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  ---  Description   ---   Supplies  ---  FAQ  ---  
Photos/Comments  ---
  DSP challenge  ---  System  ---


Supplies for LX521 construction 
and system assembly

The LX521.4 loudspeaker system can be configured from different components. In all cases the acoustic platform must be as I designed it, as well as the crossover/equalizer transfer functions, which determine the signal level and phase for each driver in the acoustic baffle. Baffle shape and drivers define the all-important polar response of the loudspeaker. 

April 2017 - Here is an update to show where I am with my two LX521.4 systems, which are set up in different homes and rooms. I remind you that I design and build loudspeakers for my own satisfaction and pleasure, striving for a maximum in realism in a normal living space from a well done recording. The DSP System below was my starter. The crossover/equalizer transfer functions where derived from acoustic free-field measurements and implemented by a miniDSP 4x10HD.

The ASP System below came about when I had exactly duplicated the miniDSP 4x10HD derived transfer functions in analog circuitry using operational amplifiers, resistors and capacitors. It started as an engineering exercise to eliminate the digital signal processor and have a loudspeaker, which operates from L and R electrical input to acoustic output in the analog signal domain with which I am most familiar. Of course the input signal itself has most likely gone through digital processes these days. But at least there is a clear dividing line.

Performance wise my reference has been the PB6 System below. The ASP.4 and PB6 systems match to a degree where it is difficult for me (I am not the youngest person anymore) to hear a difference or to say which I prefer. But when I play files from a hard disk HD directly into the front panel USB port of the Sonica DAC, then I am able to hear differences between a 96kHz/24bit file and a DSD file of the same recording. When I play the same files from the HD of the PC, then JRiver converts the DSD file to 96kHz/24bit and I hear no difference. The ASP System has thus become my highest resolution system.

To come to this point in my system developments took three specific components:

  • ASP.4 analog signal processor for crossover and driver equalization functions
  • AT528NC power amplifiers for complete sound neutrality and bass control
  • afi+USB isolator and reclocker for phantom image stability and sound delineation





Mako Natsume

Hello Siegfried,

My plan is to offer five PCBs as the standard configuration. Two boards are the ASP.4s, another is a power supply that can be configured to accept an external Mean Well +/-15 GP50A14E-R1B  a wall wart like the ADU160100Z5401 offered by Jameco  (US, Canada) purely for convenience and clearly not optimal, or an externally housed center tapped transformer like the Hammond 1182J15 that can also connect through the DIN plug (legends on the panel will be changed from below). The power board also has an option for balanced inputs. The other two boards are panels that will fit the Hammond case I sent to you but with neatly countersunk holes. It is intended to double as a template that users can create their own panel with.

The ASP.4 circuit diagram has been shown on the LX521.4 owner support page since 12/2016. See the LX521.4 Store. Support questions of OPLUG forum registered LX521 documentation owners can be posted at the dedicated LX521 ASP Reworked  thread.



New 2021: ASP LX521.4 v2.0 with Cascaded Topology: 

Phase issues in purely parallel crossover topologies, typical in previous ASP LX521.4 versions, prevent proper summing to a flat frequency response. Find more details on the topic in Siegfried Linkwitz' "Crossover topology issues"

Siegfried Linkwitz' statement "An optimum crossover filter topology should be the starting point when the outputs from multiple drivers with different frequency responses and sensitivities need to be combined for a flat acoustic response " was the basis for the new edition of the cascaded, precision ASP LX521.4 v2.0.:


More about improved design details of LX521.4 v2.0 can be found here.


Before shipping, the add-on circuit for alternative Lower Midrange driver LINKWITZ22MG can be activated upon request. Input and output plugs are XLR. RCA-to-XLR adaptors available.

The AT528NC Class D, 8x 200W, power amplifiers are essential to my ASP System's perfection. They have extremely low distortion even at low output power levels where much of the music detail is happening. Yet the amplifiers also have enormous output current capability to keep the woofers under full control.



Finally there is the USB isolator and reclocker. I am still surprised that it is needed in my systems. My primary source for sound is a notebook PC's USB port. The PC travels with me between the two LX521.4 systems in different locations. I thought a Sonica DAC would not benefit, but it does. The DAC performs flawlessly for files from a thumb drive plugged into its front panel USB port. It processes DSD files directly and without converting them to 96 kHz PCM files as does JRiver in my PC. I was surprised to hear differences between DSD and 96k/24bit versions of the same recording.

