Mic input transformer shielding really, really needed?
« on: April 08, 2021, 09:02:38 AM »
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.


Re: Mic input transformer shielding really, really needed?
« Reply #1 on: April 08, 2021, 09:21:04 AM »
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.

« Last Edit: April 08, 2021, 09:37:02 AM by rock soderstrom »

JohnRoberts

Re: Mic input transformer shielding really, really needed?
« Reply #2 on: April 08, 2021, 09:39:44 AM »
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
Cancel the "cancel culture", do not participate in mob hatred.

My3gger

Re: Mic input transformer shielding really, really needed?
« Reply #3 on: April 08, 2021, 10:55:29 AM »
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.

Re: Mic input transformer shielding really, really needed?
« Reply #4 on: April 08, 2021, 12:09:25 PM »
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

Re: Mic input transformer shielding really, really needed?
« Reply #5 on: April 08, 2021, 04:03:23 PM »
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.

Re: Mic input transformer shielding really, really needed?
« Reply #6 on: April 08, 2021, 04:16:30 PM »
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.

Re: Mic input transformer shielding really, really needed?
« Reply #7 on: April 08, 2021, 06:32:00 PM »
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

Re: Mic input transformer shielding really, really needed?
« Reply #8 on: April 08, 2021, 06:32:11 PM »
Not really understanding - MuMetal shields seem more a cost issue than a space problem ?

JohnRoberts

Re: Mic input transformer shielding really, really needed?
« Reply #9 on: April 08, 2021, 06:58:23 PM »
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
Cancel the "cancel culture", do not participate in mob hatred.


Re: Mic input transformer shielding really, really needed?
« Reply #10 on: April 08, 2021, 07:46:27 PM »
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

Re: Mic input transformer shielding really, really needed?
« Reply #11 on: April 09, 2021, 05:06:05 PM »
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  ?


 

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