RiPol Subwoofer

last Changes :  2011 / 03/ 04

 

RStAudio RiPol Subwoofer

 

Table of Contents

 

Introduction

For many years I think about the idea to support my 57th Stacked Quads with a subwoofer. Since 2006 I had seriously started to learn about the design of this type of loudspeakers. For a long time my favorit system was the subwoofer from the Cumulus surround system introduced in the special edition Spezial 1 - Heimkino + Surround of the German DIY loudspeaker magazine Hobby HiFi. With the time my interests changes more in the direction of the open dipol subwoofer, e.g. the designs from Linkwitz Lab and the Disub 12/2 from the Hobby Hifi magazine 4/2007.

At the exhibition HiFi-Music-World 2008 I heard at the booth of JAC Music the loadspeaker RiPolar_05 designed by Axel Ridtahler with a full range speaker and a QBMC RiPol for the bass (BMC: Blow Motion Control, QBMC: 2 stacked BMC-RiPol systems). I was directly thrilled by this bass system, the concept of the design equates the dipol systems from above. The dimension of this system is astonishing small and therefore an ideal candidate for my small listening room. The sound was exactly like the description of the dipol systems, absolutely clean and coming out of nothing and also going into nothing – very impressive !!!

Directly after the exhibition I wrote an email to Axel and got a very friendly and informative contact with him. This ended up in an order from my side for 2 SLX-315 30cm bass chassis, the dimension of a BMC ripol for that chassis and a compensation network for that design. The price I have to pay was absolutely fair and definitely worth the money.

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Description of the RiPol Subwoofer

Before I start to describe my RiPol subwoofer I want to let the inventor tell us some words about his design. Following, by courtesy of Axel Ridtahler, an abstract of his general description of the RiPol subwoofers (sorry, it is only available in German) :

Das Abstrahlverhalten herkömmlicher Bass-Systeme ist, unterhalb einer gewissen Frequenz, kugelförmig. Ein Dipol ist physikalisch klar definiert und zeigt die typisch 8-förmige Abstrahlung. Der RiPol ist die Sonderbauform eines offenen Bassdipols.

Der RiPol hat, aufgrund seines speziellen Gehäuses, nicht diese exakt symmetrische Abstrahlung, sondern, wegen der nach hinten und nach vorne unterschiedlich abgestrahlten Schallintensität, ein asymmetrisches Rundstrahlverhalten, was aber auch einen vorteilhafteren Frequenzgang zur Folge hat. Da also die Abstrahlung beim RiPol nicht der – im physikalischen Sinne – klassischen Form der 8 entspricht, ist der RiPol eben kein idealer Dipol sondern eben ein RiPol. Die Rundstrahl-Diagramme belegen hier deutlich den Unterschied zum klassischen Dipol, was somit auch eine klare namentliche Abgrenzung rechtfertigt.

Das RiPol-Gehäuse nimmt starken Einfluss auf das Abstrahlverhalten. Man kann den Effekt erkennen, wie das 'Gehäuse' verhindert, dass die Luft der Membranbewegung allzu schnell ausweicht und dadurch der Strahlungswiderstand erhöht wird. Die Eigenresonanz der Treiber wird durch den Einbau in das BMC- (2 Chassis) oder DRS- (1 Chassis) RiPol-Gehäuse abgesenkt, und die Wiedergabegüte im Tiefbassverhalten dadurch verbessert, je kleiner dieses Gehäuse wird (!!). Nach unten sind da natürlich Grenzen, weshalb die Auslegung der RiPol-Gehäuse einer iterativen Optimierung, Berechnung und Simulation unterliegen. Die Anordnung mit 2 Treibern (BMC) realisiert zudem in idealer Weise die in diesem Frequenzbereich sinnvolle Impulskompensation, und das alles in einem optimal kleinen Gehäuse.

RiPole strahlen tiefe Frequenzen also gerichtet ab. Einmal nach vorne und mit anderer Intensität und 180° verdrehter Phase nach hinten. Diese Eigenschaft reduziert schädliche stehende Wellen. Das sind Energiespeicherungseffekte im Raum, auch Raummoden genannt. Stehende Wellen können im Hörraum den akustischen Eindruck von träger, nicht abklingen wollender Tieftonenergie erzeugen. Bassstarke Lautsprecher pumpen dann den Raum mit Bassenergie, allerdings 'frequenzselektiv' voll – es bleibt der Eindruck eines lahmen oder trägen Basses. Ein RiPol erzeugt gewisserma´┐Żen tieffrequente 'Antimaterie' die diesen Effekt gar nicht erst aufkommen lässt. Ein Bassimpuls wird abgesetzt und durch seine Gegenwelle wird – quasi von hinten – das 'stehen bleiben' verhindert. Das Signal wird nicht durch seine vielfachen Eigenüberlagerungen verwaschen. Diese Eigenschaft macht den RiPol zu einem der musikalischsten Sub-Systeme.

