The product offered here is provided by Watts Abundant, formerly Professional Audio Services.  Professional Audio Services was recognized as an authorized Phase Linear service provider in February 1984.  We continue to support Phase Linear products, primarily the amplifiers to this day. 

The Phase Linear amplifiers of the 70's and early 80's were popular at home and in rock and roll PA's.  50 years later and they're more popular than ever.  At this age reliability concerns are valid.  Often times a failure results in 80 volts DC into the speaker with the result of a failed speaker and a failed amp.  Unlike most power amps, the 700 and 400 did not have an output relay to disconnect the speaker upon amplifier failure. 

The Phase Linear 400 output relay kit offered here accomplishes the following design goals:
Fast detection of DC voltage on the output.  Fast means faster than the blink of an eye.
  1. Upon detection, the relay has to disconnect the speaker and put a short across the speaker in case of relay failure.
  2. When the relay detects a failure, it stays off until power is cycled off and back on for additional protection. 
  3. Installation must not require drilling or modification of the chassis.  Instead, the circuit board bolts onto the output binding posts.
  4. Easy installation with detailed instructions.

Like hundreds of receivers and amps, the circuit uses a tried and true integrated circuit, UPC1237 for detection.  The difference here is all of the other designers compromised and did not crowbar (short across the speaker) which resulted in welded relay contacts and still damaged speakers.   

The relay used here has both a normally open and a normally closed contact for each channel to disconnect the speaker and short across the speaker's terminals simultaneously.  This provides dual protection to both channels.  It is possible the relay could be damaged.  Therefore it is mounted in a socket.  Compared to the speakers, the relay is cheap.  Extensive prototype testing, that intentionally caused amplifier failure, was performed with neither speaker or relay failure.  Thus far there have been no reports of relay failures to date. 

As mentioned above, the circuit board mounts on the existing speaker jacks.  No modification/drilling of the chassis.  Installation is greatly simplified.  If someone wanted to, they could remove the relay circuit and return the amp to stock form.  If you know which end of a soldering iron to hold on to you can likely do the installation.

Note:  There are lots of relay boards available on EBAY at a much lower price.  Here are a few things to consider:
  1. They don't drop into your amplifier.  You have to drill the chassis.
  2. They are not compatible with the higher voltages in the amplifier.  You have to modify the design.
  3. They use cheap relays and do not offer the dual protection this kit does.
If you can modify the design, drill the chassis and accept the limitations of the the relay you should buy the cheap ones found on EBAY. 


Tools required are modest.
  • 7/16" nut driver or deep socket for replacing the binding posts
  • #1 Phillips screw driver.
  • 1/8" wide screw driver for terminal connections
  • High quality 60 watt soldering iron
  • High quality wire strippers for #16 and #22 wire.
  • Pocket knife (to remove oxide prior to soldering)
What's in the kit? 
  • Assembled and tested high quality circuit board.  The board is double sided with plated through holes for long term reliability
  • New binding posts.  The old ones are hard to reuse because the threads have solder in them.
  • Red, black, and white hook up wire.  Occasionally a color may be substituted if we are out of stock.
  • Ground lug
  • Ty-wraps
Complete step by step installation instructions with pictures available for download.  EBAY does not allow a download link to be inserted here.  Contact us after the purchase (ask seller a question) so we can send the latest installation procedure.  Once purchased we can send instructions if you provide an email address.  That's EBAY's rules, not ours.

There are more than 350 of these kits installed.  Every now and then someone writes to thank us as they got carried away and the the relay did it's job.  And they're happy about it!

Final note:  If your amp has not been serviced recently there are legitimate concerns about the electrolytic capacitors in the amp.  These electronic parts are used in every electronic circuit.  They have  a defined life and there is tons of data available to support replacement before they fail.  In recent years the small signal transistors on the main circuit board have shown consistent failures.  Consideration should be given to replacing them after installation of the relay board. The relay will likely avoid damage due to a mistake made when replacing the transistors.  See the transistor service bulletins at watts abundant.

White Oak Audio, no affiliation, sells a variety of upgrades including the main driver board.