This device points to problems with the digital data and spurious signal streams coming from the USB port of my PC. I will investigate this. The engineer in me wants an answer. The writing in German or in English on the afi website is not very clear about the internal workings of the machine. And I do not know enough about digital bus traffic. I cut open a USB printer cable and found loosely twisted data lines and shielding that is only connected to the metal connector shell at the source end (as specified!). That makes the "shield" a nice antenna for EMI injection! The data lines are probably poorly terminated, which will cause reflections and thus jitter.

But in the meantime I am happy that the afi+USB does such a great job.




The LX-Store informs you about the components that are available for building or buying an LX521.4 System.

The LX521 Construction Plans are your license and access to up-to-date Owner-Support information and the closed pages of the OPLUG owner support forum.

More information is found in the older part of Supplies for LX521 construction and system assembly below:




Supplies for LX521 construction 
and system assembly

What do you need?

  1. Construction Plans
  2. 2 x Top baffle for midrange and tweeter
  3. 2 x V-frame for woofer
  4. 2 x Woofer bridge
  5. 2 x 6 Drivers (2 tweeters, 1 upper mid, 1 lower mid, 2 woofers)
  6. 2 x Passive crossover (4 mH inductor, 12 mF capacitor)
  7. Wire connections to the baffle mounted drivers, crossover and terminal connectors
  8. Analog Signal Processor and separate preamplifier with remote volume control, or:
    Digital Signal Processor with built-in IR remote volume control capability
  9. 6 x 180 W ATI power amplifier, or 12 x 60 W ATI power amplifier, or equivalent
  10. Speaker cables and shielded interconnect cables 

Details for above items, what to build yourself or where to buy parts and assemblies, are given below. 
I sell Construction Plans and blank printed circuit boards for the ASP.  I can order and have ATI power amplifiers shipped to you from the factory at a special price. An order form is at the bottom of this page. Everything else can be supplied by others, if you own Construction Plans. For cables and interconnects see FAQ8.

The LX521 was designed for easy DIY construction and without sacrifice to acoustic performance. It is important not to change the baffle contours, the frequency response of the ASP, or the passive crossover. If you want to experiment or have ideas about improving my design, then I recommend that you first build and listen to the LX521 having followed the instructions in the Construction Plans. Support of the LX521 loudspeakers is provided via a web page to which owners of the LX521 Construction Plans have access. Additional construction and test information, revisions and updates are posted on that page. Check it out before you start building.

When building the speaker cabinets yourself, you will need to have some woodworking capability. At  minimum a circular saw and a saber saw are required, but not a router. Alternatively, NC machined wood panels and partially assembled cabinets are available from different suppliers. For the crossover you must load and solder components onto a printed circuit board, and connect wires where indicated. The finished assembly must to be tested electrically to avoid using the costly drivers as fault indicators. A digital voltmeter, a 20 Hz to 20 kHz sinewave signal source and AA-cell batteries are sufficient for spot checks.  Acoustic measurements are not necessary, because of the consistency in production of the SEAS drivers used.


Construction Plans

The 34 pages of the LX521 Construction Plans contain:

  • Drawings for all baffle parts 
  • Mechanical assembly instructions
  • Driver and crossover wiring instructions
  • ASP crossover/equalizer circuit description and diagram
  • Material list for electrical components
  • Loading chart for printed circuit boards
  • Electrical assembly instructions
  • Electrical test instructions
  • System turn-on and test
  • URL of the LX521 Owner Support Page

The LX521 Construction Plans are the license to build the loudspeakers for personal use. They are also required when subassemblies are purchased from other suppliers.