Und noch ein Effekt sei an dieser Stelle erwähnt:
Aufgrund der gerichteten Bassabstrahlung entstehen seitlich der RiPole Zonen, die frei von tieffrequenten Schallanteilen sind. Diese Bereiche sind der rechte Platz für mikrofonieanfällige Geräte wie Schallplattenspieler, Röhrenelektronik usw. Diese werden, neben einem RiPol, nicht von tiefen Frequenzen bei ihrer Arbeit gestört!

Der RiPol wird (wie jeder sinnvolle Sub) aktiv betrieben. Zudem ist für eine korrekte Funktion des RiPols eine passive Kompensationsschaltung zur Anpassung der Chassis an das Gehäuses erforderlich.

Everyone who is also interested in such a subwoofer system can contact the inventor directly under the following address :

www.ridtahler.de

Patent No. :    DE 198 30 947 A 1

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SLX-315 Bass Chassis & Compensation Circuit

End of November / beginning of December 2008 I got the ordered parts from Axel. Below you find the photos of the 30cm bass chassis

SLX-315 30cm Bass Chassis       SLX-315 30cm Bass Chassis

SLX-315 30cm Bass Chassis

and the associated compensation circuit

Compensation Circuit

Compensation Circuit for 2× SLX-315 30cm Bass Chassis

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Assembly of my RiPol

After I reached the construction details from Axel, I used a 2D CAD program (QCad with Linux) to make the drawings for the needed wooden parts of the RiPol and ordered it at a carpenter nearby.

21mm Birch-Multiplex for my RiPol

21mm Birch-Multiplex Blank for my RiPol

The drillings and cutouts I brought in afterwards with the help of a Datron M35 CNC milling machine.

Side Plate of my RiPol       Center Plate of my RiPol

Side and Center Plate of my RiPol

Thereafter all parts are purchased and preprocessed, now it was time to assemble the subwoofer. Although the design was not very complex the complete time for the assembly took around 4 days. During that time the wooden parts are glued, two aluminium brackets to hold the speaker terminals are produced, the screw holes for the spikes in the bottom boards are mounted, aluminium round rods to fix both halfs of the subwoofer together are sawed and on both sides screw threads are cutted, the finally glued wooden parts are coated with glaze, the drivers are mounted and wired and afterwards all parts are assembled.

Bottom Boards with Srew Holes for Spikes

Bottom Boards with Srew Holes for Spikes

Brackets for the Speaker Terminals

Brackets for the Speaker Terminals

Glueing of the Wooden Parts

Glueing of the Wooden Parts

RiPol Assembled

RiPol Assembled

 

At this point I have to thank – once again – my father, without his active assistance I was not able to produce the subwoofer cabinet in that perfection.

 

Afterwards I coated the cabinet with wood preservation glaze.

Cabinet Coated with Glaze

Cabinet Coated with Glaze

After the glaze was dried – I waited 24 hours for that – the final assembly of the subwoofer could started.

Mounting of one Driver

Mounting of one Driver

Wiring of the Driver

Wiring of the Driver

Front View of the Finished RiPol

Front View of the Finished RiPol

Rear View of the Finished RiPol

Rear View of the Finished RiPol

I use van den Hul CS-122 speaker cable for the wiring of the drivers. The same cable I have also in use between the power amplifier and the subwoofer.

This was my first speaker project and I learned that it is not so easy to work with wooden materials. Finally with the help of my father I got a very nice subwoofer which is an eye catcher in my listening room.

The subwoofer fulfils all the expectations I have. Certainly it may not be the right woofer for a video playback system with a lot of low frequency effects, but as a high end audio subwoofer for electrostatic speakers it is exactly the right partner. The bass is clean and fast and it comes and goes out of nothing (e.g. no ventilation noise). I'm very stoked and it was worth the effort to build this subwoofer.

Addendum:
Since more than 30 years I'm busy with audio equipment, but such a dramatical change of a device after some time of first use as with the RiPol I never experienced before. This subwoofer definitely needs some time, but thereafter it is

phenomenal !!!

All the above described positive characteristics have become much better and the range towards the lowest frequency is enlarged again.

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Electronic needed for the Subwoofer

Power Amplifier

A very long time I thought about the right power amplifier for this subwoofer. I started with a clone of the Pass Labs X600, but at that time I felt that it was to complex. Therefore I began to look for digital power amps and my choice was the Hypex UcD 400 module from the Netherlands. With his 400W at 4Ω it is an ideal partner for the RiPol. Anyhow I couldn't got used to the idea of such a technology within my audio system. I started to design a class AB amplifier with the help of the service manual of the Pass Labs X5 power amplifier with an UGS3 module at the input. In March 2009 I was able to buy a very cheap module of the Hypex UcD400HG and therefore the decision was made to give this technology a chance for a subwoofer power amplifier.

All further informations to this power amplifier you will find at the description of my Hypex Class D amp.

The compensation circuit for the RiPol is directly housed in the case of the power amplifier.

Active Frequency Cross Over Device

To use a subwoofer you need also a cross over network to divide the frequencies in the different parts for the loudspeaker systems which are in use. An active version for my design you will find here

The actual cross over frequency between the RiPol and my stacked Quads is 100Hz and 24dB for both Linkwitz-Riley filters (low and high pass).

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