schematic for the differential amplifier without the cathode follower output stage. In the LTspice simulation, the input signal is connected to the V+ terminal and the V- terminal is connected to ground. There is no feedback loop between the output and either input terminal making this a high-gain open-loop configuration. However, adding a resistor from V- to ground and one from the output terminal to V- would accomplish the same results as a non-inverting opamp.

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I wouldn’t say that adding a cathode follower “improves” the output stage of the amp. It was more an experiment in comparing different solid-state output stages with a vacuum tube stage operating at very low voltages. However, I will say that this thing sounded amazing with the couple of guitars I tested through it. Putting a potentiometer in the feedback network allowed me to play with different gain settings. It’s a very bright sound overall giving lots of high-end sparkle, but the breakup was quite remarkable. I suspect this might be a very usable configuration for a tube mic preamp or a number of audio applications. Hopefully, I will get an opportunity to revisit this before too long.

For those of you interested in the ins and outs of this experiment, you can download our full report here.

Already Dead Tapes releases Document II

“As we march along on our mission to create a diverse and expansive creative community, we reach a new milestone: 200 releases. To this end, we bring you ‘Document II’, a retrospective cassette that comes with a Golden Ticket good for downloads of our last 100 releases. This is a one-time limited edition of 200 golden cassettes with hand numbered gold fold j-cards and golden tickets.

The mix itself is a collection of just a few of our favorite tracks from the last one hundred releases, a diverse collection of tracks that span genres, from artists all over the world. As always, thank you for listening.”
– Joshua Tabbia, Already Dead Tapes, 2016

3-stage amplifier analysis

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analysis of a 3-stage amp. working up to delivering some power to a load. the CC section is not worked out here, but the final design delivered a whopping 18 mW to an 8 ohm load. i found better results with a class AB configuration than this class A design as expected. this is a good exercise at running through the small-signal analysis.

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Pixie: 40m QRP CW transceiver

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first post of 2015. it’s a small pocket size 40m CW transceiver that i got off of eBay for a mere $8. i’ve mainly used this while i’m at school to flex my cognitive abilities and decode the communications of extra class morse pros. the transceiver has a tuning range of 7.023-7.026MHz (maybe) which puts it in that tiny 250kHz portion of the 40m band reserved for amateur extras. at 5:30pm PST, it’s virtually an endless cacophony of signals. however, i was able make out a signal 565 miles south this afternoon from KI6JD using only the support railing as an dipole antenna. i don’t think he heard my response, but i would expect that considering my total radiating output is somewhere around 0.25 watts. also, my morse skills are lackluster at the moment, so i might have just sent gibberish out on the air. who knows.

DOD FX55B SupraDistortion – BigBuff Mod

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Here’s another in a line now of DOD pedal mods. For this FX55B, I decided to go for more of a BigMuff sound. Playing around with this pedal initially, I found it to be like most mass-produced distortion/fuzz effects. It had a very thin sound and that notorious volume drop that you may have heard on a high-school band’s first album. The lows were almost non-existent when the effect was engaged. I found the following schematic and, with a few simple changes, came out with a distortion pedal with considerable gain, massive lows, and a smoother, more rounded square wave.

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Overall, the idea here was to get more from the pedal by employing some germanium to smooth out the harshness and to increase the overall output. The output is now considerably higher as long as the tone knob is set closer to 10 o’clock. At noon, the mixing is practically useless. At the 4 o’clock position, you get more of a thin, trash punk type of sound with an excessive noise floor. The sound sample demonstrates the effect with the tone set at about 10:30-11 and first contrasts the distortion setting. The third set shows off the tone below 10 o’clock. The last set shows what the tone does when you sweep through it.

Class AB amplifier test

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this was an attempt at designing a simple transformer-less class AB amplifier using two BJT power transistors. i setup the USB fan to protect against thermal runaway and cool off the transistors during operation. it took me looking over several schematics to realize that the the emitters of both the PNP and NPN transistors were tied together. the collector of the PNP (TIP32C) is grounded which is obvious! the synapses just weren’t seeing the PNP symbol as upside down. it’s definitely something i’ll have to work on. i’d like at some point to have a nice 40W amp based on a similar configuration.

initial PCB milling test…

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here’s an initial PCB test for the Triode tube overdrive on the SMBA site. this picture is pre-sanding. it was milled using LinuxCNC on a Xubuntu 14.04 LTS running the Xenomai-2.6.2.1 kernel version 3.5.7. it’s taken some time to get the configuration working and compiled since i didn’t want to use the live CD. the CNC machine is a Chinese 3020T that i got off eBay. the board layout was done in EagleCAD using the PCB-GCODE ULP script.

pcb milling test from abram on Vimeo.