Research project, active speaker design and implementation

[b-ark-610873]

Hello all,

I wanted to introduce myself and an ongoing project I have, well really a whole series of projects. As a way to learn a bunch of techniques related to audio engineering, purely as a hobby to stop brain rot, I have a pair of active speakers from the 1980’s, Meridian M30, which I am hoping to have a long-term experiment with. Fortunately I have the schematics for the crossovers and amps.

As a summary, highlights of these speakers are:

• KEF T33, 180DC15 drive unit pair, both 4ohm
• Active crossover based on many 5532s with some interesting signal shaping
• 2x70W power amps based on BD139/140, MJ15003/4 output pairs
• Regulated 15v power supply using 7815/7915

So far I have done the following “basic hygiene” tasks

• Replaced all the electrolytic caps (which removed a punishing crack on power up)
• Replaced the ferrofluid in the tweeters
• Captured the schematics into LTspice (at least 98% of it)
• Taken some acoustic measurements with REW/UMIK-1 that seemed to correlate with the crossover design, including a whopping dip designed into the presence region

My aim in this project is to understand by practical activity the effect of changes in electronic/mechanical design, hopefully to produce an “improved” version by the end. Although not a “cost no object” project, I am fully expecting it to become a bit of a money pit over time! At some point this will include getting a 'scope but that's a fairly major jump.

So, I have a few questions I wanted to raise.

• I was planning to document the project as I went. In terms of diyAudio etiquette, should I put this all in a single thread, updating the first entry as I go, or should it go in a series of threads?
• As it includes parts of line level, power amp and speaker (drives, cabinet), should I raise questions for each in their own forum areas?
• The schematics are reasonably easy to obtain online in PDF form, but I think I will only provide snippets from the full diagram publicly. Does this fit with the forum’s IP/copyright approach
• If you were crazy enough to try this yourself, in what order would you consider review/upgrade? Following a source-first approach my thoughts would be to look at power supply, then crossover, then power amp.
• Is the T33 tweeter a bit long in the tooth nowadays and could be improved?
• Are there more modern output devices that could produce interesting changes or are there other places to start.
• I replaced the electrolytic caps and the remainder are mostly polypropylene and a couple of tantalum ones. Would these normally be replaced too? Any other components that would tend to age and should be replaced as a matter of course?

Thanks for any support you can give!

Ian

 

[indianajo]

The last question: 40 year old electrolytic caps with significant hours use in audio devices are invitations to bruise the ears. Unused ones can last 100 hours or so even with cracked seals, but the end is near. A test with an ESR meter is one non-audio test that can detect failure. Tantalum caps, some brands/types are similarly useless at 40 years of age. All the tantalums in a 1968 Hammond H182 were failed. But I replaced the electrolytics in a 1980 Allen organ that had gone silent in 2018 with over a hundred tantalums left in it I did not change, and it still is sounding weekly. Polyprophyene caps, unlikely to have suffered any damage unless in high temperature high voltage switcher power supply service.
The interest in redesigning a 40 year old box of unknown dimension and port size with 40 year old drivers is limited. The relevant driver parameters are unlikely to be available on-line. I've not seen any KEF product in any of my frequent visits to resale shops. Unless you measure thiele parameters yourself and publish them, your test data is not much use. You describe your microphone, test room (anechoic?), test procedures with any data or get sand-bagged by the usual complaints.
As for the 40 year old tweeter, yes there have been major developments. See the cask05 post of this thread https://www.diyaudio.com/community/threads/3-octaves-400hz-3200hz-whatcha-gonna-do.417819/ about some radically new compression drivers. If for example the tweeter is a 3" paper cone, that technology is so reviled no replacements are available. I bought one for my KLH23 speaker just in time in 2008. The above link describes a Multi-Entry-Horn which since Danley enterred the market, may have made the rectangular box obsolete as a speaker type.
About the copyright infringement of 40 year old schematics, look at the corporate history of the source. Most publishers of such diagrams are moribund. Dead companies do not pay lawyers. Some current successful corporations complain vigorously about publishing their diagrams: stay away from Behringer for example. KEF is still around.
Individual projects described in the appropriate forum topics may elicit more discussion.