Since I started being interested in audio, I’ve seen all kinds of speaker designs, weird arrangements and weird geometries. From all those speakers, in terms of extremely low frequency reproduction the best ones where the infinite baffle subwoofers. Those setups go easily down to frequencies like 10Hz. You may say these figures are useless with a musical program and normally i would agree but if we are in a quest for perfection i would say it’s mandatory. Even if our music doesn’t have much information below 27Hz (lowest note on a grand piano, though pipe organs go even lower) it is important to have the roll off region in frequency response at least an octave lower.

The reason for this is that the speaker and the cabinet box form a high pass filter and like with all filters it will add lots of delay in the slope region which may affect the quality of the reproduction. For Home Theater applications response down to teen frequencies becomes a necessity. If you look at the spectrogram of a .50 Barret rifle shooting sound you will see the most information is in the 10 to 20 Hz area. You may not hear the 10Hz but you will certainly feel it, much like if you would be standing next to the rifle.

The principle of IB (Infinite Baffle) is to install the woofers on a panel that is wide enough so that the waves coming from the back of the woofer never meet the waves coming from the front of the woofer. Looking at the wavelength of a 10Hz sine you will see you need a very big panel. However IB can also be done if you put the woofer in a box big enough so that it has zero damping. Custom HT rooms have the woofers mounted on a sealing or floor and have the attic or basement act as that big cabinet box.

In this project I’m presenting the author build a speaker on a panel to fit a door way and have an entire room to isolate the back waves from the woofers. He is referring to the Hoffman’s Iron Law that says: having efficiency, low frequency response and box volume one must sacrifice one of the three to have the other two. IB of course sacrifices box volume. There are some who argue this Law, because it actually holds true if you place the subwoofer in an open environment. Closed spaces have gain that will  your low frequencies even if your speaker loses efficiency, and here by speaker i mean the woofer plus cabinet. Woofer’s efficiency is constant.

Another thing is nowadays Hoffman’s Iron Law can easily be defeated with powerful woofers and equalizers. There are woofers that can take tons of power and have huge excursion and can go really low in small enough boxes. Returning to the IB setups you will also need high excursion woofers and high pass filter on your amplifier with a corner frequency between 5 and 10Hz, sometimes higher depending on the woofer your using. You will need high excursion because the woofer won’t be damped at all so it will be free to move. At high powers and low frequencies it will move allot.

In this project the author didn’t built a subwoofer but a full-range infinite baffle speaker. I don’t see the point of this expect for a center channel. He used 8 x 12″ woofers for the low frequency part. If the woofers aren’t such quality it’s better to use many of them, this way the stress will be divided between them. When i first saw the baffle i thought it is too thin to hold the eight woofers. But then i saw the bracing on the back, and that will make it more rigid for sure.

The tweeters are piezo which in my opinion are not that great. Also the crossover i think could use more work. I always liked higher orders crossovers because they give you allot more dynamic range. The whole system reaches 96dB/W/m, which is very nice. Is a nice and fun build, and even if it won’t live up to your expectations at least you will have a sonic weapon.

Infinite Baffle Loudspeaker: [Link] – [Via]

I’ve always thought certain sound pressure levels need to be achieved in order to be engulfed in music. This effect is most noticeable when music comes from loudspeakers and that is because when sound is played at low levels, the perception is affected by ambient noise. Even late at night when things are quiet and you don’t want to disturb people around you, there is still noise. A clock on the wall, a pet, you computer, a cooling fan, some kind of noise will disturb your listening experience. Things are not the same with headphones though. You put it on your ears and outside world is gone.

There are plenty of good headphones on the market, giving good level of detail, good frequency response, and good sound-stage. But there are some drawbacks to using headphones. You will not feel the same impact and if you don’t chose your headphone carefully you will feel fatigue after long listening time. Another thing to consider is that with speakers you can move them around the room to get the response that most suites your tastes, something that you can’t do with headphones. This is way the amplifier that drives them plays a huge role in a headphone setup.

Everyone who has a passion for audio knows about the qualities of tube amplifiers. The problems with these amps is that they are big and heavy, pretty hard to build and operating at dangerous high voltages. This usually discourages most Do-It-Yourselfers. However Mark Houston built a headphone amplifier that is battery powered and it’s small enough to fit in a pocket.

Mark used Raytheon JAN6418 Tubes in his project. These miniature tubes are pentodes made in USA and you can find them in a kit sold by Oatley Electronics. The amplifiers uses one tube per channel in common cathode configuration followed by a buffer to be able to drive lower impedance headphones. The buffer is a low distortion IC PT2308, a class AB CMOS headphone driver.  This IC has an impressive SNR of 110dB so it’s well suited for this application. The output impedance of tubes in common cathode is high so higher value resistors in the driver’s feedback loop are needed. The 6418 tubes need 1.2V to heat their filament. A 5V regulator chip heats the two filaments connected in series through a 270 ohm resistor.

