I picked up a pair of vintage three-way Acoustic 807 cabs for keyboard use. Each cab is rated at 8 ohms and contains two ceramic-magnet 12"s (one is ported in the cabinet, the other is closed off), one 4"x10" mid horn, and two piezo tweeters. The two 12"s and the two piezos are paralleled. As measured by a DMM, the two 12"s and the mid horn are 16-ohm (they aren't marked). The guy I bought them off of told me one of the "tweeters" had been replaced but on inspection it's the mid horn, and it was replaced with a Gem Sound unit that's 8-ohm.
Here is a simplified schematic of the crossover. I have not drawn in the circuit breaker or the shunt resistor that's in parallel with it; if I had, it'd be in series with the capacitor, to its left.
One problem is that the replacement 8-ohm horn seems to have the effect of nearly shorting out the tweeters; they're speaking but just barely when I feed the cab white noise.
I started going over these cabs with the idea of bringing them to spec as their designers intended, which is complicated by the replacement horn being the wrong impedance. But in the crash course I've been giving myself on crossovers (EE degree but I specialized in digital; practical application of passive crossovers was out-of-school education only), nowhere have I seen discussed the idea of just throwing a low driver across the input and letting its highs be diverted parasitically like this.
The original mid horns and the 12"s appear to be Panasonic/Matsushita; I have tried and failed to get any specs based on what little info the stamped codes provide.
I know there is no one "right" answer for what to do here, but I would like to make these cabs serviceable for keyboards (analog synths and VST instruments). I have even thought about circumventing the original crossovers entirely and just bi-or tri-amping these things; I do have a second power amp that I could press into service and I can spare 1U of space in the rack case for an active crossover. But if I biamp (versus triamp) I still have to work up some sort of passive crossover between the mid horns and the tweeters; it's not clear to me that doing that is better than sticking with one power amp and giving the cabs new 3-way crossovers, perhaps along the lines of this.
Anyway, what do you think?
Here is a simplified schematic of the crossover. I have not drawn in the circuit breaker or the shunt resistor that's in parallel with it; if I had, it'd be in series with the capacitor, to its left.
I started going over these cabs with the idea of bringing them to spec as their designers intended, which is complicated by the replacement horn being the wrong impedance. But in the crash course I've been giving myself on crossovers (EE degree but I specialized in digital; practical application of passive crossovers was out-of-school education only), nowhere have I seen discussed the idea of just throwing a low driver across the input and letting its highs be diverted parasitically like this.
The original mid horns and the 12"s appear to be Panasonic/Matsushita; I have tried and failed to get any specs based on what little info the stamped codes provide.
I know there is no one "right" answer for what to do here, but I would like to make these cabs serviceable for keyboards (analog synths and VST instruments). I have even thought about circumventing the original crossovers entirely and just bi-or tri-amping these things; I do have a second power amp that I could press into service and I can spare 1U of space in the rack case for an active crossover. But if I biamp (versus triamp) I still have to work up some sort of passive crossover between the mid horns and the tweeters; it's not clear to me that doing that is better than sticking with one power amp and giving the cabs new 3-way crossovers, perhaps along the lines of this.
Anyway, what do you think?
I figured that's one way to go in hopes of at least getting the two cabs to sound about the same and probably at least close to their original condition. I can test that with some ganged-up low-wattage resistors and see what happens.
I've got a plan. I'm going to operate under the assumption that the original crossovers are insufficient. I've ordered a all-built-up replacement crossover from Parts Express along with a couple of L-pads which I'm going to put between the crossover and each of the mid horn and the tweeter part, using the cabinet that has the replacement 8-ohm mid horn. I'm going to undo the phone jack in the back and run all the speakers' wires out of the hole so I can just jumper-wire everything together hanging off the back of the cabinet. Once that's done, I'll mess with the L-pads while some music program is playing and see if I can get it sounding something like "good." 🙂 If I'm successful that far, I'll order a second crossover, two more L-pads, and a new pair of 8-ohm mid horns in the interest of getting two that are the same.
That's not how I read your linked article, though. It seems to be saying, "Here's how you might crossover a piezo tweeter," not "A 'conventional' crossover won't work with a piezo tweeter." As long as what the piezos are getting is suitably high-pass-filtered, how much difference does it make if it's a piezo or not?
I don't disagree, but at the same time, with a crossover and an L-pad in front of the piezos - and assuming they are correctly connected to the highpass output of the crossover - where's the problem?
