Hello all
I have built a little 1W amp using a TDA7267 (schematic: http://www.st.com/stonline/books/pdf/docs/1569.pdf (http://www.st.com/stonline/books/pdf/docs/1569.pdf)). It worked like charm. I then decided to add a volume control, so instead of the input capacitor at pin 4 I connected a 100k log pot between pin 4 and ground. I then connected the input capacitor at the pot's input. The layout is like this:
INPUT -||-|-----
|
|---- PIN 4
|
|---- INPUT GND + PIN 5 (CIRCUIT GROUND)
(sorry for the terrible ASCII scematic, I have no EDA software on this computer)
The circuit did not work anymore. Instead of 10mA of quiescent current it drew before, it drew 60mA afterwards. It produced no sounds at low volume and a low sound, heavily distorted, at high volume. I removed the pot and reverted to the original design, It worked normal again.
I have build this circuit twice with different stripboard layouts, different caps. I am fairly positive that the issue is the log connectivity and not somewhere else (e.g. faulty caps).
Any thoughts?
Hi Autir,
At a guess try inserting the Cap *After* the pot. :tu:
Phil.
Fully agree with Phatt/Phil.
I think the amplifier self biases, and the pot's resistance to ground alters that.
It *needs* that separation capacitor between it and the outer world.
Yes, that fixed it. Well, sort of. The pot worked alright, but at high volumes it would draw 40-60 mA and pick up hum from the environment. It was soldered directly on the stripboard, and connected to the input jack with good quality cable. Plus it was noisy when adjusted (rotated). I removed the pot and the circuit worked perfectly.
The pot was plastic and bought from an electronics store. Should I go for an expensive, quality metal pot from an electronic guitars shop ? Or is it something else? Could it be its value (100kΩ)?
No, that pot is fine, you probably have a grounding problem.
What happens when you use your guitar volume and tone controls?
I use a star topology, with the connecting point inside the chip. The ground of the pot and of the input were connected to pin 5 of the chip (one of its four ground points). The pot was directly soldered to the stripboard and for its connection to the input I used a short (~3cm) shielded cable.
My guitar has no tone, only two volumes (one for each pickup). With them the circuit works fine.