Feedback
Some recent designs have attempted to remove feedback following a trend that feedback is 'a bad thing'. It is true that this principle is not as understood as it well might, even among professionals. The author holds that if used correctly 'negative feedback will not turn a badly designed amplifier into a good one; it can however turn a basically good design into an excellent performer'.
An excellent article 'Negative feedback' appeared in Elektor (E 23, March '77, 3-38 to 3-45). This also made reference to the treatment given by Klein and Zaalberg van Zelst's 'Precision Electronic's (Philips Technical Library).
In order to prevent TID (transient intermodulation distortion) in their Beomaster 4400 (type 2417) tuner amp, B&O used a 50kHz filter before the power amp and feedback techniques to limit the system's bandwidth. The Technical Product Information book, which accompanied the service manual for this model contained a very detailed explanation of these which can be useful for those interested. Images derived from same are set out below.
Bessel filter
Output Amplifier and Internal feedback
External feedback
Additional notes include
DC feedback and Constant-current generator for no-signal current
Constant current generator for differential amplifier and Dual Transistor output
Overload indicator
Current limiter
Speaker protection
Excess temperature protection
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