Bob,
A "Q multiplier" is just what the name says. It is a tuned circuit
(either a narrow bandpass filter or a narrow band notch filter) which
incorporates a means of "multiplying" the filter Q.

"Q" of a tuned circuit is defined as 2*pi*the ratio of energy stored to
energy dissipated per cycle in the tuned circuit (or inductor or
capacitor...), which for the usual parallel tuned circuit is essentially
equivalent to the ratio of the circuit inductive reactance to the
resistance of the inductor.

The Q increase is achieved by hooking up an amplifier to provide
positive signal feedback, from the filter's output back to it's input.
The amplitude of the positive feedback is set just high enough to cancel
most of the losses in the filter, but not high enough to make the
circuit oscillate.

By canceling the losses in the filter, the effective Q of the passive
filter is increased, and therefore the bandwidth is decreased, giving a
very sharp frequency response.

This doesn't come for free, however. Since there is an active amplifier
involved, there will be some noise output from the filter along with the
desired signal.

Bob
WB4TGG


bobinphx wrote:
To All,
I just purchased a Lafayette kt-200 and research on the net says it has a
jack for a Q multiplier. Further research on a Q multiplier shows that is
some how acts like crystal filter, only variable and it appears that it can
also boost poor signals??? Can someone give me a readers digest, not overly
technical explanation of "how do it work" ????? and how well does it
work????

In addition any suggestions as to which model, of the many that heathkit
sold, would be, arguably, the best for this application.....

Bob in phx