UPDATE: Have picked up a sheet of fiber/rubber gasket material that's labeled as 1/32" thick. I will have to do some more precise measuring to determine if I need one layer or two.
Last edited:
I've cut one gasket but after reassembling one horn with the gasket in place, it sounded terrible - no longer like a kazoo but with voice coil rub.
Long story short: the index holes in the new diaphragms don't really fit properly onto the pins that jut out of the magnet assembly. I've drilled those index holes out somewhat on one diaphragm so that I could slide it around and while running a 10-20Hz sinewave through the voice coil at a level such that I could see the dome move, I was able to scoot it around until the voice coil was not contacting anything. I managed to add the gasket, phase plug, and metal lens to the magnet and diaphragm and it appears like I have a second working unit. I'll want to test it some more before I decide whether I want to repeat the process on the other horn.
I had hoped that during the low-frequency test I would know that I had positioned the diaphragm properly when I could see the dome move but there was no sound, yet I could still hear a flapping sound when the dome didn't stop moving from being in contact with the magnet. I am assuming this is not a big deal because I was operating the diaphragm way, way outside of normal usage and that it's not representative of what happens post-crossover in the cabinet; otherwise there'd be a lot of distortion in the form of extra harmonics just from mechanical shenanigans in the diaphragm. I set up both horns side by side and ran sinewaves through them of varying non-insane frequencies and I was getting similar results from both but the one with the new diaphragm seemed a bit louder.
Long story short: the index holes in the new diaphragms don't really fit properly onto the pins that jut out of the magnet assembly. I've drilled those index holes out somewhat on one diaphragm so that I could slide it around and while running a 10-20Hz sinewave through the voice coil at a level such that I could see the dome move, I was able to scoot it around until the voice coil was not contacting anything. I managed to add the gasket, phase plug, and metal lens to the magnet and diaphragm and it appears like I have a second working unit. I'll want to test it some more before I decide whether I want to repeat the process on the other horn.
I had hoped that during the low-frequency test I would know that I had positioned the diaphragm properly when I could see the dome move but there was no sound, yet I could still hear a flapping sound when the dome didn't stop moving from being in contact with the magnet. I am assuming this is not a big deal because I was operating the diaphragm way, way outside of normal usage and that it's not representative of what happens post-crossover in the cabinet; otherwise there'd be a lot of distortion in the form of extra harmonics just from mechanical shenanigans in the diaphragm. I set up both horns side by side and ran sinewaves through them of varying non-insane frequencies and I was getting similar results from both but the one with the new diaphragm seemed a bit louder.
The best way to cut holes in gasket material or diapragms is a gasket punch set from some import tool shop. You select the right size, put the gasket on a piece of soft wood, and hit the tool with a hammer. You local auto supply as oriellys will have a selection of gasket materials they can order. Cork is soft, paper is medium, vinyl flooring material is hard. For more exotic materials like silicon rubber or buna rubber of various hardnesses, try mcmaster.com .
I'll remember that, thanks - although cutting the material with scissors and a utility knife and using a spring punch and drill for the screw holes worked just fine.
Earlier today I tried running some program through the one horn in which I've installed the new diaphragm and the single layer of gasket that's between the diaphragm and the phase plug. The class-D Crown amp I'm using has front panel controls that let me interpose a highpass filter so I was running with that set somewhere around 900Hz. I had it turned up fairly loud - loud enough for me to have to wear hearing protection even at a distance - and I couldn't hear anything wrong. I was listening for harshness associated more with transients that would indicate voice coil rub but no such. I also hooked up the other horn with the one good original diaphragm in it for comparison and again, couldn't hear a problem. I ran sine sweeps through both and the one with the new diaphragm was noticeably louder. I was starting the sweeps at about 500Hz (probably too low) and the one with the new diaphragm did sound overworked until it got to about 800 where it kind of "barked" and then settled down.
So I think I'm going to run with this and repeat the process on the other horn. Then I suppose I'll test the two pairs of piezo tweeters and if they're all sorted I'll put everything together and see how they sound. I still have those two L-pads that I might choose to install for the midrange. However, it has also occurred to me that I could add an equalizer to the rig and I know I can remove a digital reverb that I'm not using or needing to make room for one.
