Infinity Alpha speakers
7: Replacing bad tweeters
Of course I later found that the tweeters has been burned in the Alpha 50 main speakers, that was the reason that the crossover resistor was burnt. I already know earlier that one tweeter had a dent on the dome so anyway I had to replace them.
Normally there is no problem to find replacement speakers. But this France made speakers was a bit tricky.
The tweeter part number is A1201B-H, 9747202. I found new ones but they were expensive, maybe this old speakers are not worth it. In the spare part list they say it's a 4 ohm impedance tweeter, could that be correct?
If I now rebuild the speakers to be better I think I shall spend some time to see if I can find ones with a better and a more modern design.
A simple drawing I made to help me understand what tweeters I can replace the original ones with, I have recently found a Russian drawing with slightly different components values, strange.
For me it looks that the 1.2 ohm resistor and the 8.2 uF capacitor makes a Zobel circuit to the mid range speaker. By calculate backwards I found the the speakers DC resistance should be 0.96 Ohms, and the Inductance 7.6 mH. Good to know if there is a need to replace it.
For the bass speakers I did the same backward calculation and found that the DC resistance is 1.76 ohm and inductance is equivalent to 0.18 mH. Or 3.5 ohm and 0.36 mH for each because the two bass speakers are connected in parallel.
To make it easy for me I used the web calculator you find at the link below.
A Zobel circuit makes it easier to have control over the speakers impedance, then the filter works better and the load to the amplifier isn't that evil. With this we also see that the crossover to the tweeter is a 12 dB / octave filter, and the others only 6 dB filters. One good thing about that is that the phase shift isn't that high at the crossover frequency but it makes a heavier demand on the speaker elements.
Later, maybe I buy an instrument so I can measure the inductances of L1, L2 and L3.
After some research I found Dayton have one model that could fit both mechanically and electric as a replacement to the tweeter.
It says it's a 4 ohm impedance speaker, the Infinity system impedance is 8 ohm. But what I know the impedance of the original tweeter is 4 ohm and when calculate the crossovers frequency of 3000 Hz it looks to be correct. The Dayton tweeters have extra cooling fins, good.
If you want to get deeper in filter calculation you find it here:
I will come back later and tell how I solve this problems with the defective tweeters.
How to replace the tweeter:
Cross reference list:
Check carefully that above list is correct before you buy anything!