On Tue, 28 Jan 2014 10:56:29 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
John Williamson wrote:
Another alternative would be to install a 100 volt speaker system with
independent volume controls in each room, but that would mean buying a
new amplifier,and some matching transformers and volume controls.

I've used a 100 v line transformer I had lying around to adjust the level
of a speaker fed from a normal low impedance amp. It worked rather better
than a pad. It had five level taps on it - but it would be a bit of luck
if it gave the amount of attenuation wanted. Pads always seem to upset the
bass end - even on speakers which aren't exactly Hi-Fi.

I'm not familiar with the design of attenuator pads used in 100v and
70v line driver setups but presumably it must be raising the 'source'
impedance as seen by the speaker.

I suppose the reduction of "Damping Factor"[1] is allowing the bass
cone resonance to more prominently dominate the frequency response.

[1] Forty years ago, "Damping Factor" was an oft quoted figure of
merit for amplifiers which was derived from the output impedance
(ideally, zero ohms) of the amplifier versus the impedance of the
speaker loads it was designed to drive. Figures of 400:1 for 8 ohm
loads being not uncommon (implying an output Z of 0.02 ohms).

Since over 90% of the typical bass driver's "Voice Coil Impedance"
was made up of its ohmic resistance, the actual "Damping Factor" was
more like 1:1 using such an amplifier (the voice coil resistance being
effectively in series with the amplifier's output impedance).

In practice, the damping effect was only of importance at the
resonant frequency of the drive unit which is reflected as an increase
of impedance which, for an 8 ohm driver, could climb to as high a
value as 30 or more ohms.

In the case of a 32 ohm impedance at resonance, the actual damping
factor applied by our "400:1 Damping Factor" amplifier would actually
be a mere 4:1, just 1% of the claimed 'figure of merit' implied.

It's been a good 30 years or more since I last looked at such Hi Fi
advertising 'blurb' so I don't know whether this misleading practice
has stopped and the more useful "Output Impedance" figures are being
quoted instead (e.g. "Output Z = 20 milli ohms", rather than "DF=400:1
for 8 ohm loads"). Quite frankly, the difference between 500 milli
ohms and 20 milli ohms output impedance would be barely measurable let
alone audible.
--
Regards, J B Good