So I think I'm going to run with this and repeat the process on the other horn. Then I suppose I'll test the two pairs of piezo tweeters and if they're all sorted I'll put everything together and see how they sound. I still have those two L-pads that I might choose to install for the midrange. However, it has also occurred to me that I could add an equalizer to the rig and I know I can remove a digital reverb that I'm not using or needing to make room for one.
Taking a pause to get the aluminum lenses of the Heppner drivers powder-coated by a guy who has done some work for me before. I had tried to clean them up with soap and water but they still looked a little ratty:
BTW I also brought along the phase plugs to get them blasted. Both were kind of rough with corrosion - not surprising for being 40+ years old and kept part of the time on a houseboat at a lake. 🙂 Once I get everything back, I'll buff them up with Maas metal polish and that should afford them a certain amount of protection going forward.
My powder-coat guy said that the finish defects apparent in the horns is due to the aluminum metal outgassing. It stands to reason that, as is the case with bullets (the metal's just for the weight), no one sweated the quality of the metal being used very hard.
My powder-coat guy said that the finish defects apparent in the horns is due to the aluminum metal outgassing. It stands to reason that, as is the case with bullets (the metal's just for the weight), no one sweated the quality of the metal being used very hard.
Sadly, I'm not going to be able to go forward with the original Heppner horns even with the new diaphragms. I think I've been able to make one of them serviceable but the other one has a problem: its mounting flange at the throat end has a warp in it, I've tried to shim near the screw that has the warp but even with that, whereas I am able to position the diaphragm such that the voice coil isn't rubbing without the lens involved, as soon as I start tightening the screws with all the parts together (including my layer of gasket material that's needed to get the phase plug away far enough away from the diaphragm's dome), I can hear changes to the white noise I'm putting through it during assembly. I suppose I can try to put it all together with the one working original diaphragm and no gasket but including the shim, but it and the other horn won't be a sonic match. I was able to find a few sources for horns that resemble the GEM Sound unit the previous owner had replaced the blown horn with; I went ahead and ordered a pair given that I know they'll fit and that they're adequate. If it turns out that they're too sensitive for the rest of the cab, I can either add the L-pads I already have on hand or fix it in EQ.
Updating via reply to the above message. The cabs have repainted with two coats of Exohyde, applied with a 4" foam roller. The texture is more rough than merely lumpy but it looks good. I have reassembled and tested one cab; the other will need to wait a few weeks.
After getting the original Heppner horn with the blown voice coil back, I had the lenses powder-coated and I got hold of replacement diaphragms. Then two things went wrong: 1) one of the lenses had gotten warped at the rear mount plate - not sure when that happened but it was a dealbreaker 2) the registration holes in the diaphragm were not properly located to fit cleanly on the magnet assembly. I ultimately had to abandon using those. I ordered a pair of plastic-lensed horns from a vendor that just taped together a couple of retail boxes made of thin cardboard. Both arrived damaged and the vendor was simply unable to do anything but refund my money. So even though they were weirdly peaky I went back to the metal-horned replacement horns I'd ordered, took them to a machine shop to mill the long edges of the front mounting flange so they'd fit into the recess at the top of the cabinets, and carried on.
I hooked all four 12"s up on my workbench, connected them all up series-parallel to one side of that Crown power amp, and ran subsonic sinewaves through them just to make sure I had good cone movement. I did but I was getting some rubbing-like noise. I was partway through trying to source a quartet of new 12"s when I got curious about something and activated the Crown's built-in lowpass filter. The rubbing noise mostly went away; it was due to my cheap function generator.
One of the four piezos tested weak so I replaced them all - for less than $3 each new. Turned out one of the originals was an actual Motorola.
Plastic corner protectors and 1.5"-wide rubber feet have been added to the one cab I've finished.
Okay, now for the cool part. Last night I took a couple of songs, GTR's "When The Heart Rules The Mind" and Rush's "The Anarchist" (the latter of which has some tasty Moog Taurus pedal stomps (those things were AMAZING) and mixed them down to mono in the laptop which was connected to an audio interface feeding into one side of the Crown. The cab was up on a folding table and I was sitting on a stool at my workbench about eight feet away. The bass response was poor and the midrange was harsh. Then I stood up and the midrange calmed down. Then I took a few steps back in the garage...and the bass came up.
