Arthur Kelm knows a thing or two
about power and electronics. He has built studios across the country,
served as chief engineer at many others, and now consults on power and
infrastructure as the owner of Ground One in Guerneville, Calif. He
has a 43-year legacy that takes him back to when Aphex introduced the
first 900 series modules, and he is credited with coining the termLunchbox
in the late 1970s when he and Niko Bolas were working at Record One.
Today, he works at Capitol Studios
in Los Angeles as Director of Engineering and is overseeing the overhaul
of the entire facility, and recently he joined the team that has just
started the process of resuscitating the famed Chess Studios in Chicago,
in its 1964 glory with a 2013 digital backbone. We asked him to comment
on power and how it works in the range of 500 Series racks on the market.
Power distribution seems to be the hot topic
in 500 Series today
Its only about in the last
year or so when this talk started picking up, in casual conversations
at AES or in studios. People started talking about how different racks
sound different, and everybody had an opinion about which units sounded
better. Me, being a power guy, I started to think, Oh, thats
interesting. Before, there was no comparison. People might compare
an API 10-space to a Brent Averill 11-space, but no one even thought
about it. Now, there are a lot more options.
So what are the primary challenges,
from an electronics point of view?
Headroom. Its all about
headroom. And headroom is related to dynamic current draw. Obviously
a kick drum, snare drum or any percussive instrument with really high
transient requires dynamic current draw. That becomes a peak current
draw on the power supply. So lets say a module is rated at 1 amp;
peak current draw with a dynamic could be as high as 4 amps for a few
milliseconds. Times 4 is a good rule of thumb.
Would you recommend thats
what a buyer first look at?
I would say the first thing you
want to do is add up the current draw of each of your modules and then
compare that to the current rating of your power supply. Thats
your first bit of homework for due diligence. Then apply the 4x calculations.
This should be the current rating of your power supply. Then,if you
can, find out from the rack manufacturer whether they use current limiting
in their power supply. The reason for using current limiting is that
in case of a short circuit the power supply shuts down. But there has
to be a threshold there, right? That will be your limit for peak current
draw. Most power supplies use current limiting, and most manufacturers
use off-theshelf product, rather than design their own circuitry. I
prefer using power supplies that do not implement current limiting.
A slow-blow-fuse approach on a power supply with no current limiting
is a much better approach. This will give you maximum dynamic current
for high transient demands.
Are there advantages to either
an internal or external power supply?
Usually internal PSUs have an
EMF field issue that you have to guard against. Because its a
transformer inside the box, depending on what module you put next to
the power supply and including how well shielded the power supply is,
you will have more or less hum. If the manufacturer did their due diligence,
its not an issue. But it does raise a flag. Thats why most
people went external. It takes your EMF interference issue away. But
that creates a dynamic current draw problem, which is the whole basis
of my conversation. Now you have a length of cable and at least two
connections, if not three, between your modules and the power supply.
Depending on the surface area of the connection, the length of cable
and the size of wire of the cable, that has current-limiting possibilities
written all over it.
What happens, then, from power
supply to rack?
If we go into the minutiae, starting
at the module the first thing you have in your path is your edge connector.
All manufacturers use the same pin out and standard connector, but the
amount of gold and the amount of tensile strength matters. So all edge
connectors are not created equal. Secondly, is the connector hard-wired,
or is it PC board? PC board assemblies are cheaper to manufacture, but
the drawback is the size of the trace going to the power distribution.
If they have really thin traces, they will limit dynamic current draw.
Ideally, like in the old APIs, they were all hand-wired, bus-wired going
through the connectors. When you start looking at what youre buying,
youre looking at the number of connectors, the quality of the
connectors, the surface area of the connectors
then you start looking
at wiring. The gauge of wire should equal current draw. For instance,
you dont want to have 16-gauge or 18-gauge wire feeding 10 amps
to your rack. It may carry the current, but theres no dynamic
current draw. You can compare it to speaker wire. The longer distance,
smaller wire, less headroom, less dynamics, softer sound.
The rack itself
with 2, 3, 6 or 10?
More slots in the rack would
dictate the use of a heavier wire gauge or larger traces. More modules,
more current draw. The whole thing is based on dynamic current draw.
Thats why racks sound different. The whole goal with 500 Series
power is to not restrict the modules ability to perform the best
it can. All
modules use amp blocks, whether discrete or IC. You want to supply enough
dynamic current so the amp blocks can perform to the best of their ability.