Hi all, I'm new. Hopefully this won't annoy anybody. I promise I've searched and searched but can't figure it out.
I have a pair of JBL 6208 active reference monitors. They were made in the late 90's to early 2000's. They are known for having mains hum when brand new. The transformer has a slight mechanical hum when nothing is connected to the amp. With the drivers connected to the amp, but nothing connected to the input, there is a hum through the speaker when the amp is powered on. It's not very loud, but as these are meant to be used as near field studio monitors, every little thing counts.
I did some experimenting and noticed when the input jack (Neutrik XLR/TRS combo jack with chassis ground) is removed from the panel, the hum reduces. The jack is physically located right next to the transformer. Moving it away from the transformer gets rid of the hum altogether. I tried placing a metal shield in between them, but that didn't do the trick.
The service manual with schematics is here:
http://www.jblproservice.com/pdf/Studio Monitor Series/6208.pdf
Any advice for how what to look at next would be greatly appreciated. Replacing transformers is definitely on the table as an option, if it would help.
I have a pair of JBL 6208 active reference monitors. They were made in the late 90's to early 2000's. They are known for having mains hum when brand new. The transformer has a slight mechanical hum when nothing is connected to the amp. With the drivers connected to the amp, but nothing connected to the input, there is a hum through the speaker when the amp is powered on. It's not very loud, but as these are meant to be used as near field studio monitors, every little thing counts.
I did some experimenting and noticed when the input jack (Neutrik XLR/TRS combo jack with chassis ground) is removed from the panel, the hum reduces. The jack is physically located right next to the transformer. Moving it away from the transformer gets rid of the hum altogether. I tried placing a metal shield in between them, but that didn't do the trick.
The service manual with schematics is here:
http://www.jblproservice.com/pdf/Studio Monitor Series/6208.pdf
Any advice for how what to look at next would be greatly appreciated. Replacing transformers is definitely on the table as an option, if it would help.
"The jack is physically located right next to the transformer. Moving it away from the transformer gets rid of the hum altogether."
Can you not arrange that as a permanent thing ?
Can you not arrange that as a permanent thing ?
Well, because it's a plate amplifier with a substantial part of the exterior covered with heat sinks, placement options are limited. Also, it looks like the jack grounds the shield to the mounting surface, so I was concerned about whether removing that ground would be OK.
Tried replying yesterday but computer said No... :|
Oh well was worth a try.
Maybe a shielded toroidal instead of the ei transformer ?
Or, better still. Remove the transformer, put it into an external enclosure, changing the mains input socket to a low voltage AC socket.
Don't forget a fuse on the primary and I'd also fuse the secondary to allow for possible pinched or shorted wires from the new external supply to the speaker cabinet.
If you can work out the size (Voltage and current) of the transformer, maybe even something like a ready made switch mode brick.
That would save a lot of work and bypass the legal side of mains wiring.
Oh well was worth a try.
Maybe a shielded toroidal instead of the ei transformer ?
Or, better still. Remove the transformer, put it into an external enclosure, changing the mains input socket to a low voltage AC socket.
Don't forget a fuse on the primary and I'd also fuse the secondary to allow for possible pinched or shorted wires from the new external supply to the speaker cabinet.
If you can work out the size (Voltage and current) of the transformer, maybe even something like a ready made switch mode brick.
That would save a lot of work and bypass the legal side of mains wiring.
Looking @ the schematic and the panel layout you may have Primary AC coupling from the wiring to the signal inputs. It may not be the power transformer. Try moving/routing the AC wires away from the signal inputs and twist the AC wires to reduce the magnetic field. I all else fails move the signal input off the chassis. Good hunting Duke.🙂
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