Billfort's Strange Audio Obsession
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A few years ago, I needed to get my Conrad-Johnson push-pull tube amp fixed. I found a great shop in Toronto that does tube equipment repairs and while there, experienced a major paradigm shift in how I view home audio. While digging deep in my pockets to fix the CJ, I heard amazingly lifelike music coming from the repair shop and asked what was pulling off such a convincing illusion. To my surprise, it was a simple 300B single ended triode amp, putting out a meager 7w and driving high efficiency Lowther rear horn loaded speakers. The system was hooked up with low dollar wire, used an inexpensive turntable and was just sitting on the workbench - definitely not a great listening environment. I remember sitting there for the longest time mesmerized by the music - this was the best sounding system I ever heard - regardless of price - and to my surprise, not that expensive. That was it for me, it had to be SET and properly matched high efficiency speakers.

As much as I liked Lowthers, I wanted bass and thought there might be another route to SET bliss - in a word - Altec. For my small room, big VOT systems where out but I kept reading that the 604 series, originally intended for studio monitoring use, might fit the bill nicely. These should have 100db/w efficiency, be an easy load for an SET, have decent bass and are a true point source design that should possess good imaging - even with near-field listening. I got lucky and found a pair of 604s for sale close by - even better, they where cheap - the guy just didn't want something that big in his living room anymore. The homemade boxes where big (25"x20"x38"), ugly and a little beat up but the drivers where the prize - mint Altec 604-8Gs with the original crossovers. Aside from the K series, this was the model I lusted after most. These have alnico magnets and I found out much later - the tangerine phase plugs that extend treble out to 20kHz. I was amazed at how good these sounded with my Golden Tube Audio 300B SET amp - a match made in heaven.

Altec 604-8G driver

These drivers are from the later G series production - Alnico magnets and tangerine phase plugs.

I had the drivers, now I wanted new cabinets. The ones that seem to be regarded as the best sounding cabinets Altec made for 604s where the 620As. These are about 9cu.ft./40hz tuning so after playing around with various parameters in winISD, I decided to go with these numbers. I wanted the driver centers to be up around ear level (38" from floor) but wanted the cabinets to be under 48" in height to make efficient use of 96" long veneer and since I'd be using 1 1/2" pointed feet, total height ended up at 48 1/2". I originally played with the idea of a narrow baffle but started to read warnings of thin sounding midbass and lower midrange with these. I never noticed this problem with the old 25" wide boxes and since the 620As are 26" wide, decided not to mess with the tried and true - cabinet width would be 26".

My current listening room is extremely small (11' x 12') so I tried the old boxes pushed tight into the corners and aimed into the room at a 45 degree angle. Results where very encouraging with full bass and great imaging. I now wanted the new cabinets to have chamfered rear corners to accommodate corner positioning - sounds better in my room and I realize the nice fringe benefit of more efficiently using my limited floor space. I used these considerations with the above front panel size, played with the numbers to get the right volume and make efficient use of raw materials, and came up with this design.

Here is my cabinet drawing

The cabinets are 3/4" Baltic Birch - the 1" stuff was too hard to get so I figured I would use stronger corner construction and lots of internal bracing to compensate for the thinner material. I used bottom side venting because I wanted to lessen the midrange noise and huffing I might hear through them - I also thought this might help a little bit in the bass. The large triangular shape was to keep vent mach down (huffing again), for aesthetics, and for ease of manufacturing.

We cut all the parts on a good table saw and used a high-speed plunge router for mortises, stiffeners, vent/speaker openings and panel trimming. 1/4-20 T-nuts where added for spiked feet and driver mounting.

Parts where assembled using carpenters glue, clamps and brads with an air nailer. All edges where then trimmed if needed with a bearing guided trimming bit in the router. A little filler was used here and there.

Wood backed cherry veneer (from www.tapeease.com) was then added and trimmed, one panel at a time, with solvent based contact cement. Although time consuming, this step went very well thanks to good sharp edges on the birch plywood box and the thick, easy to work, veneer. Trimming was carefully done with the router, a flat file for the 135 degree edges and a sanding block (I guess that explains the sawdust on the camera lens here).

Finished cabinets where sanded up to 220 grit and oiled with Watco natural Danish oil. 4 heavy coats where wet sanded in up to 400 grit. I then rubbed in a coat of Watco satin finish wax.

Here's the finished speakers in my room. Grills are friction fit, ratshack grill-cloth covered MDF rings with neoprene gaskets on the inside diameter to grab the drivers.

I think the best binding posts are no binding posts, so I ran the speaker wires direct from the drivers to the external crossovers. The wires I currently use are single runs of 24ga., solid core copper stripped out of CAT-5 plenum grade computer network cable. I tried this a while back and was amazed at how good it sounded - very nicely balanced with these drivers and the low power involved.

There is a lot I like about Lowthers (no crossover!) but these Altecs excel in other areas that are ultimately more important to me. Bass is fantastic - effortless, fast and musical - you can feel it in your chest, hear the body and wood of the instruments. I don't think these play flat much below 40Hz but I'll take bass quality over bass "numbers" any day - these are as far from flabby, sloppy, booming and banging home theatre subs as you can get, short of a REAL bass horn (I'm thinking Edgars here). These drivers have a reputation for having a forward, aggressive midrange and this does come through at times but for the most part, I love them - they really let the SET magic envelope you. Source seems super critical - CD is junk, SACD can be very good but vinyl (with the right recording & pressing) is heaven. The sound-field the system throws at times is incredible - the image floats, breaths and fills the room. Turn the lights out and the sense of someone playing or singing in the room can be spooky. My next step with these will be bi-amping with a tube crossover and a second 300B SET amp - maybe I'll get comfortable with this crossover thing yet.

All speakers are a compromise I suppose, and I'm comfortable with the compromises you make with Altec 604s. High efficiency, point source and truly full range speakers that make sweet love with low power SET's - I've found my mates!

Billfort