Mic input transformer shielding really, really needed?

[mkiijam]

I have a few other threads running on modding a Trident series 65. I'm thinking about adding some input transformers, but really don't have enough space without heavy mods to include the mu-metal shields. How important is it really?

I know there are a lot of dependent circumstances that are not known at this time, but:

1. The Trident doesn't have any internal AC currents that I can really think would make a big difference. No internal power supply etc.

2. Are mic input transformers as sensitive as say single coil guitar pickups? I have been in plenty of rooms and studios where you simply could NOT record single coil guitars, too much hum.

 

[rock soderstrom]

I have a few other threads running on modding a Trident series 65. I'm thinking about adding some input transformers, but really don't have enough space without heavy mods to include the mu-metal shields. How important is it really?

It is very important to shield the input transformers, because the signal is amplified very strongly afterwards. Any interference can become a big problem. I have not even been able to successfully run one mic preamp channel in a controlled test environment with a unshielded input transformer.
I have never seen a mixer with an unshielded input transformers, not even the inexpensive ones do that.

Really no room for shielding? That takes up almost no space.

 

[JohnRoberts]

Maybe try one channel each way and see what results you get.
====

old story, last century when I was designing fixed install background sound systems, I had to put a mic input transformer inside a small box with a 70/100V output transformer. The low power amps were harder than the higher power models, because of similar voltages but smaller chassis.

mu metal shielding helped in that scenario.

JR

 

[My3gger]

The only mic input transformers i've seen functioning properly without shielding were used inside metal cases with active DI electronics. Preamps need mumetal or similar shielding, i don't think you can get away from that.

 

[gridcurrent]

2. Are mic input transformers as sensitive as say single coil guitar pickups?

depends on the transformer design.
plenty of devices fitted with UTC input transformers minus additional shielding.

 

[mjrippe]

depends on the transformer design.
plenty of devices fitted with UTC input transformers minus additional shielding.

Key word here - "additional".  As your image says, they have built in shielding.  The mu-metal shields offered for the O and A series add an extra layer of protection.

 

[rock soderstrom]

Key word here - "additional".  As your image says, they have built in shielding.  The mu-metal shields offered for the O and A series add an extra layer of protection.

Exactly, humbucker winding plus shielding makes a quality transformer.

 

[gridcurrent]

31267 / Carnhill VTB 9046 are examples of input transformers not requiring "additional" shielding.
connect just one winding and you have a magnetic field "sniffer".
wire both primaries and they become super quiet, thanks to the hum-bucking construction.

 

[Newmarket]

Not really understanding - MuMetal shields seem more a cost issue than a space problem ?

 

[JohnRoberts]

If mu metal was absolutely necessary you couldn't buy a transformer without mu metal.

Like I said try both ways and see if it makes a difference worth the cost for your application.

JR

 

[Tubetec]

I think if your operating in the vicinity of a mains transformer both core orientation/geometry and mumetal sheilding would be nessesary for best performance  , even on something like a tube mic where the PSu is remoted , you may be miking up something with a mains tx such as an amp , the extra noise reduction afforded by a mu metal sheilded transformer is worthwhile . Agreed also bi-limboid or humbucking transformer construction adds an extra level of immunity from outside sources but I do recall miking up combo amps with U47's and having to play around to find a spot with minimum induced noise from the amps mains transformer.

 

[Newmarket]

Maybe try one channel each way and see what results you get.
====

old story, last century when I was designing fixed install background sound systems, I had to put a mic input transformer inside a small box with a 70/100V output transformer. The low power amps were harder than the higher power models, because of similar voltages but smaller chassis.

mu metal shielding helped in that scenario.

JR

Yes - basically tales from the EMC lab - we are dealing with predominantly H-field here. Distance is your friend and orientation is important. Take a single coil Hi-Z guitar pickup and move it around near a mains transformer and you'll get the idea.
Clearly there will be differences here - European 50Hz and related harmonics are much more euphonic than those North American 60Hz related artefacts
If muMetal ( and let's not forget it comes with varying permeability / saturation characteristics ) is not economically viable then steel will also help to a degree.
Either way I still don't see it as a space issue  ?

 

[gyraf]

I'm pretty sure that the major part of the cost of mic input transformers is from shielding..

Which again means that if it made any sense skipping on it, it would have been done long ago

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

/Jakob E.

 

[Newmarket]

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

Sounds like you have a story there...?

 

[merlin]

F.ex: Neve 1073 does not have mu-metal shielding, just the steel case the transformer comes with. They don't play nice with switch-mode supplies, but anything else doesn't seem to matter as long as you don't have power transformers too close of course.

 

[Winston OBoogie]

F.ex: Neve 1073 does not have mu-metal shielding, just the steel case the transformer comes with. They don't play nice with switch-mode supplies, but anything else doesn't seem to matter as long as you don't have power transformers too close of course.

And in the case of console amplifiers, each channel in also encased in its own metal box. 

The earlier iterations of the 10468 and 31267 by Marinair (that could sometimes be found on the exterior rear of the module, were in a heavier gauge steel canister with an octal plug. 

 

 

[Marcedona]

A cheap clone of 1272 made here in Buenos Aires, with Oep AE262 (shielded) and linear source in the same body (one rack unit) has had hum problems, which diminished when I reoriented the power transformers (two common 12v) and they disappeared when I put them in another cabinet. I imagine what it would have been "without" mumetal.

Another vote for mumetal here.

 

[Moby]

If you use the small step up (up to 1:4) maybe you can ommit the mumetal shielding, but for higher step up ratios it's highly recommended.

 

[Lucas Rosso]

If you use the small step up (up to 1:4) maybe you can ommit the mumetal shielding, but for higher step up ratios it's highly recommended.

Maybe for small ratios, Iron or steel case (perhaps some nickel alloy) is enough?
Very interesting

 

[ruffrecords]

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