Ofcourse low volume...
Ok the inductance is not stable but is 1Khz not stabel for the speaker?
I mean this what I hear?
I have the Focusrite 2i2 set up with REW and measured my DIY tube preamp. I can see there are some 50HZ and harmonics interferences, maybe caused by ground loop.
But if I lift the earth pin of the IEC power socket on the preamp , all the interference disappeared. I was wondering if I can connect isolation transformers at both input and output to do the measurement? or it is common practice just to lift the earth pin of power cord?
But if I lift the earth pin of the IEC power socket on the preamp , all the interference disappeared. I was wondering if I can connect isolation transformers at both input and output to do the measurement? or it is common practice just to lift the earth pin of power cord?
Attachments
You can disconnect your DUT (preamp) from mains earth, or your 2i2, or both, in order to open the loop. Whatever is convenient and safe.
To allow a more universal approach, I isolate my USB soundcard using battery power for the soundcard and use a laptop. You may be able to make your measurement with just a laptop, that also powers the 2i2 ?
It may not be safe, or allowed by wiring rules, to disconnect the protective earth from a DUT which is not classed as double insulated. Ie. the DUT is designed to operate with a protective earth connection. Electrical service personnel may operate equipment from an isolation transformer, for a variety of reasons, and I have an isolation transformer for certain special uses, but if you don't have to go that path then imho make an effort not to.
To allow a more universal approach, I isolate my USB soundcard using battery power for the soundcard and use a laptop. You may be able to make your measurement with just a laptop, that also powers the 2i2 ?
It may not be safe, or allowed by wiring rules, to disconnect the protective earth from a DUT which is not classed as double insulated. Ie. the DUT is designed to operate with a protective earth connection. Electrical service personnel may operate equipment from an isolation transformer, for a variety of reasons, and I have an isolation transformer for certain special uses, but if you don't have to go that path then imho make an effort not to.
You should use the lifted earth if it reduces noise or hum from mains. Make sure you have a safety anti-parallel diode across lift resistor or use a high current rated NTC in place of lift resistor to provide safe grounding. This will make your amp quieter in practice and probably an audible improvement. Adding isolation transformers are expensive and they have their own distortion.
This thread has been brought to our attention because of the suggesting of disconnecting mains protective earthing to lower the noise floor.
The post in question does essentially cover the safety aspects. If you are doing this as a means of fault finding and/or testing then you obviously do so at your own risk.
To do this on equipment that is to put into normal service is dangerous and something that should not be done. It could be lethal to unsuspecting users.
Never do this on equipment designed from the outset to use a protective earth. It could be lethal.
Diodes, lift resistors and NTC's will be blown to smithereens in the event of a true live to chassis fault and offer little to no protection to the end user.
Thanks all for the advances. Actually, the 50hz noise can be seen as soon as the power cord plugged in without turning on the preamp.
As trobinns suggested , I tried a laptop to power the 2i2, that lowered 50hz peak to around 95dbFS. I think that's not too bad for a tube preamp. My analog millivolt meter only shows only 0.04mv noise floor. I'm thinking to use an isolation power transformer that has no earth wire and see if that will reduce mains noise.
As trobinns suggested , I tried a laptop to power the 2i2, that lowered 50hz peak to around 95dbFS. I think that's not too bad for a tube preamp. My analog millivolt meter only shows only 0.04mv noise floor. I'm thinking to use an isolation power transformer that has no earth wire and see if that will reduce mains noise.
Also be mindful of quality and layout of shielded connection cables, and to use metal shielding around all equipment to avoid external fields coupling in. I use large aluminium 'bbq trays' to sit equipment and cables in, and then to put on top, and also to turn off and distance all other equipment.
I think we were talking about mains isolation transformers, not signal isolation transformers. Distortion of signal isolation transformers indeed is an issue with measuring equipment.
