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-   -   LM1875 got turn-off thump??? (http://www.diyaudio.com/forums/chip-amps/113599-lm1875-got-turn-off-thump.html)

Leolabs 14th December 2007 01:42 AM

LM1875 got turn-off thump???
 
Due to DC offset or the imbalance discharge time of the filtering caps???

Nuuk 14th December 2007 08:21 AM

Could also be due to the switch!

Leolabs 14th December 2007 01:26 PM

No.It happens after the switch turn off,something around 2~3 seconds.

Nuuk 14th December 2007 06:10 PM

What filtering caps are you using?

Leolabs 15th December 2007 12:44 AM

Single 2200uF per rail.

Nuuk 15th December 2007 08:31 AM

Well I get a small noise with 10Ks but nothing with 1000 uF. It's a pain but you could try 1000 uF and see if you still have the problem!

Paswa 16th December 2007 08:05 AM

I also made a chipiamp with the lm1875. I draw +- 12V from my 800W SMPS from computer. as it, there are too noise in the output because the SMPS current is very 'dirty' and 'hummy'

now the question, what capacitor I should use per rail to get rid of the problem?

tiltedhalo 16th December 2007 10:29 PM

SMPS noise
 
First does your power supply provide enough -V's for the LM1875?
Most SMPS's only give about .5 to 1 amp not nearly enough current.
To get rid of the SM hash you need to know the power supply's switching frequency and build an LC circuit to roll it off.

PS I do believe NUUK uses two SM in series to get the proper + and - current and voltage. If your using one supply your chip is probably rolling out of balance witch will create great amounts of noise and problems.

Paswa 17th December 2007 02:10 AM

Quote:

To get rid of the SM hash you need to know the power supply's switching frequency and build an LC circuit to roll it off.
How I could know the LC frequency of my SMps

Nuuk 17th December 2007 08:39 AM

I did get away with using a single Skynet 8080 to power each channel of my system for many months. ie I was using +/-12volts and that negative regulator held up. BUT I was using very efficient speakers and never needed to turn the volume up very high.

Now, I would rather use multiple units in series to avoid using those rather limited negative regulators! :att'n:


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