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August 2006
Enjoy the Music.com Review Magazine
Bastanis Prometheus mkII
Don't Box Yourself In
Review By Clive Meakins
Click here to e-mail reviewer


Bastanis Prometheus mkII Loudspeaker  These loudspeakers are different. It is not just because they are kits or that you have considerable scope to tailor their looks to your taste. It is more due to their being an open baffle design which means they are not just yet another variation of a couple of drive units in a box. In addition they are much higher sensitivity than most loudspeakers.

Robert Bastani has worked for loudspeaker manufacturers in Germany, he understands the criteria used to design typical speakers, which is often centred around cost reduction.  He wanted to break away from such constraints and design speakers that provide a particularly high level of musical enjoyment. His quest has been successful.

Most loudspeakers are designed for use with 50W to 200W amplifiers, as this is what many people into traditional hi-fi have. It is probably fairer to say that people have 50W to 200W amplifiers because most loudspeakers demand this sort of power. The Prometheus loudspeakers sit in a very different market segment, you will know quite quickly whether Prometheus should be on your list to consider.

Prometheus will appeal to those who follow the low-watt amplifier, high-sensitivity speaker route rather than muscle amplifiers with current hungry loudspeakers. At some 100dB/watt/metre sensitivity these speakers go very loud very easily, they will take up to about 150W but I suggest you don't try this indoors! Excepting the most flea-powered amplifiers (less than about 2 to 4 watts) these loudspeakers will produce high sound pressure levels with just about any amplifier. That is not the point though, quality is the objective, and high sensitivity is just a part of the equation.


Kit & Assembled Options

Robert has found it is best to market Prometheus in different configurations for the US and Europe. The loudspeakers I am reviewing here are The Prometheus mkII Airforce version costing €1,950 from Germany at Bastanis.com this is a kit comprising a pair of "wideband" 12-inch drivers (100Hz to 11kHz), Gemini tweeters, a pair of 12-inch bass drivers and a pair of bass amplifiers. US prices start at $2,198.

The wideband driver and tweeter are mounted on an open baffle, so that means there is no box behind the drive units to muddy the sonics with internally reflected sound waves. With open baffles unless you have a very large baffle (e.g. 4 foot square), you won't get much happening below 100Hz. The Prometheus still have a large baffle but it's not so unusual at 1200mm x 400mm. There are separate bass boxes each with its own 150W amplifier, the bass boxes can either be mounted on the platform behind the open baffle or away from it as you would do with any subwoofer. The bass box is a sealed box, not a ported design. There is a non-Airforce version at €1,300 with just one bass amplifier and sporting different tweeters.

The bass and wideband drive units are based on professional audio paper cone units built to Bastani specifications; they are not standard off-the-shelf units. The 12-inch wideband has felt pads applied to it to widen the image at higher frequencies. There is also a witch's brew of oil and lacquer to stiffen the cone.  The tweeter is a 1-inch compression unit; this means that it works "backwards", the dome being at the back with the high frequencies exiting through a 1-inch diameter aperture at the front. The tweeter is MASSIVE, indeed many mid-bass units are lighter. The kit comes with a very nicely machined plywood horn to boost output from the tweeter, which has just one capacitor for its crossover; the wideband uses a single parallel resistor. This is one very directly connected loudspeaker. The bass amplifiers can take line-level or loudspeaker-level signals. There are controls for level, crossover frequency and phase (0 to 180 degrees). The suggested crossover frequency is 70Hz to 100Hz. Moving up the spectrum the acoustic crossover between wideband and tweeter works out at about 8kHz.

Single-driver loudspeakers tend to be compromised at the frequency extremes. Think of Prometheus as a single driver loudspeaker that has assistance at the frequency extremes to eradicate any sonic compromises.


