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Vintage Large Advent Restoration (1974)

4thtry4thtry
edited October 2017 in DIY


I just completed a restoration on a pair of Large Advent Loudspeakers. Will be bringing these to Grinnell to see how they sound in the Harris Center.  They are the vintage 1974 version, with classic fried egg tweeters and masonite type woofers.  They were originally designed by Henry Kloss, the K of KLH and founder of Advent Corporation.  To my ears, this was one of the best sounding speakers of this era.

greywardenBryan@MACJasonP

Comments

  • 4thtry4thtry


    Back in the day, a pair of Advents (and double Advents) were part of my reference system.  I spent many years listening to them, but, ultimately, sold my pair(s) and have not listened to them for over 30 years.  Last year when my brother gave me an old pair for restoration, I hooked them up expecting to hear that old, classic Advent sound quality.  But what I heard was quite different.  They did not sound anything like I could remember.  The bass was very boomy and muddy with none of the extension into the mid 30's that I could remember.

    You can find the schematic for my pair of Advents here:  http://baselaudiolab.com/ADVENT_LA_XO.html

    I have the 1974 crossover schematic Rev2, except that my R1 values are 4 ohms 10W instead of 3 ohms 5W.  I measured all the resistors, inductors, and caps with DATS V2 and found them all right on the money.  I also tested the 8uF and 16uF NPE capacitors for DC leakage on my Eico 950 bridge and they all checked OK.

    greywarden
  • 4thtry4thtry


    All good so far.  Until I checked the hookup wires and connectors.  I found several unsoldered crimp type connectors that had become very resistive over the years.  The lead wires were crimped (not soldered) to the speaker terminal push on connectors.  Oxidation caused the crimp point to measure 0.25 ohms in one spot.  So I hard soldered them all, bringing the connection resistance down to zero ohms.  Also, the positive lead wire going from the woofer positive terminal to the 1.6mH inductor was connected mechanically (no solder) via a small, little crimp type clip (see pic).  This junction point measured approximately 1.5 ohms on both speakers!!!  This was negatively affecting the amplifier's damping factor big time!!



    greywarden
  • PWRRYDPWRRYD
    Good find.  At least that is a very easy and inexpensive fix.
  • 4thtry4thtry
    edited October 2017


    The previous owner had re-foamed the woofers and replaced the damping material with 10 ounces of light, fluffy poly fill.  The re-foaming job was still in good shape, so I left it alone.  But the poly fill was a problem because it created a peaky alignment with a high Fc and Qtc.  So I pulled out the poly and restuffed the enclosures with approximately 1 lb per cubit foot of fiberglass.


    greywardenkennyk
  • 4thtry4thtry
    edited October 2017

    The restored speaker has a much deeper, tighter bass quality.  This is the result of the "In box" system resonance (Fc) dropping from 27 ohms at 47Hz down to 18 ohms at 41Hz. In box Qtc dropped from roughly 0.93 to about 0.7 or so.  The muddy and boomy sound quality has been replaced by punchy sounding kick drums and good extension to 30Hz.  This is how I remember them.  Here are some comparison charts:

    System impedance curve BEFORE restoration:

    System impedance curve AFTER restoration:


    Nicholas_23greywardenkennykTurn2jhollanderBryan@MACJasonP
  • PWRRYDPWRRYD
    Great work!  That's cool that just upgrading the stuffing made that much improvement.  I wonder how much improvement a variovent would make.  It wouldn't drop the F3, but it probably would lower Qtc and help reduce that peak even further.  But I'm sure you don't want to cut up a classic to try it.
  • Nicholas_23Nicholas_23
    Look at those chunky resistors.
  • greywardengreywarden
    Nice work! What's next?
    deadhorse - leviathan - harbinger - shockwave (wip)
  • Tom_STom_S
    Very cool! I have 2 pairs - Goodwill finds many years ago. But I have never heard them play music since all the woofers need new surrounds. Was the original stuffing fiberglass insulation? I know my newer pair, with the square magnet woofers, have a big square chunk of open cell foam behind the woofers and more foam behind that. It's packed in there pretty tight.
  • 4thtry4thtry
    PWRRYD said:
    Great work!  That's cool that just upgrading the stuffing made that much improvement.  I wonder how much improvement a variovent would make.  It wouldn't drop the F3, but it probably would lower Qtc and help reduce that peak even further.  But I'm sure you don't want to cut up a classic to try it.

    My guess is that a variovent, while lowering Qtc, would actually reduce the amount of deep bass output due to cancellation through the vent opening.  In addition, the lowest F3 would be with a Qtc of .707 (maximally flat).  Moving Qtc lower toward .50 (critically damped) would actually raise F3 a bit.
  • 4thtry4thtry
    greywarden said:
    Nice work! What's next?
    I think they are pretty much done at this point.  They sound very good, just the way I remember them from the 70's.  The cabinets have a few scars here and there, but this gives them character and a nice "distressed" type look.  The only thing I can think of doing in the future would be to pull the woofers out and silicone some constrained layer type damping material to the woofer baskets and Masonite ring material.   Sometimes, on some low frequency material, I can hear the woofer baskets "sing" a little bit.
    greywarden
  • 4thtry4thtry
    edited October 2017
     Tom_S said:
    Very cool! I have 2 pairs - Goodwill finds many years ago. But I have never heard them play music since all the woofers need new surrounds. Was the original stuffing fiberglass insulation? I know my newer pair, with the square magnet woofers, have a big square chunk of open cell foam behind the woofers and more foam behind that. It's packed in there pretty tight.


    Yes, the original Advents were stuffed with fiberglass.   I took mine apart, back in the day, and found that it was pressed against the side walls in such a way as to also dampen the 3/4" particle board walls.  I've spent quite a bit of time researching this on the web, and, from what I can tell, the stuffing was roughly about 1 lb per cubit foot.  They used 4 pieces approximately 16x13" inches each, two in the bottom half and two in the top half, with slits cut in the center for the crossover to speaker lug wiring to come through.

    You need to get a couple of the re-foaming kits and bring those Goodwill finds back to life :)   I think you will be glad you did! 

  • 4thtry4thtry

    Just in case you decide to re-foam, here are my free air DATS V2 measurements on one of my woofers and tweeters.  Re-foaming can alter the mass and change the TS parameters somewhat, especially if you have to replace the dust caps.  The previous owner retained the dust caps when he re-foamed my pair.

    Woofer:  Fs:  22.88Hz,  Qts:  0.4198    VAS:  6.42 cu ft

    Fried egg tweeter:  Fs:  714Hz,   Qts:  1.249,    VAS:  N/A


    greywarden
  • 4thtry4thtry
    edited December 2017

    A new thread over at PETT about the classic Large Advent Loudspeaker has a discussion about how the grill cloth and grill cloth frame affect the sound of the speaker.  I measured my Advents with and without the grills installed, but when I went to get the files, they were missing!.  Since this was such an easy measurement,  I put one of my Advents on a 25 inch high stand and re-did the comparison.  As you can see, it makes quite a difference.  This is why I played them at Grinnell with the grills on.

    http://techtalk.parts-express.com/forum/tech-talk-forum/1354571-curb-find-original-the-advent-loudspeaker


    S7910
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