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Please help, DIY amp oscillating.
Please help, DIY amp oscillating.
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Old 13th September 2021, 01:56 PM   #1
gabripeic is offline gabripeic  Croatia
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Join Date: Feb 2020
Exclamation Please help, DIY amp oscillating.

Hi everyone. I was working on a DIY amplifier with the design from old radio because I just liked the sound of it.
I know it is probably not the best circuit and probably has distortion but it just gives me a pleasant sound for non critical listening. I don't know if it is because of the germanium transistors (I doubt it) or because of the tone/loudness circuit.
Anyways I got all the parts and designed the pcb with as similar as possible layout to the original pcb in the radio but the amp is acting up. It works but sometimes it is distorted and sometimes it starts oscillating. Now it is motorboating and I noticed that with no speaker connected the voltage before output coupling cap is 6.4 v which is pretty close to the 6.8 that is listed in the schematics of the radio, But as soon as I touch the input it goes up to over 10v like the bias changes or something. I checked the pcbs and compared them to the original radio countless times and it all checks out.
So only thing that I can think of is layout problems.
Now I can't even get it to stop motorboating and it changes slightly when I touch the power supply or anything.
Here are some schematics and pictures...
Note: I am a noob and please don't roast me too hard if the design is absolutely catastrophic . I tried to lay it out as close to the original as possible and this was my first ever pcb design

Screenshot-20210728-141011 — ImgBB
IMG-20210913-153530-2 — ImgBB
IMG-20210913-153118-2 — ImgBB

Also this circuit doesn't include thermal compensation. I am not really worried about it because the radio I have was used in the workshop by my grandparents and it played for hours and amp never died and it will have a large heatsink in my amplifier , but if thermal compensation can be easily added I would gladly do it.

Anyway sorry for the long post but I would really like to build that amp. I just like how it sounds no matter how good it actually is it really compliments my vintage speakers. And I like vintage electronics.
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Old 13th September 2021, 02:29 PM   #2
jxdking is online now jxdking  China
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Join Date: Aug 2009
Output Stage Inclusive Miller Compensation (C247). <- this may cause oscillation
With in that Miller compensation loop, there is also a shunt compensation (C246).

You may try this.
Remove C246. Connect C247 to the collector of T205, instead of the output stage. Thus, the Miller cap only connects to the base and collector of T205.

Last edited by jxdking; 13th September 2021 at 02:38 PM.
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Old 14th September 2021, 10:44 PM   #3
gabripeic is offline gabripeic  Croatia
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Join Date: Feb 2020
Oh seems like the motorboating was because of one bad connection on the c246...
But there is another problem. It doesn't have any power for some reason. It sounds like it is clipping really bad as soon as I reach barely any volume... Could this also be some kind of oscillation?? Or did I fry something ?
It doesn't seem to draw excessive current and if I short the input to ground it is pretty much dead silent.
Could it be that it is oscillating when the signal is present or something ?
Also I noticed that if I try to trace the signal around that with no load on the output the signal at the base of the output transistors is cleaner. But if I connect the load the signal at the base of the output transistors sounds really distorted...
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Old 14th September 2021, 11:26 PM   #4
jxdking is online now jxdking  China
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Join Date: Aug 2009
How do you connect the load? Make sure to connect to speaker without DC blocking capacitor.
The speaker is part of bootstrap configuration for the output stage.
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Old 15th September 2021, 07:07 PM   #5
gabripeic is offline gabripeic  Croatia
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Join Date: Feb 2020
Speaker is hooked up directly to the output like in the circuit.
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Old 15th September 2021, 08:55 PM   #6
jxdking is online now jxdking  China
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Join Date: Aug 2009
I have built this kind of amp on a bread board long time ago just with 9012 and 9013, with 12V. Just as you said, it is not distortion free. However, it should play loud and clear. Better than normal radio.

You may measure the voltage based on your schematic. Good luck.
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Old 15th September 2021, 10:40 PM   #7
gabripeic is offline gabripeic  Croatia
diyAudio Member
 
Join Date: Feb 2020
Thanks! I will do some more experiments and update if I find anything.
Thing is I got it working before and it worked perfectly but now it doesn't want to stabilize.
Maybe I will record a video if I have time and post a link here so you can hear how it sounds and acts.
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