AMT open baffle project

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ed
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#1 AMT open baffle project

Post by ed »

This one is not documented on the web site, for reasons that will become clear.
ob1.jpg
I started this one after listening to Steve Shiels OBs at Owston(Nov 2013). They reminded me of the Heils I’d heard a few times, which I’d always thought had electrostatic qualities.

It’s a beyma TPL150 together with a Fane 12/200. I started with a crossover based on Andrea Cuiffoli’s design for a large Onken box. The crossover was at 1100hz which is near the bottom of the Beyma capabilities. The plots seemed to be in the right ball park so it seemed like a goer.

The first plot is the 2 drivers superimposed on 1 plot.
beyma both2.jpg
beyma full2.jpg
After a week of listening the Beymas went straight on ebay , leaving a distinct feeling of failure which lasted me for weeks.

It took me a couple of days to realise that the part of the audio spectrum that I was mostly moved by was always going to be handled by the Fane, which just couldn’t do it. More than that, the concept of a 12” or larger ever delivering that 100-1300hz area : guitar, vocal, brass, bottom end violin, flute oboe etc...well, it just wasn’t going to happen. My take was that it needed that range taken care of by a 3rd driver, such as the wide rangers that I was used to.

I dropped the project. I did consider bodging the cabinet to try an alpair in the middle and I even bought a coil winder, but it all seemed like chasing smoke. I’ve never given it another thought since, considering that my music room is really not suited to OBs because of the size(13’x13’x10’).

I do remember offering the cabinets free to anybody that wanted to collect them. I had no takers so I guess there was a concensus of opinion vis 2 way OBs with a Beyma.
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#2 Re: AMT open baffle project

Post by steve s »

Yes once I had seen what you had done after listening to mine, I felt so bad they had not worked out ,
It was a shame but from what I could see ( I did not hear them) the bass driver just would not have sounded right in the company of the beyma
It's a very low distortion driver and in my view rather special, but mating it to bass & lower mid drivers is not easy or cheap
with my present set up theres also a bit of mechanical manipulation as you have heard
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#3 Re: AMT open baffle project

Post by simon »

Interesting ed, thanks for sharing. I've mused over something similar after hearing Steve's latest OBs last Owston. I thought they sounded very good indeed and wondered if the Beymas were the secret. But I figured, like you, that it's actually the frequencies below 1k that are the most important, and there was no point playing with the Beymas unless things were sorted lower down. Steve did warn me that matching was tricky, so more thinking to be done.
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#4 Re: AMT open baffle project

Post by IslandPink »

Yes you have my sympathies Ed . Seems you definitely need a three-way for OB if you're going to use an AMT or horn higher-up. Only a light-coned WR or FR driver can keep up with the light diaphragms of those sort of drivers in the midrange - but they can't do anything useful below about 150Hz on OB. Only way to do 2-way with the Beyma would be to use something like a 208Ez on a back-loaded horn and bring in the AMT around 1500 or 2000hz. A problem with a lot of modern 12" or 15" drivers is they are designed for high power and have lots of mechanical damping and sound poor at low driver levels - this clashes badly with AMT's and compression drivers which work very well at low levels.
The best you can get is something like the Supravox 285GMF which will work quite nicely up to 1000Hz on vocals - but you lose bass by putting it up higher on the baffle, so it's still not quite stretching to be full range.
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#5 Re: AMT open baffle project

Post by steve s »

A good start would be to copy mine Simon..

Or not too far off just use the ex4 and a decent 18" driver in a cab like mine
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#6 Re: AMT open baffle project

Post by simon »

I don't have any of your drivers Steve, nor EX4s, so that's an expensive adventure. I need something smaller too ideally, so it gets difficult with OBs.

Which Fane drivers do you use? I think you said tbe 12" was rather good?
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#7 Re: AMT open baffle project

Post by simon »

IslandPink wrote: Tue Aug 15, 2017 5:51 pm Yes you have my sympathies Ed . Seems you definitely need a three-way for OB if you're going to use an AMT or horn higher-up. Only a light-coned WR or FR driver can keep up with the light diaphragms of those sort of drivers in the midrange - but they can't do anything useful below about 150Hz on OB. Only way to do 2-way with the Beyma would be to use something like a 208Ez on a back-loaded horn and bring in the AMT around 1500 or 2000hz. A problem with a lot of modern 12" or 15" drivers is they are designed for high power and have lots of mechanical damping and sound poor at low driver levels - this clashes badly with AMT's and compression drivers which work very well at low levels.
The best you can get is something like the Supravox 285GMF which will work quite nicely up to 1000Hz on vocals - but you lose bass by putting it up higher on the baffle, so it's still not quite stretching to be full range.
I've thought about using the 208Ez with the Beymas above 1K say, but I wonder if the added complexity might not justify the cost of the Beymas? Tricky without trying it to know.

