Low impedance tweeters and amplifiers

Jolsen

Member
2008-03-10 12:05 pm
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?
 
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.
 
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

Member
2008-03-10 12:05 pm
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?
 
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

Member
2008-03-10 12:05 pm
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

Ex-Moderator R.I.P.
2005-11-24 1:47 am
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
 
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.
 
tinitus said:
The 3ohm resistor is in paralel with tweeter 4ohm, which means that the total paralel resistanse will be LESS than 3ohm in series with 6ohm...thats how I see it...and I think Call is right

Not quite ;)

Anyways, pointless discussion as L-pads should be avoided at all costs. Much better with just a single series resistor if you want dampening with higher impedance, and a single parallel resistor if you want dampening with lower impedance. Choose your speakers accordingly.

And in this particular case, a series resistor is the best choice.
 
Hi.

Here is my suggestion, for what it is worth. Get the Peerless HDS 810921 - it is a cheap tweeter for what it does and it is 8 Ohm. In parallel with the voice coil fit the following LC filter - 0.82mH and 68uF in series, but across the voice coil. This is a notch filter tuned to 650 Hertz - the choke should have the lowest possible resistance. Now you can add any series resistance BEFORE or AFTER the LC filter - even in PARALLEL with the voice coil terminals. The problem with adding any series resistance to a tweeter is that it destroys damping to its Fs. The LC filter takes care of that as long as your crossover is no lower than 2KHz.

If you use first order, there is a real concern that the single series capacitor which then sees some resistance to ground after that (made up of the tweeter and any attenuation) is that the current goes negative as you approach (descending) the crossover frequency. That means a 45 degree negative phase angle at the crossover frequency assuming Butterworth, but if using 6dB summing technique, the phase angle is even worse at -60 degrees (but the Z will be higher and that helps).

So conclusions: 1) If 1st order is what you have in mind, then avoid using low Z tweeters. Keeping Z high as possible will compensate for the bad phase. 2) Higher Z (and quality) tweeters are rare but very advantageous in low order designs.

Joe R.