Making and modding a low frequency sign-wave oscillator (a PCB kit from Digit)
from Aug.2 2006 to Aug.31 2006

My portable oscilloscope - Hirel's HH973 - and a multimeter were quite good enough for modding CD players or making amplifier simply copying a schematics on web or in books. But when I tried to make a headphone amplifier based on my original design, later I found it was a variation of PRR's Tori Amp, I immediately recognized a neccessity of oscillator.
This page is my making and modding a ready-made oscillator PCB, which I found at the retailer front in Nipponbashi/Osaka, one another electronics town in Japan.

A kit, named OP20NEW, adopts wienbridge circuit for sign wave oscillation, providing 35Hz-45kHz signal by using accurate 2-channnel potentiometer. A kit is PCB only and requires +/-10VDC power supply.


fig.1 Making OP20NEW oscillator kit

For power supply, I also used a kit. It's a kit of +/-9VDC power supply with a PCB and some key parts. A big 2-ch potentiometer is for changing frequency, a switch is for changing frequency range and a small 1-ch potentiometer is for changing amplitude of the signal.

There is an opamp voltage-follower equipped for output stage. Original schematics provides two - low Z and 600ohm - outputs. I changed here to outputs two 150ohm assuming to be used as L&R.



fig.2 Successfully oscillating

It was pretty OK, except not so good wave form in + half. In this kit, amplitude is controlled by feedback loop using FET (2SK30), and instruction suggests an appropriate C and R connecting G and D of this FET can improive wave form. I tried this (see below).



fig.3 It's my first DIY oscillator

With not so good wave form in + half, though, it helped me quite lot in my designing headphone amplifier with BJT Tori Amp concept.


My first DIY headphone amp was successfully handed to my friend. And before my starting making a headphone amp for myself, I modded this oscillator, to get better wave form, even it limits the range of frequency. Actually, thru my making first headphone amp, I tend to think even a single frequency sign wave oscillator, such like 1kHz, may be good enough for my purpose.

So, I replaced 2-ch pot and caps by fixed 15kohm resistors and 0.01uF film caps to get @1kHz signal, and I tried to find optimal values for C3 and R8 below.


fig.4 Optiional C3 and R8 for better/stable wave



fig.5 100kohm pot for R8 and 33uF cap for C3. Two blue ero film caps are both 0.01uF for defining frequency

Finally, I found 33uF and @50kohm is good for 1kHz sign wave.



fig.6 Another picture



fig.7 Much improved wave form



fig.8 Much spoiled front panel orz

My future plan on this oscillator is to add the feature of providing three frequencies - 20Hz/1kHz/20kHz like my rectangular oscillator - by switching respective Rs anc Caps, expecting better front panel apperance also.


Sep.4 2006
chiaki014@mac.com

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