Crossover Help with Altec 620A/416-8B Hybrid build

Hi all,

Long post alert.

Greetings from Vancouver BC. Long time listener, first time caller. Am writing this on my phone so will try to be diligent about catching auto corrects and what not but please bear with me.

Here goes. About a year ago, after spending several years down the vintage JBL and Electrovoice rabbit holes, I decided it was time to build my own DIY, Altec speaker. I had owned pretty much the whole line of smaller vintage JBL "bookshelf" speakers with the exception of L112s and a couple others before falling into a pair of Electro Voice Interface Deltas which are truly wonderful speakers if you've never heard them.

But I had the bug for big horn speakers so decided to try out a pair of EV Sentry IIIs which came up for sale near me. They had belonged to a famous Canadian musician from the 60s who had recently passed away and I drove 2 hours from home on Easter Sunday 2022 to pick them up. Long story short, they were amazing and capable of playing incredibly loud and clear but we're missing the certain sense of airyness that I associate with large format horn speakers, particularly VOTTs and some larger Tannoy bass reflex designs I've heard. So long story longer, I sold them and slipped the Interface Deltas back into top spot while I took a deep dive into Forum Town and ended up here.

I particularly enjoyed Woody's Hiraga Onken thread and gave strong consideration to this build as I had purchased a pair of Altec 416-8Bs, however I eventually settled on the Altec Model 17 620A cabinet as I thought it would be easier for my first DIY spekaers project. I extended the depth by 4 inches to gain some additional volume and am now coming in around 9.5³ before drivers, bracing and damping.

Part of the reason I wanted to do the Hiraga Onken or the 620 is so I could experiment with various horn and CD combinations up top and had initially planned to buy a pair of 811s with drivers and crossovers from a friend. However, a pair of JBL 2380a horns and 2446h compression drivers came up for sale right down the street so I snagged them. A couple days later, a pair of 2405h slot super tweeters came up nearby so I grabbed those as well. I know super tweeters make some people cringe but I've heard them well integrated both here and in Japan and am convinced of their merit,. especially with a CD horn that breaks up like the 2380A.

So now, finally, it was time to start building. I have half-decent woodworking skills though I am by no means professional. My background is in film, photography and DJing (which is how I got into this crazy hobby in the first place but that's a nutha story for a nutha time).

Fortunately, one of my newer friends is a talented woodworker and speaker building savant who has allowed me to use his woodshop and also assisted with Sketchup renderings and some frequency sweeps, etc. That being said, he is much more into modern designs over vintage so we have at various times, been at odds over what would be the best approach.

I did up a moodboard/design guide with the guiding principle being to emulate a vintage VOTT or M19 style build in a more modern and hopefully more wife/ living room friendly approach.

We bought some of the last 3/4' Baltic Birch we could get our hands on and got to work. I had shown him a 620 DIY design from Chicago I had found online that I liked and we used that as a jumping off point.

We decided on a removable front baffle made out of doubled up MDF and also doubled up the Baltic Birch for the baffle frame. The logic behind this was that since the 620A is virtually interchangeable with the 604 and 416, that I would be able to change between the two should I ever want to go the 604 route.

So now as I approach the final stages of this build, I'm trying to decide on final cabinet tuning and crossover frequency.

Sorry it's taken so long to get to the point but I felt the front loaded preamble germane to the discussion.

It's my understanding that a 15 inch woofer is roughly equivalent to an 800hz crossover point and with the 2380a being rated for as low as 500hz, this seems like a good starting point. I picked up a very inexpensive pair for Heathkit AS-101 crossovers with the Chicago Condesner caps and Peerless attenuator and will give these a trial run to see how they work. I know the caps will likely be a bit dry to start but will run them for a couple days at least to see if they can come back to life a bit with some signal flow.

I may also try mini DSP to test some things out but to be fully honest, my favourite systems have all been passive and there is something about the sound of DSP'd active systems that I don't love. Maybe it's all psychoacoustic, I don't know but my analogue ears seem to prefer analogue components.

I'm going to be bi-amping with a Pioneer M22 Class A power amp for the woofers and a Fisher 400 up top that is presently being converted to a standalone integrated amplifier for the horns and compression drivers. I'm planning to cross to the super tweeter with an inline capacitor and l-pad.

Also, the port will be in the rear and the front baffle will be flush to the box.

I am hoping to tune the cabinets to around 29hz as per the GPA spec sheet but could use some advice around both tuner and crossover specs, tips, etc.

Apologies if anything in this post is redundant but I really didn't see anything about this particular cabinet, horn, and driver combination. Thanks in advance,


PS please find some pics attached.20230409_012714.jpg20230330_140212.jpg


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That's a pretty cool build.
Welcome to the club 🙂
Glad to be here! From my internet wanderings, this seems to be one of the better hi-fi forums I've found.

Thanks for the kind words on the build. It's definitely getting there. Need to nail down the XO and port tuning and then it will be on to finishing. Will post more pics as it moves along.
Apologies if I rambled too long above. First post and all.
My basic questions are around crossover design. As I mentioned, I'm looking to use to Heathkit AS-101 crossover to start but am also considering the Model 19 crossover or JBL 3301a which is the one that's recommended for my horn and compression driver as it has built in EQ for the horn. Am wondering if anyone has experience with these either in the canned, off the shelf or DIY versions and can impart some advice around which may be the most optimal build or design? This is my first XO build so any tips and/or pointers would be greatly appreciated. Thanks!