Sonab OA-4 (mkII) & Visatron TW 70 tweeters?

[iub]

I've discovered somwhting stange about the OA4 that I got. The two speakers seem to differ somewhat. One of them has the baffle attached with screws while the other seems to be glued on. The speakers with the screws has 2x 8 Ohm and 1x 16 Ohm MT20HFCs while the glued one has 3x 16 Ohm Tweeters. Does anyone have any idea why that is?

[Martin Dellstig]

Hi

I have never tampered with the OA-4 my self but two things spring to my mind.

Different tweeters
I have heard people talk about sometimes three 16Ohms tweeters and sometimes two 8 Ohms tweeter and one 16 Ohms tweeter.
I don't really know which is right.
However, two 8 Oms in series, connected in paralell with one 16 Ohm tweeter would result in a better crossoverfunction if the components are the same in the filter for OA-5, the filter is 0,5mH+6,8uF.
Check the filter-components, they should give a clue.

Regarding the baffle, check the serial-numbers to see if the speakers are made close together in time or not.

Best regards

Martin

[Villee]

2x 8 ohms + 1 16 ohms tweeter should be the original construction, otherwise it doesn´t make any sense .Perhaps somebody has blown tweeters (from the 3x16ohm OA-4) and replaced all tweeters to 16 ohms by a mistake (perhaps checked that one tweeter is 16 ohms and then falsely replaced all tweets to 16 ohms)? If my memory serves me right, my pair OA-4 type II (which surely has original speaker units) has 2 eight and 1 16 ohm tweeters. Of course I can be wrong.

Here somebody sells a very good pair OA-4 type 1 and there seems to be at least one eight ohms tweeter... http://www.tradera.com/auction/viewitem ... =118016723
About glued baffles it seems to be that some speakers has and some doesn´t . I don´t know does my OA-4 pair The only speaker which seems to always have glued baffle is OA-6 type II (really don´t have clue why). On OA-4, -5 and V-1 I dislike glued baffles: how can you reach the crossover?!

If you are happy with the sound and treble sounds the same on both speakers you can be happy with that. If not, changing to 2x8 +1x16 ohms configuration on both speakers can be necessarily.

[iub]

Ok, I am a bit embarrassed, seems I didn't inspect my speakers well enough . I have discovered that both baffles are actually screwed on, so restoration will be significantly easier than I thought.

However, I am still confused about the 3x 16 Ohm tweeters. One of them has a damaged membrane, which I am going to try to repair with a wood glue & water mix. After that I will conduct a careful listening test to see if there is any audible difference between the "normal" tweeter configuration and this one.

One more question though. These speakers do not seem to have an terminals mounted on the case, is this normal for this model?

[Villee]

The (grey) connection wire comes from the speaker bottom hole? OA-5 has a din-terminal but don´t know does OA-4 have any sort of terminal:D IF this is not the case, the terminal is inside of the speaker :O

[Martin Dellstig]

Hi again

As I said I have never worked on the OA-4 but I can´t see any reason why this should be different from OA-5.
I have seen OA-5 with both DIN terminals and binding posts.

Best regards

Martin

[iub]

I took some pictures to show precisely how the speaker cables are connected in the speakers:







As you can see I have cut the cables, since they will be replaced.

[Villee]

So the original speaker cable was directly connected to the crossover? Now you can add a terminal at the bottom of the speaker IF you think you need a possibility to change cables. Change the crossover capacitor to a new one (do not use electrolytics! Sounds crap.)

Check the values of the compoments (coils and the capacitor): OA-6 type 2 (which has a 2700 Hz crossoverpoint for treble) has one 0,5 mh coil for midrange and 0,5 mh coil for tweeters. Also the crossover has a 6,8 uf capacitor. If you want to modificate your crosssoverpoint to 2700, check which compoment has a different value and replace it with the same value as OA-6 and OA-5 have.

But I´m not sure about this because one of your speakers has three 16 ohms tweeters and somebody could have modificated the crossover for using these tweeters... Although I don´t think that a 2x8ohm +1 16ohm tweeter costruction changes the crossoverpoint compared to using 4x8 ohms tweeters so I think that there is some difference at the crossover in OA-4.

So simplier could be just to replace the capacitor with same value and as you said, replace the cables with better ones.
As you see, the cabinets are pretty poorly made, so you can make them a bit sturdier if you want
Btw you have pretty "first series"of OA-4 type 2, I have serial numbers about 18 000.

[Martin Dellstig]

Hi!

To me it looks like the capacitor has been replaced, it does not look like any capacitor I have seen in a Sonab-speaker.
Also the cable looks different to what I have seen in Sonab Boxes.
The feet for the box also looks like something has ben made to them.

Best regards

Martin

[iub]

the missing terminal seems to be common to some other Sonab OA-4s. I found this image on ebay



I think Martin is correct when he points out that the capacitors have been changed at some point. This is not that important, since I plan to replace them anyway. The cable seems to be cheap and (too) thin electrical wire, and I am definitely looking to replace that. I also think I might add terminals to the case.

Villee, I think you are right in just about eveything you said, however it can't give you an measurements quite yet, but will do so asap.

How do you think I could make the box sturdier? add more intenal bracing?

Btw, is it worth considering installing bass reflex tubes in the cabinet?