I've also thought about running the 285GMF up higher too, something you suggested by email. When I first had the OBs up and running with the 285GMF and Alpair 12P I think I had the Supravox unfiltered, so going quite a bit higher than 1K. It was definitely an improvement rolling them off from around 150Hz but I think that was as much to do with only having one driver covering from c150Hz up.

But even if I did use the 285s up to 1K, I'd still need something at the bottom. Like another 285 lol.

Sorry ed, there's been a bit of scope creep. (A bit like there always used to be!)
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#8 Re: AMT open baffle project

Post by steve s »

I thought you still had a pair of lowthers Simon... sorry about that
The 12's are the pa version of the famous crescendo guitar speaker, its got a very big magnet and a light cone and works well where I use it. I know a guy with two pairs he got from the fane rep in the 70's
He built them into some large cabs with just a tweeter, unfortunately he won't part with them. there's not been any on eBay... I've been watching for them for 3 years.
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#9 Re: AMT open baffle project

Post by ed »

After the disappointment and before I took the cabinets to the tip I was thinking about the Fane with a betsy in the middle and a Monacor ribbon on top. All of which I have on the shelf. All in the same OB cabinet as used for the Beymas. Each driver was to be driven by it’s own TPA3116 class D which was in turn driven by a LR active filter. The level for each individual driver could then be adjusted via CLIO.

All this thinking vanished in a puff of logic when I woke up one morning and considered the size of my music room.
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#10 Re: AMT open baffle project

Post by steve s »

ed wrote: Tue Aug 15, 2017 6:58 pm After the disappointment and before I took the cabinets to the tip I was thinking about the Fane with a betsy in the middle and a Monacor ribbon on top. All of which I have on the shelf. All in the same OB cabinet as used for the Beymas. Each driver was to be driven by it’s own TPA3116 class D which was in turn driven by a LR active filter. The level for each individual driver could then be adjusted via CLIO.

All this thinking vanished in a puff of logic when I woke up one morning and considered the size of my music room.
The electostatics work well in there Ed ?
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#11 Re: AMT open baffle project

Post by ed »

there is something intrinsically different considering the SPL necessary to get a dynamic result from a pair of OBs and a pair of electrostatics. My electrostatics have always worked superbly in my room, IMO better than anything else I've used, and to date thats an awful lot of speakers, and an awful lot of types of speakers......

maybe its just a fluke of symmetry but all the OBs I've tried have been overpowering, go figure cos it's the same dipole approach.

There is a similarity to the way I discovered class A in the 1980s when I bought an MF A100. The music seemed to be much more present at lower levels. For ages I associated this presence with class A when in reality it was probably much more to do with the synergy between the MF and the speakers...never did prove that one.

Anyway, I've had the electrostatics for nearly 15 years and nothing has usurped them yet, in that room. I do remember my previous music room was 6mx6m and I think I listened a lot louder then. OBs would have probably been ideal.

diet: to be fair Steve, your SPL and mine are so different that any comparisons or justification are meaningless.
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#12 Re: AMT open baffle project

Post by pre65 »

ed wrote: Tue Aug 15, 2017 7:49 pm
Anyway, I've had the electrostatics for nearly 15 years and nothing has usurped them yet, in that room.
What electrostatics do you use Ed ?

I'd love to hear some in a home environment.
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#13 Re: AMT open baffle project

Post by IslandPink »

simon wrote: Tue Aug 15, 2017 6:54 pm Sorry ed, there's been a bit of scope creep. (A bit like there always used to be!)
See my thread now !
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#14 Re: AMT open baffle project

Post by andrew Ivimey »

My tiny 2p here. I've only used ESS Heil AMTs and thought about OBs but gave up the idea due to size - now I feel I have a couple more excuses. However with ESS cabinets that use a 12 inch speaker and a 12 inch parasitic bass radiator all is ticketty boo to me. WAS did paper cones originally and I much preferred them to the think black polyprop cones of the later speakers.

One chum decided on just an inductor to gently slope off the 12 inch woofer at around 1200Hz. But his little bit of magic dust was to use 12 inch BBC (Rogers) woofers as used in BBC LS5/8s. I never got round to this mod but a sizeable number of listeners to his Much modded ESS AMTs have praised them highly - to a man though not valve users.
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