Estimated cost of materials (prices of 2013)

Construction Plans .................................................... $155
4 Tweeters, 4 Midranges, 4 Woofers ....................  $1600
Wiring, connectors, passive xo, speaker cables ..  $200
Wood, screws, glue, paint .......................................  $200    
miniDSP 4x10HD ....................................................  $520
----------------------------------------------------  Subtotal   $2675
6x180W Power amplifier ........................................  $1850
---------------------------------------------------------- Total   $4525


Blank Printed Circuit Boards for the ASP - check with <www.LINKWITZ.store>

Printed circuit boards are essential for assembling the Analog Signal Processor electronics. The ASP divides the incoming left and right channel audio signals into woofer, midrange and tweeter channels for driving the power amplifiers of left and right loudspeakers.
The LX521 ASP is assembled on the most recent ORION ASP printed circuit board. It supports balanced and unbalanced inputs. Tweeter, midrange and two woofer output connectors are unbalanced. Outputs can be attenuated to match power amplifier voltage gains if they are not identical. Silk screened component outlines and numbers on the pcb simplify loading the board. The board material is FR4 with two layers of 1 oz copper, has plated through holes, top and bottom solder mask and top silk screen. It is 0.062" thick and measures 7.6" x 7". This industrial strength circuit board makes for easy soldering of components. A praxis session is strongly recommended in order to learn temperature and timing to get to wet and shiny looking solder joints. Cold solder joints, solder bridges or burnt pads and damaged components can turn into a nightmare when trouble-shooting a dysfunctional circuit. Two boards are required for the ASP. (These boards may serve either for your DIY  ORION ASP or "vintage LX521.3-ASP" project)

           LX521 ASP proto



   Assembled DSP and LX521 Configuration File

Instead of the discontinued ASP a Digital Signal Processor can be used to generate the appropriate woofer, midrange and tweeter drive signals for the power amplifiers, which are connected to the speakers. 
A Configuration File is provided with the LX521 Construction Plans, which needs to be loaded via USB from a computer into the DSP unit. The miniDSP 4x10 Hd is ready made with both analog and digital inputs and has volume control. The unit can be trained to recognize Sony/Philips IR remote controls. It thus also replaces the preamplifier, which is required in front of the ASP above. But the DSP lacks an indication of volume level. 

You need to purchase:
1 x  miniDSP 4x10 Hd
1 x  4x10 plug-in firmware to structure the DSP for XO/EQ applications


   Rear of miniDSP 4x10 Hd

   See DSP challenge for alternatives



ATI Power Amplifiers

For many years I have been using ATI power amplifiers. In their product line the 12-channel AT6012 power amplifier is well suited to the LX521.  Each driver would have its own 60 W stereo amplifier, which can provide as much peak current or peak voltage as I would ever want in my 100 m3 listening room at The Sea Ranch. 

The AT6012 is rated at 60 W per channel into 8 ohm FTC full bandwidth output power, with all 12 channels driven simultaneously. The power supply has high current reserve and a 2000 VA transformer. A pair of LX521 speakers utilizes 8 amplifiers, the LX521.4 would take 10 amplifiers. A separate amplifier channel is allocated to each of the 10" dipole woofer drivers to obtain output capability that is commensurate with the midrange, yet minimizes bottoming and the risk of mechanical damage to the woofers, or having to reduce the speaker's low frequency extension. At very high sound levels the amplifiers clip first, which gives a clearly audible warning to turn down the volume. 

The AT6012 exhibits good low power intermodulation distortion performance as seen in the graphs above. It is primarily 3rd order at 1 W in response to equal amplitude input signals at 1 kHz and 5.5 kHz driving an 8 ohm resistor. Harmonic distortion products are always of lower amplitude than intermodulation products. Unlike for many Class A/B amplifiers the distortion does not increase as power goes down. A 100 mW output level is not unrealistic. Early transistor radios had 50 mW output stages and made quite a bit of noise.

The AT1806 is a six-channel power amplifier rated at 180 W per channel. It can drive the 3-way LX521, if the two 10" drivers in each woofer baffle are connected in parallel to one amplifier. The parallel drivers present a low 2 ohm impedance to the power amplifier around 100 Hz. The power amplifier driving this load must be capable of at least 200 W into 4 ohm.  The AT1806 is capable of producing higher bass output volume from the LX521 than the AT6012 and handle extreme loudness peaks. The DSP crossover/equalizer requires no modification for its use. 
But, the higher power has the potential to bottom out and damage the woofers below 40 Hz and to overheat or stress LXmini woofer/ midrange and tweeter with high level test or transient signals. I never had a failure with program material even at extreme volume levels.

The intermodulation distortion performance of the AT1806 at low power levels is exemplary.

