I'm building a preamp using one transformer for the high and low voltage sections. If the build doesn't use the typical steel/conductive chassis, does the transformer case need to be attached to the grounding buss?
The transformer case should be connected to safety ground, unless you can guarantee that it is impossible for anyone to touch it or anything connected to it.
All conductive parts of the preamp that a user can touch must be connected to safety ground. Also make sure the case has a securely-fastened bottom.
Thanks for all the helpful replies.
Blast these powder coated chassis. It's a Hammond knock down. I'll have to scrape paint off the corners on all six panels with my Dremmel.
I can put a bolt with a star nut inside rear where the power cord comes in and use that for the AC ground line and a line to the transformer case. I'll mount a buss near the front machine screwed into the chassis to handle grounds from the power supply board and the tube board. I'll also run a wire from the shielding on my signal lines to the buss also.
Any issues with my thinking here?
Blast these powder coated chassis. It's a Hammond knock down. I'll have to scrape paint off the corners on all six panels with my Dremmel.
I can put a bolt with a star nut inside rear where the power cord comes in and use that for the AC ground line and a line to the transformer case. I'll mount a buss near the front machine screwed into the chassis to handle grounds from the power supply board and the tube board. I'll also run a wire from the shielding on my signal lines to the buss also.
Any issues with my thinking here?
Last edited:
Let me give an opposing (and probably unpopular) opinion.
If I were building a commercial product, then absolutely, I would ground the transformer case. Just to protect myself from any liability in the case that the transformer insulation were to break down. If you were going to get an agency approval (UL, etc.) it would be mandatory.
But building a DIY amp for myself? I wouldn't bother going out of my way to get the transformer grounded. At least if it is from a reputable manufacturer that has agency certifications (like Hammond - their transformers bear an ETL mark). You are probably more likely to get hit by lightning then run over by a truck on the same day than to suffer a shock from the transformer insulation failing.
Is it possible? Of course. But extremely unlikely.
By the way, these same considerations apply to output transformers. But a probably more important consideration is what happens if there is a primary to secondary short in an OPT - you certainly want the secondary grounded.
As a side note - something I find amusing is that the safety standards that electronic equipment are held to are so strict as compared with standard household items. You can unscrew a light bulb and stick your fingers in. You can cram a fork into a toaster and contact line voltage. Those are OK, but build a computer and you have to comply with 500 pages of safety specs that ensure that it is impossible to hurt yourself...
Pete
If I were building a commercial product, then absolutely, I would ground the transformer case. Just to protect myself from any liability in the case that the transformer insulation were to break down. If you were going to get an agency approval (UL, etc.) it would be mandatory.
But building a DIY amp for myself? I wouldn't bother going out of my way to get the transformer grounded. At least if it is from a reputable manufacturer that has agency certifications (like Hammond - their transformers bear an ETL mark). You are probably more likely to get hit by lightning then run over by a truck on the same day than to suffer a shock from the transformer insulation failing.
Is it possible? Of course. But extremely unlikely.
By the way, these same considerations apply to output transformers. But a probably more important consideration is what happens if there is a primary to secondary short in an OPT - you certainly want the secondary grounded.
As a side note - something I find amusing is that the safety standards that electronic equipment are held to are so strict as compared with standard household items. You can unscrew a light bulb and stick your fingers in. You can cram a fork into a toaster and contact line voltage. Those are OK, but build a computer and you have to comply with 500 pages of safety specs that ensure that it is impossible to hurt yourself...
Pete
I am more concerned with a mains wire coming loose and touching some other part inside the equipment.
That usually means the equipment apparently goes "OFF" and your first reaction is to go and fiddle with wires, connections and knobs. Any exposed metal in those connections could be connected indirectly to that loose live wire.
The Safety Earth is connected to Protective Earth (PE). It's name describes it's purpose. Protection and it does this by blowing the Mains Fuse if any conductive internal part becomes LIVE due to a fault.
That usually means the equipment apparently goes "OFF" and your first reaction is to go and fiddle with wires, connections and knobs. Any exposed metal in those connections could be connected indirectly to that loose live wire.
The Safety Earth is connected to Protective Earth (PE). It's name describes it's purpose. Protection and it does this by blowing the Mains Fuse if any conductive internal part becomes LIVE due to a fault.
Thanks Pete. I have lots of older amps, tube and ss, that only have a two prong AC cord. I don't fear using them at all.
An ungrounded transformer could have audio implications, quite apart from safety. There will be capacitive coupling from the windings to the frame, and then from the frame to nearby circuitry. Hum may be the result.
Grounding does no harm, and may do some good.
Grounding does no harm, and may do some good.
Looking at how the paint on the transformer covers 100% of the housing, is it standard practice to have scrape paint off the bottom of the feet anyway for good conductance to a steel chassis?
BTW, I don't have an EE Degree, only a passionate interest in HI-Fi and the DIY world. I really do appreciate all the important issues discussed here.
BTW, I don't have an EE Degree, only a passionate interest in HI-Fi and the DIY world. I really do appreciate all the important issues discussed here.
Last edited:
If they have done a good paint job then yes you need to scrape a bit off. Often the screw will do this as you tighten it. Use a continuity tester to check.
Here in SA we have earth leakage trip switches, so yes I ground my transformers to chassis earth should a wire come loose which will then trip the mains before anyone gets electrocuted.
Mac.
If you guys are worried about your mains wire coming loose from the transformer, how exactly are you attaching them? I solder the mains and the primary side to solder lugs which are bolted to the case. Only an axe would get them loose.
Murphy. Soldering is no guarantee that they won't come loose, that's why the wiring in your car is never soldered.
A severely overheating joint could get hot enough to melt solder.
A poorly connected soldered joint could crack.
It is generally recommended that solder should not be used for mains connections. Mechanical fixings only.
A soldered joint should be mechanically fixed before soldering. This "rule" is broken when using PCB. I wonder if that is why tag strip and other point to point are reputed to sound better. Mechanical fix before soldering?
A poorly connected soldered joint could crack.
It is generally recommended that solder should not be used for mains connections. Mechanical fixings only.
A soldered joint should be mechanically fixed before soldering. This "rule" is broken when using PCB. I wonder if that is why tag strip and other point to point are reputed to sound better. Mechanical fix before soldering?
I was always taught that a connection was to be first made mechanically sound via crimping, twisting, friction, bending, or similar. Then solder was added to enhance and maintain conductivity.
A very good article on grounding can be fiund here:
http://www.diyaudio.com/forums/showthread.php?t=163575
He recommends that transformers are isolated from the chassis, but earthed using a lead from the tx directly to safety ground.
http://www.diyaudio.com/forums/showthread.php?t=163575
He recommends that transformers are isolated from the chassis, but earthed using a lead from the tx directly to safety ground.
- Status
- Not open for further replies.