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Thread: Altec 604 8G and 604 8K throat sizes.

  1. #1
    Senior Member eso's Avatar
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    Altec 604 8G and 604 8K throat sizes.

    It's been a while since I've been here, but I need help from some of the brains here.

    For many years I've been playing with and building Urei 801B duplex drivers and custom horns. The parts are pretty readily available make make a great sounding system.

    People keep asking me for my custom horns for the Altec based duplexes, but I hadn't made any because I didn't have an 604 to work with to make my tooling.

    I just came up with a pair of 811A monitors, one of which needs a recone so it was a perfect choice to dissect and reverse engineer my horns with. After after removing the 801A horn, I find the throat is a 7/8" bore.

    With the B or C series duplex–neither of which have any Altec parts–the throat at the horn is ~1 1/4".

    Cross section illustrations of earlier 604 drivers also show a conical throat flare from the compression driver through the woofer center pole.

    Does a Stock 604 8G have a conical throat flare?

    Did Urei add a restrictor to the 604 8K throat? if so, was this also a feature in the original 604 8G based Urei 801?

    This is a very curious discovery to me.

    Any tech insight will be appreciated.

    eso







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    Senior Member eso's Avatar
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    I'm surprised no one here has any input.

    Having these 801AA duplexes to study has raised more questions than they have answered.

    I can certainly clone this horn and mounting in my wood horns, but I'm going to need an earlier 604 to compare to. After some web crawling it is looking like the 604 8K/Urei 801AA has a different center pole than earlier Altec 604 models. The 604 8K had the Mantaray horn which also had the tiny throat, a 7/8" cylindrical bore from the phase plug through the center pole the the horn mount. By contrast the 801B duplex using the PAS CX1580 woofer and JBL 2425H compression driver has a conical flare from 1" at the compression driver to 1 1/4" at the horn mount.

    That conical flare is also shown in cut-away illustrations of earlier Altec 604 models so it is what I expected to find.

    Unique to the 801AA horn is an open-cell foam plug at the horn throat–probably added to help pattern control and limit beaming from that long narrow cylindrical bore in driver.

    Another item unique to this model are the small circular Helmholtz resonators in the horn sides.


    All of these oddities really seem like a series of bandaids applied to compensate for some core shortcoming.

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    Sorry, no experience with the 604 versions of the UREI monitors, I've got the C version. But it looks like the original 801 was based on the 8g, but could have been sold with 8h or 604e (all Alnico) with the added Mantaray horn. UREI apparently had networks for each. Perhaps based on what was available?

    Then the 8k (ferrite) was the 801a with the foam edged horn and modified throat because of the magnet change. There's also mention of the new horn being available as a retrofit to 8g and 604e models. Perhaps one of your pictured horns is from the 8k and 1 is the retrofit horn for Alnico? I've not seen mention of an 801AA previously.

    http://www.technicalaudio.com/pdf/Du...04/DUPLEX.html

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    Senior Member eso's Avatar
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    Quote Originally Posted by rusty jefferson View Post
    ... I've not seen mention of an 801AA previously.
    It's slightly confusing, but the original version of the Urei duplex was based on a 604 8G driver and was called an 801A. It was used in the 811, 813 & 815 monitors.

    The 811A, 813A and 815A monitors used a duplex based on the Altec 604 8K and it was called the 801AA.

    The B series was more consistent in naming with the 811B, 813B and 815B all using an 801B duplex.

    eso
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    I get it. I knew the system model numbers and assumed the 801 numbers followed suit.

    Since the 811 (801a) had the Mantaray horn, don't you think one of your pictured 800 horns (foam edged) is the mentioned retrofit horn for 801a?

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    Senior Member eso's Avatar
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    The 801A used an Altec 604 8G and had the original blue Urei radial horn, not the Mantaray.

    The mantaray horn was introduced on the 604 8K which was the platform for the 801AA, hence the tiny throat on the horn for the 801AA

    eso
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    Quote Originally Posted by eso View Post
    The 801A used an Altec 604 8G and had the original blue Urei radial horn, not the Mantaray.

    The mantaray horn was introduced on the 604 8K which was the platform for the 801AA, hence the tiny throat on the horn for the 801AA

    eso
    Apologies for playing fast and loose with the term Mantaray. I'd not seen a pair of 811/813 (801a) with anything except the original blue UREI exponential horn (without foam lining or diffraction foam on edges) that sort of looks like the Mantaray horn. I assumed your lower imagine in post #2 was the 801aa and the upper imagine was the upgraded horn made available for the Alnico 801a drivers. I see now there were likely several iterations of horns for (I assume) the 801a I wasn't aware of. Didn't mean to derail.
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    Altec 604 Duplex throat size and Mantaray(tm) introduction

    Quote Originally Posted by eso View Post
    It's been a while since I've been here, but I need help from some of the brains here.

    For many years I've been playing with and building Urei 801B duplex drivers and custom horns. The parts are pretty readily available make make a great sounding system.

    People keep asking me for my custom horns for the Altec based duplexes, but I hadn't made any because I didn't have an 604 to work with to make my tooling.

    I just came up with a pair of 811A monitors, one of which needs a recone so it was a perfect choice to dissect and reverse engineer my horns with. After after removing the 801A horn, I find the throat is a 7/8" bore.

    With the B or C series duplex–neither of which have any Altec parts–the throat at the horn is ~1 1/4".

    Cross section illustrations of earlier 604 drivers also show a conical throat flare from the compression driver through the woofer center pole.

    Does a Stock 604 8G have a conical throat flare?

    Did Urei add a restrictor to the 604 8K throat? if so, was this also a feature in the original 604 8G based Urei 801?

    This is a very curious discovery to me.

    Any tech insight will be appreciated.

    eso







    hello eso,

    Long-time UREI 811 / 811A / Altec 604 user here...

    The Altec 604 Duplex changed throat size from 1 inch to 7/8 inch between the Alnico and ferrite magnet models because of the need to maintain a correct expansion rate in the shorter axial length of the ferrite magnet structure. To my knowledge and measurement of both Alnico and ferrite) Altec HF drivers, both have conical throats.

    The final Alnico model (and the first to feature a Mantaray constant directivity horn) is the 604-8H, used in the Altec Model 18.

    Confusingly enough, the first ferrite magnet Duplex is the 604-8K, which also features the Mantaray. This is the driver used in the UREI 800 A-series.

    The horn-throat adapters between the Alnico and ferrite 604s are different because of the different throat size. So far as how this affected the design of the UREI A-series horn compared to the original UREI horn, you can read how this was handled in this dB Magazine article by Dean Austin, one of the UREI Time Align project engineers:

    Time-Aligned Loudspeakers Revisited, page 27:

    https://worldradiohistory.com/Archiv...DB-1981-08.pdf

    What's apparent from the article is that the foam diffraction buffer (foam lips) are to compensate for the shorter path in addition to providing a smoother transition from the reflective horn to air. Interestingly, the size was determined experimentally as the horn mouth is of two different materials.

    What's also apparent is the role of the foam vertical walls in the A/B/C- series horns to not only accommodate the smaller throat size and attendant flare rate change but to absorb the corner reflections between the horizontal and vertical horn walls that cause transient distortion (the tone burst figures in the article are very interesting).

    BTW, Dean also authored an introduction to the UREI Time Align in the same magazine a couple years earlier:

    Time-Aligned Loudspeaker Systems, page 58:

    https://worldradiohistory.com/Archiv...DB-1979-03.pdf


    Hope that this helps,
    Robert
    Last edited by joyspring; 10-20-2020 at 08:35 PM. Reason: grammar

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