Chapter Ⅸ "External OPS Stability"

Zobel Network ＆ Damped Inductor

  Fig.7.4 illustrates two methods of radically improving external OPS stability .The first of these is called a "Zobel network". Its purpose is to counteract the effects of inductive reactance within the speaker load. For the most part, EF OPS designs seem to be rather immune to this problem, but CF designs are much more susceptible to high-frequency instability from inductive loads. In either case, the Zobel network certainly help to clean up the output rail. Remember, a clean, stable output rail is important for both external OPS stability and global NFB stability.

  Continuing to refer to Fig.7.4, the resistor value of RZ is virtually always chosen to closely equal the value of the nominal load. With rear exception, this means it should be about 8 ohms (10 ohms works just as well, in case you have some trouble finding higher-wattage 8.2-ohm resistors).Typical values of CZ range from 0.047uF to 0.15uF, but 0.1uF functions well in almost every situation, and is the more or less universally accepted value. In other words, what we have here is a circuit that works optimally for just about any size or type of power amplifier. At first thought, it might seem unusual that one circuit configuration would be optimum for a wide variety of amplifiers. However, we must remember that a Zobel network is actually a best-fit design configuration based on the output load, not the amplifier. since nominal load characteristics are going to be considered the same for all power amplifier designs, it should not be surprising that the Zobel network changes very little from one design to another.

  スピーカ負荷は、動的に容量成分や誘導成分、抵抗成分等で構成されており、
位相補償、及び出力段に対する超高域発振防止対策として付加する必要があります。

(負荷Z補償素子) Zobel Network
・Rz … 4.7～10Ωの酸化金属皮膜抵抗 (出力電力により2～5W程度)
(出力Trがパラ接続時は実効的gmが倍加する為、上記の1/2程度とする)
・Cz … 0.022～0.1uF 大型フィルム・コンデンサ(印加する電源±電圧以上の耐圧)

(安定化コイル) Damped Inductor
・L1 … 1.2～1.6mmφのエナメル線等の絶縁被覆線を内径15mmで10ターン前後の空芯コイル(1～2uH)
・Rd … コイルのQを落とす為、パラに10Ω抵抗を入れる。