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Old 3rd May 2017, 08:53 AM   #1
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Default ER Audio 505 hybrid electrostatic build

Hi guys, I know there's already a thread on these electrostatic panels, but I wanted to document my build of hybrid electrostatic speakers based on E R audio 505 electrostatic mini panels.

This journey started when I came home with a pair of Martin Logan Electromotions and received an extremely low WAF rating! In my wife's defence the speakers did completely dominate the room in no small part due to their heights. I then went looking for something that might have similar or better performance but with a smaller footprint.

This search lead to me discovering the E R Audio 505 mini panels, so called because of their 505 mm height A good start given that the Martin Logans are 1.8m high!

Rather than boring you with the rest of my journey I'll jump straight in to the design and build process. Before doing so though I would like to acknowledge two mentors in particular throughout this project. The first is Rob Mackinlay from ER Audio who has been incredibly supportive, always responding to my countless questions with detailed and thorough responses. The second is John Watkinson who I am fortunate enough to be friends with. He is a highly published and well respected guru in all things audio and digital. I highly recommend reading his Slaying Dragons articles in Resolution Magazine.

I'll be the first to admit that I designed and learned as I went along during this build. I knew very little when I started and while I have much to learn still I am far more informed than I was a year ago!

The aim of the game with this build was to build active hybrids that stayed as phase linear throughout the frequency range as possible, trying to match the speed the supporting woofer with that of the electrostatic panels. The above two points excluded the use of transmission line, reflex port, and bass radiator designs.

One of the first things I learned from John Watkinson (and later Siegried Linkwitz) was that the best practical shape for a pressure chamber is a cylinder. They are far more structurally rigid than boxes and are far better suited to handling the fluctuating pressures experienced in a speaker cabinet. This is why submarines and aerosol cans are cylindrical.

When I first conceived this project I wanted to use 8"drivers to keep the speakers compact. The problem I ran into though was it was going to be difficult finding an 8" driver with a large enough xmax to hit 25hz in a sealed enclosure at a reasonable volume. I therefore opted for 10" woofers - 1 per side.

Click the image to open in full size.

The next step was to find a cylinder for these to go into as I didn't want to have to build up the cylinder with layer upon layer of ply. I looked into bendy ply but in the end took inspiration from Linkwitz' Pluto and went with 12" PVC pipe (10" was not readily available). I ended up finding a recycled length of storm drain pipe to use. Classy, I know...

Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.

Initially I went with SB Acoustics woofers as they are great value for money, but the speakers turned out so well that I later upgraded to Morel Titanium series woofers which are very expensive but very good. One of the attractions was that they have huge voice coils which minimising distortion of the woofer cone as the force is better balanced. The voicecoil is also aluminium which helps the woofers stay flat up to an incredible 10Khz.

Click the image to open in full size.Click the image to open in full size.

Orientation of the woofers was one of the decisions that I made as I went along.

Click the image to open in full size.

After speaking with John he advised that I fire the woofers upwards with the panels mounted centrally over them.

Click the image to open in full size.

The reason for this is that at the frequencies being reproduced by the woofers the response perpendicular to their axis was the same as the on-axis response. You therefore get an omnidirectional polar pattern which while not identical to the figure 8 pattern of the ER Audio panels is an acceptable compromise.

Click the image to open in full size.

The reason you mount the panel centrally over the woofer is that it ensures that both sides of the panel see the same pressure from the woofer, minimising distortion. A simple yet effective solution.

Last edited by Bosco Birdswood; 4th May 2017 at 11:02 AM.
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Old 3rd May 2017, 09:58 PM   #2
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Not sure why my linked photos aren't showing here. Should public Dropbox photos work?


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Old 3rd May 2017, 10:43 PM   #3
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I can only see them by right clicking; when opening in new tab dropbox shows up.
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Old 4th May 2017, 11:03 AM   #4
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Originally Posted by pieter t View Post
I can only see them by right clicking; when opening in new tab dropbox shows up.
Fixed
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Old 4th May 2017, 12:20 PM   #5
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Click the image to open in full size.

The frames for the panels are made out of bullnose oak skirting which I then profiled with a roundover router bit to reduce edge diffraction and for aesthetics.
Click the image to open in full size.
The crossover is a miniDSP 2x4HD using FIR filters to create a phase-linear Linkwitz Riley 12dB/octave slope. The panel has a bit of a low shelf boost added to the FIR to counteract the effects of dipole cancellation. The net effect just changes the shape of the crossover slope rather than boosting as such. The idea is so that the net audio effect is just that of the LR filter rather than the filter plus the effects of dipole cancellation.

The design of the base of the panel frames is such that it sits centrally over the woofer and can pivot on axis in the rebate it sits in over the woofer. I've then got bolts and captive nuts to secure them when toe in has been set. This is largely to stop my little guy from pulling the panels off the woofer enclosures!

Click the image to open in full size.
Click the image to open in full size.
The bases were made by painstakingly routing 32mm MDF in a series of processes that basically makes a ring with a centre strip to which the oak panel frames are screwed.

The speakers are closer to the wall than is optimal but they sound great. I reckon all up they owe me circa AU$3k but that puts them at about half the cost of the Electromotions, and I think they sound way better.

Click the image to open in full size.

Last edited by Bosco Birdswood; 4th May 2017 at 12:30 PM.
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Old 4th May 2017, 04:37 PM   #6
HankF is offline HankF  United States
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Nice woodwork!
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Old 4th May 2017, 09:36 PM   #7
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Nice woodwork!


Hah thanks! The main thing I impressed myself with was the panel base rings. The frames were easy once I bought a good quality mitre saw.

The cabinet boxes are far from perfect and I am already planning on some mk2 cabinets that are a bit less bulky and a bit more stylish. These also weigh about 40kg each...

Finally, the iron on veneer was a nightmare. Apply enough heat to make the glue bond and you dry and split the veneer.

I'm going to try spray contact adhesive next time.


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Old 5th May 2017, 06:06 AM   #8
WrineX is offline WrineX  Netherlands
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Cool !!! The idea of putting the woofer underneath the panel I thought about a few times but never tried(except for a tweeter), cool you made a pair , how do they sound? I hoped the way you made them the esl an woofer blend a little easier. Are they ?

Nice woodworking !!!!
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Old 5th May 2017, 01:51 PM   #9
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Originally Posted by WrineX View Post
Cool !!! The idea of putting the woofer underneath the panel I thought about a few times but never tried(except for a tweeter), cool you made a pair , how do they sound? I hoped the way you made them the esl an woofer blend a little easier. Are they ?

Nice woodworking !!!!


Hey thanks! Yeah the sound integrates really well between woofer and ESL because their axis are physically aligned and because their polar patterns are similar. The problem most speakers have is that their acoustic centres aren't aligned and the polar patterns are really different meaning they blend well in only a few positions relative to the axis of the speakers.

I highly recommend giving it a go. They sound great to me.


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Old 5th May 2017, 03:03 PM   #10
WrineX is offline WrineX  Netherlands
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Will try ! Thx
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