UPDATE: I've received the crossover and a pair of L-pads from Parts Express. I removed the input jack and I ran the driver's wires out through the hole so I could wire everything together external to the cab. The two paralleled 12"s are connected straight to the crossover while each of the mid horn and the two paralleled piezo tweeters have an L-pad between them and the crossover
Here is how PE describes the crossover I bought:
Tonight I've connected my cheap Chinese digital function generator to the Class-D Crown XLS1000 power amp that is driving the cabs and I've also done this with a vintage QSC Model 1400 amp. I'm finding that the cabs drop off hard below about 90-100Hz. I can jack the gain up and get visibly wiggling cones down to 15Hz or lower but if I keep the gains constant and adjust the function generator's frequency up and down they both really just give up under 90-100Hz.
One thing I've read about "bass horn" designs like Voice of the Theaters and these Acoustic 807s (I think there are some Heil Sound cabinets that are a bit similar) is that their low end comes off differently at far-field as opposed to near-field, and I've been testing these things from mere feet away in a fairly small room. I've also read that the folded-horn 1x18" Acoustic 361 bass cabinets (of Jaco Pastorius, Jean Millington, and John Paul Jones fame) seemed louder the farther you got away from them (one description talked about feeling pinned to the wall at the back of an auditorium).
After messing with the Parts Express crossover and L-pads I'm finding that no mattter what I do, the other cab has better-sounding treble. I'm going to try swapping the mid horns between the two and see if that makes a difference.
Here is how PE describes the crossover I bought:
Tonight I've connected my cheap Chinese digital function generator to the Class-D Crown XLS1000 power amp that is driving the cabs and I've also done this with a vintage QSC Model 1400 amp. I'm finding that the cabs drop off hard below about 90-100Hz. I can jack the gain up and get visibly wiggling cones down to 15Hz or lower but if I keep the gains constant and adjust the function generator's frequency up and down they both really just give up under 90-100Hz.
One thing I've read about "bass horn" designs like Voice of the Theaters and these Acoustic 807s (I think there are some Heil Sound cabinets that are a bit similar) is that their low end comes off differently at far-field as opposed to near-field, and I've been testing these things from mere feet away in a fairly small room. I've also read that the folded-horn 1x18" Acoustic 361 bass cabinets (of Jaco Pastorius, Jean Millington, and John Paul Jones fame) seemed louder the farther you got away from them (one description talked about feeling pinned to the wall at the back of an auditorium).
After messing with the Parts Express crossover and L-pads I'm finding that no mattter what I do, the other cab has better-sounding treble. I'm going to try swapping the mid horns between the two and see if that makes a difference.
Don´t think they are fully separated, probably a horizontal shelf to make enclosure more rigid.
Maybe, if you measured that, so be it.
The original unit was the MOST sold one way back then, under 1000 different brands, but actually made by Foster/Fostex Japan.
Gem Sound is also a generic one and most probably a copy, so it should work.
We need pictures.
What is the circuit breaker rating?
Not sure it works after all these years, I would replace it by a 1A polyswitch or a 12V 12-15W car lamp in series.
Does it attenuate the Piezos o simply overpowers them?
Not the same (the first would indicate some kind of short, very dangerous),if it just overpowers them add an 8 or 10 ohm 15 to 25W resistor in series with horn.
Whi8ch is needed anyway, both to balance sound and for safety.
It is very common, allowed by Piezos being very high impedance and behaving as if they always had a built-in crossover.
I would expect early CTS (Eminence predecessors), even square magnet ones, we need pictures plus any labels/stamps/codes closeups.
They "should" start with 67
On the contrary, being PA they are 90% there.
Not impossible but not much justified: the woofers are fine for 100/140W total; 50/70W each, tops), but those horn drivers are very weak, 15W RMS tops and Piezos take literally "no" power.
Forget triamp, just parallel Piezos wih shown hi-pass crossover INPUT so they are NOT attenuated together with the horn driver.
See above suggestion.
Leave as-is.
Custom designed crossovers beat random generic ones any day of the week.
That tubecad article is excellent but very wide, it does not specifically address YOUR cabinet, leave as is.
You will not outwit the original designer.
Why?
Unfair and unjustified.
Bing bing bing!!!! alarm bell ringing!!!
Those Piezos are anything but 8 ohm.
I'm finding that the cabs drop off hard below about 90-100Hz
That´s what they do.
If important, use your active XOver and spare power amp to drive a separate subwoofer.
It does not apply here, you have a poor imitation of a Bass horn there.
That should tell you something 😉
Serious, return cabinet to "original", you will not "improve" it, even less throwing random parts at it.
I listened to Diana Ross in Buenos Aires using 4 of them live inside a Theater and they did sound acceptably well.