Turns out these cabs are crap at near-field. Running the finished one in the garage was the farthest I've ever been away from it running and I feel like the result at 50-75 feet would be even better. Now, ultimately my use case for the cabs is for a keyboard rig, not general PA, but what I discovered has shown me that I was right to believe that the original designers at Acoustic weren't fools. They're no Voice of the Theater I'm sure, but they tried to make something good by being a bit unconventional, as was their wont back then.
I'll post a photo of the one completed cab soon once I've cleaned up the grilles; they're pretty gross but I tried cleaning a patch in the middle of one and it looks like new there.
So the Saga nears its end? 🙂
Yes, way back then these cabinets were actually used for PA duty, and they held their own.
Yes, way back then these cabinets were actually used for PA duty, and they held their own.
I would like to thank all of you for the information provided in this thread. Your knowledge and experience made it possible for me to restore my Acoustic PA speakers back to serviceable condition. My story starts over a decade ago when I was gifted a pair of these cabinets that eventually found their place as the dual center channel speakers in my home stereo system. I know that might sound odd to many of you, but it was the clarity and sensitivity of these speakers and their ability to accurately reproduce the human voice that consistently beat out every other speaker I tried over all those years.
About six months ago I brought them back into my system after yet another failed attempt to find something better and it was during this time that I noticed the high end frequencies were lacking somewhat. A quick test of the mid range horn and the paired tweeters confirmed that they were no longer functional. My guess is they had been this way from day one and I just never noticed it until now. Thanks to Parts Express I found the 3" tweeters that were an exact copy of the originals and a set of Pyle 4 x 10 horns that I hoped would fit.
As WattPusher as already noted in his previous posts the 4 x10's don't fit into the cabinet using their supplied plastic horns, so I opted to go a slightly different route. I unscrewed the plastic horns and using my 3d printer made an adapter to fit the original metal ones. It seemed the easiest possible solution for me and works just fine for my purposes. The one major problem I had is that over the years I had lost track of the original wiring setup (due to an untold number of modifications on my part) and that prompted me to start searching the Net in hopes of finding a wiring diagram of the crossover network. I had zero expectations of finding anything on a set of speakers built in 1967, but hope springs eternal in a fools heart.
My second search result brought me to this page and for that I am eternally grateful. I know more about these PA's than I could have ever hoped for and I would like to thank you WattPusher and the rest of the crew on here for your knowledge and experience. For the first time in God knows how many decades these speakers are back up and running again! It's going to take me some time to get used to the new sound, but I can assure you that will be easiest part of this journey.
Thanks again!
About six months ago I brought them back into my system after yet another failed attempt to find something better and it was during this time that I noticed the high end frequencies were lacking somewhat. A quick test of the mid range horn and the paired tweeters confirmed that they were no longer functional. My guess is they had been this way from day one and I just never noticed it until now. Thanks to Parts Express I found the 3" tweeters that were an exact copy of the originals and a set of Pyle 4 x 10 horns that I hoped would fit.
As WattPusher as already noted in his previous posts the 4 x10's don't fit into the cabinet using their supplied plastic horns, so I opted to go a slightly different route. I unscrewed the plastic horns and using my 3d printer made an adapter to fit the original metal ones. It seemed the easiest possible solution for me and works just fine for my purposes. The one major problem I had is that over the years I had lost track of the original wiring setup (due to an untold number of modifications on my part) and that prompted me to start searching the Net in hopes of finding a wiring diagram of the crossover network. I had zero expectations of finding anything on a set of speakers built in 1967, but hope springs eternal in a fools heart.
My second search result brought me to this page and for that I am eternally grateful. I know more about these PA's than I could have ever hoped for and I would like to thank you WattPusher and the rest of the crew on here for your knowledge and experience. For the first time in God knows how many decades these speakers are back up and running again! It's going to take me some time to get used to the new sound, but I can assure you that will be easiest part of this journey.
Thanks again!
I'm so glad you got some value out of my postings! Let me update:
First, the cabs are put together and are in use. In fact, they saw service Halloween night - outdoors, playing my daughter's Halloween playlist for the trick-or-treaters. I ran audio through a Dynaco preamp from 1976 that has been sent to me for restoration; the tone controls helped bring out the bass and compensate for differences between the two cabs with respect to treble response.
