Thanks Marcel, I will think about this. You are right, I think MC cartridge are usually insulated: Not sure if this is true also for MM. But the cartridge provides differential signal per design. Maybe I try to use this as a benefit, good proposal.
The best you can do Is do 2.class with "silent" earth to keep safety and avoid questionable rdc.
If you want maximum safety, use transformer with earthed shield primary, earthed chassis. All mains double insulated, and than connect amps gnd with earth at audio input connectors. No RDC and minimum earth-loop involved. If I understand this is phono, than connect earth and gnd at preamp output connectors, not input.
If you want maximum safety, use transformer with earthed shield primary, earthed chassis. All mains double insulated, and than connect amps gnd with earth at audio input connectors. No RDC and minimum earth-loop involved. If I understand this is phono, than connect earth and gnd at preamp output connectors, not input.
By the way, common sense tells me that this is not safe:
(Fuse and mains switch not drawn for simplicity.)
Assuming the "poorly insulated transformer" has good enough insulation for class I equipment, it would be safe if there were a connection from signal ground to protective earth or if there were no cinch or other connectors to touch. As drawn, touching the cinch connectors that are mounted insulated from the metal enclosure as well as the metal enclosure will give you quite a shock when the transformer insulation fails.
(Fuse and mains switch not drawn for simplicity.)
Assuming the "poorly insulated transformer" has good enough insulation for class I equipment, it would be safe if there were a connection from signal ground to protective earth or if there were no cinch or other connectors to touch. As drawn, touching the cinch connectors that are mounted insulated from the metal enclosure as well as the metal enclosure will give you quite a shock when the transformer insulation fails.
Last edited:
I read about it. Do you know why it's necessary to connect PE at the output GND of the amplifier? I guess it's related to the insulated cartridge at the input and the earthed amplifier at the output, isn't it?
That's my understanding and drives me a little crazy. Maybe basic insulation (your poorly insulation) is only allowed for a device that provides no interface to the outside? If you have an interface, you may need a double insulated transformer. Or will reinforced be fine as well? More and more questions, I'm sorry about that! 🙂
Interesting link, sounds like Reinforced Insulation is necessary between primary and secondary and Basis Insulation between primary and earth. That should be valid in mostly all small audio devices and makes sense for me and matches to the things discussed above. This matches to the chargers I mentioned as well.
https://www.xppower.com/storage/documents/technical-articles/High-Pot_Testing.pdf
They specify that if the secondary voltage is below a maximum voltage, there are no test requirements related to the seconary. This let me assume, that if somebody like to have a GLB, it's important to know that your transformer is at least Reinforced Insulated. But be aware, that for a retrofit it may be difficult to know the safety concept the manufacturer chooses and know that they do everything right. If it's legal in your country is another topic to check, but from a safety aspect, know all details of the safety concept and the transformer before doing something critical.
I'm still not sure about the Double Insulation. When do we need this?
https://www.xppower.com/storage/documents/technical-articles/High-Pot_Testing.pdf
They specify that if the secondary voltage is below a maximum voltage, there are no test requirements related to the seconary. This let me assume, that if somebody like to have a GLB, it's important to know that your transformer is at least Reinforced Insulated. But be aware, that for a retrofit it may be difficult to know the safety concept the manufacturer chooses and know that they do everything right. If it's legal in your country is another topic to check, but from a safety aspect, know all details of the safety concept and the transformer before doing something critical.
I'm still not sure about the Double Insulation. When do we need this?
Last edited:
As far as I know, double insulation and reinforced insulation are both allowed between the mains and points you can touch when there is no protective earth (class II).
Actually there is no issue, yet. I thought about adding some options to the circuit if there will be issues with hum. I'm working at the schematic, layout of power supply is ready but no physical circuit, yet.
I'm afraid of the higher gain. My MMs are really silent, no hum and no issue. The new design is a puristic JFET design with higher MC gain. I try to be careful, consider the design recommendations I know, and I very often read about a GLB recommendation but the legal questions. Therefore I ask to get a better understanding.
Last edited:
AES, equipment grounding tutorial in slide presentation format by, Bill Whitlock of Jensen Transformers.