Heil AMTs Resitance and Impedance

I have 4 pair 0f AMT 1s and each has a different resistance and so I assume a different impedance. I have 3 pair in my living room with 1 pair in use. They range in resistance from around 4 ohm to around 8 ohm. The ones I have inline now are just over 4 ohms so I built some high pass Xovers using 6 ohms as the measure to base them on. I don't know if this is correct, but I doubt there could be such a range in the resistance without there being a difference in impedance as well.
All the diaphragms are original 70s as far as I know. I also have a pair of the smaller front firing AMTs here in the computer room made in Dec 77 that are original and they have a resistance of 6.8 ohms.
Any thoughts, experience, opinions or anything else on this? I want to get the Xovers correct.
I use them with Altec 416s which are plenty fast and a bit more efficient than the Heils, which is the way I like my woofer/tweeter arrangement to be.


Paid Member
2010-12-11 12:39 am
Current replacements diaphragms,Part No.689-1107, tend to range from 3.6 up to 4.1 ohm in their D.C. resistance measurement. It would be prudent to base your design on new diaphragms as all the early polyethylene types would be suspect by now. The smaller AMT units,as I understand it did have a higher resistance,but I have not used these.When ordering replacements you can ask for them to be resistance matched. The ESS(Great) HEIL AMT has a resistive impedance above its' resonant frequency and the sensitivity is 92dB at 1kHz(2 V input level).The response tends to rise with frequency and this is easily flattened with a parallel network of a 65uH and 22 ohm resistor fitted in series with the AMT if you find the HF,s are too bright.You do have a problem if you wish to use a conventional passive xover with the Altecs,given the impedance mismatch.
The low pass is based on 16 ohms. I am not worried about that, and the old diaphragms sound just fine and I will keep them.
As to the resistance on the diaphragms on the 3 pair I can easily get to the measurements are 4.3 & 4.7 on the pair I am using with a high pass based on 6 ohm impedance.
The other a pairs measure at 6.1 & 6.2 and 6.8 & 7.2. The other pair are on the back of a high shelf in my garage and not easy to get to. When I had them in use the high pass was based on 8 ohms impedance but I do not remember the resistance, but seem to remember it being in the 6 to 7 ohm range.
Do I figure impedance like one does with other drivers? Or is it the actual resistance?