DIY Visaton BG 20 Single Driver Full Range Speakers

This page is about design and construction of full range DIY loudspeakers with Visaton BG 20 speaker drivers. The BG 20 high efficiency full range drivers are excellent choice for homemade single driver bass reflex speaker designs. Read how to build the speaker box enclosures and notch filter circuit for the frequency response correction.

Visaton BG 20 Single Driver Full Range Speakers

Visaton BG 20 full range drivers:
  • 40 W nominal power (70 W max)
  • relatively smooth frequency response
  • manufactured in Germany
  • affordable
  • very good value compared to Visaton B200
  • high efficiency (92 dB)

The Visaton BG 20 full range bass reflex speaker design began from the necessity to relocate my leisure corner where I had previously done all my music listening. This wasn’t my first DIY speaker design as I had previously constructed some 3-way loudspeakers. They were an open baffle design with closed cabinets for subwoofers.

Those 3-way speakers were fine (Visaton BG 20 full range speaker design is superior tough) but you are mistaken if you think that you can build acceptable 3-way speakers without excessive testing. I actually spent hours and hours listening the subwoofer, midrange and tweeter until I had good crossover circuit built.

Due to the semi open baffle design the individual elements of the speakers were physically quite far away from each other. Audiophiles consider it to be really bad thing if the listener is near the speakers and its elements are placed far from each other, but I didn’t find it to be a real problem.

The time it took to get the 3-way speakers to sound good however was a problem, so I decided that single driver full range speakers is the way to go with my next DIY loudspeaker design.

Full range speaker drivers are designed to reproduce whole spectrum of audible sounds. Some brands and models are better at this than others, as the full range drivers are a compromise by default. As I had decided that full range speakers were the best solution for my DIY speaker design I starter to source for good speaker drivers.

I quickly found a local supplier who had some Visaton drivers in stock, and I started to search the web for more info. I quickly found out that Visaton BG 20 full range drivers would be perfect for the project (I now think they would be perfect for many homemade loudspeaker designs).

There are other good full range drivers in the Visaton product selection, namely the superb B200 speaker driver, but the BG 20 had a decisive characteristic for may project: it seems that the BG 20 offers excellent sound quality with superb value – perfect for my low-budget full range speaker project.

Visaton BG 20 Full Range Speaker Driver Visaton BG 20 Full Range Speaker Driver

Visaton drivers, including BG 20, are actually manufactured in Germany (not in China!) and the asking prize is very reasonable. BG 20 speaker drivers are only little worse in the highs than the B200 full range drivers but BG 20 are almost five times cheaper than Visaton B200 drivers.

There was no question for me: heart of my DIY speaker design will be good and affordable Visaton BG 20 wide band speaker drivers.

I chose to build the single driver bass reflex speakers as I had never built one before, and because I wanted a stronger bass response. It worked out great! In my opinion, bass reflex speakers were even easier to design than than sealed box acoustic suspension speakers.

You are welcome to copy my design for non-commercial purposes, or maybe you just get some ideas for your own plans. I will walk you through my DIY project starting with the next item:


Homemade Single Driver Speaker Cabinets

I started to plan the cabinets by choosing outer dimensions for the speaker box. I actually needed to fit the speakers to a specific location in my room, so some of the design decisions was made for me. Nevertheless, I think the size and proportions of the finished speaker boxes are excellent! The homemade single driver speaker front baffle measures 74 x 23 cm, depth of the cabinets is 34 cm.

Visaton BG 20 Single Driver Full Range Speakers_Front Baffle Detail Drawing

This HiFi speaker design has:
  • excellent compromise between modern narrow speaker design paradigm and good inner volume
  • pleasant proportions
  • inside volume 39 liters
  • good match for the Visaton BG 20

As I wanted to build superb HiFi speakers, I chose to use 18 mm board for the project. I didn’t want to use any protection grille for the Visaton BG 20 drivers and opted to use thick front baffle (double board material thickness).

Double thickness (36 mm) front baffle in this project is extremely rigid and enabled me to flush-mount the Visaton BG 20. That way the BG 20 speaker drivers are somewhat protected even without the use of the grille. Width of the enclosures is excellent for the Visaton BG 20 because there’s very little leftover space.

Modern speaker enclosures are very narrow in relation to the driver size because narrow front baffle has better acoustical properties than wider baffle. Viston BG 20 driver measures 205 mm so the 230 mm front plate is good match for it.

This design has very good compromise between material thickness, speaker box rigidity, outer dimensions and inner volume. And I really like the the proportions of the completed loudspeakers.

