Hi All,
The story - I aquired some SB-AA2 Technics speakers (8 ohm, 160W music, 80W din). The person I got them from said one of the Tweeters were blown. After searching for a tweeter replacement, I found an original one and purchased it. In hindsight, I maybe should have tested first. I connected the tweeter and still no sound coming from it, then realised, no sound was coming from the Mid speaker. Interestingly, I noticed that the tweeter is connected to the mid speaker (seems to be common in some of the Technics models Ive looked at). When I disconnected the Mid speaker, the tweeter came to life. I measured the ohms on the mid speaker and it was reading open.
So, I thought on my working mid speaker, I would get that mid speaker and try it in the speaker box that had the blown mid speaker. It didnt work and upon checking that "working" mid speaker, it now reads 5 megaohms, rather than the 8 ohms I measured before testing it and of course, doesnt work anymore.
I have bought 2 replacement mid speakers, but I dont not want to connect them before I figure out why the good mid speaker blew.
I took the crossover out and checked all components. They are fine, Cap is measuring correctly, inductors are reading (but I dont know the actual value of them), and the resitor is bang on the value.
As there is no technical information for this speaker model, I reverse engineered the crossover to make a circuit diagram for it. I did find a very similar crossover design from the SB-F555 speakers, but a few components missing compared to mine (but had the same wiring colors as my speakers).
However, what I cant understand is that the mid and tweeter polarities are crossed, meaning the Positives are wired directly to the negative input from the Amp, and the Negatives are on the components side of the Positive input. The diagram will show what I mean.
My questions are:
1) Can anyone see a reason what would cause the mid to get destroyed from this cross over design?
2) How does this work correctly with the +/- side of the speakers reversed (is this something to do with "Linear phasing"?
Thanks all...
The story - I aquired some SB-AA2 Technics speakers (8 ohm, 160W music, 80W din). The person I got them from said one of the Tweeters were blown. After searching for a tweeter replacement, I found an original one and purchased it. In hindsight, I maybe should have tested first. I connected the tweeter and still no sound coming from it, then realised, no sound was coming from the Mid speaker. Interestingly, I noticed that the tweeter is connected to the mid speaker (seems to be common in some of the Technics models Ive looked at). When I disconnected the Mid speaker, the tweeter came to life. I measured the ohms on the mid speaker and it was reading open.
So, I thought on my working mid speaker, I would get that mid speaker and try it in the speaker box that had the blown mid speaker. It didnt work and upon checking that "working" mid speaker, it now reads 5 megaohms, rather than the 8 ohms I measured before testing it and of course, doesnt work anymore.
I have bought 2 replacement mid speakers, but I dont not want to connect them before I figure out why the good mid speaker blew.
I took the crossover out and checked all components. They are fine, Cap is measuring correctly, inductors are reading (but I dont know the actual value of them), and the resitor is bang on the value.
As there is no technical information for this speaker model, I reverse engineered the crossover to make a circuit diagram for it. I did find a very similar crossover design from the SB-F555 speakers, but a few components missing compared to mine (but had the same wiring colors as my speakers).
However, what I cant understand is that the mid and tweeter polarities are crossed, meaning the Positives are wired directly to the negative input from the Amp, and the Negatives are on the components side of the Positive input. The diagram will show what I mean.
My questions are:
1) Can anyone see a reason what would cause the mid to get destroyed from this cross over design?
2) How does this work correctly with the +/- side of the speakers reversed (is this something to do with "Linear phasing"?
Thanks all...
Technics will have decided that the midrange/tweeter combination be connected in opposite electrical phase to the woofer.
This will be to help provide an in-phase acoustical response at the crossover frequency - linear phase if you like.
The tweeter in this circuit is just piggy-backing on the same high pass filter as the midrange, extending the frequency response.
This will be to help provide an in-phase acoustical response at the crossover frequency - linear phase if you like.
The tweeter in this circuit is just piggy-backing on the same high pass filter as the midrange, extending the frequency response.
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I can't see how the mids could be destroyed by this crossover. Perhaps the "good" mid just happened to be ready to fail?
I would get any old, disposable speaker driver and wire it in place of the mid to see if it continues to work properly.
Also, if accessible, check the flexible braid wires which carry the current between the terminals of the mid and its cone (voice coil) in case a solder connection has failed.
I would get any old, disposable speaker driver and wire it in place of the mid to see if it continues to work properly.
Also, if accessible, check the flexible braid wires which carry the current between the terminals of the mid and its cone (voice coil) in case a solder connection has failed.
That puzzles me, I would expect the tweeter to work if the mid was open circuit, but be shorted out if the mid were short circuit.
Thanks @Galu
Thanks for the info. Its been a while since I dabled in crossovers and I found some Technics Service documents for different model of speakers that werent crossed polarity, so I was wondering more if someone was tinkering with them. Im thinking that the Mid that was working as you mentioned, was about to fail anyway. They are closed casing, so cant get to the braided wires as you suggest.
I thought about finding a speaker to test first... I only have a very small one, but since Im not driving it that much, it should be OK to test.
I was puzzled why when I disconnected the first dead mid, that the tweeter came to life as well, which made me suspicious of the crossover, but, that has checked out perfectly fine.
