How to make the most of 420A

[mfaughn]

A few days ago I picked up a pair of old DIY speakers. The are about 5.5cubic feet and have 8 small holes (no tube, just holes) in the baffle board. They are about 1.5 inches in diameter. They result in some chuffing but add bass. I covered them with duct tape and the speakers sound a lot better. The speakers also had some tiny radioshack horn supertweeters (alas, not the decent ones made by Fostex). I disconnected those and instead sat some SEOS-12 horns with DNA-360 compression drivers on top. I'm using a miniDSP to attenuate them and implement a high-pass filter. The Altecs are still running full range. Things are sounding pretty darned decent now and I'm wondering if I can re-implement a better ported design in these boxes that won't chuff or if they are just going to sound best sealed. I'm thinking about getting serious with these cabinets -- new baffle boards, etc.. I also have some smaller ~4ft Barzilay-ish cabinets I could use. They are nicer but obviously smaller and aldo shorter so they wouldn't get the drivers up at a seated person's ear level and would have to be tipped back a bit I think. Thoughts?

[mfaughn]

No love for the 420A.

I'm now trying them out with some Morel MDT-12 tweets. Using miniDSP to try different XO points / slopes. Still having fun. Still liking these bi-flex drivers. Still wondering about how to get optimal bass out of them (in either of the sets of cabs I have available).

[RMC]

Hi mfaughn,

I think you'll find enough love below.

These Altec 420A drivers appear to be from the early seventies based on spec sheet. Remembering a few things first can be useful for better understanding later.

The driver power rating is 25 W max; response indicated 25-14 khz; sensitivity given 97 db/1w/4' which translates to the more recent db/1w/1m by adding 1.7 db = 99 db/1w/1m (rounded), and the EIA rating of 50 db/30'/1mW which is converted again to db/1w/1m by adding 49.2 db = 99 db/1w/1m, same result; Cone resonance 27 hz.

I took the Thiele/Small driver parameters, to model some boxes, from the Unofficial Altec web site (2016 T/S list). Note the latter indicates driver Fs 22.6 hz and the driver spec sheet mentions resonance 27 hz. I took the former number to model (22.6 hz), while keeping in mind the stated 27 hz. Driver EBP (Fs/Qes) gives 53 using Fs 27 hz (EBP 44 using Fs 22.6), suggesting a sealed box would be more appropriate, but it could still be used in a vented box if you're willing to accept more bumpy bass response.

Also keep in mind the 99 db sensitivity is more likely applicable to the extended range of frequencies instead of the LF/VLF range. The 1.7 % reference efficiency number from the driver's T/S sort of betrays the 99 db sensitivity number.

Eargle (note 1) gives a "table for converting efficiency directly into 1-W, 1 m piston band sensitivity" based on Half-space driver efficiency. And 1.6% efficiency = 94 db , 2% = 95 db, and it would take 5% efficiency to reach 99 db... So at 1.7% efficiency the Altec 420A is likely to be at about 94 db sensitivity in the piston band, which makes more sense to me LF/VLF wise. The 99 db number is probably more applicable higher up, indicating a possible driver rising response in the mid/high, similar to Musical Instruments woofers.

On the other hand, because driver Qts and Vas numbers are both relatively high (0.47 and 799 L respectively) you are bound to get a pretty large box for optimum bass response. My advice is to compromise, i.e. accept some LF response ripple in exchange for more acceptable cab size.

I would be very hesitant to tune a vented box with that driver at less than 30 hz or a little more, considering its an old driver, with limited power input and cone travel capability. More so at higher power levels. As a precaution, I'd rather think of an Fs at 27 hz instead of 22.6 hz, considering the difference between these two is almost 20%, double the modern 10% or so tolerance. And it can go either way, lower or higher.

Note that somewhat bumpy bass is not necessarily a bad idea here since it raises the LF level making it a little closer to the mid/high output, therefore minimizing perception of weak bass level. Similar help may also come from box placement in the room instead.

RE "either of the sets of cabs I have available" Well, I see 5.5 cu.ft. mentioned here but don't recall any other size. I have also assumed 8 cu.ft. in the few modelings I did, both sealed and vented boxes. Most done with QL 5 (box loss assumption) not only because of box sizes, but also because older "coaxial" type drivers often tend to have more air leaks... Let me know if interested in seeing some of these.

btw chuffing may well mean the ports are too small, area wise when each considered individually.

Regards,

Richard

P.S. I inherited many years ago from my father of a pair of E-V SP12B "coax" drivers from the late sixties, similar type to the Altec 420A...

Note 1 : John Eargle (JBL), Electroacoustical Engineering Fundamentals, in Loudspeaker Handbook, P. 13

[mfaughn]

Wow, RMC, thanks!!! I went and wrote a long reply already and then my browser ate it. Let's try again, slightly abbreviated. I really like these drivers. I'm liking them with the Morel MDT-12 tweeters. Less efficient than the horns but I think I like them better. Trying different XO points b/w about 3K and 6K. Having a clue about the T/S params is a big help. I'm glad you chimed in on the power handling. I've been driving them, to fairly high volume, with a lot more than 25W. They still seem fine but I'll not do that again. The bass is really quite nice as is in the sealed (albeit with duct tape) ~5.5cf cabs. I'll try a bigger port. The baffle boards in these cabs need to be replaced anyway so cutting holes in them to experiment is no problem. It was night and day though when I closed them up. They went from "meh" to "wait a second, maybe these need to stay". I think what I'd like to do is try them with some more efficient tweeters (domes or planars) and fiddle about until I decide what I like in terms of XO and then build some passive XOs, fix the boxes up nice and put them into heavy rotation.

