Phase Linear 400 Ouput Relay Speaker Protection

Description

The Phase Linear amplifiers of the 70's and early 80's were popular at home and in rock and roll PA's.  They were not intended for road use.  Driven hard without fans they could fail.  Often times a failure resulted in 80 volts DC into the speaker and the speaker failed.  Unlike most power amps, the 700 and 400 did not have an output relay to disconnect the speaker upon amplifier failure.
It's 40 years later and the Phase Linear 400 is still popular.  However it needs some help.  If it fails, the speaker needs to be disconnected, fast, before the speaker is damaged.  The most reliable means to disconnect the speaker is through a relay.
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.
Upon detection, the relay has to disconnect the speaker and put a short across the speaker in case of relay failure. The relay is sacrificial if it protects the speaker.
When the relay detects a failure, it stays off until power is cycled off and back on for additional protection.
Installation must not require drilling or modification of the chassis.  The board bolts onto the output binding posts.
Easy installation with detailed instructions.
The circuit uses a tried and true integrated circuit, UPC1237 for detection.  Boat loads of receivers and amps used it.  Virtually all of them went the cheap route and didn't crowbar (short across the speaker) which resulted in welded relay contacts and still damaged speakers.
Note: There is no economical relay with contacts rated for
100 VDC
that is small enough to fit in the amp.  Relays are rated for 250 volts AC, but only 32 volts DC
.
The relay used 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.  Although the relay could fail, there have been no reports of relay failures to date due to a blown channel.
Here's the best part:  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.
Tools required are modest.
1/4" & 7/16" nut driver or deep socket
1/8" wide screw driver for terminal connections
High quality 40 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.
There are more than 200 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!