Make Or Buy The Baffles & Bass Cabinets

When I built these loudspeakers Robert was still planning the baffle/cabinet kit, it is now available. I created panel-by-panel diagrams for a woodworking company to make up the panels. Most people won't have the tools to make a decent job of the woodwork starting from scratch, especially as the holes for the drivers really should be CNC machined. In Europe you can now purchase birch plywood kits for around €750 and you can pay extra to have the wood assembled and prepared ready for you to wax, oil, paint or veneer. It might be that shipping costs to some countries become prohibitive, if they do you can find a local company to work to the plans from Robert's site. The plans you'll find there are produced by myself and are what I used with my local wood machining company.

You lucky guys in the United States have some extra options; cabinet kits and assembled loudspeakers are available from Bill Allen at BaulsAudio.com ("Wood" and "Black" versions) or custom-made hardwood cabinets from Tony Landry at AudioSpecialtiesOfNM.com. I sourced the wood from Rockingham Display based Leicester, England. They ship all around the UK and are used to cutting and machining wood for loudspeakers as they supply a major loudspeaker kit company. The obvious options for materials are MDF or birch ply. Either way 25mm thickness wood is recommended. The open baffles and bass boxes can be made from different materials, it is your choice. Ply is a little harder to work with but requires less finishing whereas MDF needs veneering or painting. The shape of the open baffle is not sacrosanct so there is considerable scope for your own interpretation of the visual aspect of Prometheus. I built them strictly according to the suggested design just to make sure I would not compromise the sound in any way.  The main guideline is that the baffle around the 12 inch wideband driver needs a minimum of 8cm from the edge of the paper cone to the baffle edge. This means that all sorts of curvy shapes can be accommodated and I am rather partial to curves but that's another story! I have thought about have the baffles machined from Corian, this could look really special.


Assembling & Finishing The Baffles Plus Bass Cabinets

I am not a skilled woodworker, in fact far from it so the pre-cut wood from Rockingham was a godsend. All I needed to do was to glue it all together and use a detail sander to smooth off any small excesses. When I say, "all I needed to do was to glue", well it ain't that simple. I don't have large clamps to hold together the panels while they dry so I need a different approach, I used wood dowels that were 6mm x 30mm to position the panels. I drilled holes for the dowels with a hand drill, as I said, I am not a skilled woodworker so the holes I drilled were sometimes 1mm to 2mm out. I found that the dowels could be easily shaved with a craft knife such that the panels could be made to line up well enough. Interconnecting all the panels this way meant I could test assemble all the wood fully prior to gluing, what is more the interconnected panels did not require clamping as everything was a good firm fit. As I have said, I am not a woodworking expert so this may not be the ideal approach but it was quite quick and it worked for me. Once the wood was glued and it dried overnight all I had to do was use a sander to make sure the joints were smooth to touch as well as lightly sanding all the edges to make them less vulnerable to damage. Do not sand the edges if you plan to use veneer. If you are experienced with using veneer you will not need my novice woodworking advice, so just ignore the above!


With ply you can use oil or wax as a finish, I used Ronseal Brushing Wax. I've since built a ply plinth for my Garrard 301 turntable, I used Danish Oil for this, it needs at least 3 coats with drying time in-between and a very fine sanding between coats. I preferred the oil finish; it brings up the grain a little more, is slightly richer in color and is smoother to touch. There are lots of ways to finish these loudspeakers; you can find plenty of photos at the Bastani and US websites for inspiration. When you have created your own masterpiece add some pictures to Robert's collection.

Finally you need to position the drivers, screw them down, fix the binding posts to the baffles, solder the padding resistor to the wideband and solder the crossover capacitor with its bypass to the tweeter. Good quality hook-up wire is provided.


The First Sounds

This review has been the longest-term review I have done, these loudspeakers have been in my system for over a year now so my impressions are most definitely from long-term listening. This is just as well as I was advised that Prometheus needs at least 200 hours before they come on song and possibly more like 1,000 hours before the process is fully complete, if indeed it is ever complete! The oil and lacquer are no doubt responsible for much of the break-in time.

