Sansui AU D9 Service Manual

Page 2

1. BLOCK DIAGRAM

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2. OPERATION

2-1. Super Feedforward Amplifier

This amplifier was developed by combination of conventional nega-
tive feedback and feedforward techniques.

This feedforward technique helps to eliminate distortions which can
not be achieved by the NFB.

The feedforward technique is a technique to nullify, In principle, all
types of distortions at the output point by adding distortions of
reverse phase but of equal amplitude as those generated in the
power amplifier.

The NF 3 makes use of distortion generated by the output stage and
feeds it back to the input, the output stage is bound to have distor-
tion. Therefore, in order to reduce distortion to zero an infinite
amount of NF B is required and as a result an amplifier of infinite
gain becomes necessary, which is not available in reallty.

by NFB or Feedforwerd

Improved Value

Fig. 2-1. Shows how distortions an be nulled by

application of feedforward.

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System B

Fig. 2-2.

On the other hand, the feedforward is unlike NFB, finite amount of
feedforward can theoretically reduce distortion to zero over a wide
frequency range as the feedforward amount does not have to be re.
. duced at higher frequencies.

2-2. Operation of Super Feedforward Amplifier

Fig. 2.2 shows a block diagram of super feedforward amplifier. This
circuit consists of voltage amplifier (Al ), power amplifier (A 2), error

correction amplifier (A3) and summing network (21 & Z2).
Distortion marked A is generated by A2 and reverse phased distor-

tion is produced by NFB and fed to A2 and A3.

Reverse phased distortion fed to A2 reduces the form of distortion
after NPR and reverse phased distortion fed to A3 is amplified with-
out phase changing.

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