They have new corners and feet and three coats of Exohyde paint. As mentioned before, the two 12"s in each one are original and both pairs of piezo tweeters have been replaced with new production because one of them had weak output.
The real problem has been the horns and I don't have a satisfactory solution in place yet. The two metal-lensed new-production replacements - the ones I took to a machine shop to have the long sides of the mounting flanges milled down - just don't sound good in the cabs and much of the problem seems to be that they're 8-ohm whereas the original Heppners are closer to 16 ohm (the DC resistance of the one good one is 15.4). I have the bad Heppner horn; it has two issues: 1) voice coil is open-circuited (no one makes a usable replacement diaphragm assembly) 2) the back end of the lens where the diaphragm assembly and magnet mount is warped. I can't remember if I'd said here, but I found a source for horns just like the GEM Sound one that the guy I'd bought the cabs from had put in as a replacement for the bad Heppner. They both arrived with damage because the vendor did a terrible job of packaging; I wound up effectively getting them for free. I've put one of those in because I was able to work around its damage. It works but that cab doesn't sound very good; I think the problem is that being 8-ohm, it's just taking more input power than the designers intended and so the cab winds up too mid-dy. The good Heppner is in the other cab and, other than the disappointing bass, it actually sounds pretty good. It's got a nice texture just playing music through it and really, playing music through them both isn't terribly objectionable especially if using the Dynaco's tone controls (nice that the two channels have separate bass and treble controls; when do you ever see that?). So I'm left really with three options: 1) replace the bad Heppner with a good one off the used market (yeah, wish me luck) 2) actually fixing the bad Heppner - rewind voice coil and somehow bend the flange back into shape (although I suppose 3D-printing a whole damn lens is one possible way to go but that's waaaay out of my ability) 3) put a power resistor in front of the too-loud replacement horn. Of those, 3 is the most practical; it's still going to be a compromise because power that's supposed to be going into sound will instead go into heat. I think what I will wind up doing is experimenting with values of low-power resistors (or more likely combinations of low-power resistors) hoping to get the two cabs to more or less sonically match up and I think that'll put me in a good place.
I've often thought of replacing the dual 12" drivers with a modern version (more excursion) to get a little better bass response or converting the cabs into subs by going with full blown subwoofers...neither option has really appealed to me as I always hoped to bring them back as close to original as possible. The fact that all four 12's are in remarkably good condition considering their age has tempered my desire to do so. Over the years I have tried to drive them in various configurations in the hopes of getting more bass out of them, but IMHO they're simply not designed for the task. Besides the other 12's and 15's in my mains plus the dedicated dual 10" subs (rarely needed or used) in my system more than make up for any lack of bass response in these cabs.
As for the horns....I think I've found a viable solution that you might consider. Granted they've only been in place for a day and I haven't given them nearly enough listening time, but to my untrained ear they're doing everything I had hoped for. Since both of the original voice coils were open circuited I had no way of knowing their impedance, so taking a chance on the Pyle 4.4"x10.5"'s was a no brainer, especially at $19 apiece. I thought about trimming them down to fit but that didn't seem like a viable option as I wasn't sure the body of the horn itself would even fit in the enclosure. Once I had them in hand and realized I could just unscrew the driver from the horn body I decided to take a different tack.
I have to correct myself because in my previous post I said I used my 3d printer to make an adapter....that wasn't exactly true. I have three of the darn things and hundreds of hours making CAD models of all sorts of stuff, but I was in a hurry and wanted something...Right Now! So, instead I went out to the shop and cut out two doughnut shaped pieces out of 3/4" particle board using hole saws and a drill press. Yep, it was quick and dirty and I was too ashamed to admit it publicly...hence the fib. Be that as it may it took almost no time to hack the two pieces together in a reasonable fashion and get on with the hard part of the equation and that was figuring out your schematic and my crossover.