Using a test CD I found that brand new the bass cones did not move when using a 17Hz test tone, wow they were tight! After a few hours there was considerable movement. The break-in requirement is for real but the first few hours are just the beginning. Playing music at normal levels is mostly sufficient for break-in although it is advisable to pump some real power through on a few occasions to get the suspensions loosened up.

What I hear during the break-in period? For the first 15 hours the central image was quite diffuse and there was little emotion portrayed. The sound was a clinical. Don't panic! The sound will develop. At 30 hours I found the bass came and went. At 70 hours the loudspeakers started to sound "right". So I thought the sound couldn't change much more. I was wrong, I reckon that after a year they continue to improve but now the changes now are small. I would certainly suggest that no one forms a definitive opinion on these loudspeakers until they have 200 hours on them and 500 hours would be more like it.

Most of my listening was done with 300B single-ended triode amplifiers of 7W, 8W and 9W. The 7W was the Welborne DRD, the 8W was the Diyhifisupply Ladyday '91 and the 9W is the Ladyday original, or Billie to be precise. I also used a WAD KaT 34, which is a 32W EL34 push-pull amplifier with which I also use with KT88 power tubes. 32W is way more than these loudspeakers need but it was useful to push them to high levels for break-in purposes a couple of times, even though I had to leave the room as the sound pressure levels were too high! The EL34 amplifier was quite a surprise; its performance seemed to be lifted considerably by these speakers. During the initial break-in period I much preferred the EL34 amplifier with its Audio Note Copper PIO coupling capacitors, the warmth they gave was desperately needed. The Ladyday 9W with Audio Note Silver coupling capacitors was much too clinical during this period as the loudspeakers were very bass-shy. Not to worry, the expected pecking order was restored later.

When I reached 200 hours it was time to assess the speakers properly. By the way, how nerdy is it to keep notes of how many hours loudspeakers have been used for? Anyway, I found that in my room the bass was best balanced with the bass boxes against the back wall and the open baffles a little over one meter from the back wall. Any changes I made to the system electronics were phenomenally easy to hear. The sound from Prometheus floods the room, it is rich and detailed with treble detail of a quality that is for sure up with the some of best tweeters. Bass performance was a step up from my two REL Storm subwoofers. On Dylan's Lily, Rosemary and the Jack of Hearts from Blood on the Tracks there's a fast underlying bass line that is often mushed out of existence, not so with Prometheus. Compared to the best subwoofers the very deepest bass is missing, I believe this intended so as to make room matching easier. In reality I noticed just a few deep bass notes missing on maybe just 4 or 5 albums. This is of very little significance.

Vocals are superb. I can make out words on certain songs when previously I couldn't but much more importantly the micro dynamics are there to deliver emotion. With easy drive and high sensitivity, these speakers enable low-powered amplifiers deliver wonderful performance even with complex orchestral music or with more contemporary music such as Goldfrapp (Train from Black Cherry) exhibiting tremendous power and drive. Throw away any pre-conceptions about low-powered tube amplifiers; many would assume the sound was from a solid-state muscle amp but with surprising speed and delicacy. To be fair much of the bass is provided by the 150W amplifiers, so it is not all down to low-powered tube amplifiers.

Even though it's easy to hear any change made to the system these loudspeakers are not especially unforgiving. The sheer amount of treble information being presented made me worry a few times such as with Tchaikovsky's Trepak (Dutoit on Decca), I felt I knew what was coming and it was going to hurt; it didn't. There are bags of top-end information and energy without discomfort. Wonderful.


Time To Tweak

Exchanging a few emails with Robert revealed some tweaks to try. Whether or not you hear a big difference with some of these tweaks will depend on your room, your system and how they interact. Some of these tweaks are just sensible measures to take, even if they don't have much effect you'll feel all the better for doing them. Some are fiddly or even impossible to reverse so I cannot be totally sure of the extent to which they improve the sound.


Most amplifiers have their negative loudspeaker terminal connected to ground. What you do is to attach a wire to the metal tweeter housing and to the metal basket of the wideband, there are some useful small holes in the wideband basket. Ensure some paint is scraped away to allow electrons to flow. Finally connect the wire to negative loudspeaker binding post. I can't honestly say I noticed a big repeatable different but it is easy to do and cannot do any harm.


