Pluto Loudspeaker for PA

[mayfly]

Hey Folks,

I'm currently off scratching a major itch of mine - designing and building active loudspeakers.  Myself and a co-worker are doing a bunch of work with drivers, boxes, active crossover designs, amplifier designs, measurement tools (mics, mic pres, noise generators, analysis software) with the goal building some truly world class stereo loudspeakers.

As part of our research we decided to get into some designs by a gentlemen named Linkwitz.  Although he never worked in the audio field (he was a microwave communications engineer with HP) Linkwitz is well known in snooty audio circles, with circuit topologies and many other 'classic' designs named after him.  He's also the designer of a famous loudspeaker called Orion which is used in classical music mastering and recording houses.  Interestingly you can get a kit to build a Pair of Orions, but even in kit form they cost as much as a freaking car to build.  For example just the drivers cost over $2000:

http://www.madisoundspeakerstore.com/3-way-speaker-kits/orion-3.4-linkwitz-lab-3-way-kit-pair-drive-units-only/

Fortunately he also designed something called the Pluto.  Plutos are such wacky things I suspect that he designed them almost as a joke.  Fortunately he also designed them to be inexpensive and easy to build.  Reports out there in internet land all say that they are great - almost as good as a set of Orions. Sounded good to us.

So we decided to build a pair.  More to come.

 

[Cagey]

Looking forward to this one. Always curious about speaker design - such a bottomless pit of variables.

 

[mayfly]

ok - Linkwitz's main thing is eliminating the box problems (resonance, beaming, phase alignment, response peaks) by eliminating the box.  His designs are open baffle speakers.  Doing this allows you to skip worrying about any box 'issues" and let you concentrate on just the driver issues.  So that's what Linkwitz does - he works on measuring the drivers in free space, then comes up with an EQ curve to compensate for any strangeness in the driver.  Phase differences between drivers (something called 'group delay' that I confess I only have a rudimentary understanding of) is compensated for at the crossover points.  Add some 24db per octave crossovers and some full bandwidth delay on the tweeters to time align them with the woofers and you should have a good sounding unit.  Or so the theory goes.

Sounds pretty complicated, and it is.  Because of this, all of Linkwitz's designs use active crossovers employing several stages of op-amp filter units.  Just the crossover and active EQ units of the Orion loudspeakers are thousands of dollars to buy.  Add those expensive drivers, six 'audiophile' power amps (remember we have 6 drivers to worry about) and some woodwork made by a guy who would normally make fine furniture for a living and you've got a set of speakers that cost you $20K.

As you can imagine the guy received some criticism for this.  I'm guessing some folks were saying something like "at $20K they better sound good" or maybe "those drivers would sound good in a shoebox - what's so special about what you do?"  But as a guitar player, the Orion always appealed to me because it was an open back design.  We all know that a Marshall stack has massive beaming problems and the off-axis frequency response really suffers.  But we also know that an old Fender super reverb does not seem to have the same issues; the sound is about the same everywhere in the room.  Translating to snooty audio land, a typical box loudspeaker is that Marshall stack all over again - leading to guys with one chair right in the "sweet spot" and speakers that are not really useful for ordinary people who have the music on while doing something else.  This is why the Orion always made sense to me - it's the fender super reverb of the audiophile world.  Of course I was never going to actually build a pair because of the stupid high cost.

I apologize for all that background, but I think it's important because it really lets you understand the impact of finding out that Linkwitz had another design that used drivers that cost a few hundred dollars, an electronic crossover/amplifier design that used 'chip amps' which you could buy the raw PCB for fifty bucks, and an enclosure that was made out of plumping pipe.  And the rumour mill said it was as good as the Orion.  This was cool.  On a whim I ordered the plans from Linkwitz's website.

 

[mayfly]

Here's what you get when you order the 'kit' From Linkwritz:  A set of plans, some raw PCBs, and a parts list using DigiKey part numbers.  That's it:

Here's what it looks like.  There are two drivers for each speaker; a tweeter and mid-woofer.  The electronics (active crossover and amplifiers) are in the base.  The design calls for 60W for the tweeter, and 120W for the woofer.

Here's a schematic of the PCB:

and a big pile of theory of operation:

 

[swarfrat]

Beamy they may be, but a good solid woody cabinet thunk is a beautiful thing.

 

[Cagey]

Those are some unusual drawings.

 

[mayfly]

Just wait until you see the speakers 

 

[mayfly]

ok the Pluto uses an active crossover with separate power amps for woofer and tweeter.  Linkwitz not only designed them, but he also designed a PCB for them and made them available on his website.  Very cool.  Here they are:

While waiting for the PCBs to arrive, I ordered up the parts from digikey using a spreadsheet available on his website.  That was my first mistake  .  Turns out that the Pluto is very much a living design and the drivers, PCB, and the components on it changed over time.  At first I was annoyed, but then when I got down to the brass tacks of it it turns out that only a bunch or resistors and caps had really changed - maybe $30 worth of parts.  At least he's always working on improving the design, which you have to admit is a good thing.  So, If you're building these, please make sure to use the up to date parts list before placing your big digi-key or mouser order.

