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Old 13th February 2003, 05:01 PM   #1
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Location: Seoul. Korea
Default hum problem with LM1875

I recently built an amp kit called qk50 which uses LM1875 chips. Funny thing is that it makes a hum only when it's hooked up to a portable CD player. The CDP is powered by a DC walwart which is connected to the same AC mains outlet in a surge protector. All the grounds in the amp are star-grounded. Power supply is in a separate enclosure which is connected to the amp enclosure by 3-pin DIN cable (shielded). +/- rail voltages are connected to ground via 0.1 uF film capacitors. Could anybody let me know what I should do to remove this hum? Thanks.
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Old 13th February 2003, 06:42 PM   #2
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Default Re: hum problem with LM1875

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Originally posted by JAZZ2250
The CDP is powered by a DC walwart which is connected to the same AC mains outlet in a surge protector. All the grounds in the amp are star-grounded. Power supply is in a separate enclosure which is connected to the amp enclosure by 3-pin DIN cable (shielded). +/- rail voltages are connected to ground via 0.1 uF film capacitors. Could anybody let me know what I should do to remove this hum? Thanks.
Quite likely a star ground that is not properly realized.

First of all see if there's hum with the CDP on batteries.

Second, try a cable that has two wires plus shield. Connect one wire + shield to ground on the CDP end and only that ground wire on the amp end.

If this doesn't work try to make a diagram on what's connected to what.


Carlos
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