[HAM] leslie speaker impedance

[Christoph P. Kukulies]

On Sat, Nov 08, 2003 at 02:07:42AM +1100, Randal Muir wrote:
> Hi all.   Further to the speaker impedance discussion,...I have a buzzing HF
> driver in my leslie 145 and an 8 ohm HF driver in perfect condition from my
> 720.  I would like to use the good driver for gigs where a cleaner sound is
> prefered so I ran a numerical AC analysis (spice) on the crossover/speaker
> network using pure resistive speaker impedances (maybe I should use complex
> loads??).  The 8 ohm upper driver pushed the HF cut-off point higher (900 -
> 1kHz) but left the bass cut-off unaffected...... which makes sense since the

Theoretically the high pass network is isolated from the low pass filter.
They are independent from each other. But when you take the source
impedance into account, the situation is different.

You have a dissipation mismatch and an overcharge of the output transformer
at frequencies > cutoff frequency. Higher current and in the long run
danger of killing your output transformer.

Putting an 8 Ohm resistor in series cures the situation loadwise but
you have half the power in your horn driver.

> crossover circuit should largely isolate the two parts.  I figure the LF
> load impedance would still be 16 ohms but the HF (>900Hz) impedance would be
> 8 ohms.  Would this be OK since low frequencies account for most of the
> power output..... or (a)would the elevated HF current (due to the lower 8
> ohm load) in the output transformer and tube circuit still be damaging.....
> or (b)am I missing something fundamentally important in my reasoning? ... or
> (c) am I FOS and don't know what I'm talking about at all? or  (d) all of a,
> b, and c?
> 
> What do the wise, experienced members on the list think?..... apart from
> getting a 16 ohm replacement driver (involves international freight and
> customs, and more money than I can afford right now)

I'm not wise and maybe not so much experienced but I would try to repair the
buzzing 16 Ohms driver. Have you tried to get rid of the buzz, i.e. realigning
the driver? 

> Cheers -Randal
> 
> PS. Can I put an 8 ohm resistor in series with the 8 ohm driver to bring the
> load to 16 ohms?????

Not very wise (explained above),
--
Chris Christoph P. U. Kukulies