Jim Lesurf wrote in message ...
In article , steven
robinson wrote:



My problem is the lager 4Ohms 80watt speaker that is in series with the
parallel leg has burn't out. I have no idea why.

----8Ohms---- (50watts)
| |
| |
----8Ohms---- (50watts)
| |
| -------4Ohms------- (80watts)
| |

I read a similar reply recently that stated maybe its because the
wattage is unbalanced or the number of speakers were unbalanced,
resulting in the 4Ohms 80watt speaker doing double the work.

The main concern I have is that calling a speaker '4 Ohm' or '8 Ohm' is an
entirely nominal value.The *actual* value of the impedance is likely to be
very frequency dependent, and at some frequencies it may be a long way
from the quoted nominal value. Putting dissimilar speaker units in series
is a recipy for potential pun disaster for this reason...

At some frequencies the impedance of one unit may be much higher than the
nominal value, and the other(s) much lower. This mean mean the power is
far from being distributed as you might assume from the 'nominal' values.

Also: Since you are not using any crossover networks it may be that at -
for example - low frequencies *all* your speakers have low impedances
(thus drawing more current/power than you assume), and their values are
such that most of the power ends up in one unit - which can then fail.

To say more, I'd need to see impedance/frequency plots for each of the
units.

Another reason putting dissimilar units in series is often a bad idea is
that each one (for the reason given above) will experience a drive voltage
that is frequency dependent. Thus the resulting overall output sound
power/frequency response is likely to be changed. Each unit *used and
driven individually* might be acceptable as a broadband speaker, but
combined, the results may be very different.

There are also some other variables involved, so in general, I would not
expect what you produced to sound particularly good, even if it survived!
However this depends upon the units, and how they were fitted to the
cabinets, etc, etc.

Personally, I'd also use some kind of added network, just to define the
impedance values. For example, some amplifiers may not like the load they
see at LF or HF from a speaker of the kind you describe. This can also
lead to trouble if the amp is not unconditionally stable.

Also occurs to me to point out that when putting reactive components
in series, the magnitudes of the voltages on each, if added, can
actually add up to *more* than the total applied across them all.
( ;- ) So, again, this can be a recipy for problems... (The
voltages add up as vector/complex values, and may be out of
phase with each other, hence leading to this effect.)



Please help if you can, any insight into this problem would be greatly
appreciated, its driving me nuts..

Do you have impedance/frequency plots of the individual units? Any idea
how these values were altered by the cabinets and/or combined useage? What
amplifier did you use, and is it unconditionally stable?

Slainte,

Jim



Hi Jim
Thanks for the reply, a lot of what you are saying is foreign to me,
but this is what I know:
The spec of the 8Ohms 50watt speakers in the parallel leg a
Nominal power 30watts
Peak power 50watts
SPL @ 1W/1m 82db
Frequency range 100Hz to 20kHz
Resonant Frequency 130Hz
Max cone displacement 5mm
Voice coil diameter 20mm
Magnetic inductance 0.8 Tesla
Magnetic flux 200u Weber

Withount using cross overs how can I block damaging frequencies lower
than 100Hz and higher than 20kHz from the drivers? or isn't that a
concern?

Unfortunately I don't know anything about the 4Ohms speaker, except
its Peak power is 80watts.

Likewise the amp is a bit of a mistery, it is an amp called a v-series
manufactured by an amusement company in leeds. Its output is supposed
to be 70watts into 4Ohms.

I'm getting the impresion that putting dis-similat drivers in series
is a bad idea. What if I put two 8Ohms in series twice and parallel
them together, would this be a better idea? I could then use the same
speakers with the same properties. In this configuration you would
expect the output to be spread evenly across the four speakers,
wouldn't you? What do you think?

Thanks Steve