I’m not particularly happy for using the OSB board for this DIY speaker design tough. OSB board is actually great material but it has limited finishing possibilities (see below).

Visaton BG 20 Full Range Speaker_Project MaterialsVisaton BG 20 Full Range Speaker Project_Cabinet Part ListVisaton BG 20 Full Range Speaker DriverVisaton BG 20 Single Driver Full Range Speakers

The material list for TWO speaker boxes:

4 pieces of 74 x 30,4 cm
4 pieces of 74 x 23 cm
4 pieces of 30,4 x 19,4 cm
2 pieces of 70,4 x 19,4 cm (for doubly thick front baffles)

For these measures to apply the board material thickness must be 18 mm!

You also need:

2 Visaton BG 20 speaker drivers (obviously)
2 Visaton BR 15.34 port tubes


4 bracing pieces (see baffle detail drawing)

You can take this list and go straight to the shop where they cut the pieces out for you. Use only professional methods for cutting the pieces for the boxes, because for easy assembly you really need the cabinet boards to have perfectly perpendicular edges.

Please note that those dimensions don’t have any corrections for the clue joints – for OSB board you don’t need any, because edges of the OSB board are very rough and filled with tiny voids, and clue-joints will have almost no thickness.


The Boxsim Speaker Design Tool

Boxsim V1.2 is a great little program from Visaton for designing different kinds of loudspeaker boxes. There are many excellent tools for loudspeaker modeling, but Boxsim is great for speakers using Visaton drivers.

With Boxsim you don’t have to manually input different driver parameters as you can choose them from the drop down list consisting all Visaton speaker drivers, including BG 20.

Designing the bass reflex speakers is actually quite simple. While working with the Boxsim I learned, that shape of the speaker enclosure has no direct effect to the acoustical properties of the bass reflex style speaker boxes. Only enclosure volume, driver and port tube parameters matter, for simulation anyway (you get to choose the driver and port position in front baffle but at least in my simulation it mattered little) .

I chose BG 20 from the driver list, inserted my planned enclosure box volume, inputted the port parameters (there’s no place to choose even Visaton port tube parameters) and starter to analyze the frequency plots of the program output.

I quickly draw the notch filter circuit correction network in Boxsim – it was a breeze. Actually, when I first started to use the Boxsim there was a user-related problem – it didn’t output any graphs at all. Only when I was smart enough to draw a simple circuit in Amplifier menu consisting amplifier and driver did I get a fully working simulation.

After some playing with the Boxsim I concluded, that:
  • speaker box volume only matters for the lowest part of the frequency response
  • damping material suppresses resonance at tuning frequency and lowers bass response (more damping=less bass)
Visaton BG 20 Single Driver Full Range Speakers_Frequency Plot

Dotted line shows the speaker frequency response without notch filter, solid line is the corrected response

The frequency response graph shows prominent output rise in 800 – 10 KHz area. When I first started to design my homemade speakers I hoped that by using full range speaker drivers (e.g. Visaton BG 20 or B200) I’m able to get by without any crossover or frequency correction circuits. After examining the Boxsim simulated output it looked like I need one.

There are some user reports floating around Internet that BG 20 drivers don’t need any frequency correction at all. My tests with Boxsim show them to be little too optimistic.

After I did some listening with my finished speakers with notch filter circuit offline and I can confirm that BG 20 speaker driver frequency peaks do scratch the ear a little. Unfiltered BG 20 drivers sound a little hot at the upper mids and highs.

I figured that a parallel notch filter circuit is what is needed to get the frequency peaks of the BG 20 drivers to check and proceeded to make a quick component value evaluation by using an online calculator. After that I ran Crossover network optimizer in Extras menu in the Boxsim and program chose preferred components values for my network.

Calculated inside volume of the speaker box is 39 liters. BG 20 drivers and bracings will lower that value while speaker damping material will actually increase the effective volume.

I made my best guess and used 36 litres as acoustic volume in my Boxsim simulations. After applying correction network to the simulation, frequency peaks are about 6 db and I didn’t find it necessary to attempt any further corrections, because simulation predictions are already good enough.

Boxsim is a simulation program and one should not read too much into a simulated truth. Acoustic properties of your listening environment will modify the frequency response of your BG 20 powered speakers anyway, therefore it’s not necessary to try to get the theoretical frequency response to perfection.


The Notch Filter Circuit

To iron out most prominent frequency peaks from the Visaton BG 20 frequency response in this speaker enclosure, a notch filter circuit was used. It’s a classical parallel LC resonant circuit that has its Q lowered by addition of a parallel resistor.

