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soundNERD 25th October 2005 01:32 AM

DC blocking capacitors
I have found that I need to add some DC blocking capacitors to my LM3875 amplifier and LM833 Preamp.

I get large amounts of DC amplified in the preamp due to the source equipment and it causes my sub to pull in about 3/4" when on max volume. At half volume the DC causes it to pull in about 1/4"

First, where do I need the capacitors, and second, what value/type do I need?


JoeBob 25th October 2005 01:42 AM

If the DC is coming from your preamp, then all you'd need is a cap at the input of your amp, or output of your preamp. The value depends on frequency response, the -3dB point would be given by 1/2piRC, R being the input impedence of your power amplifier and C being your cap. So, bigger cap yeilds lower response, but you wouldn't need to large a value at all. And try and use a polyprop and stay away from the electrolytics.

soundNERD 25th October 2005 01:51 AM

The amount varies depending on the source. For example if I use my plain CD player it's not that bad, but if I use my active sub crossover, its terrible.

So I'm assuming it's caused by the input source, and I need one on the preamp input. But could one on the amp input hurt as well, in case some is caused by the preamp?

The input impedence of both the preamp and power amp is 500ohm/channel.

That was a mistake, and I've been planning on changing them to the 10Kohm they should be.

In that equation where does the frequency at which it rolls off come into play?

I don't want it to start rolling off until at least 20hz so sound isn't messed up at all. Using the input impedence of 500ohm, how would I set this up? I think it's wrong, but with 20=1/2pi*500*c I got C to equal .025uf. Does this sound right?

JoeBob 25th October 2005 02:08 AM

I'm at work and don't have a calculator but, you'd need a larger value of C than 0.025uF, the value of C is in F and the value of R is in ohms. With a 500R input impedance, you'll need an input cap of at least a few uF to get down under 20Hz. But, you really should change the input impedance from 500R to something reasonable, at least 10k.

Also, it would depend on your source, but it probably isn't good to drive a 500R load.

Adding a cap to the input of your amp and preamp is never bad practice, with a higher input impedance, you'll need very low values so good quality caps are still cheap at those low values.

But the first thing I'd do is change input impedence, for sure...

soundNERD 25th October 2005 02:36 AM

Thanks for the reply.

In your equation is the resistance measured in ohms or Kohms? So am I right using 500, or should I use .5? And for the 10k resistor, should I use 10000 or 10?

Thanks again,

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