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Topics - Katoda

Software / Simple budget oscilloscope for Android
November 04, 2017, 06:21:04 PM
So I just wanted to share this with you, in case anybody needs to check some audio frequency waveforms and doesn't have the money (or is just cheap like me) to buy an actual oscilloscope.

First I disassembled and stripped the wires from an expired headphones set, the one with the microphone (TRRS), and soldered the wires to a piece of perfboard. Both of the headphone outputs were loaded by 1k resistors, one of them got connected to a 100nF capacitor for AC coupling. Than was the signal output / sine generator output. I loaded the mic input with a 20k resistor and put 3 header sockets onto the perfboard for an attenuator. The signal flow into the microphone input is as follows:
Probe wire -> Coupling capacitor -> one of the header sockets, where you can swap resistors to form a voltage divider -> microphone input.

I just put in a 100nF coupling cap, which together with a 20k resistor forms a high pass filter at 80Hz. Gotta upgrade that to extend the lower range of possible monitoring frequencies.

There are a lot of oscilloscope apps in Play store, I chose SmartScope.
I had a lot more problems finding an app that would continue to play sound when it was not in the foreground.  I ended up with an app called Note Fork from siliconfish, which is a nice little musical note generator, but you can still choose a note closest to the desired frequency. And, most importantly, it is the only app I found that continues to emit sound in the background, although if you have a better phone than I, chances are you will be luckier in finding a more suitable program.

Attached is a picture of a measurement I took to look into transformer distortion. The peak-to-peak value is not to be trusted, but the waveform is nicely visible. Depending on the phones sampling frequency, you can generally measure up to 20kHz.

So if anybody wants to analyze some clipping circuits on a budget, here you go. Hope somebody finds this useful.

Also, coaxial probe leads are quite beneficial in this application.
Preamps and Effects / EVH 5153 preamp with LND150
January 03, 2016, 11:08:52 AM
Just to see how it would have turned up, I attempted to recreate the preamp from the ultra-high-gain EVH 5153 (the Fender version) with the base of research being KMGs fantastic JCM800 preamp with LND150 mosfets, implementing the diode grid current limitation and therefore closely emulating the 12ax7 tubes. I ran out of space pretty quickly, because I was using two perfboards superglued together in a recycled enclosure from a 'true' tube Pittbull preamp project. It looks ugly as ****, but it cost me about 10€ (except the LND150, I had everything at home and/or at hands reach) and it sounds awesome. Having never played the original amp I cannot claim it sounds the same, but from what I heard on YT, its pretty close.


Some sound samples - Lasse Lammerts DI tracks reamped  through the thingy, with some impulses (Peavey 4x12, i think - or recto):