John Stewart wrote:
Kevin Killebrew wrote:
Here is my question:
I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?
Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.
Kevin Killebrew
Austin, Texas
Whatever you do, don't wire the speakers in series. For a two speaker hookup,
that removes the amplifier damping from both speakers since they would be then
be sourced through each other, an intolerable situation.
The DF issue is not always important You can have 10 speakers in series and if each
has the same enclosure,
and each driver is the same, the response will be identical to a single
driver driven with the same low Rout, and this proves the
amp damping doesn't reduce.
In fact it increases.
If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10.
If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50.
People who build line array speakers using a dozen or more drives know
what I am saying to be true, or else they would be compelled to
connect all speakers in parallel, giving input impedances of very
low value.
However, where you have two different brands of speakers, different boxes
and drivers, series speakers will produce big variations in response from each
because the impedances of each are very dissimilar at different F.
So its for this reason you wouldn't want to series speakers.
Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel.
Pentode amps are a bit queer.
They have a load optimum where thd is lowest and each side of that thd
tends to rise and thd is very high when RL is very high.
But the amp NFB also becomes most effective when RL is high, when
the tube gain is high, and also the PP pentode amp works in class A when RL
is high, when it isn't producing power in class AB where RL is low.
This is an idiotic statement, but speaker Z varies for different F,
and either its working in AB or it isn't, but one has to
contemplate that the amplifier is
sending a voltage x current signal to a constantly changing load value.
Since its almost impossible to think of 3 things at once, I leave
you to contemplate the tube's operation knowing that
to win a game of chess against a naked woman and keep an erection
and do your tax return all simultaneously is downright impossible.
Better a lower Z match than higher since PP pentode amps benefit from that. The
3rd harmonic is reduced. Most speaker loads are far too high anyway at their
resonance(s). A parallel hookup tends to smooth that out provided the speakers
are dissimilar.
Using two brands of speakers in parallel with different box resonances at LF
will indeed tend to produce a more even impedance.
But also its likely that at some F the impedance will be lower then 4 ohms.
Some makers have deliberately different enclosure volumes and Fb
for a pair of bass drivers in the one speaker cabinet.
It makes the resonant Z peaks overlap, and a much flatter
Z is realised. Its all in RDH4, with a graph.
Patrick Turner.
Good Luck, John Stewart
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