another Hum issue LM3875

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
D

Dan_Steele

Member
Joined 2006
dont ask how many tries it took...

But i belive this is the one andrew:

http://travel.webshots.com/photo/2220350710086056042namwJw?vhost=travel

Regards

Dan

Edit: Hah! Just realised I could of gone straight to Album instead of trying a million different codes (slight exageration) to see how the non aurothised tag ties with the ones we can see!! :smash:

Edit2: Any the correct link had been posted already :bawling:
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
Confirm each of the following:-
red is live and black is neutral?
do you have both reds to the live terminal on the back of the IEC connector/filter?
do you have two blacks to the neutral terminal?
mains fuse inside the IEC filter/connector?
Just one connection from third terminal (earth wire back to distribution board) to chassis (safety earth)?
No other connections to chassis?

Can you measure resistance from chassis to your audio ground?
is it open circuit?
 
S

sts9fan

Member
Joined 2006
ok so I checked my voltages and they are fine. I then checked my rca jack and there was an issue. It seems that it was not isolated. So I removed it and plugged the amp in again and it was humming a lot less. I did notice that if I touched the isolation of the cable that I had disconnected from the jack the hum changed. It got louder. Why would touching the insulation of the shielded cable effect the hum? This seems to be the source of the hum in some way. Is it the cable I used? I am also getting a radio station in quite clearly through the amp. I do live in a noisy area. Were would the radio be getting in? How do i stop it? I have included some pics of the cable. It is cardas sheilded twisted pair.
http://travel.webshots.com/photo/2728737590086056042xRPXVt
http://travel.webshots.com/photo/2380330010086056042iWHLUa
http://travel.webshots.com/photo/2570428680086056042aVDdIV
Thanks Kris
 
S

sts9fan

Member
Joined 2006
To fix the radio reception, add 220pf cap from in+ to in- close to the chip...you can probably reduce this a value a little depending... on how strong the radio wafes are around you....
Click to expand...

Can I attach these right to the pins of the chip?

Is the shielded cable 2 core (i.e. balanced)? Have you used the two core for signal and ground? If so is the shield attached and where is the shield attached?
Click to expand...

yes the shielded cable is two core. The sheild is currently unattached. Should I attach it to ground? Is the sheild acting as an antenna? So the case was also acting as an antenna?

Thanks
Kris
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
the two cores in the shielded twisted pair form the signal flow and signal return.
It is common to connect both cores to the relevant terminals on the input RCA and again both cores get connected to the sig and sig gnd at the PCB.
But, that common method sometimes gives rise to more hum/buzz.
Have you connected as described or some different way?
 
S

sts9fan

Member
Joined 2006
one core is connected to the sig and the other to sig gnd at the pcb. The core connected to the sig gnd is connected to the rca tab and the sig to the center pin. Is this correct?

So if I have a hum with no source connected and the rca socket is not connected to gnd then connecting the shield may help but that will not be the cause of the hum. Correct?
 
ianpengelly

ianpengelly

Member
Joined 2006
freewebs.com/ianpengelly
sts9fan said:
one core is connected to the sig and the other to sig gnd at the pcb. The core connected to the sig gnd is connected to the rca tab and the sig to the center pin. Is this correct?
Click to expand...


That is correct. As Andrew pointed out, the shield should then be connected to the chassis, but only at the RCA end of the cable.

When checking for hum it is worth shorting the RCA.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom