Blue Circle Audio RPD 15 DC Offset Blocker

"The most common symptom of audio system DO offset is amplifier power transformer noise. The larger the transformer, the more susceptible it is to DC offset. DC offset will manifest itself as a growling or buzzing sound. Toroidal transformers are especially susceptible to DC offset. Another DC offset issue that DC offset forces a power transformer to run in an unbalanced mode resulting in a loss of power being delivered to the amplifier circuitry, not to mention the annoying audible noise.The Blue Circle RPD-15 is a simple and relatively inexpensive solution to DC offset. Like many other Blue Circle products, the RPD-15 is housed in an ABS chassis to keep costs down."
Works great !

see review 
https://positive-feedback.com/reviews/hardware-reviews/blue-circle-rpd-15-dc-offset-blocker/