Altec 420A enclosure
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Thread: Altec 420A enclosure

  1. #1


    Altec 420A enclosure
    From the Altec 420A specifications:

    a sealed enclosure, supposing standard Qtc 0.707, should have 22.8493 cu.ft. ( 0.647020 cu.meters ).
    As far as I know the inside volume of the Santana, which employs Altec 420A, is 3.736 cu.ft. ( 0.10579 cu.meters ).
    This means that in the Santana the Qtc is around 1.38 !
    I know from:

    that 420A were used also in the 872B Madrid.
    I was not able to find the dimensions of this enclosure.
    I'm planning to make an enclosure for the Altec 420A.
    Should I use the same Qtc used in the Santana or could be better to realize a bigger enclosure?
    Is the standard Qtc 0.707 not good for the Biflex speakers?
    Does anybody know the dimension of the 872B Madrid?

  2. #2


    I had mine in a 11cuFt cabinet tuned to 30Hz if I remember correctly. I have no idea how they put 420's in those small boxes. They also work well on an open baffle with a sub and tweeter.

  3. #3
    Senior Hostboard Member GM's Avatar
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    Dec 2002
    Chamblee, Ga.



    The 872 was a '50s era corner loaded cab and my SWAG is that it's ~the same Vb as what you calc'd for the Santana, so apparently they never felt the need to enlarge it. Anyway, consumer speakers back then were designed no different than today in that they used the smallest cab they could get away with.

    That said, the bi-cone was about taking decoupling ring(s) to their logical conclusion and as long as the main cone is severely excursion limited, it works somewhat better than an equivalent size 'fullrange' driver that has just the standard ring(s) moulded into the diaphragm, and I assume why they only marketed it in acoustically small cabs AFAIK.


  4. #4


    Thank you zelgall and thank you GM!
    I think I will make an enclosure of about 5 cu.ft. (bigger than the Santana).
    I will make it higher to leave them on the floor, and I will put the 420A near the ears level (when sitted).
    Does anybody have the frequency response and the impedance curve for the Altec 420A?
    I want to add a tweeter but I don't know where to cut it.
    As far as I know a good point to cut the tweeter for the Altec 420A is around 2.5 kHz.
    Thank you in advance,


  5. #5


    You can crossover much higher than 2.5KHz with a 420. The idea with the two part cone is that the high freqs come off the central area. They sound good crossed over above 7 KHz, which means you can get a decent tweeter to go with them. They really do need a bigger box unless you're going to EQ them heavily.

  6. #6


    Thank you zelgall.
    I agree with you, from the 420A specification, that this speaker would need a bigger box and a crossover over 7kHz.
    What confuses me is that I have listen the Santana speakers and they are small and the crossover is at 2.5 kHz. Their sound was very good! That's why I'm planning to do something "in the middle" between the Santanas and what coomes out from the specifications.
    Did you ever listen to a 420A in a large box with a tweeter over 7kHz? What were your impressions?

  7. #7


    As I mentioned above, I had them in an 11cuFt box, which I consider large. I tried an Aurum Cantis G1 ribbon and a small Fostex horn tweeter (individually) and crossed them at 10K. The ribbon is supposed to be 102dB but it didn't have the juice to play with the 420. The fostex had to be padded down but sounded pretty good.

  8. #8


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    Altec 420A enclosure
    Dear Zelgall, thanks again for the information.
    I just have another question for you.
    Did you used the Fostex FT17H horn tweeter or anotherone?

    I'm planning to build a sealed box (about 7 cu.ft.) with the Altec 420A and the Fostex FT17H horn tweeter. I also considered the Hi-Vi RT1-II

    I think I will use the Fostex because of the fact that you had a bad experience with the Aurum Cantus G1 ribbon tweeter.

    As far as I understand you used the 420A in a 11cuFt open cabinet tuned to 30Hz. As I said I'm planning to build a sealed cabinet. Do you think that 7 cu.ft. would be too small?


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