This amplifier can also be used as a preamp. Mark did some modifications to component quality, replacing the PT2308 with a OPA2134 IC for example. There are also some scope traces he shows in his article measured with a load of 47k. I would’ve been curious to see how it performs at much lower loads. All being said, Mark is pleased with the sound, and with tubes you can not possibly go wrong, can you?

Miniature tube headphone amplifier: [Link]

Thomas Bethe built an audio hi-fi amplifier using two TDA2050 integrated chips. I haven’t seen these chips used in projects in a while, because it’s pretty known that TDA2030, TDA2040, TDA2050 chip family is not the best so to say. I remember some years ago, a local factory was building hi-fi amps using these chips and how often i needed to replace the destroyed TDA’s. Their Pentawatt package is not very good. These ICs are more suitable for TV sound amplification, radios, applications in which they drive some small full range speaker.

The speakers Thomas wants to use with his amplifier are a pair of Klipsch RB-51, which are pretty small and well behaved. In the TDA2050 datasheet it is proudly stated that the small chip can deliver 32 Watts RMS into 8 ohms with 10% THD. At this level of distortion those Watts are actually WHATS?! From graphs we can see it can actually deliver about 18W into 8 ohms and stay in the hi-fi area of 0.1% THD.

The schematic used is the split power supply version from the datasheet with an additional feature, headphone output. Thomas used a MKP capacitor at the input of the amplifier circuit. Although it’s always best to use high quality components, i wonder if any differences can be noticed with the TDA. The construction is pretty simple, just a few external components are needed but as the author says the layout is very important in reducing noise. So follow the advices he points in his article.

The amplifier is connected to +/- 25V supply rails, therefore it is not recommended for this amp to be used with 4 ohms loads. If you have low impedance speakers use +/-18V supply. Thomas’ power supply seems to me an overkill but i guess it does the job. If you plan on building this amp yourself screen the power supply and avoid crossing the wires carrying audio signal above power supply.

As far as how it sounds Thomas seems to be happy, even calls it a “mini gainclone”. To me the TDA’s don’t stand at the same level with National Semiconductor LM chips but it’s purely subjective opinion. Anyways anything you build by yourself is ten times better than what you find in the market for the same price as your investment so happy soldering.

Audio Amplifier with TDA2050: [Via] -[Link]

Here is a project for music lovers. Although electronic tube or valve era is long passed, these devices are still used in high quality audio applications. It is true that you won’t wont find them in every home because their regarded as obsolete or too exotic. Guitar players are the ones who know and appreciate their sound as many famous guitarists use tube amps to drive their instruments. Stereo amplifiers built with tubes for hi-fi applications are still being made today and their cost can be pretty high as these amps are though to deliver high quality sound.

The reason tubes have a specific sound has to do in my opinion with the fact that tube distortion is mainly second order which is more music-friendly than the 3rd order distortion that solid state devices have. Some say this is because in tubes you control the flow of electrons and only electrons carry your information unlike transistors.

Depending on their output stage configuration tube amps can be single ended ( using only one tube working in class A) or push-pull ( like with transistor using two tubes working either class A or class AB). Because of the high output impedance of tubes they need an impedance transformer to drive the speakers. This is also the reason of their low damping factor which in combination with their usually low power output makes these amps suitable for certain types of speakers. From my experience you will need speakers with big woofers and very efficient (high SPL per watt). Also i have found that single ended amplifiers ( in class A of course) with high anode voltages applied are best when tube-like sound is desired.

Returning to the project at hand, it is a push-pull design using 6v6 pentodes connected as triodes. If you get an ultralinear output transformer you can use the 6v6 as pentodes.  The input stage using a 5965 double triode is as simple as it gets. It amplifies the signal in order to attack the final stage and provides phase inversion necessary for push-pull. The power supply uses semiconductor rectification which i find a bad thing for a tube project. There are tube diodes available for this task.

As with most audio high fidelity projects  the quality of the sound is in strong relation with the quality of the build. If you do this project just for fun then you will be happy with it just working, if you build it to experience tube sound you should take greater care at layout and components you use. Using multiple transformers is a bad thing as they introduce noise. Using one transformer with many output voltages is alot better. And screening of the transformers is a must. In the project link you will find many advices on how to build it even though the builder didn’t follow all of them.

Another thing, and a reason that keeps many diy-ers away from tubes, is that you will be dealing with high voltages. Be very careful with that. And to close this presentation with another advice don’t use it to amplify your iPod.

