September 14th, 2009

Infinite Baffle Loudspeaker

Infinite baffle speaker

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]

In-Depth FPGA Interfacing of HD44780 Based LCD

This project illustrates the interfacing of a HD44780 based LCD to the Xilinx Spartan-2 XCS200 FPGA using delayed Finite State Machine (FSM). While using a microcontroller to display text on the LCD is a fairly simple task, interfacing the LCD with a programmable logic device is a different story. The FSM controls the timing and generation of the signals required for data communication, making the process a lot easier.

The HD44780 LCD uses minimum 2.2V voltage for logic ‘1’ and maximum 0.6V voltage for logic ‘0’ for a given Vcc of 4.5 to 5.5V. These voltages are easily managed by the FPGA, using LVTTL mode. The project makes use of the write operation only in communicating with the LCD, so the RW pin is not used. The execution time delay, used to determine that the current write operation is completed, is 40ns. A 4-bit counter is used to control the instructions (the counter is controlled with the CE signal).

When electrical power is applied to the circuit the FSM enters its first state, the Pwr_Up state (check the diagram). The next state is the Pwr_Up_Delay state which lasts 45ms. The device has a multiplexer that puts the CE signal of the counter on high during these two first states. The next state is the Off_Pwr_Up_Delay, after which the FSM enters the Write_Data state (Enable pulse generation state machine). Next is the Data_Setup_Delay state, in which a delay is generated to make sure the setup time before the rising edge of the Enable pulse is adequate.

The Enable must has to be at least 240ns to be valid, and this is handled in the E_Pulse_Hi, E_Hi_Time and E_Pulse_Lo states. The E pin of the LCD is set on high during E_Pulse_Hi and E_Hi_Time states, and low on E_Pulse_Lo state. The next state is the Proc_Comp_Delay state, in which the delay for the current instruction is activated. The next state is Load_Next_Data and the FSM can either move back to Write_Data state and continue to send instructions to the LCD or go to the End_State.

The FSM and hardware layout is using the Xilinx ISE 8.1i VHDL Compiler. Check the article for a pdf file with additional instructions, diagrams and schematics and look into the datasheet of the HD44780 Based LCD for detailed specifications.

In-Depth FPGA Interfacing of HD44780 Based LCD: [Link][Link2]

Miniature tube headphone amp

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]

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