Adding an audio amplfier to the Wien bridge single pot audio oscillator
Since one quarter of the LM324 quad op amp was unused by the Wien bridge oscillator, I thought that it could be used along with a complementary output transistor stage as an audio amplifier.
I found this page of audio amplifiers by VE7BPO and thought that the Popcorn Audio Amplifier might work in this situation (near the bottom).
https://web.archive.org/web/20140321154315/http://www.qrp.pops.net/af-amp-2008.asp
However (as you might expect) instead of following the design as presented I messed around with it.
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| Audio amp on top, Wien bridge oscillator on bottom |
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| amplifier on breadboard, note nylon bolts/nuts |
One change to the design is the transistors, the NPN is a 2n4921, the PNP is a 2n4918. They don't seem to be a complementary pair as I measured an Hfe of 255 for the PNP and 80 for the NPN. But I had them on hand. They aren't available anymore from the usual suspects (Mouser / Digikey). They are both in a TO-225 package.
Another change to the VE7BPO design was to use a second set of transistors instead of a pair of 1n4148 diodes for the thermal biasing. This way the BE junctions of the 2nd set of transistors can be used as diodes. It also allows me to bolt the output transistor to the diode (with a nylon nut/bolt) to achieve a sort of thermal tracking for the bias.
I also ditched the 1 ohm ballasting resistors because I didn't have any on hand, the feedback cap is 220 pF and the output cap is 220 uF. The half VCC bias is from pin 14 of the lm324 is isolated by 10 k ohm resistor.
It "worked" right away, no smoke or heat or oscillation, very surprising ! However as can be seen from the scope photo the output is not nearly as symmetrical as that from the Wien oscillator stage. 2.4 volts peak to peak is 90 mW for an assumed 8 ohm speaker with a 9.2 volt supply.
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| audio amp output, fair bit of distortion but usable ? |
Also the leading edge glitch is present although I couldn't hear it.
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| there is a small leading edge glitch but not audible |
The amplifier sounds OK, I will have to do more measurements and possible circuit changes (change biasing, different op amp, more feedback) - more to come !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm