M-AUDIO BX5a monitors : Tweeters out of phase ?

[jives11]

Hi, a friend of my son asked if I could fix these studio monitors. They are bi-amped active small speakers. used in semi-pro work or home recording. Well reviewed within that field.

Anyway one had leaking PSU caps causing loud buzzing/hum , the other had a dry joint on the bass/mid range speaker terminal causing loss of all bass. Both are now working. Blogged here :
mending things: BX5a

When I took the speakers apart I photographed the wiring so I didn't make a mistake. On reflection the tweeters are wired in a mirror , not identically. the bass units have different sized push-fit connectors for + and - ve, so you cannot get it wrong , but the tweeters have the same size and no indication of polarity, or even which way round they go. There is a white and a black cable. However are not connected consistently each speaker.

Does anyone have any experience of these speakers or of determining absolute phase when the tweeter is not labelled ? I'm thinking that either :

1) The tweeter is somehow bipolar ? No experience of these, it appears to be some kind of soft dome

2) It was assembled incorrectly at the factory

3) It's been worked on (they were shop seconds originally) and re-assembled incorrectly.

I have posted on the M-AUDIO forum, but have not got a very detailed answer.

[Inductor]

The tweeters phase, it's my understanding from the information above, should be connected equally in both speakers. Not "wired in a mirror" (or symmetry). This may append in different cases, not here where the tweeter is aligned in the same vertical axis with the other speaker driver, then they should be xover/phase equal.

info:
Tweeter wiring and phase
This (your) case is different since its an active monitor (1 amp or two amps?). Any way connect same cables/same color to the same terminal/s in both speakers. It should be ok if electrical phase=0 with no passive components.

[pcb121055]

i hope that i am answering what you are asking here but i understand that the convention is that when a driver is connected such that the positive electrical connection is made to the positive driver's terminal and likewise for the negative lead, the diaphragm will move outward. i.e. away from the magnet structure. reversed and it will move inward.

[Ile]

It should have 4th-order Linkwitz-Riley crossover at 3000Hz, so tweeter should be connected in phase with woofer.
Active Filters

So you can measure which tweeter cable is connected to amps ground plane like woofer -terminal, I guess it's black one.

Is there any mark on either tweeter terminal, sometimes there could be red dot or something similar at + terminal.

If there isn't any markings at terminals, only way to know right polarity would be measuring or listen around 3kHz area with sweep and use polarity that doesn't cause notch to crossing area.

[Inductor]

(happen, sorry for vocabulary)

jives11, do you know the answer to your question?

[wimdehaan]

I am wondering which amplifier module(s) this active speaker uses?
Does it use the same amplifier for high and low section?
Wim

[Ile]

Originally Posted by wimdehaan

I am wondering which amplifier module(s) this active speaker uses?
Does it use the same amplifier for high and low section?
Wim

It have separete channels for both drivers. Both channels seem to have some 5-leg tda chips in jives11 picture.

Maybe TDA2020, since 7812 regulator indicates it's driven with +-12V.

[wimdehaan]

Maybe TDA2020

Tripath = TA2020.

I guess it is a TDA2030 or so. The manual claims 38W high and 38W low, but I am sure these are not the true RMS outpower numbers. So the TDA2030 comes close.
Other possibility would be the TDA2052, this amplifier IC ouputs 32 watts into 4 ohm & 1% distortion.