15 ohms (12 on ohmmeter) with 24 ohms resistor in parallel to bring to 8 ohms

[McIntosh Man]

Background informations:

I have one pair of 2 ways speakers with a 12 inches woofer and a 5 inches mid-tweeter. All drivers are 8 ohms.

One of the woofer suffers from a intermittent breakdown...it plays then it stops for a few seconds/minutes and start to play again (without touching anything).

I have interchanged the faulty enclosure with the good one and the problem still occurs on the faulty one.
Wiring and connections have been redone and the problem still occurs.
I'm convinced it's the woofer that is at fault.

Since I'm working with vintage speakers, Wharfedale from the early sixties, it' pretty difficult to find a replacement driver.

I happen to have an identical woofer BUT it's a 15 ohms (measures 12 ohms on ohmmeter)

I applied the OHM's Law:
R (equivalent) = (R1 ie: 12 ohms x R2 ie: 24 ohms) divided by (R1 ie: 12 ohms + R2 ie: 24 ohms).
Maths calculation gives 8 ohms.

Question:

If I add a 24 ohms 50 watts resistor in parallel with the woofer I should be matching the other speaker at 8 ohms.

I'm aware that we are talking impedance ... stop me if I'm wrong and please explain!

Do you think the load seen by the amplifier will have an impact on it's performance ?

Is the 50 watts resistor big enough wattage to match with a woofer rated at 30 watts RMS maximum and driven by a 50 watts amplifier ?

As somebody here ever done this type of arrangement ?

If the answer is yes can you comment of the performance.

I will be trying this tooday but I want to have your opinion.

Thanks.

[DigitalJunkie]

That will work,and the amplifier will see a proper load,but you'll be losing 2/3 of your power as heat in the resistor,and not as sound coming from the speaker.

[infinia]

Well it might and might not, it's really a simple Q but difficult to answer without all the data ie matching sensitivities and crossover circuit. I would investigate the fault more thoroughly. have you taken the suspect driver out of the enclosure yet? Some times internal connectors come loose, solder connections fail, crossover parts age and become intermittent, open or shorted driver tinsel leads, driver suspensions sag allowing voice coils to rub and short. My bet > if you localize the fault and examine and tighten up everything, reflow any solder joints in the path, and rotate the woofer 180 degrees you will have joy once again.

[McIntosh Man]

Originally Posted by infinia

have you taken the suspect driver out of the enclosure yet?

Yes, I have taken the woofer out of the box.

This afternoon I replaced the suspect woofer with the 15 ohms version and I didn't like the sound so I put the original back in. I replaced the crossover so it's presently on my workbench getting tested again...

Your suggestion of turning the woofer 180 degrees is far fetch but I will definitely try it. I have nothing to loose and evrything to gain.

Thanks.

[runebivrin]

Originally Posted by DigitalJunkie

That will work,and the amplifier will see a proper load,but you'll be losing 2/3 of your power as heat in the resistor,and not as sound coming from the speaker.

Ahem, not quite.

1. Since the nominal impedance of the driver is 15 ohms that's the value to plug in to the equation, which yields a 17 ohm resistor.

2. The power lost is inversely proportional to the resistance, so that would have been 1/3 in the example.

3. It may work after a fashion, but the bass will be unbalanced between the sides du to different power to the drivers, and general frequency response will be different between the sides.

Now, speaker drivers are really very simple in a wiring sense. There's a coil which is wound from a continous copper (most of the time) wire. At the bottom of the cone it's connected via solder joints to two soft copper braids. These in turn connect to the connection lugs via another pair of solder joints.

The problem as described is usually due to either of these solder joints working loose, and a resolder will fix the problem. Unfortunately it's not unusual for the coil-to-braid joints to be hidden under the dust cap, but there's not really much to lose, so If I were you I'd consider using a scalpel to remove the dust cap, do the soldering and then glue the dust cap back.

[infinia]

This afternoon I replaced the suspect woofer with the 15 ohms version and I didn't like the sound

yes replacing the woofer is not so straight forward as matching DC ohms. S/b the last resort and usually means alot more work even if initially sounding OK.
Test the suspected woofer by itself on the bench, playing a 400Hz tone and moving the cone in and out as part of examining things ie tinsel leads.

[infinia]

Originally Posted by runebivrin

2. The power lost is inversely proportional to the resistance, so that would have been 1/3 in the example.

3. It may work after a fashion, but the bass will be unbalanced between the sides du to different power to the drivers, and general frequency response will be different between the sides.

2) most amps are constant voltage, so no appreciable power lost to the driver. The amp may deliver more power in this case.

3) I would expect most differences due to mismatches from primarily both sensitivity, T/S, and secondarily cone materials/profiles.

[pkitt]

2. You're missing the point. With the replacement driver being 15 ohms, even though paralleling it with a resistor might bring the total impedance down to 8 ohms, at a given input voltage to this parallel combination, only part of the power will be seen by the driver (about half if the driver were paralleled by a 17 ohm resistor) especially because most amps are constant voltage sources. The other speaker whose 8-ohm driver is still okay, would be receiving ~twice the power as this 15-ohm driver at the same time. Unless the 15-ohm driver happened to be twice as sensitive as the 8-ohm driver, there will be an unbalanced output between the two.
Paul

Originally Posted by infinia

2) most amps are constant voltage, so no appreciable power lost to the driver. The amp may deliver more power in this case.

3) I would expect most differences due to mismatches from primarily both sensitivity, T/S, and secondarily cone materials/profiles.

[rjb]

My choice would be to repair the old driver.
As infinia says it is usually a break in the coil winding close to the cone terminations. That is the first place to look, and does not require any dis-assembly.
Any other fault requires dis-assembly.With many drivers from the sixties, the magnet is bolted on, and the cone, coil and basket can be removed complete, exposing the coil. If you do this, the basket and cone will need recentering, which requires care and a modium of skill, but is not overly difficult Removing the dustcap is usually not required, but makes it easier.Ask if you don't know how.
The old glues are prone to failure with age, and the so next most likely point of failure is at the top end of the coil. Sometimes it is possible to wind off one turn, (this does not affect performance much) to allow a joint to be made outside the gap. Take care as the wire is fine and easy to break. Note Wharfdale used aluminium wire in some of there drivers, (usually the full-range, rather than the woofers) and joining this is difficult to say the least.
If the fault is anywhere else a rewind of the coil is required.

[infinia]

Hi Paul
Maybe that's why talking about driver sensitivity using power is plain silly. 1st thing to do is convert it. Then it's apples to apples and hand-waving discussions are less likely.
Besides there is not enough data here to make any concrete recommendations either way.