Q: Why does my Series II 400 or 700 have a turn off "thump".

A: Series 1 amps have a very low turn off transient or "thump". Unfortunately, the Series II 400 and 700 amps had a different drive PCB and the turn off thump was louder. This does no harm to the amp or your speakers and is purely an annoyance. At the factory we would sometimes install an output relay circuit borrowed from the Model 200 Series II. Unfortunately, these relay PCBs are no longer available. See below.

NOTE: As a precaution, you should NEVER turn the amp off and on again before the power supply has fully discharged (20-30 seconds). Failure to wait will cause the amp to output a high level, low frequency transient which may be harmful to both the amp and your speakers.

Q: Why didn't the factory install output relays on the 400 and 700 amplifiers?

A: Cost. The service dept. tried to get one designed in to reduce "nusiance complaints" but were told no due to the added cost.

Q: Is it possible to build a relay control board and install it? 

A: Yes. See the Output Relay Bulletin for details on installing this modification.

Q: Why should I install in-line speaker fuses when the amp already has fuses?

A: The fuses in the amplifier are there to protect the amplifier. They may or may not protect your speakers in the event of a failure. An in-line fuse provides cheap insurance for your speakers.

Q: What is the formula for calculating speaker fuses?

A: Below is a formula from the Series 2 amplifier operation manual. I = fuse value in amps, P = maximum power rating of your speakers, and R = nominal speaker impedance. For example: an 8 ohm speaker rated at 100 watts max would have a fuse value of about 1.75 A (square root of 100/32). You could safely use a 2 A fuse. Use fast acting fuses only. No Slo-Blo or time delay fuses.

Q: I've heard that all electrolytic capacitors in the power amplifiers should be replaced with new ones. Is this correct?

A: First and foremost, as with other modern amps, all Phase Linear power amps are direct coupled and have NO electrolytic caps in the signal path. There are elctrolytic caps in the power supply, feedback network and bootstrap circuit, and protection circuit which I will address individually.

• Power supply: power supply filter cap failures are rare but they do occur. When they fail, they generally fail suddenly and there is no way to to accurately predict when one will fail. High value electrolytic caps are much more expensive than they used to be.
• Feedback network: all drive boards from the PL14A onward have a 100uF cap to ground (C 6 on the PL14) in the feedback network. The purpose of this cap is to roll off gain at subsonic frequencies. When working correctly, gain should be down about 0.5-1 dB at 20 Hz. The only time I see these fail is if the amp has failed and latched up (excessive DC offset). Failures due to any other cause are nil.
• Bootstrap circuit: bootstrapping is a type of positive feedback. The purpose of the bootstrap cap (C 11 on the PL14) is to provide symetrical drive voltage to the positive output bank. Failure will cause asymetrical clipping on the positive side. I think I've replaced one in the past 28 years!
• Protection circuit: this circuit only turns on when the amp is driven into a short circuit or very low impedance. Under normal operation it is completely out of the signal path. See C 15 and 16 on the PL 14. I've never replaced a single one. Series 2 and 3 drive boards don't have these caps.

At this point in the life of these amps, electrolytic cap failures are rare. However,electrolytic caps will eventually fail due to age. My advice is to save your money and only replace them as needed. This advice may change in another 10 or 20 years. I'll keep you posted!