Low impedance tweeters and amplifiers

[Jolsen]

I have a tweeter with a nominal impedance of 5 ohms and I'm planning on pairing it up with a single midwoofer having nominal impedance of 8 ohms. I will also be dropping the tweeter level down about 6db, probably with an L-pad(have considered a series resistor for the job but this could create damping issues apparently.)

My questions are:

1. Will the padded 5 ohm tweeter in parallel with the 8ohm woofer potentially create a dangerously low impedance for a cheap solid state amp?

2. Will the L-pad on the tweeter make the overall impedance of the speakers more friendly for the amp?

3. What should the amplifier's minimum suitable impedance be with such a setup?

[Cal Weldon]

Hi Jolsen,

I'm no expert but:

1. The low impedance tweeter shouldn't be a problem because...

2. You can adjust the series resistor in your L-pad to bring the impedance back up as you have suggested.

3. 8 ohms should be fine.

I am not familiar with damping issues by adding the extra circuit resistance or the effects. I have done what you are thinking a number of times.

[Saturnus]

Add a 5 Ohm series resistor instead of an L-pad. That will make the impedance of the tweeter 10 Ohm, and dampen it 6db.

Please note that you will see a rise in output at the tweeters resonance frequency which if unchecked and unaccounted for may drastically reduce expected power handling (ie., it will not increase lineary with dampening).

[Jolsen]

There are a lot of tweeters that have 4 ohm nominal impedance, let alone 5 ohm, so there must be tried and tested ways to use low impedance tweeters without a problem, assuming there is any problem in the first place.

[Saturnus]

Well, there is. A series resistor. They sound better than an L-pad anyway.

[Jolsen]

Thanks for the responses Saturnus and Cal, the last thing I want to do is build my speakers and damage my amp with them.

Saturnus, with the series resistor in place, what about the power handling issues you noted? How are they dealt with? Impedance compensation networks?

And what would happen if I just used an L-pad that was configured for 5-ohm nominal operation? Would that drop the impedance too low for a cheap amp once I added the 8ohm woofer in parallel? 1/5 + 1/8 = 1/x = ~3 ohm load right?

Could I perhaps put 3 ohm in series with the 5ohm tweeter, to raise it to 8ohm nominal, and then put a conventional 8ohm L-pad behind that? Would that help to minimize damping issues with the lower series resistor?

[Saturnus]

The tweeter and the woofer impedance theoretically doesn't affect eachother at all. They do however in real life, and drastic differences in impedance results in phase changes which is the real amp killer.

With a good filter configuration and a quality tweeter the slightly reduced power handling at resonance frequency is nothing to worry about, if you just set the filter frequency at least one octave above resonance frequency. That's how it's usually done in high-end speakers because most high-end speaker designers realize that adding unneeded components to a filter design degrades sound.

If you terribly worried about it then see if can't find a calculation for "impedance adjusted L-pad" which is basically and L-pad followed by a series resistor as you suggested but where the 2 series resistor have been added together.

[Jolsen]

Thankyou so much for all your help, I really appreciate it. One more small question, if I were to use the method of a single series resistor where would it be best to place it? In between the driver and the crossover components, or before the crossover components? Or doesn't it really matter? (perhaps if it were placed before the crossover components I could more easily alter antenuation levels without also having to change the crossover components.)

[tinitus]

A tweeter is not a difficult load...on the other hand many "8ohm" woofers will have DC resistanse around 5ohm

Odd phase and impedanse peaks which is often the result from crossover is much worse...a straight line impedanse doesnt present any problem

[Saturnus]

tinitus, while correct most people forget that most 8 Ohm woofers will have an impedance very close to 10 Ohm at the x-over frequency to a tweeter, therefore adjusting the tweeter to 10 Ohm impedance will result in the smoothest x-over.

This is a rule of thumb, and will naturally vary with different tweeters and woofers. But it's usually best to let the speakers do the work for you and use as few components as you can in the filter.

Jolsen, you put the series resistor where you would put the L-pad, ie. between the filter and the tweeter.