I have not found information about this subject in the forum.
I am working with a DIY BA283 NEVE with OEP A262 A3E input transformer, of course with the OEP electromagnetic shield and connect it to ground.
The circuit works very fine, but the preamplifier captures proximity transformers electromagnetic fields (even at 30 or 40 cm….!), and inclusive noise from near AC lines.
With 70dB gain I measure the low frequency noise at preamplifier output…:
- Far from AC lines and transformers = 70 uV or less, (is better than 90 dB signal noise at +10dB reference, and better than 100 dB signal noise at +20 dB reference). It’s possible to improve this figure varying the transformer position.
- At 10cm near an AC line (more or less) = 400 uV output or more , (it’s worst than 75 dB signal noise).
The noise capture is evident connecting a high gain amplifier to the output, and varying the preamplifier PCB position in reference a noise sources. (In a work bench there are a lot of possible noise sources…).
I have experimented with the OEP shield and magnets and do not know if that Mumetal is genuine or is another alloy.
1.- Does anyone have experience with these transformers and electromagnetic shielding troubles?
2.- Do you have noise comparatives with Lundahl, Sowter, Cinemag, Jensen or an others?, More expensive of course…
Thanks
Miquel from Spain
I am working with a DIY BA283 NEVE with OEP A262 A3E input transformer, of course with the OEP electromagnetic shield and connect it to ground.
The circuit works very fine, but the preamplifier captures proximity transformers electromagnetic fields (even at 30 or 40 cm….!), and inclusive noise from near AC lines.
With 70dB gain I measure the low frequency noise at preamplifier output…:
- Far from AC lines and transformers = 70 uV or less, (is better than 90 dB signal noise at +10dB reference, and better than 100 dB signal noise at +20 dB reference). It’s possible to improve this figure varying the transformer position.
- At 10cm near an AC line (more or less) = 400 uV output or more , (it’s worst than 75 dB signal noise).
The noise capture is evident connecting a high gain amplifier to the output, and varying the preamplifier PCB position in reference a noise sources. (In a work bench there are a lot of possible noise sources…).
I have experimented with the OEP shield and magnets and do not know if that Mumetal is genuine or is another alloy.
1.- Does anyone have experience with these transformers and electromagnetic shielding troubles?
2.- Do you have noise comparatives with Lundahl, Sowter, Cinemag, Jensen or an others?, More expensive of course…
Thanks
Miquel from Spain