Transformer Overkill

[miles_stevens]

Hi guys,

I'm currently building a Stereo Mnats Rev J 1176 and have a 160VA 30v CT Toriodal transformer lying about at home. I know it is total overkill, but will I run into any problems using a transformer with so much juice?

Thanks!

[rob]

no problems but it is total overkill

just remember that all mains transformers will radiate hum and buzz into your audio input transformers and such a big transformer will radiate more and give you less options for optimising its placement to minimise hum.

[The Tasmanian]

Just out of interest Rob - would mu metal make a hum barrier?

[rob]

yes, a metal shield helps and mu metal is the best sort of metal for magnetic shielding.

However, imagine the mains transformer is a speaker outputting an objectionable noise and the audio transformers are you ears hearing this noise. Simply putting a flat plate between the two will only reduce the level a little bit. Then imagine you put the lid on the box, this could "reflect" more of this noise and it might now get worse.
Just like in acoustic isolation the best thing to do is to built a sealed room and put the objectionable noise source in there, thus a mu metal can that completely encases the mains transformer is the best solution.

[The Tasmanian]

Thanks for that info Rob.

[Bent my VU]

It might be best to build an external power supply for it. I've given up buying transformer equiped gear that has a built in PS - they all induce hum, even the shielded ones to some degree.

[Drumstruck]

..... they all induce hum, even the shielded ones to some degree.

Is this true?

Are there any stepdowns that don't induce hum?

Is it better just to swap out the internal power supply?

[rob]

see my article from AT80 for a discussion about this sort of thing:

http://proharmonic.com/articles/AT80_OTB.pdf

[Drumstruck]

Thanks Rob,

I re-read that article..... and had already implemented all the things you suggest (separation of power / data cabling etc).... also run balanced cables everywhere /have recently replaced most of my TRS with XLR so atm I have only one device causing some low level hum (and I know how to fix that one).

But I often see very interesting devices f/s overseas (100/110v) and have held off buying a few of them because of the risk of inducing hum from the step downs so was wondering what option is best i.e. swapping out the power supply for a 240v one / remotely locating or shielding a larger stepdown etc....

This especially relates to some of the older pieces of equipment - do you recommend and do such p/s conversions?

Ultimately I'll have everything off the grid and running on isolated power units, but in the meantime I need the gods of the grid and don't want them humming along with the tune.


thanks

[rob]

230V to 115V stepdown transformers do create "hum" fields, just as every other mains transformer does.

The key to minimising any real world issues is distance! EM fields fall off with the square of distance so keep step downs at a good distance from the audio equipment and you shouldn't have a problem.

I have recently set up my workshop with a 115V mains system as I deal with a lot of US powered gear. So I have a 1000watt stepdown over in the far corner from which I have a extension lead to connect to a 6 way US powerboard on the workbench.

There is not necessarily anything to be gained by converting a 115V device to 230V except convienience and to avoid inadvert connection to a AU outlet when set for US power.
Indeed I have seen some US designed equipment that includes a switch to flip it between 115V and 230V and the unit has worse noise (hum) performance at the 230V setting due to poor design not accounting for the higher hum fields within the unit at the higher mains voltage setting.
And ... stepdowns also provide some filtering of the mains to reduce higher frequency mains borne noise.

Also, when you are running "off the grid" all of this will still apply .... you'll still be running your equipment off 230V 50Hz mains!

[Drumstruck]

Rob, that is brilliant - thankyou heaps - your explanation is concise and exactly at the level of detail I was after.

I see what you mean about "off the grid / off 230V 50Hz mains" as the source of power. Apart from removing reliance on the grid (basically running off solar charged UPSs), isolation of power supplies to various banks of components was one of my aims with this plan (i.e. one p/s running mixer + related devices, another p/s running amps, another running screen and ancilliaries and possibly another running 110v devices) - but ultimately there is still a source generating a frequency.....



btw - just sent you an email re another item


Rick - this man is worth more money !!

[miles_stevens]

Thanks for all the great info.

An external PSU is a good idea, though I was considering mounting the two compressor PCB's on top of one another with 25mm spacers (enough room with a few washers thrown in for good measure) with the toroid on the other side of the case. My thoughts were twofold; a), the largest distance between the audio circuits and power circuits, and b), since both PCBs are equidistant away from the psu transformer I was hoping that any noise introduced would be done so evenly on both channels.