Advice required on 8" woofer

[Avondalenaim]

Well the current situation finds me with time on my hands so I am going to use the time to accelerate the replacement of the other two tweeters in the upper cube. To save new readers the trouble of having to go back through the thread here is a basic NVA Cubix overview.

The Cubix cabinet has 2 co-joined sections each section (cube) being approximately 250mm square, which equates to approximately 12l cubic capacity per cube. The lower section has one 8" up firing woofer. The upper section also has a 8" up firing woofer. Having solved the problem of the damaged woofer my attention has turned to the other two drivers in the cabinet. The tweeter is a 1.25" Monitor Audio 25DT 52 @8ohms. There also another 1.25" driver to the right side of the cabinet on the same plane as the tweeter. There no means to identify this unit, it had a paper label that's disintegrated but I assume it's a different form of tweeter. Both drivers are circa 1991 items and I feel confident that a modern tweeter and a small 2" to 3" complimentary mid range driver will improve the sound quite a bit.

Richard Dunn of NVA was an advocate of basic shallow crossovers. The two woofers are directly connected and each of the 1.25" tweeters have a 1.8 ohm resistor and a 6.8uF capacitor. One other consideration is historically NVA amps have struggled with complex crossover hence the very simplistic design of the Cubix.

I am leaning towards replacing the front facing tweeter with a Morel Cat 378 and I would welcome your thoughts the values for the resistor and capacitor I should use. Right now I no idea which mid range driver to use but ideally I wouldn't want to spend more than £80ish per driver. Again I would welcome your thoughts and advice re the mid range driver taking in to account the other drivers in the cabinet, the cubes capacity of circa 12l and the need to keep the components simple.

Thanks in advance and please take care in these challenging times, regards Paul

Hello again. Sorry I neglected to look at this thread due to coronavirus and being obsessed with reading about my government's failings.

Since Richard Dunn started the company up again in about 2008 ? he used very cheap Chinese kevlar drivers which he doped to taste. The tweeters of choice for the NVA cubes has been Visaton G20sc (20mm fabric dome) or Visaton G25ffl (25mm fabric dome).

These are very nice sounding units. On all three cube models he used a 3.3uf capacitor and around 12R of resistance before the capacitor. The resistor had to be so high in value because the doping of the drivers reduces the efficiency of them and they are not exactly what you call sensitive to start with.

I know your early cubix speakers are different in the fact that the drivers are not doped but the general idea is the same with the high pass first order filter before the tweeters. If you chose one of the Visatons then just buy a few capacitors and find the value that sounds the best. Then try a few resistors to bring the tweeter level downby the amount you need.

With the drivers not being doped then you will not need much resistance, if any. Also because there is no doping then the frequency where the mid bass and tweeter cross over will be higher which means the capacitor value will need to get lower.

I think you have chosen a very good driver in the Monacor SPH-220HQ. It is incredibly smooth in its higher frequency output. I have made my own version of the cubix using drivers tbe same as the modern nva drivers. However, I am eager to try drivers without having to dope them so I am going to get some of the same drivers you have used for the isobaric bass units and some sph-145hq for the mid drivers coupled with the four visaton tweeters i already have which are firing up, forwards and to each side.

Do you look at the NVA users forum (hifisubjectivist) at all ? You can find me there and a lot of information in the archives about these wonderful speakers. The last version of the cubix had up to three 8" drivers in isobaric configuration all with the same box volume behind them, so you could even add another one to the bottom of yours.

Regards, Stu

 

[Avondalenaim]

Oh, forgot to add. I don't think you really need a "mid range". As the 8" mid bass are run fully open with no electrical low pass or doping then they are covering the frequency range up to where the tweeter kicks in. I would just get two identical tweeters and be done with it.

If the unlabelled 1.25" has exactly the same high pass filter as the other 1.25" then esentially they are both tweeters.