Hi-Fi Tube Amplifier: [Via] – [Link]

As i said in part 1 of this project i wanted to build a 2.1 system with good sound using most of the components i already owned and keeping the costs as low as possible.  Since the satellite speakers are not going to be used below 100Hz you can fit almost any 6.5″ woofer in there and since the box is closed. You can also use speaker filling to match the Qtc of the satellites with the subwoofer’s low pass filter. However you need to focus on voicing. Crossover between woofer and tweeter is very important and many times i found it’s making a big difference in sound. If you open most mainstream commercial speakers you will see a only a capacitor and/or resistors or something like that.

Now the subwoofer, since i don’t have much information on the drivers i have, i had to do some measurements. I found a Qts of about 0.76, Fs – 46Hz and Vas – 36l. Yes, i guess it’s made in China. No matter, it will fit a 60l (internal) box very well. The box will be sealed because the woofers are not that good and i will place it in a corner this way i will gain up to 6db in output. The thing about rooms is that their dimensions equals the wavelength for low frequency and this leads to room modes and room gain. If you have a subwoofer you might have experienced that in the room there are certain places where the bass seems to disappear. Take one step away and the bass appears again. This is due to room modes. Going lower in frequency there will be no more waves in the room and the room starts to pressurise and you have a gain going 12db per octave.

This is a generic box. You can fit in there a couple of 10 inchers too. What you must look at is strong motors in the woofers. Big magnets, thick top plates, big diameter voice coils all this will help the woofer perform in a sealed box.

Now to the build itself, first i cut the material i need using a circular saw.

After all the wood boards are cut, i use glue the boards together. Use of clamps makes things easier. Bracing the box and making the joints as solid as it possible will lead to best results. Heavy is good in subwoofers! After the glue is dried i applied the carpet on the box, corners protection, handles and connectors.

Once subwoofer construction was completed i focused on it’s amplifier. I had a car amplifier with it’s smps busted and i thought it’s just what i needed because it already has adjustable low pass filter. The corner frequency can be selected from 40Hz to about 200Hz. So i removed the defective smps and i installed a mains transformer rated 150VA which gave me +/- 35V dc after rectifier bridge. Also used 10’000uF caps for filtering. In most car amplifiers the case is used as a radiator so i had to put new radiators for the power transistors. All this and the amp board i put in a deceased Pioneer Cd player case.

Here’s the 2.1 speaker system:

Time to power the system. The satellite speakers are amplified by a JVC integrated amp and the subwoofer by it’s own amplifier with each channel to its woofer. Set the subwoofer low pass filter to about 100 – 120Hz. Sound was good however some tweaks were still needed in satellite crossover, it sounded bright. Waveguides rule! Here are frequency response graphs. Between two horizontal hard lines there is a 3db difference divided into 5 other thinner lines. The roll off after 10Khz is caused by my old mic.

– satellite speaker                           – whole system

I compared the sound of this 2.1 system with a pair of Cerwin Vega CLS-12. On the high mid-range and highs my system has more detail. I think that tweeter with the waveguide is too good to be used with this system haha. Ofcourse the CLS goes deeper and with a couple dbs louder and the big difference is seen when you turn the volume up. The 8″ woofers from my sub can’t keep up with the 12 inchers from the CLS but they do have a fair amount of excursion as you can see from this small video:

Here are the dimensions of the speaker cabinets and the crossover schematic with one modification the 6.8uF cap from the tweeter high pass filter must be changed with a 4.7uF cap.

This is it for now, I’m sure there will be more changes to the crossover and probably i will test other drivers for subwoofer as time goes by but for now i’m pleased with the result. I wanted a 2.1 system to use with my PC because the two CLS-12 speakers are too big to be placed near my desk.

Anyways if you have some woofers, drivers, tweeters, horns my advice to you is to get your tools and try and build a speaker for them and you might be surprised with their sound and you will have lots of fun in the process specially when you will blow your friends’ commercial setups.

In the first part i presented some of the woodwork on the satellite speakers and a first schematic of the crossover. These days i had time to finish the front baffles, made the edges round, cover the speaker with dark colored carpet and install the drivers and crossover for testing and voicing.

The rounding of the baffle’s edges is done using a 8mm round-over bit on the router. Make sure you get the board fixed so that u can push the router in the edge.

8 mm router bit

Once this is done i applied the first layers of black paint and glued the baffle on the rest of the cabinet. While i was waiting for it to dry i carpeted the back plate. When you chose to use carpet it’s a good idea to install the back plate a little inside the cabinet and this way to leave an edge. This will allow you to glue the carpet and not leave any visible marks at the joints. After the back plate the sides, top and bottom will be carpeted in one piece.

