Hi-Fi Stereo Amplifier gain regulation with potentiometer

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hyuna
Neuling

#1 erstellt: 16. Dez 2016, 09:20

This is my first question ever so please don't kill me icon_smile.gif

I have built a stereo hi-fi (end) amplifier 2x10W based on a IC

TDA2009. I have bought it as a construction (learning) kit ready to solder.
Schematic:

(the pictures are from the manual that came with the kit, please don't mind the Czech headers)
Parameters for the circuit (from the manual) are:

Us power supply voltage: 8-28V (I use 9V power supply)
Peak output current Io (freq. >= 20Hz): 3.5A
...and more but IMHO (now) not important values
Now, everything worked great (although I had to replace the C8 capacitor, no idea why it was broken), so I decided I want to replace the R1 and R3 resistors with a potentiometer for variable gain (gain control). I bought stereo potentiometer, linear, 1kOhm, wired it up to R1s and R3s place (first pin ground, second and third pin to the PCB). When I turned the circuit on, the pot started smoking and burned.
If I get it right, I understand that the potentiometer couldn't handle the load when set to little resistance. I didn't really had a choice when selecting the pot as the one I used was the only stereo 1kOhm pot they had in the shop.
So I need to know:

Is my approach correct here? Can I replace the two resistors with a stereo pot?
Am I right thinking that the pot couldn't handle the current/load, or was it just a bad piece?
How do I calculate the power that can be on the resistance R1/R3?
Thank you for help!

Monohuhn
Hat sich gelöscht

#2 erstellt: 16. Dez 2016, 22:00

First of all: I'm not an expert electronics technician. But let me ask you what you are actually trying to do there. Do you want a volume control? In this case, just connect a stereo potentiometer to the inputs of the amp and leave the gain as is.

audiophilanthrop
Inventar

#3 erstellt: 16. Dez 2016, 23:44

Indeed. Get a 10k log or something. The ends attach to input signal and input ground (the one where the wiper is if turned all the way counterclockwise must be the ground side), the wiper goes to the amp input.

Trying to use a pot in the feedback there is a Bad Idea for various reasons. These amplifier ICs are not unity gain stable, the datasheet for this one says minimum permitted closed loop gain is 26 dB = 20. You could set a pot no higher than 50 ohms out of 1 kOhm, very tricky to do with a linear one. Even if done properly, adjustment range would end up being quite limited, like 20 dB tops. Not nearly enough for a typical volume control.

[Beitrag von audiophilanthrop am 16. Dez 2016, 23:46 bearbeitet]

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