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Simple audio synthesizer part 5: schematic showed but encountered parasitic oscillations



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Please read the textbox for more info. Sorry for the sudden stop of the video, the 2 MB memory card stopped. Part 5 of the audio synthesizer project. Schematic of one unit is showed now.

It was made with a NE 555 Chip, a BF 256 FET, and a standard NPN Silicon transistor, the BC 547 B. It works as 1 unit (very) properly.

Though: with 2 units on the same board I encountered the problem of parasitic oscillations (!). Could be that the second oscillator unit (same schematic by the way…) has a flaw, that has to be tested out.

Also the earth wiring could be not made properly or the impedances of all the amplifier stages (output NE 555, input FET BF 256 b) are too high, thus making the oscillator unit too sensitive for external effects (capacitive, inductive, etc.).

It will surely be cured in a future video.

My idea is that the hay wiring causes parasitic oscillations, though with 1 oscillator unit there are no problems.

Anyway, sorry for the certain stop in the video (!).

What I wanted to tell was this, parasitic oscillations can be stopped by:

1. Proper earth wiring and everywhere a zero potential to the “0”, the mass. In cases of (audio) amplifiers the minus (-) is often on ground, mass, shielding, cabinet. All (-) wires in that case have to go to the 1 and only mass point where all the electronic circuits (in this case with all their minus) are connected to. Solder this mass contact to the metal cabinet, preferred: steel, no aluminum. Steel has much better shielding properties.

2. Keeping the whole circuit low impedance at all the “sensitive” electrodes: so the gates of FETS, the base of bipolar transistors. Say: go to the 1 K-10 K (4K7) range.

3. Keep audio input wires far away from audio output wires

4. Keep wires that supply DC or AC current far away from lines that have to transport audio signals or other signals (EM influence)

5. There are many video’s about these issues on my YT Channel, e.g. go to my Channel trailer (Radiofun232 on You Tube) and search with keywords via the “looking glass” there. Words like “shielding”, “ground”, "earth", etc.

Earlier video’s are here:
Video 1 https://youtu.be/Zt0widtCGRU
Video 2 https://youtu.be/Ud2WjJGgaHE
Video 3 https://youtu.be/atfLEFhQ49E

Good first video (31 december 2021, showing the principles and the parameters) is here https://youtu.be/KisdQ0mednI

Good earlier video (how to make a basic synthesizer with 3 Astabile Muvibs, generating square waves) is here https://youtu.be/Zt0widtCGRU

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in.

My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.
https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=Ko+Tilman

I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 14 January 2022.
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Audio
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