Are these speakers worth refurbing?

[Mothman]

Right guys thanks for all the advice, it would appear these are worth spending some time on.

Andre I'd like to take you up on your kind offer of help in sourcing the parts. The first job I suppose is to remove the crossover board. I'm assuming that to desolder the 6 wires I use a soldering iron which I don't have, so if someone can recommend a soldering iron starter kit (nothin to fancy) that would be suitable for DIY audio work then that would be a great start.

Lots more dumb questions to follow.

[Rare Bird]

The Antex XS25..only 25W irons but they are very good i use one for small work, you can get a massive selection of pop on tips for em.

[Mothman]

On order, thanks Andre.

[Kaiman]

Hi Mothman, hi all. Sorry to resurrect this old thread, but everyone's opinion and advice will be much appreciated.

I bought a pair of Audiomaster MLS4's, like Mothman's pair, in 1979 or 1980, IIRC. They gave me sterling service for several years, and I gave them to my parents in the late 80's. They were scarcely used while with them. Now my parents have passed away and I got the speakers back (with the home). I'm not selling them, they are not worth much except sentimentally. They remind me of my youth and they were the first thing I bought with my own money, so I'm keen on resurrecting them, along with the lovely brown Sugden A48II that drove them, as an office or bedroom system. The veneer has held up admirably through the years, and I don't really mind the back that's finished in something that looks like industrial cork, as long as it's hidden from view.

The original driver setup was the HD20B25 H4C12 8" bextrene woofer and the HD100D25 1" cloth tweeter. The woofers had to be replaced in the mid-80s after an accident that had them tear apart. The replacements were supposedly drop-ins: they look exactly the same, except for a white circular strip stuck on the front of the frame rim, right between the rubber surrounds and the four triangular lugs that the fastening screws go through. I suspect it's the HD20B25 J series, but I'm not sure. Of course I'll find out as soon as I take them apart, but in the meantime if anyone knows what model the white paper strips stand for, please let me know.

The speakers had one flaw that was identified in measurements at the time, and I had found mildly audible: there was a response depression in the presence region, between 2 and 3 kHz, and a prominence around 10-15 kHz. The latter would probably be welcomed at my age, whereas the former was a bit unpleasant and I had devised a couple of electrical workarounds back then (playing with capacitive loading for the cartridge, or with the Sugden's comprehensive tone controls).

What I definitely need do is replace the crossover, or the capacitors in it in the very least. This should be fairly straightforward to do, as the crossover is stuck on the speaker's back wall and can be approached after removing the woofer.

In decreasing order of importance, and increasing order of ambition, I could concoct a new crossover, ideally one that takes care of the presence region problem, or I could replace the tweeter altogether, or even replace both drivers. I am not going to mess with new screwholes, as the veneer looks good and I'd hate to ruin it, being a total ignoramus in the woodworking department.

I googled about replacement parts and found that several Audax tweeters are claimed to be "drop-in replacements" for the venerable HD100D25, namely the TW025Ax series (x being 0, 1, 2 etc). It is not clear to me whether there is much difference between these TW025Ax variants, or much difference between them and the HD100D25. I have seen some CSD plots of them on the Web, and by gosh do they look atrocious - by today's standards anyway! So I wonder if possible replacements are limited to Audax models, or there are other tweeters that can be substituted. There is a number of modern soft domes, from the likes of Tymphany etc, that are so much better, and I'd like to use them if I could.

The woofer matter is more difficult. The HD20's are heavy and insensitive. Furthermore, there are cutouts on the baffle for them to fit in, and I don't want to do any sawing or sanding of the woofer hole. If I am to replace the woofers with something more modern, it should be a driver with the exact same screws at the exact same diameter. If need be, I could add some sealant underneath, but that's just about it. Any suggestions are welcomed.

As for the crossover, what I plan to do is work out the schematic, study it, and see if I can play with the element values in order to cover that 2-3 kHz response trough. I have a rudimentary understanding of electrical filters, so I plan to educate myself on the subject a lot more. If I manage to replace one or more of the drivers with something newer, there could well be lots of crossover solutions available on the Web for them, so that might work well.

