General: |
The Phase Linear 400 is very powerful amplifier. It will deliver about 250 watts / channel into 8 ohms. Into 4 ohms it delivers about 350 watts. The amplifier will shutdown in the event that the heatsink temperature exceeds 90c (194F degrees). Considering the age of this amplifier the sound quality is quite good. |
Input: |
The input to the amplifier is a typical differential pair. The current source is
generated by a simple 200k resistor tied to the negative supply rail. The differential
pair is supplied about 375uamps via this resistor. From there the signal pass into a
second differential amplifier that is used to generate more open-loop gain. Following the
second differential pair the audio signal is passed to a class-A gain stage. The class-A gain section was nicely implemented by utilizing a static current source, which supplies about 3.5 m-amps to the class-A amplifier under low signal conditions. Between the Class-A gain transistor, and the static current source resistor, is a normal adjustable bias generator composed of a single transistor and a 3 diode temperature sensor. |
Driver: |
The pre-driver section is realized by using a 40327/MM4003 followed by a direct coupling to high-gain SJ2741 transistors. The driver stage is directly coupled to the output stage. |
Output: |
The output section utilizes (2) X (3) PL909 power transistors. These are reasonably powerful transistors for when this amplifier was designed, but now days these transistors are considered very weak based on the output power of this amplifier. You will get differing views on this comment but using only 6 -100 watt transistors in a 250 watt amplifier tends to make this amplifier unreliable under high-power delivery. |
Protection: |
The output SOA protection of this amplifier is a typical current limiting design. Based on the weak output transistors don't even think of disabling the SOA! If you ever need to replace the SOA protection transistors, keep in mind that theses transistors are not silicon, but instead geranium, so be careful. |
Power Supply: |
The DC power supply is rectified by a bridge rectifier made from a 25 amp bridge diode. From there, the DC power gets a healthy amount of smoothing from a 5,600uF capacitor on each power supply rail. The rail voltage is about 75 volts. In addition, before the DC voltage goes to the amplifiers, a fuse has been placed in-line with a value of 5 amps. |
Final comments: |
Each amplifier has 18 transistors with in it. It has good power output, low
distortion, and reasonable output transistor protection. I think that Bob Carver should
have added the following though, when Thais amplifier was designed:
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