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Altec Phase Plugs - Radial (Tangerine) vs. Circumferential?

jaaronb

jaaronb

Active Member
Hey folks - I happen to have a pair of Altec 604-8gs, one of which has the newer radial/"Tangerine" phase plug, and the other has the older circumferential.

I DO hear a distinct difference between the two, and despite having swapped diaphragms, crossovers, R/L sides, amps, pre-amps, sources, and rearranged the room entirely, the audible difference between the two persists. They've been remagged and gone over by GPA recently too, so one isn't demagged.

It's enough to my ears to be distracting, so before I send the older one to Bill at GPA to update with a Tangerine phase plug, I thought I'd take some measurements of the two speakers, using the same diaphragm and crossover.

I'm posting all this over at the Altec Users Group on Hostboard, but thought more folks might see it here and find it to be of value. I plan to do the measurements this weekend if I can get around to it.

Here's a copy/paste from Hostboard of my perception of the audible difference between the two speakers and some other stuff:

As to the differences I hear between the two speakers in a stereo setup, in short, the older circumferential driver sounds louder in the ~2-8k range by ~1-3db (I'm guessing at these values). The output differential is enough to be noticeable during stereo playback. Because the circumferential driver is louder in the midband, I can't really tell if the radial/tangerine driver is more extended above 15k or not.

If the radial/Tangerine driver IS more extended in the highest octave, AND is indeed down a db or two in the ~upper-mids like I think I'm hearing, it would sort of correlate to the predictions/findings of Altec engineer Clifford A. Henricksen in his presentation to the Audio Engineering Society at the 1978 convention ("Phase Plug Modeling and Analysis: Radial Versus Circumferential Types" - http://www.nutshellhifi.com/library/Henricksen_1978_Tangerine_PP.pdf)

Henricksen's model on page 17 predicts a ~1.2db loss in efficiency in a radial phase plug vs. circumferential at ~9.2k (but with some efficiency loss seemingly extending back into the midband based on the chart on page 17). He also predicts a ~5db gain at 20k for the radial/Tangerine vs. circumferential. He tests his predictive models in the same paper and validates them.

So the difference I've been hearing between the two is significantly more output/information in the ~mid/upper-midband in the circumferential driver vs. the Tangerine driver. The Tangeringe'd driver sounds like it has less gain and midrange energy than the older driver, and sounds polite by comparison.

Are there other factors I'm not thinking of that could cause these two 604-8gs to sound so different, remembering I've swapped crossovers/diaphragms/room position/room orientation, etc?

I'm using 23744 diaphragms with the simplified Markwart 604-8g crossovers. I went through four 23744 'phragms to find the two with the closest output.

As to measuring equipment, I'm afraid I only have The Studio Six Digital RTA app for my iPhone. It does apply a calibration curve to my iPhone's mic. I can also take measurements with a another RTA app I have for a separate Android phone.

Question: The iPhone RTA app lets me choose Flat/A-Weighted/C-Weighted SPLs as well as Flat/A-Weighted/C-Weighted Plots. Any thoughts as to which I should set it to for these two settings?

I plan to run a digital pink noise file at approx 90db and measure on-axis at 1 meter. I'll use the same crossover and diaphragm on both speakers for testing.

Anything else I should be thinking of?

Anyone else here ever had GPA put a new phase plug in an older driver?
 
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I'm looking forward to seeing your measurements, it will be interesting to see if they match the designer's measurements on page 19 of the linked pdf.

Radial vs. Circumferential.png

I think I'd prefer to have matching Radial Tangerine phase plugs.
 
I'm looking forward to it too..

Like I said, the most noticable difference between the two is that the older driver sounds significantly louder in the midband than the Tangerine'd driver.

I'm especially curious to see where in the midrange the circumferential is louder and by how much.

Has anyone else noticed a midrange prominence in the circumferential drivers vs. the radial?
 
I took some quick measurements the other day using pink noise and an iPhone RTA app...the results seem to make a pretty strong case for the superiority of the Tangerine phase plug vs. the circumferential in the 802 compression driver. Keep in mind I'm using the 23744 diaphragms, but the grey colored radial phase plug that GPA uses, according to Bill, is identical to the Tangerine plug - it's not orange because the dye process makes them much more expensive to produce - so I expect the results would be similar on the newer GPA diaphragms. (I've swapped the diaphragms in the past and gotten the same/similar results).

Radial RTA.PNG Circumferential RTA.PNG
 
There may be something else going on with your measurements. Look at the low end - the left most 5 bars are way out of band for the horns but the levels are quite a few dB different. Then the 125 to 1K area looks the same on both.

I had several 604 variants at the same time and they all measured differently whether radial or circumferential slots. Also, the piece to piece differences were sometimes more than the model to model ones.

It's interesting that you can switch the plugs between the two versions. I was not aware that you could do that. The radial is plastic while the other is metal. I've re-glued a bunch of radial types and have disassembled a circ type that was poorly packaged for shipping and broke - see photo. The phase plug has 3 ribs that lock into grooves as a press fit. On the radial types, the phase plug is basically sitting in the cone shaped area and glued to that surface. I used crazy glue. 20190207_145726_HDR.jpg
 
kyle said:
There may be something else going on with your measurements. Look at the low end - the left most 5 bars are way out of band for the horns but the levels are quite a few dB different. Then the 125 to 1K area looks the same on both.

