A problem with AC Power you may not have considered.
My posting is not about a stereo system but it is related to
AC Power, from which all stereos draw power.
Read on, I am sure you will find this interesting. I certainly did and it caused me to rethink
and replan AC Power to my stereo.
At my real job as an electrical engineer, I manage a cross-disciplinary
engineering team for a large energy company.
We make large, residential green energy management systems, a size that
borders between most large homes and utility companies. A few months back, we released a new product
to the 230VAC single-phase market (Australia, Europe, etc.) and recently
introduced the same product to the 240VAC split phase market (USA, Canada,
etc.). In addition to a slew of UL,
IEC, IEEE, CSA, TUV, and other safety codes, we also had to meet FCC Class B emissions
(which all your digital audio equipment must also meet) and also meet FCC Susceptibility
requirements (which digital audio does not have to meet, unfortunately).
Since the two products are almost identical, I thought we could
leverage what we learned for the 230VAC unit onto the 240VAC unit. Well, this is where the impact of grid power
to our stereos comes into our interest.
The emissions requirement is of two parts, of which you may
be familiar. One is radiated emission,
which is the noise the product broadcasts into the air. The second part is conducted emissions, which
is the noise the product injects onto the power lines and runs throughout your
house and probably into your neighbors as well.
The 230VAC unit passed emissions, which I expected as we did
a lot of design work to make it pass.
The conducted part was a concern, since that injected noise is from the
equipment our vendor produces, not something we designed in house. Well, when the certified testing house tested
conducted emissions, it failed. A couple
of weeks of debug later, at 2K$ per day, the problem was solved when I
suggested they test with the grid connection running through 8 feet of steel
conduit, since all installations have at least 8 feet of conduit.
Fast-forward six months to the 240VAC testing, which took
place here in the USA. Surprisingly, the
unit failed conducted emissions, even though we used the same 8 feet of steel
conduit. Another week of debug, again
at 2K$ per day, we stopped testing since it was clear a new design is needed to
fix it. I designed a 50 Ampere Balanced
LEMP Filter that had over 50-dBm isolation in the affected frequency
range. Problem solved. So, why did 8 feet of conduit fix the
problem one time and not the next? A good
I took the same 8 AWG THHN wire we used to connect the unit
to the grid, ran it through the same 8 feet of 1 ½ inch steel conduit, and
rented some high frequency test equipment.
In the conduit we had two 8 AWG wires for Line 1 and Line 2, one 8 AWG
wire for Neutral, and another 8 AWG for Earth ground. I ran a bandwidth test from Line 1 to
Neutral and tied the conduit and Earth wire to earth, while the other Line wire
floated. The test started at 60 Hz,
which I referenced as 0 dBm and I ran the test all the way to 30 MHz. The generator produced 10Vrms, the level I
checked at each step, and fed a 50-Ohm load.
To my great surprise, I had a 2-dBm rise at 10 MHz where it began to
roll off and was only 2 dBm down at 30 MHz, the limit of the test
generator. In other words, that length
of pipe and THHN wire had a bandwidth of +/- 1 dBm from 60 Hz to 30 MHz! Whoa!
We are allowing a ton of injected noise into our systems!
To prove that, I grabbed the power supply from an analog stereo
amplifier and fed the test signal through the cord, fuse, transformer, and measured
the bandwidth on the secondary. In
spite of a UL/CSA approved transformer, it was surprisingly transparent to the
test signal. Throughout the test
spectrum, it was never more than 6 dBm down and it peaked in a couple of areas,
Our homes usually don’t have grounded conduit, what most
homes have is Romex wire. That stuff is
transparent to radiated emissions and we live in a world of radiated emissions. Think cell phones, FM and AM radio, TV
broadcasts, all the communication frequencies, plus who knows what we have for
the dirty noise injected by electric motors.
Think your fridge, your AC unit, your furnace, ceiling fans, light
dimmers, electric vehicles (that is the reason they don’t usually come with an AM
radio these days!), the list can go on for a long time.
For my stereo system here at the house, I built a smaller
version of the LEMP filter, added additional suppression, along with 20,000
Amps of surge protection. I am also
installing a dedicated earth ground as well.
However, you don’t have to home brew – you can purchase equipment that meets
the local safety codes and is LAB certified to meet multiple suppression standards. These units have strong filters in them to
clean up line power. There are replacement
AC line cords on the market that contain RF suppression. I don’t
suggest you get a new mortgage just to buy AC noise suppression equipment or
new line cords, but I do suggest you do something to kill those RF demons.
Look for equipment that has at least 30 dB of suppression
from 100 KHz to 15 or 20 MHz. Thirty to forty
dB is the range where most emission problems fade away, so that is a good
starting point. Some equipment has
lightening suppression as well; look for an IEEE spec stated in joules of
energy, the more the better with a test pulse of 8/20 microseconds. Don’t be afraid to stack some of the
equipment in series.
The lighting in your listening room can also matter a great
deal. Stick with plain, old school incandescent
bulbs; avoid the CFL’s, LED’s, neon’s, light dimmers, and other lights that
require power supplies to run.
Incandescent bulbs are very quiet, which is why they appear regularly in
emission anechoic chambers. Although
digital equipment is less sensitive than analog equipment, it is not immune to
susceptibility. Vacuum tube equipment usually
has an edge over solid state, too.
I hope what I wrote is of help to you in your quest for