The AT1202 is a two-channel power amplifier rated at 120 W per channel. It would be needed to drive left and right LX521 tweeters, if the  AT1806 powers each dipole woofer driver individually and also the LX521 lower mid upper midrange drivers with their passive crossover. The AT1202 is a little safer for the LX521 tweeter than the AT1806. In any case caution is advised with such high power amplifier setup. A second AT1202 would be needed for the 10-channel LX521.4 to drive the upper midrange drivers.

The intermodulation distortion performance of the AT1202 at low power levels is very good.

Note that my power description is for peak voltage equivalent power. A 2 Vpeak sinewave at 1 kHz and a 2 Vpeak sinewave at 5.5 kHz superimpose and fluctuate at a 4.5 kHz rate as seen on an oscilloscope. The fluctuation has 4 Vpeak amplitude, which I call equivalent to "1 W" for the conventional single frequency sinusoid of 4 Vpeak or 2.83 Vrms across 8 ohm. Similarly the "100 mW" description is for 1.26 Vpeak of the superimposed sinusoids. The actual dissipated powers are 500 mW and 50 mW into 8 ohm. 
Harmonic distortion products are below 0.01% for the three amplifiers measured. But musical signals are rarely single sinusoids. Intermodulation distortions are always higher than harmonic distortions and their spectral distribution is in my opinion a better differentiator between amplifiers than a single harmonic distortion number.

ATI making an amplifier -- ATI sales story



Other Power Amplifiers

A wide range of power amplifiers can be used for the LX521. The primary requirement is that they all have the same voltage gain, otherwise you need to adjust the gain defined by the LX521 configuration file for the miniDSP 4x10HD. I have heard good things from a trusted source about the Outlaw 7075 7-channel amplifier, which measures similar to the AT6012.  Here are examples for which I have some measurements.

amp-parasound-a51.jpg (72933 bytes) amp-Pass-proto.jpg (78739 bytes) amp-pluto-90mW.jpg (79342 bytes) amp-pluto-470mW.jpg (73141 bytes) amp-UPA500-100mW.jpg (79478 bytes) amp-UPA500-470mW.jpg (75159 bytes) amp-miniX-100mW.jpg (74360 bytes) amp-miniX-480mW.jpg (76218 bytes)
Parasound A51 Pass Labs proto Pluto LM3886 Pluto LM3886 Emotiva UPA-500 Emotiva UPA-500 Emotiva Mini-X Emotiva Mini-X
amp-Outlaw-7075-1W.jpg (61732 bytes)
Outlaw 7075
amp-Hypex-UcD180.jpg (76780 bytes)
Hypex UcD180
BehringerA500-1W.jpg (45489 bytes)
Behringer A500
BehringerA500-100mW.jpg (51661 bytes)
Behringer A500
amp-test2.jpg (85364 bytes)


Internals of Outlaw 7075 and Dave Reite's UcD180 based 8-channel DIY power amplifier:


Power amplifier related links:

Amplifier and/or Driver limited Sound Pressure Level

Amplifier Distortion

Speaker cables and interconnects



Drivers, Wood Flat-pack & assembled ASP

https://LINKWITZ.store in Germany offers a complete, cosmetically improved CNC machined wood flat-pack and parts kit, including all drivers, for building the LX521 Fully hidden cabling. Different wood finishes are available.
They also offer turn-key LX521 systems with everything you need.
Just connect your music source (CD-player, turntable, ...) <

Contact Frank Brenner after having studied the www.LINKWITZ.store website for your needs. LINKWITZ.store ships world-wide with DHL, UPS and other carriers.

Madisound, the distributor of SEAS drivers in the US, offers a complete LX521 Driver Kit of woofer, midrange and tweeter drivers.  Madisound offers a NC-machined flat-pack of wooden parts for DIY assembly of the baffles. 
Contact Adam Johnson.
Contact Dave Reite about availability to build a fully assembled and tested ASP crossover/equalizer.


Buyers of cabinet kits, pcb or assembled pcb's must own LX521 Construction Plans ($155). 

See below for ordering, shipment cost and payment.


Ordering information 

To order the LX521 plans, choose from plans, CDs downloads (or navigate to LINKWITZ.store or Madisound)





What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the streams of superimposed air pressure variations 
at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

Last revised: 02/15/2023   -  1999-2019 LINKWITZ LAB, All Rights Reserved