It was 1970 something.
Sound Technology has advanced a lot since.
Thank you for taking the time to respond. No, they really are; the upper 12" is in its own little box
The DC resistances of each of the 12"s was around 7.4 ohms and the original horn was like 15.1
I'll get some pics next time I have the cabs open, which won't be until next week.
Haven't been able to tell; I haven't removed it and I haven't been able to make out any markings yet. Would you advocate for putting the lamp at the same location as the circuit breaker and leaving the big power resistor(s) that is(are) connected in parallel across it as-is? I was surprised the breaker wasn't right at the speaker input ahead of the crossover.
I'll certainly take that into consideration. Could I just use those L-pads I bought in front of the mid horns?
Will do. Circular ceramic-magnet arrangement in the back. I've forgotten the code stamp but looking it up told me Matsushita/Panasonic.
Understood. One thing about the original crossover is that its coils are really big - much bigger than the ones on the PE crossover. Like, one of them looks like they just used a spool that wire comes on to make it out of. 🙂
That was kind of my original approach but I wanted to see if I could reasonably improve on it any, thinking that the original crossover may have been just barely adequate (perhaps tipping into inadequate) in order to meet a price point.
OK, certainly not at DC or freqs below their rating, but what about in the PE crossover's >5kHz high output? Or is this simply a bad match?
The Crown xls1000 I recently bought has a built-in selectable crossover that can low-pass. One thing I could do is use my vintage QSC 1400 Series One amp to run the Acoustics and take my keyboard rig's line mixer's mono effects output and run that to the Crown in bridged mode, and drive a sub with that. It's certainly an option worth thinking about.
It's a good suggestion; I'll reconsider my plan.
Interesting.
If you don't mind, I've got some questions as far as going forward with them as designed.
- Would it be a good idea to replace those 3.3uF capacitors with prejudice? I mean, I could lift one leg and test them but if I'm going to do that, why wouldn't I just replace them? If I do, are there any formulations of cap to avoid (I know not to use electrolytic)?
- I'm still stuck with the two dissimilar 4"x10" mid horns. The guy I bought the cabs from told me that he thinks he still has the original that he said was blown; I'm willing to pick it up from him in the interest of having its voice coil rewound if that's what it needs. If that's unworkable, though, I'd need a path forward to buy and install two new horns - I presume 16-ohm if I'm going to keep things original. What's a shame is that the original horn's lens is made of metal, not plastic like the replacement's. if a couple of new units isn't going to be readily sourceable I may want to push to get the broken original back.
As long as the basic crossover calculations have been applied (= specified load & emission of the transducer) the results are to be expected. Two woofers(but also one) low passed do produce less output because of BSc, yes, there 's an efficiency loss which doesn't apply to the mid&tweet because of their higher DI. The original design suggests that : mid and tweeter are reinforcement to the unfiltered woofer.
Another one notion missed is that a horn needs EQ (generally dual slope) and that the original CL net worked with the horn... Same for the R in series to the piezo.
Another one notion missed is that a horn needs EQ (generally dual slope) and that the original CL net worked with the horn... Same for the R in series to the piezo.
Eminence has several horn drivers with 16 ohm nominal impedance. N-314T-16 1.4" throat model is to crossover above 800 hz and has 100 w AES rating. Plumbing to your original mid-range flare or replacing with another horn is up to you. It has 4 tapped 1/4"x20 tpi holes on a 4" bolt circle. They do sell plastic horn parts: I didn't dive into that.
I owned some 2 ways that crossed at 1200 hz with titanium diaphragm & 1.4" throat, a Peavey RX22 driver in a SP2-XT cabinet. I was quite pleased with the sound on piano tracks particularly. The burglar was so pleased he carried them off for me.
I owned some 2 ways that crossed at 1200 hz with titanium diaphragm & 1.4" throat, a Peavey RX22 driver in a SP2-XT cabinet. I was quite pleased with the sound on piano tracks particularly. The burglar was so pleased he carried them off for me.
I appreciate your response; thank you. I looked at that Eminence; the problem is going to be one of matching up a 4"x10" horn and a 16-ohm driver, looks like. It's just going to take some more looking, I feel like.
This is the modern version of your plastic Fostex horn which was used by everybody (including Acoustic) back in the day:
https://www.simplyspeakers.com/replacement-speaker-tweeter-horn-t-122.html
Even better, modern replacement diaphragm should fit yours, so get one:
https://www.ebay.com/itm/130940580849
by the way, I never ever saw a 16 ohm one, all I found and replaced,including in Acoustic cabinets, were 8 ohm.
https://www.simplyspeakers.com/replacement-speaker-tweeter-horn-t-122.html
Even better, modern replacement diaphragm should fit yours, so get one:
https://www.ebay.com/itm/130940580849
by the way, I never ever saw a 16 ohm one, all I found and replaced,including in Acoustic cabinets, were 8 ohm.