As I mentioned before I've had these things for a long time and over the years I've made any number of modifications to the wiring including ditching the 1/4" jacks on the back. Not unexpectedly I'd lost track of where everything was supposed to go on the crossover lugs and it was your hand drawn sketch that gave me a glimmer of hope that I could find my way back. With some head scratching and a bit of testing I think I finally arrived at the correct connections, at least everything seems in balance, and for now I'm one happy camper!
I do have one question about that diagram. In it you show the mid range horn connected with a reverse polarity...is that what you intended? I assumed it was a mistake and did not go that route, but I was curious if I missed something.
FWI it wouldn't be too difficult to 3d print an adapter...maybe twenty minutes in CAD and an hour on the machine. A horn on the other hand would be a little more CAD time and a lot more printer time...but it's definitely doable.
Thanks again WattPusher for a well done thread and your timely updates.
Midnitmike - I never did respond to your question; sorry. I drew the diagram with the polarities the way I found them. I've tried switching it around on the horn and I'm not sure it matters all that much, to be honest. The low, mid, and high drivers are all at different depths from the front of the cabinet...what I figure is most important is, which polarity produces the most even response far-field or, to put it another way, which polarity is making the best use of amplifier power? I find it interesting that the original Heppner horn appears to be 16-ohm when the two paralleled 12"s present 8 ohms and the cab itself is rated 8 ohms; it's like they were adjusting driver level by power-splitting via impedance. This is one reason why I turned away from replacing the original crossovers; the same people who designed them were the same people who specced a 16-ohm horn.
BTW I wanted to buy one of those Behringer Ultracurve Pro equalizers to automatically read the speakers' responses and flatten it, but those things are unobtainium right now so I bought a used Alesis third-octave stereo EQ from Music-Go-Round and a new Behringer measurement mic (that I'd use for the Ultracurve Pro anyway) so I'll just use that and read the spectrum in software or something to make the adjustments.
A couple of ideas I've had:
BTW I wanted to buy one of those Behringer Ultracurve Pro equalizers to automatically read the speakers' responses and flatten it, but those things are unobtainium right now so I bought a used Alesis third-octave stereo EQ from Music-Go-Round and a new Behringer measurement mic (that I'd use for the Ultracurve Pro anyway) so I'll just use that and read the spectrum in software or something to make the adjustments.
A couple of ideas I've had:
- In the upper reaches of the EQ's range, if I can't hear those bands, I doubt most anyone else can either so why don't I just cut them down? Why waste the amp power on content that's inaudible?
- Look at this photo of various models of Altec-Lansing Voice of the Theater speakers. Note the big boys that have baffles out to the side. A-L did that for bass range extension purposes. I'm thinking about doing something similar - just some free-standing plates to sit to either side of each cab - see if it makes a difference. There's also the idea of pushing the two cabs together side by side to make an exit that's twice as wide but I think it most applications I'm going to want to space the cabs far apart.
Very short horns with not much compression
it more for emphasis or efficiency of mid bass.
I would only guess EQ is your friend.
Is what it is.
When I had my Rhodes electric piano
would have loved these guys.
Specially now with so many stereo effects.
Pickups are in series
I already had a split in the pickups and extra jack to run 2 amps.
One amp for low register and another for top end
it more for emphasis or efficiency of mid bass.
I would only guess EQ is your friend.
Is what it is.
When I had my Rhodes electric piano
would have loved these guys.
Specially now with so many stereo effects.
Pickups are in series
I already had a split in the pickups and extra jack to run 2 amps.
One amp for low register and another for top end
That's the plan - these are mostly going to be used with keyboards and Arturia's Stage-73 plugin is definitely going to be featured. In fact, the first verse of Gary Wright's "Dream Weaver" with its stereo tremolo is one of my Rhodes target sounds.
Here's what that article about the A-L VotT speakers had to say on the subject of phase:
I expect the engineers at Acoustic were of similar mind.
In the 70s and ealy 80s I designed and made a lot of Stereo Rhodes amps,obviously with Stereo Tremolo (LDR based) for our Argentine Rock stars.
Rhodes pianos were the thing (it was Chick Corea´s heyday, an idol for them), as well as funky Ckavinets and creamy Hammond+Leslie combinations, so also made loud aggressive Claviner amps and biamped 200W power packs for Leslies.
Had to replace original speakers and drivers with EVM15L and custom horn drivers; originals expected about 40W from a pair 6550