Output DC protection relay ciruit boards are now available for all versions of the
400 and 700 amplifiers. A complete kit which includes the assembled and tested circuit board, new OEM binding posts,
input jacks, hookup wire and hardware is available for $99.95 in the U.S. The cost also includes priority mail shipping.
Unlike many high power amplifiers, all versions of the Phase Linear 400 and 700 did not provide any
speaker protection against amplifier malfunction. The relay circuit senses a DC voltage at the output of each channel
and quickly disconnects the speaker. The circuit also eliminates turn on/turn off thumps by delaying the relay at turn
on and immediately disconnecting the relay at turn off.
One of the primary design goals was to avoid altertion of the chassis. This would greatly simplify
the installation by eliminating the need to drill mounting holes. With the help of good instructions, most enthusiasts
can do the installtion themselves.
The circuit board is designed to mount on the output binding posts. In the stock
amplifier the amplifier output connections are soldered to the binding post stud along with the R/C network. This
usually damages the threads on the studs. So the binding posts are replaced during the installation.
The output R/C (Zobel) network was originally connectd to the binding posts. The network
is now located on the relay board so that the network is allways connected to the amplifier, regardless of the status
of the relay.
OEM RCA input jacks are included with the kit because the ones on most amplifiers have become loose
and make intermittent connections. The installation of the board makes for a good opportunity to replace the input jacks.
Some amplifiers have been previously modified with alternative binding posts. In most
cases the new posts have a larger stud than the 4-40 stud on the originals. The relay board can not be drilled
to accept a larger stud. For these situations contact us for assistance before ordering.
For those persons with advanced skills, the blank circuit board is available for $29
including priority mail. A parts list with Mouser part numbers is available to make ordering parts simpler. The 400
board uses an integrated circuit that is not readily availalbe. The chip will be shipped with the board. It is
the responsibility of the builder to properly identify the rotation of the pins.
When building the board, a good quality soldering iron in the 40 watt range is all that is needed.
Solder with water soluble flux is recommended. This will allow the board to be cleaned following assembly and leaves
a much better appearance. Note that some water soluble flux is conductive and therefore must be cleaned.
The parts used on both the 700 and 400 boards are common parts that are readily available. The
exception is the IC used on the 400 board. Many DIY types will have the board components. All 1/4 watt resistors
are specified as 1% because they're cheap. If the exact value is not available, an alternate value within 5% of the
specified value is acceptable. Alternate capacitor values should not be used.
The 700 board mounts to standoffs with 4-40 x 1/2" screws and lock washers.. Phillips head screws
are preferred. The minimum quantity to purchase at Mouser is 100pc. It is suggested that these be purchased
at a hardware store. Buy a few extra in case one is lost.
Make sure to check the links at the bottom of this page.