Take some old fashioned carpet felt or some thing similar, cut it into shapes resembling the main struts of the wideband basket and glue these inside the struts with a something like a PVA-based glue. I remember telling myself, don't get any on the paper cone or anywhere that isn't metal and do not let them rub the cone either. I expected a slight increase in clarity and this is what I believe I achieved but it is difficult to reverse to verify this.


Magic Twins
These are plug-in or solder-in devices that are presumably a zobel network or something similar, they can be plugged into or soldered into preamplifiers, power amplifiers and loudspeaker binding posts. They are Radio Frequency filters, reducing RF should smooth the sound, I found they made an improvement. Whether they work for you will be highly dependent on your environment. Some assembled loudspeakers come with a pair of Magic Twins, for others and kits they are an option.


Dipole Tweeters
If you have the Gemini tweeter you can run these as dipoles. As standard they have a plastic back attached with 4 hex bolts. These bolts can be removed to expose the tweeter diaphragm on the back of the compression unit. Be very careful! Do not damage the delicate back of the tweeter. The normal mounting of the Gemini tweeters with plastic backs uses a wooden disc to clamp the tweeter to the front-facing horn. Removing the rear cover from the tweeters results in needing a different clamping approach. There are some lovely donut shaped supports I have seen photos of. If you can make these then this looks like a good approach, but make sure it is a donut shape with a hole in the centre, not a jam filled one!

I used a cruder method but it is easy to do. I bought some bolts of a similar type to the original threaded studs that clamp the wood discs to the tweeters. 30mm washers are placed under the bolt heads, these sit at an angle to hold the tweeters in place. The bolts should be cut to length so that they don't punch holes in the front of the horn. Place washers against the back of the horn and secure these with a nut. The bolts will splay outwards without this and may wreck the wooden horn.

Standard Tweeter

Dipole Tweeter

Does going dipole make a difference? YOU BET! Their already very good treble becomes excellent. There is loads of "air", treble floats around the loudspeakers as though it has been set free. It has been. Be warned, Robert tells me that if your system does not have top notch treble this could a tweak to far. I didn't have any problems, with vinyl or CD.


Crossover Frequency & Jupiter Capacitors

If you find you need more treble you can add capacitance to the 1uF capacitor on the tweeter, you can try adding 0.1uF or 0.2uF, this will result in significantly more treble. The standard value should be fine in the first place. My kit came with Obbligato capacitors, which are very good quality. Robert suggested trying Jupiter beeswax capacitors and I had the correct value. I would not classify the difference as huge but tonality and clarity were slightly improved with the Jupiters.



I have not tried this yet but it sounds eminently sensible. On the widebands there is a gap between the magnet and rear of the basket. Fill this gap with epoxy. Making the basket more rigid cannot be a bad thing. I must do this sometime soon.


Final Set-Up

During the break-in period it is useless trying to set-up the loudspeakers properly, they keep changing their characteristics. With at least 100 hours under your belt you can start but be prepared alter the set-up as they break-in some more.

The bass units can be housed on the platform behind the open baffles with the bass drivers pointing inwards or they can be placed in a traditional subwoofer position.  In my room I find best sound is best with the bass units in the rear corners of the room, the main difference is that the bass goes deeper. Your room will be different to mine so make no assumptions based on my experience except that if you already have an ideal position for a couple of subwoofers then this is likely to be best for the Prometheus subwoofers too. Robert will advise on setup by email, he is very helpful indeed and frighteningly knowledgeable. You can also use his support forum on their website.

The general advice for the subwoofers is to start with level, frequency and phase controls at their top centre position and tweak from there. I've ended up with phase at 180 degrees with level and frequency at about 4 minutes to the hour. I run the open baffles 1.3 meters from the rear wall, the distance from the rear wall defines the image height and depth of image. I now run the open baffles totally straight-on. Before dipoling the tweeters I needed a slight toe-in. In general the set-up is very forgiving, there is no need to be paranoid about positioning Prometheus to the last inch.