Parallel resonant LC notch filter circuit has high impedance at resonance. Impedance depends of the Q-factor of the circuit. Visaton BG 20 full range drivers have wide moderate amplitude rise from 800 Hz to about 10 KHz.

Visaton BG 20 Single Driver Full Range Speakers_Notch Filter Circuit

Notch filter circuit with component values

Frequency correction notch filter circuit must be tuned somewhere in the middle of that band and it must have appropriate Q-factor. I did use the Crossover network optimizer of the Boxsim to find the optimal component values. For this project they are:

C = 0.68 µF (nonpolar)
L = 0.8 mH (0,3 Ω)
R = 22 Ω

The component values mean that the filter circuit is tuned to 6,8 KHz – higher than the center frequency of the excessive output of the BG 20 driver. Boxsim shows that the notch filter tuning center center frequency is 4,5 KHz. Boxsim probably takes driver parameters into consideration, hence the lower tuning prediction (4,5 KHz is about ideally centered in the BG 20 excessive output band).

I used thickest wire for the coil I had in hand and the computed coil loss resistance is only 0,3 Ohm. If I were to do this project again I would use much thinner wire for the coil because 1,4 mm seems to be overkill.

I’m sure there are lot of audiophiles who will think that the coil should have even smaller resistance. The same crowd thinks that big gauge wire is needed for the speaker cables. Well, I don’t, not anymore. Actually I prefer the HiFi audio system to have some resistivity at the output stage, so that the sound of a solid state power amplifier could resemble little more the sound of a vacuum tube amplifier. More on that later.

The capacitor of the notch filter circuit should have voltage rating at least few hundred volts if you’re gonna run the full power Visaton BG 20 can take. As the Q of the notch filter is low thats probably overkill too, but it’s better to have some headroom.

Parallel resistor of the filter circuit is 22 Ohms and there’s the question of its power rating. Power of the parallel resistor in the notch filter circuit is actually many-faceted problem. Necessary power rating depends of the LC circuit Q but also from the spectral content of the signal the notch filter is meant to alter. And, of course, from the applied power and the resistors resistance. It also depends of the spectral power spread around the notch filter circuit tuning frequency (6,8 Khz in our case). It actually depends of what music you listen and how loud. If you use more than 10 W resistor in this speaker design you should be fine. I have much smaller resistors there and I haven’t managed to burn one out.


Notch Filter Circuit: Making The Coil

  • coil former – empty PTFE thread seal tape reel (large)
  • wire thickness 1,4 mm (SWG 17)
  • wire length about 23 meters

Coil former dimensions: length – 20 mm, diameter – 31 mm, anything similar will do

If you want to make a coil that has as low as possible loss resistance then you need to wind the coil with optimum proportions (shape). There’s a simple empirical rule that describes the optimum multilayer coil shape, but I used an advanced online calculator to try to do even better. My suggestion is that you don’t bother with this and just use the empirical rule (Google “multilayer coil shape”).

Visaton BG 20 Single Driver Full Range Speakers_Coil FormerVisaton BG 20 Single Driver Full Range Speakers_Coil Former

If you are gonna use wire with different gauge from that I used, then you are on your own to design the coil. I used an advanced multilayer coil calculator and found out the wire length that was needed for 0,8 mH coil. I can’t tell you how many turns I had on the coil former, but there was 23 meters of wire. If you use exactly that same coil former that I did, you’re fine. Just use the emptied large roll of PTFE thread seal tape as a coil former.

To wind the coil:
  • attach one end of the wire to something solid and other end to the coil former
  • attach coil former with a DIY jig to a cordless drill
  • wind the coil by moving with the drill until all wire is wound
  • wind one layer at a time, cover each layer with couple of turns of electrical tape
  • you need to use something to strengthen the empty PTFE roll (see images)


Making The Speaker Boxes

If you did use the professional shop to cut out the OSB board pieces (or pieces of whatever board material you chose to use) necessary for the Visaton BG 20 full range speaker project then the assembly of the boxes is easy. First thing to do is to assemble the double thickness front baffle.

  • cut the round hole for the Visaton BG 20 to the outer part of the front baffle
  • attach inner front baffle piece to it

Visaton BG 20 Single Driver Full Range Speakers_Making Front BaffleVisaton BG 20 Single Driver Full Range Speakers_Making Front BaffleVisaton BG 20 Single Driver Full Range Speakers_Making Front BaffleVisaton BG 20 Single Driver Full Range Speakers_Front Baffle

You can use wood screws to attach the two OSB board pieces together. I did use some liquid wood glue between the baffle pieces for additional strength – probably an overkill. If two pieces are attached then mark the BG 20 mounting hole to the inner part of the front plate and cut it out.