Thanks again. I'll follow up with that guy about picking up the other original broken horn. Puzzling about the ~15 ohm DC resistance I measure on the one original. I'll post pics of what I have soon.This is the modern version of your plastic Fostex horn which was used by everybody (including Acoustic) back in the day:
https://www.simplyspeakers.com/replacement-speaker-tweeter-horn-t-122.html
Even better, modern replacement diaphragm should fit yours, so get one:
https://www.ebay.com/itm/130940580849
by the way, I never ever saw a 16 ohm one, all I found and replaced,including in Acoustic cabinets, were 8 ohm.
Update with pics:
I stand corrected about the upper 12" sitting in its own enclosure. the "wings" to either side are open in between, so both 12"s are in fact communicating with the vent across the bottom front of the cabinet.
I stand corrected about the upper 12" sitting in its own enclosure. the "wings" to either side are open in between, so both 12"s are in fact communicating with the vent across the bottom front of the cabinet.
Ok, it was weird having them fully separated.
As of the stamped codes, "130***" is an internal Acoustic parts/warehouse code; the speaker SCREAMS CTS/Eminence (same thing, different year)
Some manufacturers ,won´t say lie but obfuscate parts origin,specially speakers, to be able to claim "some" uniqueness, doubly so when "one" manufacturer supplies "everybody" he he, such as Jensen in old Fender days or CTS>Eminence later, until today.
We are talking the cutthroat sold-by-the-container OEM market, not more limited or high flying ones.
For example, I just found a German Forum dedicated to Acoustic amps and cabinets, where they talk about your cabinet´s "bigger brother" with 2 x 15" (I guess it was 811 or 813) and it states on a speaker which has TWO code pairs, Factory and Acoustic´s own:
As of the horn, it has the 130*** thingie stamped, all it tells us it´s original, and certainly must be a Fostex/Foster product (none other in that specific market way back then)
So for now we can discard the Matsushita/Panasonic connection and focus on original parts, which are available (I kean replacement diaphragms).
Find it weird the 16 ohm driver and somewhat weird the 12" speakers in 16 ohm , but of course an important Manufacturer can have custom parts made.
Money rules 😉
As of the stamped codes, "130***" is an internal Acoustic parts/warehouse code; the speaker SCREAMS CTS/Eminence (same thing, different year)
Some manufacturers ,won´t say lie but obfuscate parts origin,specially speakers, to be able to claim "some" uniqueness, doubly so when "one" manufacturer supplies "everybody" he he, such as Jensen in old Fender days or CTS>Eminence later, until today.
We are talking the cutthroat sold-by-the-container OEM market, not more limited or high flying ones.
For example, I just found a German Forum dedicated to Acoustic amps and cabinets, where they talk about your cabinet´s "bigger brother" with 2 x 15" (I guess it was 811 or 813) and it states on a speaker which has TWO code pairs, Factory and Acoustic´s own:
another also showed the dual code and is DEFINITELY Eminence:
As of the horn, it has the 130*** thingie stamped, all it tells us it´s original, and certainly must be a Fostex/Foster product (none other in that specific market way back then)
So for now we can discard the Matsushita/Panasonic connection and focus on original parts, which are available (I kean replacement diaphragms).
Find it weird the 16 ohm driver and somewhat weird the 12" speakers in 16 ohm , but of course an important Manufacturer can have custom parts made.
Money rules 😉
Here's the original horn's motor and plug, disassembled. there's a bit of a pillow of fluffy material atop the magnet in the area circumscribed by the gap - the fluff pulled in two when I removed the diaphragm; some is within the diaphragm's dome. reassembly won't be a problem as long as all of that is neatly kept inside of the voice coil. BTW, I've measured its DC resistance as 13.4 ohms.
Looks like I'm facing two possible paths, both based on using both cabs' original crossovers but replacing the 3.3uF caps for good measure:
- Try to find another one of these CTE/Eminence horns, to include potentially getting the blown one from the guy I bought the cabs from and replacing the diaphragms in both (might as well replace the working one too).
- Buy two of the T-122 horns from Simply Speakers (the cut-out holes in the cabs are 3" X 9.5", just like the horn specs call for).
I may hang on to the two L-pads I bought, thinking I might still want to use them for the mid horns.