Quote:
Originally Posted by Gabe001
This reminds me of when I built my first amp a few months ago. I ran into the same issues, how much smoothing, how to stop the damn thing buzzing, how to apply negative feedback (if necessary), what size cathode resistor etc. It's all part of the learning curve
A couple of things which you may know already but which may help if you dont.
1. The position and orientation of the output transformer is crucial to avoid magnetic coupling. Google the "headphone test for output transformers" to work out where best to out it.
2. Usually a smaller volume pot will give you less hum. As has been already mentioned 47k will probably be quieter. If you connect a low impedance device to the input eg phone it may short out the hum and the amp will be silent which may solve the problem for you (sort of)
3. Make sure the negative feedback is connected right way round for negative feedback . There is 50% chance of getting this right first time and it's unpredictable (to me). If you get oscillation it may be that you're getting positive feedback instead. Flip the ground and signal wired on the output transformer secondary and try again.
4. 100hz hum is a pain. As others have said it is mostly down to smoothing especially in V1 and V2 but it's a balancing act if you don't have enough volts. Once you build the amp put your ear to the speakers you intend to use and see if the hum you can hear (if there is any) is acceptable to you or not. If not you may need to up the resistor value or add capacitance. As such you're building an amp to match the speakers you're using with it, if that makes sense.
Good luck, it's great fun once you get it to work as it should. Your next build will be easier.
Gabriel
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Thank you for your comments. All of them are going to be considered, especially when I get done with the experimental setup and start a final form of construction.
The principles of feedback learned from tinkering with regenerative receivers, helped me understand NFB and importance of phase for proper connection.
For the volume potentiometer, the only stereo pots I have now are either 10K or 100K. I will use the latter till I can procure a 50K pot, which means I have to drive to downtown, an experience ususlly associated with psychological trauma.
I will soon report the results.
Regards.