lm1875 PS/Transformer problem :/

[fr1s]

Hi everyone

Lately I've started getting my amp togather into a case, then I hit a problem, for some reason my amp started giving loud humm.
My Ground connections were connected all togather and bolted to the amp case just like my lm1875 chip. Then i've blown couple caps on my PS , but @ the end I figured out that my grounds TX CT and PS CT got -13v and -18V and when I didconected both grounds from the case the loud humm disapered but the CTs remained connected. Isnt the CT's supposed to be -0V???

Is there any way I can get rid of this problem?

Thank you!

[pacificblue]

Originally posted by fr1s
Isnt the CT's supposed to be -0V???

They are supposed to be at 0 V. You force them to that voltage, when you connect them to the case that in turn should be forced to 0 V through an earth connection.

Originally posted by fr1s
Is there any way I can get rid of this problem?

Yes. First you have to find out, what causes it.
- After disconnecting the transformer center tap and the power supply ground from the case, can you measure a voltage between ground and case?
- Can you check, if you have a proper earth connection?
- Does the hum disappear, if you have no source connected?

[sasmit]

a pic please....of the guts..

[fr1s]

- Does the hum disappear, if you have no source connected?

yea it does in most cases, it's playing ok when I've hoocked up my mp3 player

- After disconnecting the transformer center tap and the power supply ground from the case, can you measure a voltage between ground and case?

I belive it's between the power supply ground and case ground it's -18v, between transformer and case it's -13v

does it have anything to do with the chip it self connected to the case?

[fr1s]

Originally posted by sasmit
a pic please....of the guts..

ok

also can I ground more then once
I have feeling like it's shorting when I connect all the GND's to the case but it looks like i got nothing touching the case exccept the ic

Another question, is the metal peace on ic GND??

[sasmit]

Another question, is the metal peace on ic GND??

The metal piece on the IC is hot i.e. ( at -Vee ) you need to isolate that from the heatsink using a mica sheet or any kind of thermal padding.

[sasmit]

even though it's not very clear from the pic ... don't use so many ground wires. Connect amp to PS board with a single very thick wire..at one point. i.e. ( only one ground wire from amp to PS board ). Simmilar approach from PS board to transformer..not mulitple ground wires..but a single thick wire from PS board to transformer. connect the chasis earth to body of transfomer...wind it around the mounting bolt of the transformer.
If you are using a 3 pin power plug ..from the AC wall outlet to your amp...connect the wall outlet Earth to the chasis. ( this requires additional circuitry depending upon laws in ur place )

But all the above is secondary ( though imp for hum less operation), first u need to isolate the chips from heat sink, that probably is the reason for short circuits...you're not having burnt chips may be because the contact between the IC's and heatsink may not be good...

[fr1s]

ok thx so if I buy mica and put it in it should fix it? what about the screw?? it's still metal :/ what should I do with that?

[fr1s]

before I had ALL Gnd's connected to the the transformerand the case so I guess that was good idea

Well i have two GND wires comming out of PS to amps, my amp is two separate amps in tottal I got 5 GND wires

1PS+ 2 GND's to my amps
Transformer(TX) CT
GND from wall socket (one with 3pins)

I guess the problem is my ic isnt isolated from the heatsink

[sasmit]

unless you're having hum issues....you're grounding is most probably ok..( but this a very ..crude approach..ideally an amps wiring and component placement is a bigger issue than it seems ). you can use plastic sleeves on your screws....in this part of the world i.e. ( in India ) we get Isolation kits..for TO-3 transistors and power devices..I used one of them...it comes complete with screws,plastic sleeves, mica sheet. The 2 notches that are on the sides of the IC have little metal protrusions from the die ( need to look very carefully ) if these protrusions are touching the screw..then u need to isolate the screw with a plastic sleeve...u can use the ones used for insulating component legs or for wiring purposes.If you have a nylon screw, you can use that .

Below is a link for farnell heat sink acessories.I guess in your part of the world farnell is called newark or something.

http://in.farnell.com/multicomp/mk33...-kit/dp/520251