TDA2611A Hum

[cathoderay57]

Hello, I've built a small audio amplifier using a TDA2611A (see data sheet attached). I've built it as per Fig 3. The power supply is a 9V mains transformer and 4 x 1N4001 diode bridge with 2200uF capacitor delivering about 13.4v DC. To test it I have attached a 2M potentiometer via a length of coax as temporary volume control. Problem is, with the control at minimum there is a 50Hz hum from the speaker. As the volume control is advanced, the hum decreases until at max volume it is virtually hum-less. I get the same hum if I simply short-circuit the input. Now then - if I replace the AC supply with a 9v battery it works perfectly, no hum at all. I have checked the obvious such as a duff smoothing capacitor and faulty diode. I can mitigate the problem to some extent by inserting a resistor between the 0.1uF input capacitor and the slider of the pot. A 100k resistor quietens it a bit and a 1M seems to do the job but I haven't tested it yet with an audio input to see what effect the series resistor has. The purpose of this amp ultimately is to act as power amplifier for an R1155 receiver, located inside a separate power unit box. There will therefore always be quite a long length of coax feeding the audio in; not ideal but I wanted to keep it separate and not inside the R1155 chassis. Any ideas welcomed. Cheers, Jerry

[dragonser]

Hi,
I just wonder if you could try a volume control of less than 2M Ohms ?
I " think " with the volume control being half way round the impedance of the circuit at that point might be about 1M Ohms.
with a lower value volume control ( 100K ) then the impedance would be less...
I don't know off hand what audio output the R1155 receiver was designed to drive ?
If it was designed for headphones than I suspect that a 100k volume control would be ok.
Of course if you were wanting to use it with a crystal set then it probably wouldn't work !

[Nuvistor]

50Hz or 100hz hum, if it’s from the PSU it will be 100hz, 50Hz will be stray pickup from mains wiring, transformer etc?

[cathoderay57]

Hi Peter and Frank, thanks for the quick replies. The circuit is I think intended for the 0.1uF input capacitor to be connected to the slider of the vol cont pot and so regardless what value pot I use the hum problem always appears at minimum setting. The alternative of connecting the slider to the input signal is probably a bad idea generally as it would short out the input signal at zero setting. However, looking at the R1155 it has its own volume control prior to the final triode amplifier valve so I don't need another pot on the final amp. The audio output from the R1155 comes from a coupling transformer at an impedance of about 150k I think so maybe I just have to connect up and see what happens. I don't have the R1155 ready for power yet. Frank, you may be right it is possibly 100Hz hum definitely higher pitch than obtained with "finger on input".

[ms660]

I powered a loudspeaker from the R1155 that I had, for "matching" I just used a bog standard valve radio output transformer connected to 'phones out so far as I can remember.

Lawrence.

[peterpiper]

Hi CR, watch where you connect the co ax outer of your input lead. Try taking it to the neg of the reservoir/smoothing cap. These TDAxxx chips have very high gain and are prone to hum leaking in at the input. Ive had to resort to suck it and see methods to get the hum down

[cathoderay57]

Peter Piper, you were spot on. I built this on veroboard and the input from the coupling capacitor was running the whole length of the board strip causing pickup. Cutting the strip and connecting coax directly to the cap and braid to pin 4 (gnd) completely silenced it. Very many thanks. Jerry
PS Mods: thread can be closed