Once the PCBs arrived I had ordered the correct parts.  The unit uses Burr-Brown op amps and LM3886 "chip amps" for the active stuff.  The chip amps are very interesting.  It's an integrated amplifier that can put out 50W with the right supply, and with 3db down points from 10Hz to 100KHz.  A pair of them can also be run in a kind of push/pull arrangement for 100W of power - pretty cool.  This is just what's done in the Pluto - a single LM3886 for the tweeter for 50W, and two of them at 100W for the woofer.  All that along with the power supply on a single PCB.  Once I was done it looked something like this:

That fits into the base of the unit.  Just plug in our RCA jack and plug it into the wall and you have power!

 

[mayfly]

Now onto the rest of it.  These things are made of plumbing pipe - one pipe for the tweeter, one for the woofer.  Here's mine:

The woofer literally screws to one of those 4" to clay rubber adapers:

The woofer is actually specially made for the Pluto by Seas:

Here's a shot of the electronics in the base.  I used plexiglass side panels so you can see the guts:

 

[swarfrat]

These things are made of plumbing pipe

Gives a whole new meaning to 'put that in your pipe and smoke it' doesn't it?

 

[TBurst Std]

This thread is awesome!

 

[AndyG]

I'd be really curious to know what your thoughts are on the sound of these speakers.

I've used SO many different speakers over the years, that I've either become cynical or immune ....  :glasses9:  In fact, I'm probably one of the few who actually likes the sound of NS10's!

Every design has its merits, and this one will probably come as close to a "source point" system as any.  The fact that the bass driver points up may look a bit strange, but bass frequencies diffuse much more than treble.  The high-frequency drivers will probably still need the 60 degree angle to create the true "phantom center", and therefore a good stereo image.

Cocentric designs never sounded right to me, and electrostatic speakers (which eliminate the box from the equation) I've always felt had a really "smeared" stereo image.

Looking forward to reading more!

 

[mayfly]

Hey everyone,

I'll do my best to describe how the speakers sound, but with the usual caveats of everyone's ears are different, everyone has different expectations and experience, and different hearing abilities etc.  Finally, words don't work very well to describe sound.

Having said that - here we go!

The speakers are really great.  They do all of the things that a good loudspeaker should do: They have a nice flat frequency response, no phase distortion that I can hear, they sound very natural and easy to listen to, very accurate, with good bass response.  But there's something more.  Because of the upwards woofer, these things have an omni-directional playback pattern in much of the audio range.  Not the dipole pattern of the super reverb or those ribbons that Andy talks about.  I suspect because of this the resulting soundstage of a well recorded piece sounds FREAKING HUGE.  You can literally be in any part of the room and it sounds like you're in a concert hall, but just moving around in the hall.  Get closer to one speaker, and the instruments in that speaker are louder, but you can hear with great detail the instruments in the other speaker.  Move back, things get quieter but still sound natural and in balance, move closer, things get louder but are still natural and in balance.  I don't hear any of the ribbon 'smear' that Andy talks about (although I have limited experience with ribbons).  Gone is the sweet spot the size of the chair.  It's really cool.  You have to hear these.

I've listened to a lot of material through these and I've discovered that they work best with things that are very well recorded and mixed.  Hack mixes don't sound so great.  I've got an old Telarc release of Holst's "the Planets".  Now that sounds great.  Katy Perry - no so much.  One rock recording that sounds amazing is Tom Petty's "Damn the Torpedoes".  The song "even the Losers" is one of my favorites because of how well the drums and percussion are recorded.  Play that song through the Plutos and it sounds like you've got those guys in the room with you.  You can walk around and get closer to the bass, and it sounds like you got closer to the bass.  Walk closer to the organ, and it sounds like you got closer to the organ.  Really, it sounds like you're present during rehearsal and can move around.  Another song that has the same effect is Joel Plasket's "Drunk Teenagers".  Fantastic.  However stuff that's been pretty compressed sounds kinda flat - like Foo Figher's "Everlong" or something.  It sounds fine, but you don't get that out of body experience that you can get with something really expertly recorded.

After doing all that, It was natural to try these puppies as the PA!  So I did.  First it was just my vocals and the guitar (via the AxeFX of course).  OK - the AxeFX through these things sounds fantastic.  Very natural, very expressive, and the sound is everywhere.  A complete win.  Then I used them for rehearsal with 4/5ths of the band (drummer was not present).  I left the 'old' PA in the room (but turned off) and didn't say anything to the group.  I was loving the sound, but I didn't comment.  Then halfway, Dosh (the bass player) said "I just have two questions:  Did you change something with the PA, and what the hell are THOSE things?"  .  Then I explained what they were and the girls were very enthusiastic.  Theresa (the singer) was especially keen "it sounds so full and natural".  Dosh liked the industrial nature of the design and the "it sounds like it's everywhere" effect. 