Sorry for the length of the post. Any thoughts, suggestions, ideas, hints, tips etc will be very, very much appreciated!

[Reid Malenfant]

The speakers had one flaw that was identified in measurements at the time, and I had found mildly audible: there was a response depression in the presence region, between 2 and 3 kHz, and a prominence around 10-15 kHz. The latter would probably be welcomed at my age, whereas the former was a bit unpleasant and I had devised a couple of electrical workarounds back then (playing with capacitive loading for the cartridge, or with the Sugden's comprehensive tone controls).

Hi Kai, i think you'll find this is probably due to the crossover & drivers & it may be rather difficult to cure

It sounds as though it's around the crossover region which is why i suspect it's an interaction between the tweeter & woofer being out of phase with each other. You see while it's difficult but none the less not impossible to get the drivers in phase there is one rather massive problem to overcome. That problem is the distances that the voicecoils are from your ear...

What tends to happen near the crossover point if this isn't taken care of is that the way the two drivers interact beams the sound down towards the floor rather than straight out in front of the speaker. I reckon this is what is causing the dip in response, try it out & see if that dip disappears if you move nearer to the speaker & move your head up & down in front of it with a nice load of material that is in the region affected

[Kaiman]

Thanks, Mark, I'm still in "idea-gathering" mode anyway, so there's no rush. In due time I think I'll be able to do some gated measurements using my omnipotent Metric Halo. It seems likely to me too that the response dip is a cancellation of sorts, because it looks too sharp to be anything else. Two way speakers with an 8''er are tricky anyway. I think I should try to fit a slightly larger tweeter and use a lower XO frequency, but will a larger tweeter fit the hole? Wish I knew at this stage.

I need to find out what the crossover topology is, above anything else. Second order filters are easier to fix phasewise, as you say,; fourth order are unlikely to have been used; third order ones start with phase at quadrature, which means that they are hardly fixable: usually they either work or they don't.
If I'm not mistaken, the picture Mothman took shows six capacitors, three resistors and three coils (two of which are aligned next to each other, rather than being at right angles!). Could be second or third order filters, or both, with or without baffle step compensation, who knows?

[Reid Malenfant]

The only passive way of fixing these things tends towards adding a second bass mid driver above the tweeter, the D'appolito arrangement (MTM).

Doesn't really help you though!

You'd need to dig the crossover out & draw the circuit out to see what it's doing, as i'm sure you aware the crossover could well be second order but with extra components to tailor some midrange peaking due to the intrinsic breakup modes of the bass/mid driver..

Like you say, you need to dig deeper

[Kaiman]

Yeap!
I dug up this BTW:

The text says that the trough is a destructive interference issue but I'm not quite so sure. It does worsen below axis, but doesnt' correct itself above axis as it should; if anything, it becomes broader and shallower. I remember compensating for this by using the Sugden 4 kHz, 6 dB/octave filter and a generous twist of the treble control that produced a shelf of about 6 dB above 2 kHz. This killed the 15-20 kHz range, but sorted out the presence region. Overall it was a pleasing compromise, though not very accurate. I then removed the additional capacitance I had initially stuck on the cartridge terminals. This gave a droop at 15 kHz and a peak at 20 kHz or so, and it was pretty flat by ear. Some residual spitting from the HD100D25 remained, which I guess was a breakup/distortion issue. That's why I'd rather replace the Audax with a Scanspeak for example, if possible.

[Kaiman]

Correction: the text says it is not a phase anomaly, so my hunch was right after all. The HD100D25 is supposedly flat through this region, so I guess it's good ole bextrene breakup. One more reason to try to experiment with an LR4, ideally with a larger tweeter.

[Reid Malenfant]

You might get away with tweaking the crossover When these things go into breakup they usually start to get a peaky output response rather than a dip. I have a suspicion that they may well have overcooked the rolloff of the crossover by doing it too early (which normally gets rid of the peaky nature). Overdone would cause the dip.

Of course fitting a tweeter with a lower resonance & tweaking the crossover to a lower xover would also work, probably better if you can get it right