I'll do some testing later and get back to you the outcome. Thanks again for some thoughts.
Thanks for the info. Its been a while since I dabled in crossovers and I found some Technics Service documents for different model of speakers that werent crossed polarity, so I was wondering more if someone was tinkering with them. Im thinking that the Mid that was working as you mentioned, was about to fail anyway. They are closed casing, so cant get to the braided wires as you suggest.
I thought about finding a speaker to test first... I only have a very small one, but since Im not driving it that much, it should be OK to test.
I was puzzled why when I disconnected the first dead mid, that the tweeter came to life as well, which made me suspicious of the crossover, but, that has checked out perfectly fine.
I'll do some testing later and get back to you the outcome. Thanks again for some thoughts.
I may have a set of those drivers here, branded Akai but I think they are the same. Could you post pix of the speakers and the drivers you have pulled out?
I'd have to have a look but I think my Akai tweeter has a 1uF cap soldered to the input tab.
I'd have to have a look but I think my Akai tweeter has a 1uF cap soldered to the input tab.
Yes, a 1 uF bipolar capacitor wired in series with the tweeter would make sense.
The cap would allow only the highest frequencies through to the tweeter rather than having it reproduce all the frequencies reaching the mid.
The cap would allow only the highest frequencies through to the tweeter rather than having it reproduce all the frequencies reaching the mid.
@Moondog55 - The tweeters are piezo style and apparently the mid is meant to be a closed cone tweeter, so really, these arent 3 way speakers. There is no 1uf cap in series with the tweeter.
I have replaced the mid in both boxes (as both were dead) with a Visaton TW70 (recommended in these forums but for a different model of Technics Speakers, that used the same Mids as mine). The TW70s do both the Mid Range and Tweeter frequencies, and the speakers sound great. However, the tweeters in both are not producing any sound (maybe I cant hear anything that high but they do when I disconnect the mid - so same situation as mentioned above, but the sound is very low). Anyway, the TW70 covers all the ranges so those tweeters are not needed now. I know these are not anywhere near a good set of speakers, but for my application the are perfect.
Here are photos of the Mid and Tweeters that @Moondog55 requested - Sorry, I dont have any other photos.
Tweeter
Mid
I have replaced the mid in both boxes (as both were dead) with a Visaton TW70 (recommended in these forums but for a different model of Technics Speakers, that used the same Mids as mine). The TW70s do both the Mid Range and Tweeter frequencies, and the speakers sound great. However, the tweeters in both are not producing any sound (maybe I cant hear anything that high but they do when I disconnect the mid - so same situation as mentioned above, but the sound is very low). Anyway, the TW70 covers all the ranges so those tweeters are not needed now. I know these are not anywhere near a good set of speakers, but for my application the are perfect.
Here are photos of the Mid and Tweeters that @Moondog55 requested - Sorry, I dont have any other photos.
Tweeter
Mid
Strange one. My first thoughts after reading the first paragraph were unreliable connectors. Other thoughts include amplifier misbehaviour (DC, instability), or circuit board shorts.
If I could maybe build on Charles' post with a simulation. It is not the direct equivalent here (trying to work within the simulator)... It does suggest a filter becoming unloaded and its damping dropping significantly. In this case increasing the signal by 12dB, with the amplifier seeing a low impedance.
I read that if a 2nd order filter is unloaded, it will present a zero ohm load to the amplifier at the crossover frequency (or some multiple of the crossover frequency), leading to the possibility of amplifier damage.
Hi Allen... I have checked/cleaned or contacts and checked connections. They didnt look the best.
Hi All,
So I tested the 3 Piezo speakers today (2 original and the 1 replacement I purchased) and they are measuring about 12nF and I can here clicking when testing (nothing fancy, just a multimeter on capacitance). However, I have read that Piezo Tweeters should read about 200nF.
Does anyone know for sure?
Thanks..
So I tested the 3 Piezo speakers today (2 original and the 1 replacement I purchased) and they are measuring about 12nF and I can here clicking when testing (nothing fancy, just a multimeter on capacitance). However, I have read that Piezo Tweeters should read about 200nF.
Does anyone know for sure?
Thanks..
This is heavily depending on the type of piezo used. And since it is a lossy capacitor the measurements can be off depending on the equipment used.
Regards
Charles
Edit: Here is a link to a manual for the old CTS piezos: https://docplayer.net/22178076-Cts-piezoelectric-tweeters.html
Regards
Charles
Edit: Here is a link to a manual for the old CTS piezos: https://docplayer.net/22178076-Cts-piezoelectric-tweeters.html
Thanks Charles for the document link.
Interestingly, even that document seems to show that the Piezo should be reading around 120uF or 10x more than mine are.
SP1-super twitter (ceramic). SP2-twitter(dinamik). Common crossover (connected in parallel). Just in case, replace the electrolytic 3.3 (non-polar). Perhaps the old speakers can not handle the power of your amplifier. Or there are high frequency vibrations.
SP1-super twitter (ceramic). Plays ultrasound - you may not hear 🙂
SP1-super twitter (ceramic). Plays ultrasound - you may not hear 🙂
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