Transformer Hum 1U pkg.

[Harry Bissell]

Harry Bissell writes:

JH. could you be more specific. Do you mean transormers (flat) like the Signal
Transformer "Flathead"? These have a (QUOTE) "semi-toroidal" construction that
minimizes radiated magnetic fields. Doesn't work ??? or is the transformer you
were referring to a different core (like an E-I)?

It might be that the problem with the small single rack transfromer is that
the core is so small that the flux density is right in the edge of saturation,
and so magnetic leakage.... ???


(SOT)
Real toroids rule. I power my studio with a 1200VA 120pri/120VCT secondary in
the "balanced power" mode... hot and neutral are 60VAC from earth ground. The
center tap goes to earth ground (ROD). No hum problems... but I have good
grounding practice anyway. Most of the benefit is probably from the real earth
ground, and second from the balanced hot/neutral currents. It cost about $200
to get the custom wound, shielded, vaccuum impregnated Xfmr. And to this day,
years later.... The Xfmr rep. still calls me to see if I need another ;-)
Harry



jh <jhaible at primus-online.de> wrote:
Well, it was just an idea, and an example of how to add to
a standard module without endangering the standard (;->).

I've been using both methods before, though if I go for low
voltage AC I usually get it form wall warts (they are so cheap, 
2 Dollars sometimes). And I'm not so sure that several
rectifier / voltage regulater circuits fed by one AC source
will work under all conditions. Keep in mind that there is
always a second connection over signal ground. I wouldn't
like to see AC currents flowing between the modules just because
one rectifier has lower drop voltage than the next one ...

The drawback of distributing low voltage AC is that you're
creating large magnetic fields along the cables (higher
current for same power), and while I haven't actually calculated
the fields, from experiments I know that these cables can
induce more hum into sensitive circuits than a decent power 
transformer. 
There are *some* mains transformers, however, which have
increadible stray field, especially these flat pcb mount types.
Avoid these by any means. (You can often find them in 1U
rackmount boxes because they fit in so nicely, but they can
ruin the SNR of all modules in the whole rack !)

JH.


-----Original Message-----
From:	Per Mattsson [SMTP:per.mattsson at mkv.mh.se]
Sent:	Sunday, May 09, 1999 11:06 AM
To:	jhaible at primus-online.de
Subject:	Re: RE: polymodular package / cables

jhaible at primus-online.de skriver:
>
>I'll probably build a small PSU for each of these physical modules,
>
>so I won't have an overall +/-15 DC bus, but 240V AC for each module.
Wouldn't 24V AC be cheaper = no transformer for 240V AC> 18V AC per module?
You would still get very good electrical insulation between modules. Just
the cost of 240V AC cords + connectors will be rather steep.

But also, yes, the modules would not be selfsupporting if you want to
remove it from the rack.

---Per




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