Visaton BG 20 Single Driver Full Range Speakers_Cutting HoleVisaton BG 20 Single Driver Full Range Speakers_Cutting Hole

I used ordinary jigsaw to cut out all the holes (including the smaller ones for the port tubes). One can’t make perfect holes with a jigsaw but quite good holes anyway. If you have a router, use that, or maybe they will cut the holes for you in the shop where they cut out the boards.

Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets

If you have the front baffles finished then everything else is easy. You should use plenty of glue when making your DIY speaker cabinets. The boxes should be hermetical just like the sealed acoustic suspension loudspeaker enclosures.

Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets Visaton BG 20 Single Driver Full Range Speakers_Making Cabinets

It’s kind of a counter-intuitive, that the boxes should be hermetical, because there’s a obvious big port tube in the bass reflex speaker cabinets. Why the joints have to be airtight then? Well, a bass reflex vented box enclosure is a resonant system, much like a wind instrument. You wouldn’t think that a tuba, for example, would sound right when there’s cracks or punctures in its tubing, right? Well, anyway, you should always make sure that joints of your DIY bass reflex design were airtight (or at least mostly airtight). If you assembled the speaker boxes, it’s time to start to finish them.


Finishing the OSB Board Speaker Cabinets

I chose to use the OSB (oriented strand board) for my full range speaker box design because it is cheaper than plywood. Now I wish I had used the plywood anyway. When I chose to use the OSB board I didn’t know that it’s almost impossible to apply any kind of decent finish to it. What I didn’t know was that:

  • you can’t paint the OSB (you can but it will look awful)
  • OSB surface is rough and you cant use wood filler to smoothen it out
  • you can apply lacquer to it but it makes almost no difference

Basically, theres no decent way to easily finish the OSB board. I decided to use the lacquer but I wish I didn’t. The result simply isn’t worth the bother. Sanding the OSB speaker boxes and applying the lacquer and sanding again and applying second coat and so on. Know what? If you’re done with all of this it looks almost exactly the same as the OSB board natural surface.

Visaton BG 20 Single Driver Full Range Speakers_Finishing OSB Board

OSB Finishing: sanded (sides) and lacquered (top)

OSB Board Finishing_Natural_Lacquered_Sanded

OSB Finishing: natural (left corner), four coats of lacquer (left), four coats of lacquer, sanded (right)

OSB surface closeup sanded and lacquered

OSB Finishing: sanded and lacquered (one coat)







If you’re gonna use the OSB board for this homemade speaker design then just use the OSB board as is, don’t attempt to ‘beautify’ it. It actually doesn’t look bad as it is and I kind of like how my Visaton BG 20 speaker boxes look. But just want to warn you – you really can’t have a nice finish with OSB. If you want to paint your speakers for example, you’r much better off with using the plywood.


Damping The Speaker Boxes

It is wise to use at least some amount of damping material inside the speaker boxes. Sound absorbent damping material lessens the standing waves and unwanted reflections inside the speaker cabinet.

Fiberglass Heat InsulationFiberglass Heat Insulation In Speaker BoxFiberglass Heat Insulation In Speaker Box

Damping doesn’t generally lessen the acoustical volume inside the boxes and may even increase it when used in moderation. The consensus seems to be, that you should use any damping material very sparingly in vented box bass reflex designs because damping lowers the Q of the box.

The bass reflex design relies on good acoustical Q (it’s a resonant system, remember), use only enough damping material to kill the most prominent reflections inside the speaker box. Basically you should use the damping only near the driver(s).


Damping Material

I make it simple for you: best material for killing unwanted reflections in the speaker box is the ordinary fiberglass heat insulation. Other options (natural wool etc) may equal it sometimes but never surpass it. Don’t worry about the fiberglass – it’s not the same awful stuff it was 40 years ago.


Mounting The Drivers

After you completely finished your speaker boxes it’s time to mount the Visaton BG 20 drivers. You need to use some sort of seal or gasket for mounting the BG 20, because you don’t want to lower the Q of your bass reflex boxes. I used ordinary tape-on window seal because it’s cheap and easy to use.

Visaton BG 20 Single Driver Full Range Speakers_Drivers Ready For Mounting Visaton BG 20 Single Driver Full Range Speakers_Driver Seal Gasket Visaton BG 20 Single Driver Full Range Speakers_Seal Gasket Detail

You must cut small holes through the seal gasket before mounting the BG 20 because otherwise the screws will roll the seal around them. Now you can connect the drivers with the notch filter and mount them to the cabinets. Your BG 20 full range speakers are almost done!