Further Listening

With more like 1000 hours of music played through the loudspeakers I can report that they improved from their already very high standard. I found that dynamics can be explosive, they have the ability to scare, more than once I replayed the start to Ugly Papa by Dana Gillespie just to sure I was not imagining the dynamics. Micro-dynamics are also very present, subtle inflexions in vocals are impressive, this develops emotion and significantly adds to musical involvement. The soundstage extends all across the room, way outside the loudspeaker boundaries with good focus in the centre. Prometheus really achieve that audio cliché; a disappearing act. One more trick they perform is to go loud without this being obvious, that is until you try to talk to someone in the room, and then you find you have to shout.

Sound-wise Prometheus has everything, dynamic, emotional, detailed and easy to drive. Looks-wise they could be more attractive but there is a lot of scope for individual tailoring of their looks. The way I chose to build Prometheus will not be ideal for many homes or marriages but you can make your own choices about baffle shape, materials and finish. I chose simple and effective, you might want something more aesthetically pleasing.

I could quote lots of other music that impresses with these loudspeakers, that is because it all sounds fabulous. What I want to get across is the overall effect of setting the sound free when compared to traditional box loudspeakers. The effect is very similar to the spacious sound created by Quad Electrostatics. I could try to describe this in detail but until you have heard the effect you won't totally appreciate what I'm talking about. Whatever, it works for me!  Add impressive dynamics, deep, powerful bass and airy treble to the Quad sound and you aren't so far away from knowing how Prometheus Airforce mkII performs.

It is easy to hear small system changes through Prometheus but strangely if something isn't quite right in the system it is possible to ignore this. This seems to be to do with the soundstage and space these loudspeakers produce. They major on producing a natural sound, not an impressive "hi-fi" sound. They do not add a tonal character of their own, some loudspeakers always produce sounds with a particular character, Prometheus do not. When changing recordings it is possible to go from a bright sound to a dull sound, this is totally dependent on the recording, not the system. Rather than listen to how impressive the sound is I find I marvel at the music. Oh yes, they are easy to drive too. I am hooked on open baffle, high sensitivity loudspeakers and the way that they release the best sounds from my single ended tube amplifiers. I would have liked to try Prometheus with 2A3 and 45 SE amplifiers. I'm sure that 3W to 4W of 2A3 power would be plenty, whether 1W to 2W of 45 power would be enough I suspect would be a closer call.

If Prometheus will fit with your living environment then I wholeheartedly recommend them. If they don't fit in your environment or with your marriage then still buy them and fix the problem. They are that good.



Sub-bass (10Hz - 60Hz)

Mid-bass (80Hz - 200Hz)

Midrange (200Hz - 3,000Hz)

High-frequencies (3,000Hz on up)



Inner Resolution

Soundscape width front

Soundscape width rear  
Soundscape depth behind speakers

Soundscape extension into the room


Fit and Finish

Self Noise


Value for the Money



Type: DIY Loudspeaker Kit

Tweeter: 1 inch compression, horn loaded, 16 ohms

Wideband: 12 inch in open baffle, 100Hz to 11kHz, 8 ohms

Woofer: 12 inch in sealed cabinet

Sensitivity: 100dB/W/m

Power Handling: 1 to 150 Watt

Weight: 68lbs for mains, 36lbs for woofer.

Subwoofer Amplifiers: 150W, 30Hz - 160Hz crossover, 24db/octive, 3db boost at 40Hz, subsonic filter at 25Hz, 24db/octive
Auto switch-on, line and loudspeaker level inputs, 115V and 230V operation,
Phase 0-180 degrees

Price: Varies, see article


Company Information

Robert Bastani
Am Pfaffenbrunnen 7
55296 Lörzweiler

Voice: +49 (0) 178-7117925
E-mail: info@bastanis.de
Website: www.bastanis.com













































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