This week it's the full band with drummer.  I have concerns that they might not keep up with the drums, but we'll find out.  There's about 150W per speaker which is not really all that much so we'll see what happens. 

Trevor

 

[Cagey]

Ok. These have to be the most bizarre speakers I've ever seen. So... wtf? What's the theory? You're saying the reality is magical...

Lucy? You have some splainin to do!​

 

[mayfly]

It's pretty involved.  Try here first:

http://www.linkwitzlab.com/Pluto/Pluto-2.1.htm

 

[mayfly]

Had the rehearsal with drums.  Sadly the speakers could not cut it.  They could play loud enough, but during peaks they would overheat, the protection circuitry would kick in, and the bass drivers would shut off for a second or two.  Just not enough jam.

I had such high hopes too.  Not ready to give up on the concept though, because the sound is glorious.

I sense that I have a long and adventurous road ahead of me here.  Here we go....

 

[SustainerPlayer]

Bummer. Hope there is more in the pipeline so to speak. 

 

[MortenB]

Ok. These have to be the most bizarre speakers I've ever seen. So... wtf? What's the theory? You're saying the reality is magical...

Lucy? You have some splainin to do!​

The theory is, that you have omnidirectional radiation of the sound, as mentioned this gives a huge sound, and you can be in any location around the speaker and still get good and even response. Not like from a conventional speaker where the sound is only optimum in a small ''sweet spot''... The designer of these is one of the pioneers of this, and he is really, really talented.

 

[MortenB]

Had the rehearsal with drums.  Sadly the speakers could not cut it.  They could play loud enough, but during peaks they would overheat, the protection circuitry would kick in, and the bass drivers would shut off for a second or two.  Just not enough jam.

I had such high hopes too.  Not ready to give up on the concept though, because the sound is glorious.

I sense that I have a long and adventurous road ahead of me here.  Here we go....

I looked at the specs, and the little woofer has to cover the frequency range all the way down to 60hz... And that is your problem..! Just about all the energy in music (energy = meaning what is causing the biggest movement of loudspeaker cones) is on the range from 60hz and up to around 180hz. These 2 - 3 octaves cover all the ''fundamentals'' in music also sometimes referred to as ''the power zone'' because this is where amps need to deliver most of their energy. And speaker drivers ''convert'' most energy into cone movement.

One solution could be to cut off the low frequencies in the electronic cross over so that the small woofer in the speakers comes in at say 100 - 120hz or so instead of 60hz. You will then need a subwoofer to cover the range below 100hz. Relieving the small woofer of the lowest range will give you more headroom and you can go louder. The higher the cross over between the subwoofer and the speakers, the louder you can go because the sub covers more of the power zone.

To do this you will need a high pass filter option somewhere in the chain, to cut the low end before the woofers. Maybe there is already an option for this on the board from Linkwitz..!? Or maybe he can sell you a small ''add on board'' with this function.

 

[mayfly]

Had the rehearsal with drums.  Sadly the speakers could not cut it.  They could play loud enough, but during peaks they would overheat, the protection circuitry would kick in, and the bass drivers would shut off for a second or two.  Just not enough jam.

I had such high hopes too.  Not ready to give up on the concept though, because the sound is glorious.

I sense that I have a long and adventurous road ahead of me here.  Here we go....

I looked at the specs, and the little woofer has to cover the frequency range all the way down to 60hz... And that is your problem..! Just about all the energy in music (energy = meaning what is causing the biggest movement of loudspeaker cones) is on the range from 60hz and up to around 180hz. These 2 - 3 octaves cover all the ''fundamentals'' in music also sometimes referred to as ''the power zone'' because this is where amps need to deliver most of their energy. And speaker drivers ''convert'' most energy into cone movement.

One solution could be to cut off the low frequencies in the electronic cross over so that the small woofer in the speakers comes in at say 100 - 120hz or so instead of 60hz. You will then need a subwoofer to cover the range below 100hz. Relieving the small woofer of the lowest range will give you more headroom and you can go louder. The higher the cross over between the subwoofer and the speakers, the louder you can go because the sub covers more of the power zone.

To do this you will need a high pass filter option somewhere in the chain, to cut the low end before the woofers. Maybe there is already an option for this on the board from Linkwitz..!? Or maybe he can sell you a small ''add on board'' with this function.

Actually Linkwitz does offer a sub 'kit' for the plutos.  Will probably build these 'just because' in a few months.  I agree that the speaker has a lot of range that it needs to support, but we were just running vocals and guitars through it - not really sub frequency stuff.  But I'm going to try it anyway.