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How to build a Gain module for Subwoofer Amp without hum problem (TDA8950)
How to build a Gain module for Subwoofer Amp without hum problem (TDA8950)
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Old 25th March 2020, 03:03 PM   #1
dannyjmf is offline dannyjmf  Malaysia
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Default How to build a Gain module for Subwoofer Amp without hum problem (TDA8950)

hi members ...

i need to build a gain module to compensate for the input sensitivity of my TDA8950 (dont know why it has low input sensitivity). But now the problem is after the low pass filter the signal will pass into an inverting op-amp section which has gain of around 8db and this has given me Hum problem when the subwoofer volume potentiometer is turned more than 60%.

Low pass filter is 2nd order LR and there is an inverting summing opamp stage that combine the Left and Right channels and this stage has unity(1) gain ... only the final stage operates with gain while inverting back the signal phase

thanks
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Old 25th March 2020, 10:53 PM   #2
Ken Newton is offline Ken Newton  United States
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I suggest that the first thing you do is to determine exactly why the input sensitivity of your amp seems low. The datasheet for the TDA8950 specifies that it features a fixed gain of 30dB. Which seems like it should be sufficient.
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Old 26th March 2020, 02:35 AM   #3
dannyjmf is offline dannyjmf  Malaysia
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hi

As you can see in the photo there is a pot that controls the gain of the TDA8950 module ... the sensitivity now is reduced because i turned it down ... if i increase this pot more than 50% i will start to run into problems of Hum ... so it is not the problem of the low gain of the TDA8950 but i've reduced it ..

Further checking yesterday i found that the hum is coming from the low pass filter module and not the amp module ... if i am using a balanced output from the low pass filter and balanced input for the TDA8950 will it cure the hum problem? thanks
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Old 26th March 2020, 02:38 AM   #4
dannyjmf is offline dannyjmf  Malaysia
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Quote:
Originally Posted by Ken Newton View Post
I suggest that the first thing you do is to determine exactly why the input sensitivity of your amp seems low.
i didnt notice this till you mentioned ... i've previously turned down the gain due to hum problems ... i have some gain on the low pass filter module section ... sorry about that
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Old 26th March 2020, 11:16 AM   #5
sgrossklass is offline sgrossklass  Germany
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You should not need to used balanced connections internally, no. Track down the presumed ground loop instead.

Now externally, that may be another matter. Is the sub built as an IEC Class I or Class II appliance?
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