Instrument jack wiring

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Deepdark

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Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Hi

2 questions. I want to add a di to a pre, basically the tip goes to the grid and from there a 1M resistor to ground. It's a switch plug, so i sent the high secondary of the input tranny to it.

1. I use shielded wire to connect the plug. Does the sleeve connect to the shield, and go to chassis, just like pin 1 of xlr? Or i should send the sleeve to signal ground?

2. I use a trs plug because that was the only one i got at hand. Should i strap the ring to the sleeve (so to ground)? I got some hum when connecting a guitar inti it (note that i use a trs cable instead of a ts cable).

Thanks
 
Just a thought about teh hum, maybe the fact I use a TRS cable between a guitar and a TRS plug (with the ring not connected) cause the hum. Probably I should strap the ring with the sleeve at the plug, sounds right? Is it possible the ring signal (cold) act as an antenna?
 
Deepdark said:
Hi

2 questions. I want to add a di to a pre, basically the tip goes to the grid and from there a 1M resistor to ground. It's a switch plug, so i sent the high secondary of the input tranny to it. 

1. I use shielded wire to connect the plug. Does the sleeve connect to the shield, and go to chassis, just like pin 1 of xlr? Or i should send the sleeve to signal ground?
You don't mention whether your jack is insulated or not. If not, the sleeve goes to chassis anyway and you can only pray  :eek: .
Ideally, the sleeve should go to the reference ground of the input stage - that's the "ground" where the cathode resistor (and cap) is attached. But EMI/RFI gurus will object, saying that sleeve should go to chassis. Huh? In fact, for EMI/RFI protection, the sleeve should have a strong RF connection to chassis. My suggestion is to connect the sleeve:
A)  to audio ground via a ferrite bead
B) to chassis ground via a 0.1uF film cap with a 100r across with the shortest possible connection
This way you get a low-Z path to audio ground at audio frequencies for signal integrity and a low-Z path to chassis ground for EMI/RFI protection.
2. I use a trs plug because that was the only one i got at hand. Should i strap the ring to the sleeve (so to ground)? I got some hum when connecting a guitar inti it (note that i use a trs cable instead of a ts cable).
You need to strap ring and sleeve for the instances where a TRS jack is used. Your analysis is correct. If not connected, the remaining floating conductor picks all sorts of interference.
 
thank Abbey. I strapped the sleeve/ring to the chassis. I still got some hum, but it's way better than it was the first time. Maybe the cap/resistor would help to minimise it. It has to be around it because once disconnected, the pre works silent dead with microphones so it's only when inserting a jack into the plug. I used a switched plug just like Neutrik NMJ6HC-S . I let the ring and sleeve switch unconnected
 
Deepdark said:
thank Abbey. I strapped the sleeve/ring to the chassis. I still got some hum, but it's way better than it was the first time. Maybe the cap/resistor would help to minimise it. It has to be around it because once disconnected, the pre works silent dead with microphones so it's only when inserting a jack into the plug. I used a switched plug just like Neutrik NMJ6HC-S . I let the ring and sleeve switch unconnected
All that shows that the problem is with the jack's sleeve being connected to chassis instead of audio ground.
 
For DI Jacks on tube pres I have always used a standard two conductor  (unbalanced), non shorting jack, non insulated.  For the wire connecting the tip to the grid I have used both shielded coaxial cable and just a straight wire and had excellent results.  An unshielded (unterminated) wire of most any length on a grid is generally a big No-No and I wouldn't advocate it but it has worked fine before.  I generally use shielded coaxial and tie the shield to the 1st stage signal ground.  Since the jack is non insulated that also means its tied to the chassis on one end.  If you do get a ground loop you may have to use an insulated jack, but I've never had an issue with that.

If you get stuck, I would switch to the two conductor jack (commonly called speaker output jack) and see if you still have the problem.  The TRS jack is just making redundant connections with no real benefit since you're dealing with an unbalanced instrument signal.

As always, make sure the overall lead length is as short as possible.
 
That is the kind of jack I used. I sent the shield to the chassis front plate (in fact, straight to the xlr body just beside the jack. The front panel is aluminium, and the xlr isn't connected to the circuit, I used the xlr on the back of the cases., but it has ground continuity). Good chances my trouble sit there. At the grid, there is a 1M resistor to circuit ground first stage. 

nmj6hc-s.jpg


 
Deepdark said:
That is the kind of jack I used. I sent the shield to the chassis front plate (in fact, straight to the xlr body just beside the jack. The front panel is aluminium, and the xlr isn't connected to the circuit, I used the xlr on the back of the cases., but it has ground continuity). Good chances my trouble sit there. At the grid, there is a 1M resistor to circuit ground first stage. 

nmj6hc-s.jpg
You should definitely take advantage of the fact that the jack is isolated.
 
abbey road d enfer said:
Deepdark said:
That is the kind of jack I used. I sent the shield to the chassis front plate (in fact, straight to the xlr body just beside the jack. The front panel is aluminium, and the xlr isn't connected to the circuit, I used the xlr on the back of the cases., but it has ground continuity). Good chances my trouble sit there. At the grid, there is a 1M resistor to circuit ground first stage. 

nmj6hc-s.jpg
You should definitely take advantage of the fact that the jack is isolated.

Can you elaborate on this? Isolated from the chassis?
 
Deepdark said:
abbey road d enfer said:
Deepdark said:
That is the kind of jack I used. I sent the shield to the chassis front plate (in fact, straight to the xlr body just beside the jack. The front panel is aluminium, and the xlr isn't connected to the circuit, I used the xlr on the back of the cases., but it has ground continuity). Good chances my trouble sit there. At the grid, there is a 1M resistor to circuit ground first stage. 

nmj6hc-s.jpg
You should definitely take advantage of the fact that the jack is isolated.

Can you elaborate on this? Isolated from the chassis?
It's in Reply #2
"My suggestion is to connect the sleeve:
A)  to audio ground via a ferrite bead
B) to chassis ground via a 0.1uF film cap with a 100r across with the shortest possible connection
This way you get a low-Z path to audio ground at audio frequencies for signal integrity and a low-Z path to chassis ground for EMI/RFI protection."
 
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