I had several 604 variants at the same time and they all measured differently whether radial or circumferential slots. Also, the piece to piece differences were sometimes more than the model to model ones.

It's interesting that you can switch the plugs between the two versions. I was not aware that you could do that. The radial is plastic while the other is metal. I've re-glued a bunch of radial types and have disassembled a circ type that was poorly packaged for shipping and broke - see photo. The phase plug has 3 ribs that lock into grooves as a press fit. On the radial types, the phase plug is basically sitting in the cone shaped area and glued to that surface. I used crazy glue. View attachment 1412328
Click to expand...

Aww rats Kyle... thanks for noticing. ;) You're right about the lowest couple octaves. I'm going to take some more measurements, but I'm hoping the difference there is due to the very sub-optimal 614 enclosures I have the 604s in right now... one of 'em doesn't even have the back screwed on - its just sitting there threatening to fall out. Need to fix that.

Roger that on the phase plugs. Bill told me what you discovered with your broken driver - that the process for changing the phase plugs out is not as simple as removing one and gluing the other back in. Still, it's doable.

I love the 604s, but I sometimes wish I had held out for a pair of Model 19s. In fairness, I've never heard the 604s in anything approaching an adequately sized enclosure, so it's probably not fair to pass judgement. Also, my room is only 15x14, although that was fine for my resto-modded 9844s, which use the same top end and crossover as the Model 19. But perhaps the low-end of the 19s would overwhelm the room.

The thing is, the Model 19s might *sound* better/more exciting than the 604s, but it's hard to deny the fact that 604s are more accurate and let me hear further into the recording than I believe the Model 19s are capable of (at least from 1600hz on up, where I've compared them both in the same room). Using the phase-corrected MarkWart crossovers + the point source coherency of the 604s is really, really hard to beat.

A longtime, well-heeled audiophile friend of mine who has a long-standing Klipsch-induced prejudice against horns came over the other day to hear the 604s (having never heard them). He was visibly surprised and impressed - declaring them the only other speaker he'd ever heard that matched the Quad 57's ability to reproduce acoustic tones like guitar and piano. I think I'll be much happier with the 604s once I have them in right-sized enclosures.

BTW, I was able to at least get the output of the two speakers at equal db levels overall, and very similar 2-8k output levels by turning the midrange L-pad down to about 8 o'clock on the circumferential driver, leaving the radial at about noon. I can live with that until I get the circumferential driver to Bill and get a hold of some decent cabinets.
 
I've had 604-8Gs in 620/model 17 cabinets and model 19s in a room about that size and I thought both sounded fine. I never understood why people say that you need to sit 20 feet or so away from horns for them to integrate. I like bigger rooms for any speaker but they work near field as well. The 604 sound better in a 10 cu ft box or even larger. The other thing that makes them sound really good is wide baffles. It really fills in the low to mid-range and with the horn's directionality there's no issue with the highs.
I bought the pair of 604s from Jeff Markwart that he used to develop his crossover. Nice guy, very knowledgeable. It's a good crossover for a lot of different speakers with a few tweaks.
I have a pair of model 19s that may be leaving me if I can bring myself to part with them. I've been paring down my Altec stuff for the last couple of years. I made the mistake of collecting for years without a thought of what to do with all of it.
 
kyle said:
I never understood why people say that you need to sit 20 feet or so away from horns for them to integrate.
Click to expand...

I understood that such people had little experience with horns and simply didn’t know what they were talking about. Some pellethead hears a pair of KHorns at the HiFi Hutch and thinks he can apply what he heard to horns in general.
 
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I sit 11 feet from my Big Red Supers, and there are no issues with the integration of the horn/woofer(s). There is about 15 feet of open area behind me though.
 
I use an ~8-9 foot equilateralish triangle with my 604s in the 14' x 16' room, and it sounds fantastic.

Same setup with my 802g/811b + dual 414z cabinets on the Model 19 crossover also sounded great. I loved the 802g/811b sound at that distance.
 
Another thing I've been wondering - since I'm using the small (by Altec standards) 614 cabs, which are not properly tuned, should I consider trying them open-backed?
 
jaaronb said:
Another thing I've been wondering - since I'm using the small (by Altec standards) 614 cabs, which are not properly tuned, should I consider trying them open-backed?
Click to expand...

I suspect you’d have a considerable decrease in the depth of the bass. But you can always reinstall the backs.
 
kyle said:
The 604 sound better in a 10 cu ft box or even larger. The other thing that makes them sound really good is wide baffles. It really fills in the low to mid-range and with the horn's directionality there's no issue with the highs.
Click to expand...
Just curious, how wide is wider, and how large is larger?? I’m researching cabs and am interested as what a bigger box brings over a 9.0 ft3 620 variant.
 
My Super Big Red boxes are 15.5 cu ft, exterior volume, but also include the Utah aux woofer. I have no idea if they are internally separated.
 
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