Ground lift in passive DI.

[nerol1st]

How?

 

Is it merely a bypass switch on pin one?

[Pushtone]

A passive DI uses a transformer to balance the signal, change the impedance and lower the level to mic-level.

 

The transformer also decouples the signal ground which effectively blocks hum & buzz created from ground loops.

 

What this means is the "ground lift" switch is really a "ground re-connect".

The default setting for the lift switch is open (lifted) so the transformer can do it's decoupling. If you close the lift switch (not lifted) you have just connected the signal ground around the transformer and thus defeated it's ability to block ground loop hum.

 

Lifted is normal. Only rarely would you want to re-connect signal ground in a passive DI. One case I ran into was a battery powered preamp (for acoustic guitar) that needed a connection to ground at it's output or it wouldn't pass signal.

 

Now ya now.

 

BTW - active DIs don't have transformers and can not block ground loops to prevent hum. Active DIs actually lift the signal ground at the 1/4" input. If an active DI lifted the ground at the pin-1 on the XLR then phantom power would not work.

[Pushtone]

 

How? Is it merely a bypass switch on pin one?

 

 

 

Not a bypass switch.

It's either open or closed switch.

 

Hope this helps to answer your question.

[nerol1st]

Yes thanks I meant "on/off switch"

[jafair]

where is the ground lift placed? i am building a passive direct box based on a jensen jt-db-e transformer http://www.jensen-transformers.com/datashts/dbe.pdf ... it looks like it goes from the ground circuit to pin 1, then inbetween pin 1 and the casing/shield. is the ground symbol on this schematic mean pin 0 (the connection to the xlr housing)?

[Sporky McGuffin]

 

BTW - active DIs don't have transformers and can not block ground loops to prevent hum.

 

 

I'm not sure that's right. You could have a buffer driving the transformer, or an op-amp based ground loop inhibitor (Geofex has a schematic).

[WRGKMC]

jafair, that circuit looks right to me. I considerd buying some of those awhile back but opted out. If i had all the junk parts around it would be ok. (Which I probibly do) but you add the cost of connectors up, the transformer, switches and the case its more expensive than to just buy them on ebay for $10~20 like I do. Lot less trouble too even if I have to repair it its less hassel.