mitsos
Well-known member
- Joined
- May 4, 2007
- Messages
- 2,886
I wan't going to mention (or am I doing that right now) proper shielding, as I know Ben has successfully built lots of gear, but here are my thoughts on the subject:
1. IEC ground pin connected to chassis.
2. Should have continuity between IEC ground and every panel of the chassis. Check by touching your DMM to the threads of different mounting holes, because, obviously, the paint is not conductive. If any panel does not have continuity, you will need to scrape away some paint where it touches the next panel or your case will not work as a shield.
3. EVERY XLR Pin 1 connected to chassis (some may want to stick a switch between output XLR pin 1 and chassis to allow lifting the ground... your input XLR pin 1 MUST be connected to the case or your phantom won't work).
4. If you use shielded cables for your I/O, connect the shield to the XLR pin 1 ONLY. DO NOT connect it to pin 1 AND your PCB.
5. Methods of grounding the circuit may vary a bit, but generally you will either take everything individually to one ground point (the so-called star ground) and then connect this star point to the main chassis ground through a switch of some sort OR you can run everything individually to your PSU and connect the PSU ground to chassis ground, again, through some sort of switch so you can break the connection if you ever need to.
Unless I'm forgetting something, that should do it.
Unshielded cables will not make or break a well grounded/shielded system. Unless your PSU trafo radiates noise into the circuit. To give some perspective, a while back I bought a DIY312 which had NO shielded cables on I/O, even the DI. It did have an external PSU, but in any case, it never had any problems with noise.
Assuming your grounding/shielding is OK, as I assume it is, and your opamps are known to be working, I would look for problems where the signal is balanced. Basically check from the XLR input up to and including the input trafo, and output trafo through XLR out. Check DCR on your trafo windings to make sure you don't have an open winding. Check the input pad resistors and switch, as well as the whole daughterboard for cold solder joints. And don't forget the 5-pin molex that connects to the smaller board.
Hopefully you'll find it quickly and it is something simple to fix. If not, then inject a signal into the circuit and probe around with your scope or a signal probe (if you don't have one of these, here is a link to DIY instructions by a fellow member here, Mark Burnley:
http://www.diyfactory.com/data/mbsignaltracing.htm
good luck!
1. IEC ground pin connected to chassis.
2. Should have continuity between IEC ground and every panel of the chassis. Check by touching your DMM to the threads of different mounting holes, because, obviously, the paint is not conductive. If any panel does not have continuity, you will need to scrape away some paint where it touches the next panel or your case will not work as a shield.
3. EVERY XLR Pin 1 connected to chassis (some may want to stick a switch between output XLR pin 1 and chassis to allow lifting the ground... your input XLR pin 1 MUST be connected to the case or your phantom won't work).
4. If you use shielded cables for your I/O, connect the shield to the XLR pin 1 ONLY. DO NOT connect it to pin 1 AND your PCB.
5. Methods of grounding the circuit may vary a bit, but generally you will either take everything individually to one ground point (the so-called star ground) and then connect this star point to the main chassis ground through a switch of some sort OR you can run everything individually to your PSU and connect the PSU ground to chassis ground, again, through some sort of switch so you can break the connection if you ever need to.
Unless I'm forgetting something, that should do it.
Unshielded cables will not make or break a well grounded/shielded system. Unless your PSU trafo radiates noise into the circuit. To give some perspective, a while back I bought a DIY312 which had NO shielded cables on I/O, even the DI. It did have an external PSU, but in any case, it never had any problems with noise.
Assuming your grounding/shielding is OK, as I assume it is, and your opamps are known to be working, I would look for problems where the signal is balanced. Basically check from the XLR input up to and including the input trafo, and output trafo through XLR out. Check DCR on your trafo windings to make sure you don't have an open winding. Check the input pad resistors and switch, as well as the whole daughterboard for cold solder joints. And don't forget the 5-pin molex that connects to the smaller board.
Hopefully you'll find it quickly and it is something simple to fix. If not, then inject a signal into the circuit and probe around with your scope or a signal probe (if you don't have one of these, here is a link to DIY instructions by a fellow member here, Mark Burnley:
http://www.diyfactory.com/data/mbsignaltracing.htm
good luck!