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The RPD-15 DC Offset Blocker
What is DC Offset?
The simplest explanation of DC offset is an imbalance in the AC. A more technical explanation is as follows: DC offset is when an
AC signal, in this case the 120 V 60 Hz line voltage, is offset by a constant DC voltage. This should not be confused with noise
distortion. Noise distortion is when unwanted AC components are added to the supply voltage, such as through radio-frequency
interference (RFI) or electro-magnetic interference (EMI).

What Causes DC Offset?
The causes of DC offset are varied and in most cases occur outside of your home. There are some residential sources of DC offset,
such as the half-wave rectifiers used in halogen lamps, CFLs and LED lighting, but industrial processes are a much greater source
of DC offset. They load the mains asymmetrically (in an unbalanced way) resulting in a DC offset. It should be noted that DC offsets
are often transient and can increase, decrease, and disappear randomly, depending on the load that is being applied to the grid in
your area.

What Are The Symptoms of DC Offset
The most common symptom of DC offset in an audio system is noise production from the power transformer in your amplifier.
The larger the transformer, the more susceptible it is to DC offset. DC offset will manifest itself as a “growl” or buzzing sound.
Toroidal transformers are especially susceptible to DC offset. There are other concerns with DC offset that are not so obviously
apparent to the listener. DC offset forces the transformer to run in an unbalanced mode, and can even cause core saturation. This
can result in a loss of power being delivered to the amplifier circuitry.
Some topics in audio are just kind of dry and techie. In order to explain some of these
issues, without getting too technical, we often fall back on analogies. In this analogy,
we will assume that your audio system is an internal combustion engine and the 120
V AC mains supplying power to your system is the fuel. If the fuel that is feeding your
engine is polluted in any way, then the engine cannot perform at its optimum level.
For many years we at Blue Circle have been researching and building a variety of
powerline conditioners and filters to clean up your AC, and our PLCs and line filters
do a great job removing noise from the supply line. But the Blue Circle RPD-15 DC
Offset Blocker is designed to handle another growing problem that filters do not
address, namely DC offset.
The Solution
The Blue Circle RPD-15 is a simple and relatively inexpensive solution to the problems caused by DC offset. Like many other BC
products, the RPD-15 is housed in an ABS chassis to keep costs down. It has a standard IEC inlet and an industrial grade duplex
outlet. At a cost of under $200.00, it is a very affordable solution to this growing problem of DC offset.
The RPD-15 is 4.5” long and 3.5” in diameter. It weighs one pound.
*Power cord not included.
Demo: Click here.....  For a video of the RPD15 demo at AXPONA 2015.
Hi Gilbert,

I thought it was about time now that I give you a quick 'review' of the results of your work.
Well, they seem to have fixed the transformer buzz! Not a 'peep' since Iinstalled them all. And that includes listening through the '
Critical periods' - Thursday and Friday, late afternoons - when I am sure the 2 local farms and/or the local metalworks fabricant
were using machinery which put 'crud' on the mains. Saturday nights as well - all the local folks at home running TVs and
dishwashers, etc!
But, on top of that there's been a real benefit to the sound quality of audio system. I believe that this may be because I run an
entirely differential balanced system. From the outputs of the DAC, and the tone-arm/phono cartridge, right on through the phono
amp, line-stage/pre-amp, mono-bloc power amps and up to the speaker terminals the signal runs in true balanced-mode. And I
would have thought this makes it more susceptible to the problems of DC offset.
Anyway, whatever - with your units installed before the 4 'Pure Power' isolation transformers I use, the entire system is protected
from any DC off-set. And there is a distinct improvement in the bass! It is 'clearer' and better-defined (it sounds as if it even
'times' better). Right the way down to drum thumps and other normally 'woolly sounding' notes. Most unexpected.

I am well pleased. Money well spent, IMO!

Thanks for your kind help.


A customer from Luxembourg
Contact:  bcircle@bluecircle.com  /  519-469-3215
The following email was received  in July, 2016. Satisfied customers make the best reviewers.