[RMC]

Hi mfaughn,

Thanks for your input. Want to try some nice sounding more efficient dome in a horn tweeters at about the same price as the Morel? Morel $34. and the one below $36.80 USD.

Try these: Scan-speak Discovery H2606/9200 Horn Loaded 1" Textile Dome Tweeter

They are sold in the USA by well-known to audiophiles Madisoundspeakerstore.com

You would gain 6 db sensitivity (Morel 89 db, Scan-Speak 95 db) and their Fs is the same so could be crossed over also around 2.5 khz depending on slope (min. 12db/oct). And they can handle 100-200 watts.

Scan-speak is a European (Denmark) driver manufacturer and they, along with Peerless and Vifa, belong to Tymphany Audio.

Along with some others, I've had for many years a pair of the Vifa version (H25 TG 35 with silk dome) similar to the above tweeter and would NOT let them go for three or four times what I paid... Very nice and sweet sounding for this type of driver.

RE: "The bass is really quite nice as is in the sealed (albeit with duct tape) ~5.5cf cabs. I'll try a bigger port."
Well you DON'T just make any size port, bigger or smaller, it has to be evaluated in speaker design software along with driver/box volume, to see if it makes sense acoustically. I did some modelings of that, let me know if you need that type of info. Regards,

Richard

[mfaughn]

RMC, Thanks again!

I'm familiar with ScanSpeak/Vifa/Peerless. I've had a few speakers in my day and drivers made by those manufactures have been in some of my favorites. Not enough to have a favorite tweeter picked out from among them though. I'll grab a couple of the tweeters you recommend.

I've played around with some design s/w before. I used to use WinISD online but that seems to be gone now. I don't have a Windows machine (or license) so I'm sort of stuck with online options. I can't find one that both lets me put my own box parameters in and for which I have all of the requisite parameters to make the calculator work. Most of them just tell you what size box you should use (which in the case of the 420A is a very, very big box). If you have any advice on what might work for me, I'm all ears. I know it won't be optimal but I figure that I can at least get a ballpark idea of what things might sound like by just trying some different port sizes / shapes. I should at least be able to decide if I want to nail down an optimal port or just go with a sealed box.

[RMC]

Hi mfaughn,

Here are a couple of speaker software modelings I did to try to help you decide which way you want to go.

Pic # 452

Closed box 5.5 and 8 cu.ft. on the same graph for comparison purposes. The less peaky curve is 8 cu.ft. and the other is 5.5 cu.ft., + 1 or + 2 db or so.

Pic # 458

Vented box 5.5 and 8 cu.ft. both tuned to 30 hz also on the same graph for you to compare. The lower reaching curve is 8 cu.ft. and the little more peaky is 5.5 cu.ft. Both in the range of + 3 - 4 db or so. Not a big difference between the two (about half a db) in peak amplitude, though the 8 cu.ft. peak reaches about 10 hz lower.

Note on this pic, at bottom right, the software recommends a min. vent area of 11.7 sq.in. or close to a 4" dia. vent (12.57 sq.in.). This is relatively small for driver size, and due to the driver's low power rating of 25W. Since you already indicated having fed the driver with a lot more power, plus most people like to beat a little on their speakers, then I would suggest a min. vent dia. of 5" would be more appropriate (almost 20 sq.in.). The Winspeakerz software indicates a 5" dia. tube would need to be 7.1" long or TWO 4" tubes (25.13 sq.in.). would require 9.6" long to resonate the box at 30 hz. That should take care of the port noise experienced before...

Pic # 461

This is a simulation simply trying to illustrate what the partial "total" response of LF with Mid/high may look like considering the different sensitivity levels of the low-frequency part VS the mid/high part of the spectrum covered by driver. It is not an accurate representation, but still gives an illustration of the different driver output levels.

As explained previously, the 99 db line shown on the graphs is more likely applicable to the mid/high part of the coax driver, and the 1.7% driver efficiency = about 94 db is probably more applicable to the LF part, therefore may create a sound perception of lower level bass compared to driver's Mid/high section, in absence of bass amplitude from box design or EQ, or more boundary box placement. Also depending here on choice of sealed (flatter bass) or vented box (more bumpy bass). Regards,

Richard

[mfaughn]

That helps a lot. The bigger, sealed cabs are the way to go. The 5.5 cabs they came in are fairly well built, good enough to work with at least, but they smell bad and don't look great. I actually had to put them in the garage today because I was tired of the smell. I think they are gonna have to go. 8cf cabs are big though. Zero WAF so relegated to the man cave, which is OK. It is gonna take a while to get around to building some. I've got the tools and skills but not the time. More likely is that I could have some built. I know myself though and what I fear is that this is gonna be hard. I'm the king of the unfinished project. I think I might tuck these away for a bit and try to clean up enough to make the physical and temporal space to give them the attention that I'd like to give to them. Who knows, maybe I'll come across some big cabs already built...