Advice required on 8" woofer

yespsb

Member
2019-05-15 6:17 am
I own a pair of NVA Cubix omni directional speakers circa 1991/2. The speakers set up correctly can sound absolutely wonderful, unfortunately I have damaged one of the 8" Electro Acoustic Industries woofers and as the original part is in all probability no longer available I am using my misfortune as an opportunity to upgrade. The NVA Cubix are a bit unusual in so much as there's no crossover, two 8" woofers, the lower one is totally enclosed with the 2nd sitting directly above. The tweeter and mid range employee a resistor and capacitor and that's about it. Any replacement woofer must work ok without a crossover as I want to remain faithful to the original design and as I said earlier with the correct stands, cable and positioning they are a very involving pair of speaker's.
The speakers are driven by NVA monoblocks with an NVA passive pre amp.
I am a complete audio DIY novice but I can follow instructions ok, use a soldering iron etc. My other question is will I be ok just upgrading the upper woofer in each speaker and leave the original lower woofer in situ or will I have to upgrade all four woofers?
Any and all advice is much appreciated. Thanks Paul
 
NVA Cubix omnidirectional speakers

I would first measure the T/S parameters of one good 8" speaker, and based upon the values, determine if there is a similar new speaker or if the original 8" has crazy-weird specs. If there is a close match, I would probably purchase a pair to replace one in each cabinet for symmetry.

There are databases of speakers with their T/S parameters in addition to manufacturer catalogs.

There are instructions for measuring T/S parameters with simple equipment.

Measuring Loudspeaker Driver Parameters

To measure these parameters using the method outlined, you'll need to have the following items:

A power amplifier, rated at 1-10 Watts (RMS) or thereabouts (must have low output impedance <0.1 ohm)
Audio frequency oscillator (PC based is fine)
Digital multimeter (with frequency measurement), or PC based instrument
An accurate test resistor (any value, although I suggest 10 ohms) A ½W component will be quite sufficient.

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This is a good time to study how the in-room listening experience differs for speakers with: omnidirectional vs. dipole vs. horn vs. standard-baffle radiation patterns.
 
NVA Audio are still running in the UK. The website is NVA Hi-Fi | Specialist Audio | Handmade in England

Under the support tab/returns is an email link that you could use to see what they could do to help you.
I assume that they are unlikely to have a stock of replacement drivers, but if you don't ask you wont know. At the same time you could ask that they provide you with the Speaker T/S details.
LineSources advice would be the way to proceed if you cannot get the information.

Maybe somebody here knows something about Electro Acoustics Industries and their speaker specs
 
Electro Acoustics Industries sounds like Elac to me!

In the 1980's the UK company made midbass drivers like this:

694487d1533019191-kef-carlton-iii-resurrection-seas-22t-af-kef_cara_loudspeaker-jpg


694488d1533019191-kef-carlton-iii-resurrection-seas-22t-af-kef_cara_bass-jpg


Falcon Acoustics used to have a database of old UK Elac drivers.

These days, Elac is a German company making something entirely different and more modern in loudspeakers. But with a head Engineer called Andrew Jones.

Photos would help. The paperclip thingie in the reply box which attaches photos.
 
Don't underestimate the kindness of strangers! :D

Whilst we are not entirely sure what we are talking about here with what seem to be Elac woofers, I happen to have a pair lying about that are available.

714063d1541548033-restoring-monitor-audio-r300-bookshelf-speakers-ma-r300-md-current-4th-build-jpg


By the way, here is Falcon Acoustic's archive of Elac drivers which is under the IMF and TDL archive: The Falcon Acoustics Archive

The RAM archive is interesting too. Sounds like the distributors ran off with the cash! :eek:
 

yespsb

Member
2019-05-15 6:17 am
Hi thanks for your reply, I wanted to replace both 8" units in each speaker however I simply couldn't extract the lower one as Richard used glue as well as the normal bolts. I have literally taken everything out apart from the said lower driver and upgraded the wiring, resistors, capacitors,binding posts, grilles and the upper 8" drivers are now Monacor SPH 220HQ. The Cubix now sound better than ever and I am very happy with the way things have worked out. Onward and upwards :)
Ps big thanks to System 7, your advice and generous offer is the hallmark of a top guy
 
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Happy days. Did you dope the new monacor 8" or just use it as is ?

Richard will probably have used bison kit contact adhesive to secure the driver in. It can be prised apart but you need to get leverage some how. Maybe a hairdryer would work by gently heating.

Anyway, if you're happy with them then might as well call it a day :p
 

yespsb

Member
2019-05-15 6:17 am
I didn't dope them because the originals were not doped. My Cubix are circa 1991 and I think Richard started doping speakers a bit later. Internally the wiring was not the prettiest but it's all hunky dory now. I think I will leave them alone for the rest of 2020 and then maybe next year look at the possibility of upgrading one or both of the tweeters.
 

yespsb

Member
2019-05-15 6:17 am
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
 
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AllenB

Moderator
Paid Member
2008-10-18 11:31 am
If I cast my mind back I seem to recall that this kind of crossover uses the tweeter impedance peak to exacerbate the tweeter response at the region of the first woofer breakup mode, to compensate for it beaming and then dipping. So perhaps consider a tweeter with a similarly positioned and proportioned impedance peak and response in that region.

This will be compounded in magnitude when choosing the resistance to manage the sensitivity, and in frequency if compensating when choosing the capacitor value.

A sim might be helpful if original tweeter information is available.
 
I think we are struggling to visualise this speaker, Paul! Some pictures would help. You can upload them to the forum with the paperclip thingie inthe reply box.

It's an omni box with an isobaric bass setup, which I think is two 8" woofers in parallel making 4 ohms, but more bass than you might expect from a given box.

There must be a couple of 1" soft-dome tweeters ponting sideways or something. Have you measured the faceplates? Usually 104mm if round.

6.8uF and a 1.8R resistor is quite a difficult circuit for an 8 ohm nominal tweeter. 4.7uF would be more expected.

And two in parallel might be getting into low impedance.

Some substitutes here possibly:
Buy Treble Units Online, UK

The Morel CAT378 is a 94mm waveguide, so is quite directional. That might not be right for an omni.
 
INTRODUCTION TO THE ELAC CATALOGUE ( CIRCA. 1987)

Founded in 1946, ELAC is today the major British specialist manufacture of original equipment loudspeaker drive units supplying the international automotive, high fidelity, telecommunications, public address and general electronics industries.

With more than four hundred employees and with factories in London and South Wales, ELAC produces in excess of three million drive units annually and provides its customers with an unrivalled service in the design and development of customised products meeting many different performance requirements and applications.

TDL & IMF Drive Units ELAC Datasheet - The Falcon Acoustics Archive - Archive/Links/History/Tips

I can only go by what I read... :)
 
25DT52 might be an Audax derivative tweeter, so you could investigate its parameters and frequency response.

I fi understand things correctly both tweeters share the same value of resistance and capacitance, that suggests to me they may be the same type?

As system 7 has said, some images might help us recognise the tweeters.

Typically 8" drivers start getting directional at 1.5 -1.7Khz, old tweeters from that era may be good down to 2Khz or so, you have two of them driving into the room so you will not have too much to fill in between 1.5Khz and 2Khz at a wild guess. Without real measurement in room we are blind unless somebody can find the a good review that defines the ideas behind the speakers.

Pictures, and maybe somebody to carry out some measurements or post a review may be the next stage.

I assume NVA didn't want to support you with info or replacement drivers?