Quad 405 Brochure

Page 2

Current dumping is the name given
to an entirely new audio amplifier output
cirCuit developed by ACOustical. in
which the linearity of the main Current
carrying trans:stors(theCurrent dampers)
has no bearing on the overall amplifier
performance. hence the need for biasmg
and allied problems associated With
crossover are eliminated.

The basis of current dumping is
feed forward error correction', first
proposed by Harold Black in 1928, in
which a derived error signal is added to

the output signal of an amplifier in
exactly the right ariiOunt to cancel the
distortion produced by the amplifier.
While it is relatively easv to add error
current to signal current Without
interaction. as in Black's invention, it is
very much more difficult to do so in
voltage terms. Loudspeakers reciuire a
stiff source and so feed forward' has
t0und no successful application in audio
amplifiers.

What Quad has done - not quite as
easy as it sounds - is to produce a Current

controlled amplifier. apply a special type
of feed forward Current error correction
and then apply Overall voltage feedback
to convert the whole to a voltage
anipllfier. The reSult is that the duality is
solely dependent upon the performance
of a low powered high quality aii'pi-"ier
together with f0ur passwe components.
When correctly designed. all distOrtion
in the Output stage is reduced to zero.
The advantages ofthistechnique are
that it is possible to produce an amplifier
of very high performance vvith0ut using

Current Dumping

carefully matched relatively fragile
Output dewces. There are no crossover
biasmg problems, no alignment or
adjustment IS required to obtain
OptlmU'Tl performance and nothing
can go out of alignment during life. In the
event of component failure. replacement
can be effected and performance
restored Without realignment.
Forfurther details see Current
Dumping Audio Amplifier" by
P.J.Wallier and M.P.Albinson presented
to the 50th convention of A.E.S.