Mic input transformer shielding really, really needed?

[Newmarket]

I have built several lunch boxes with four of my tube mic pres and a power supply in them. I used quality input transformers (Cinemag/Jensen) and each mic pre was built in a screened plug in enclosure. Even so, I could measure hum from the toroid transformer feeding the heater supplies and it got better the further you got from the toroid. It was not audible but it was enough to worsen the measured EIN by a few dB. As soon as I switched to an SMPS for the heaters the problem disappeared.

Cheers

ian

Hi Ian. What material were the plug in screened enclosures made with ?

 

[Heikki]

..just try a Neutrik NTE10/3, that'll show you

Neutrik NTE 10/3 without shielding. Gain at max (62 dB) and 150 ohm source, output noise is around -62 dBu.

0dBFS = +20 dBu in the picture.

 

[abbey road d enfer]

Neutrik NTE 10/3 without shielding. Gain at max (62 dB) and 150 ohm source, output noise is around -62 dBu.

0dBFS = +20 dBu in the picture.

This figure, without the actual gain and the source impedance, is meaningless.
What makes me suspicious is that the relative amplitude of the 50 Hz bin is not dominant as it should be if it was the main source of noise.
Can you produce an FFT with a 1kHz tone at 0dBu at the output?

 

[gyraf]

Well done, but.. ..I don't see any power supply in that box?

If you just move the NTE10/3 far enough away from any power transformers or -switchers, it off course won't pick up any crap

This also why it's very usable for microphone transformer despite its lack of magnetic shield

/Jakob E.

 

[ruffrecords]

Hi Ian. What material were the plug in screened enclosures made with ?

Aluminium, so no help against magnetic fields.

Cheers

Ian

 

[Winston OBoogie]

Well done, but.. ..I don't see any power supply in that box?

If you just move the NTE10/3 far enough away from any power transformers or -switchers, it off course won't pick up any crap

This also why it's very usable for microphone transformer despite its lack of magnetic shield

/Jakob E.

I believe the lower green pcb is the power supply. Looks to be 2 X DC converters with outputs of +/- 15V (op-amps) and 200V (H.T.).
I assume the raw DC input is used for the valve heaters.

 

[Newmarket]

Aluminium, so no help against magnetic fields.

Cheers

Ian

I see. Yes no H field protection from that. Cheers.

 

[My3gger]

I have built several lunch boxes with four of my tube mic pres and a power supply in them. I used quality input transformers (Cinemag/Jensen) and each mic pre was built in a screened plug in enclosure. Even so, I could measure hum from the toroid transformer feeding the heater supplies and it got better the further you got from the toroid. It was not audible but it was enough to worsen the measured EIN by a few dB. As soon as I switched to an SMPS for the heaters the problem disappeared.

Cheers

ian

I get better good results with toroids having ~50% more power than needed because they radiates less (Cinemag, Jensen, Sowter are inputs), voltage doesn't drop under load which is important for heater regulators, etc. They are custom made with additional shielding for phantom and similar, so cost is almost the same as for lower power. Such toroid is probably not practical for more than a channel or two...
Some people have problems with saturating back to back transformers as used in Gyraf G9. I got same problem with unknown EI ~230:15V transformer being too small, then switched to toroids because local winder knows what's needed for audio psu applications and is cheap. Wish he could also wind outputs going higher than 20kHz.

 

[Winston OBoogie]

I just received an EI custom power from Majestic Transformer here in the UK. This is for valve gear: 1 X H.T., 2 X L.T., + a phantom V winding.
So far, same power to power ratings, it radiates far less field than the custom toroidal jobs I've recently tried. Being a fairly low bandwidth device, it also blocks a lot of crap from the mains that gets through with the toroids.

On my next lower current experiment I'll probably have another stab at using DC : DC converters for the H.T. such as Hiekki did above.

 

[abbey road d enfer]

I get better good results with toroids having ~50% more power than needed because they radiates less (Cinemag, Jensen, Sowter are inputs), voltage doesn't drop under load which is important for heater regulators, etc. They are custom made with additional shielding for phantom and similar, so cost is almost the same as for lower power. Such toroid is probably not practical for more than a channel or two...
Some people have problems with saturating back to back transformers as used in Gyraf G9. I got same problem with unknown EI ~230:15V transformer being too small, then switched to toroids because local winder knows what's needed for audio psu applications and is cheap. Wish he could also wind outputs going higher than 20kHz.

The main issue with off-the-shelf toroids is that they are designed for maximizing power, which implies running them too close to saturation. they are often used at more than 1.5T.
I had custom xfmrs made for my products running at 1.2T and the radiated field is about 20 times less. Of course, they were a little more bulky, but it was worth it.