The front baffle was cut with 3mm more on each side so that the carpet would go right at the same level

The satellite speakers are almost done at this point, it is time to install everything inside for the first tests. I chose to fill the cabinet about 70 to 80 % with wool and this way lower the total Q of the speaker to somewhere around 0.8. This can be a high value but since i will use active filter on the subwoofer matching will be easier.  The first version of the crossover network was done in air just for testing. It seems the waveguide gain was slightly higher than expected so the initial L-pad had to be changed. Also the cut frequency was little too high for the tweeter. Since i wanted the option of bi-amping i decided to add a tweeter protection circuit i had in my schematics notebook which I’ve seen used in some professional speakers. After these changes in crossover came another session of listening tests. There was still a part in the upper midrange (voice sibilance) i found to be too emphasized. Adding a resistor of about 1.5ohms in series with the inductor in the high pass filter lowered the Q and smooth the sound.

In the last picture you can see the Zobel network which is an impedance equalization

These are 2nd order filters so you need to reverse polarity of the tweeter, connect the plus of the tweeter to the minus of the filter and vice versa. Otherwise you will have a big dip in the frequency response caused by the phase shift of the filters. Another thing to be careful about is not to connect ground of the high pass and low pass filters if you’re using bi-amp connector. The strap at the connector takes care of that. The radiator i used on the TIP transistors may be an overkill but better safe than sorry.

I did an experiment with a baffle step circuit. From a certain frequency the directivity of the speaker becomes focused and this may appear as an increase in sound pressure level. A baffle step attenuates the response a little from that frequency up. Usually i don’t use it for i didn’t find it necessary. In this case however i got better response on bass at the expense of overall efficiency. However once i will add the subwoofer to the system this circuit might not be necessary. In this case the circuit is made of a 2.7mH inductor in parallel with a 8 ohm resistor connected before the crossover. Baffle step circuit is impractical when using bi-amping though.

Here is simulated baffle step responses using Edge software:

To be continued:

- Measurement of on-axis and off-axis frequency response

- Building the subwoofer

Yes, it is an Arduino board, but I omitted to mention in the title not to discourage those who doesn’t have one, because the project needs just a few passive components beside the ATmega168 you can even build it on a breadboard.

The project is a great example of the AVR micro controllers speed since it does a real-time audio signal sampling and output on the PWM channel, it has also a couple of audio effects like reverberation, overtone. Because it isn’t a dedicated audio DPS it has some limitations, like 15kHz sampling rate, 512byte ring buffer, 8bit PWM which handles the digital to analog conversion, so don’t expect for high quality sound output. Consider this a scaled down audio effect generator, it covers the basic effects and is a great introduction for more complex audio processing

ATmega audio processing: [Link]

I’ll post some of the most important features and specifications here:

Dimensions:

• motherboard: 81×88 mm.
• matrix: 236×72 mm.

Power:

• 7,5 V 1 A DC (the maximum current draw is 600 mA).
• power is supplied to the board trough a 2,1 mm DC power connector.
• you can get this type of adapter from Youritronics shop.

Input signal:

• signal level 200 mV
• Audio input is supplied to the board trough 2 x RCA connectors.
• you can get this type of cable from Youritronics shop.

Features:

• 20×20 led bargraph display (red or green).
• 9 display modes selectable trough button 2 present on the display board.
• on/off (standby) control selectable trough button 1 present on the display board.
• ISP (in system programming, if you have an AVR programmer that supports this feature, you can update the firmwire if new version is released)

First units were shipped,  and I’m curious about what you think related to the project so visit the Youritronics forum and share some of your thoughts also if you have any questions you can ask them there.

You can now chose between 9 display modes and you have on/off(standby) control, My favorite is the first mode

As you can see there are now 4 displaying modes, and on/off control trough two pushbuttons that sit next to the LED’s. In this version I used a cable to connect the two boards, but the boards can also be connected one on top of each other.

I’ve checked some shipping rates for united states and here is what I got:

Romania to USA, 1 kg:

UPS Express Saver 60 USD 1-5 business days
UPS expedited 62 USD – 2-5 Business Days
DHL – something like 110 USD with 2 days delivery time
Express Mail Service 41 USD probably 10 days
FedEx -  117 USD 2 business days

At the moment it seems that UPS, DHL and FedEx require proforma invoice for the product. Because I’m not a registered company I have no means to provide that so I’ll have to find a way around this problem.

later edit: The solution is Express Mail Service which is the cheapest and I can send the item as a gift thus avoiding any other papers. Delivery time to US usually under 10 days