功放 = 功率放大 = power amplification
倒备 = backup
单元 = 器 = a unit, a device or maybe an equipment

功放倒备单元 = power amplification backup unit, power amplification backup device ---> backup power amplifier

http://www.audio-workshop.co.uk/pdf_systems/pages rx3/SB.htm...
The PA Fault function is used to monitor the operation of the zone power amplifier and will, upon detection of any malfunction, report to the CP 3100 central processor and, if configured, activate and insert a backup power amplifier.


--------------------------------------------------
Note added at 3 hrs (2008-08-14 03:58:17 GMT)
--------------------------------------------------

http://scholar.ilib.cn/Abstract.aspx?A=xbgbds200309014
调频广播停播报警倒备器的设计
...
五主一备式调频发射系统 FM transmitter system with five-host and one-backup
http://www.dlpabx.com/Chinese/Bs_ProductShow.asp?ArticleID=2...
http://www.hzsdgb.com/products_show.asp?productID=150
一台备用功放对7台主输出功放同步倒备

===> Correction:
倒备 = backup switching
功放倒备单元 = (power) amplifier backup switching unit/device/equipment