For the mains side fuse you should use a 500mA anti-surge type. All wiring at mains potential
should be adequately insulated, secured well and protected from straying fingers.
There is no need to fit an AC standby switch since you will be fitting a proper mains switch in
series with the transformer primary coil. So you should link S1A to S1R, and S2S to S2S, on the
RPSU PCB.
Toroidal transformers are in theory much easier to mount than ordinary EI transformers, they
simply need one large bolt to secure the various parts provided. However, there are two important
considerations involved when mounting a toroidal in a 1U high rack case. The first is that the
transformer and the mounting bolt must fit inside the case without the metal mounting bolt or top
mounting plate touching the metal case. The mounting bolt must only be in contact with the lower
panel. If it touches the top this will short circuit the transformer and it will probably catch fire.
Secondly, fitting a large bolt through the lower panel will mean the bolt head will stick out proud
from the underside surface, and the case may not now fit into a rack without touching other rack
equipment mounted below it. Commercial rack cases that use toroidal transformers often have a
upward indented section on the underside panel on which the transformer sits. The hollowed out
section can then easily hold the screw head without making the case exceed the 1U height format.
Earthing
Remember it is up to you, the builder of the equipment, to make sure that your item is safe and is
built to the required safety standard in your country. These notes are only a guidance and it is up to
the reader to establish the exact obligations required in their own country.
It is essential that everything you build, that has both live mains inside and a metal case or panels,
has a safety earth fitted. UK legislation requires that any metal panelling should be adequately
insulated, ie. double insulated, or connected to earth. Since making a double insulated case is not
practical you should ensure that any exposed external metal parts be properly earthed.
The case should be bonded to earth using an M4 screw, toothed washer, washer, solder tag and a
securing nut (or two) bolted through the case and then via a thick piece of wire back to the earth
tang of the IEC power inlet. It is useful to mount this earth bonding point on the rear panel of the
unit. Remember that all other parts of the case must be earthed too. Painted metal parts of the case
must be dealt with so that they too are earthed. This may involve using secondary bonding points or
scraping back the paint at the appropriate point.
The RPSU board should be securely mounted (using all four mounting holes) onto the earthed
casing using appropriate screws and toothed washers. You should also solder a thick wire from the
solder pad marked 'CASE' on the RPSU to the earth bonding point on the rear panel.
You will also need to provide earthing to any exposed transformer core. This does not apply to
toroidal types but EI types should have their metal frame earthed.
It is possible that by earthing the case and local ground you may introduce earth loops when you
connect your mixer to the sockets of the SRE330. The outcome of this is audible humming at
50/60Hz and its harmonics. It is produced by currents travelling down the screen of the connecting
cable(s). This can be avoided by careful studio wiring and/or by using balanced audio lines to pipe
signals to and from the unit and mixing desk. Most mixing desks and sound cards will have
balanced outputs and inputs.
