February 26th, 2011

Audio Spectrum Analyzer V2.1

I’ve been pretty busy lately working on the latest spectrum analyzer V2.1. Its available in the online shop if you are interested in getting one. Here are some videos bellow with the new blue LED’s and with a half green/half red display.

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I have also been working on the new expansion board pictured bellow. The Expansion Board was created to help you create bigger custom displays for the Spectrum Analyzer. The Expansion Board works by switching the signals from the spectrum analyzer with the help of high current MOSFET transistors thus giving you the possibility to output higher voltages and currents for your custom setup. You can read more about it on the forum.


There is also a new assembly guide for the audio spectrum analyzer V2.1 that will be helpful to those that get the kit unassembled.

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]

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]

August 23rd, 2009

Audio Amplifier with TDA2050

Audio Amplifier with TDA2050

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]

August 21st, 2009

Hi-Fi Tube Amplifier

Hi-Fi Tube Amplifier

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]

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