Finishing The BG 20 Speaker Project

Only thing left to do now is to mount the port tubes to the speaker boxes. Before you do that you might want to do some listening without the tubes installed. If you followed my speaker design then you have 36 mm thick front baffle in your full range DIY speakers. That 36 mm long hole you cut for the port tubes is actually working as a port by itself. My Visaton BG 20 project speakers were tuned to about 52 Hz without the port tubes installed and tube holes acting as ports.

That kind of high tuning gives bass reflex speakers their pronounced mid bass response. You may even prefer to tune your vented BG 20 speaker boxes a little higher for stronger “kick” bass. Depends on what you like to listen. I preferred to tune my Visaton BG 20 speaker design lower than usual.

After installing the Visaton BR 15.34 port tubes the tuning will drop to about 35 Hz (one of my BG 20 speaker box was tuned to 33 Hz, other one to 35 Hz). I did no attempts to match the tuning because it’s simply not necessary – you wont be able to hear the difference. Tuning bass reflex speakers so low has several advantages:

  • more natural bass
  • less boomy (the cheap plastic vented boxes are usually tuned high and are very boomy)
  • lower steep bass roll-off point
  • detailed, concrete bass, much like sealed box speaker design has

There’s an extremely simple way to check the tuning of your vented loudspeakers. I have plenty of electronics equipment here (including scope) and I’m capable of using it but I prefer the simplicity of things.

For quick check method you need to download some kind of signal generator utility program and feed the continuous tone to the speakers through a power amplifier (make sure your program outputs pure sine wave). Touch very lightly an edge of the membrane of a BG 20 with your fingernail. When you vary the frequency slowly while touching the speaker driver membrane you can quite easily detect the lowest amplitude point of the membrane. That’s the resonant frequency (tuning frequency) of your homemade speaker boxes.


Thoughts On Completed Speakers

It has been extremely satisfying project for me. DIY projects will usually leave you cold after you completed them because, well, they never really live up to their expectations compared to professional gear. This project is different, for me. I sometimes find myself listening music just for the sake of running those beautiful Visaton BG 20 drivers.

The sound of the described speaker design actually isn’t perfect right out of the box. It is slightly lackluster. Maybe it’s because my planned notch filter circuit is tuned slightly higher than the Boxsim suggested. That may well be if the coil I built is less than 0.8 mH. I never bothered to measure it. Why? Because the speakers work extremely well the way they are. For superb HiFi audio and getting full range out from the Visaton BG 20 you need slight correction with the equalizer.

I have a fully analog power amplifier here with fully analog controls and for perfect sound I need to turn my controls for

25% more bass
50% more highs

You may have to use your equalizer anyway because the BG 20 drivers have slightly less output in the highs than the perfect B200 full range drivers. Remember, a Visaton BG 20 driver costs substantially less than a Visaton B200.


What I Would Do Differently

  • would use plywood instead of the OSB board for the speaker cabinets
  • would make the notch filter coil from thinner wire

That last statement sounds like a blasphemy for the audiophile community but there’s an explanation. The main difference between tube and solid state amplifiers is that tube amp works more like current source, opposed to the solid state amp that works mostly like a voltage source. In other words, a solid state amp has much lower output impedance than a vacuum tube amp. Well, one may wonder why vacuum tube amplifiers have such a warm and pleasant sound. It’s probably just because they resemble more current sources than solid state power amps. You can mimic the tube amp with a solid state amp if you introduce some resistance to its output.

Doing so will lose some power, but if your power amp has some headroom and you don’t run your speakers at full power then it may be an option. Thinking so enables you to throw out those horribly expensive novelty speaker wires and cables and use cheapest and smallest speaker wire there is. That’s why I would use thinner wire for the coil of this speaker design notch filter circuit if I would do my Visaton BG 20 project again.

It is much easier to wind the coil from thinner wire and if you actually find some resistance to be a good thing, then there’s just no necessity for thicker wire. I guess it does sound like a blasphemy but, on the other hand, it may be your cheapest option to HiFi tube amp sound. Visaton BG 20 drivers are so sensitive that you can afford to lose some of that solid state amp cheap wattage.

Things I got right with this project:
  • choosing single driver speaker approach
  • using the Visaton BG 20 full range drivers
  • use of 18 mm material for good vibration properties
  • doubly thick front baffle for strength and driver flush mounting
  • using bracings for even more vibration suppression
  • good proportions and inner volume of the enclosures
  • extremely rigid speaker boxes
  • low port tuning frequency
  • excellent HiFi sound quality