Re: Advice on speaker wire needed.

On Tue, 06 Jun 2006 13:18:02 GMT, nospam@xxxxxxxxxx (Don Pearce)
wrote:

On Tue, 06 Jun 2006 14:08:07 +0100, Serge Auckland
<serge.auckland@xxxxxxxxxxxxx> wrote:

Pooh Bear wrote:

"Mr.T" wrote:

"Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
news:4485579F.B9D784DD@xxxxxxxxxxxxxx

I do hope you're not making a classic mistake here of thinking you can
simply lump in the cable resistance with the speaker > impedance.

Care to explain exactly what it is you are claiming, and a reference of any
sort.

It's quite simple. An 'ideal amplifier' has zero output impedance so a speaker
connected to it with cable of minimal resistance gets highly damped. If the
cable is several ohms then that damping is reduced. Consider the amplifier
output as a short circuit ( i.e. zero impedance voltage source ) for simplicity.



Let us assume that the output impedance of the amplifier is 0.1 ohm, the
cable resistance is 0.2 ohm and the loudspeaker's resistance (note, not
impedance as we are talking about the low frequency damping) is 6 ohms.

The damping factor is 6/ 0.2 + 0.1 = 20

Now, if the cable resistance increases to 2 ohms, the damping factor is
6/ 2 + 0.1 = 2.86

Thus, the cable resistance has increased by 20 times, whilst the damping
factor has reduced by 7 times.

Considering that the loudspeaker's dc resistance is always in series
with the amplifier's output impedance in conventionally designed
amplifiers, it shows the folly of marketing specs that stress
vanishingly small output impedances, more often stated as very high
"damping factors"

S.

No, this isn't right. What we are damping in a speaker is the
fundamental resonance. Although that is a spring and a mass, we can
represent it by Lr and Cr, making up the tuned circuit. That is damped
by a parallel resistance. That resistance comprises, in series, the
voice coil resistance, the cable resistance and the amplifier output
resistance (note, output impedance is *not* the term we need, as the
reactive components do not contribute to damping).

The damping factor is Rtot/Xl, where Rtot is that sum - somewhere
around 6 ohms for most speaker/cable/amplifier combinations.
Increasing the cable resistance by 2 ohms would make the total 8 ohms,
reducing the damping factor by an amount not exceeding 33%, not the 7
times you state here. I say "not exceeding" as there is considerable
mechanical damping involved, probably nearly equivalent to the
electrical damping - you can not decrease the total damping factor
below this, however poor the cable.

d

Corr: Xl/Rtot in the last para.

d

--
Pearce Consulting
http://www.pearce.uk.com
.

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