10
Apr 13

My Tube Amp build

In addition to designing speakers, I have been designing and building a low power tube amp over the past year.  The goal was to make a (relatively) inexpensive nice sounding, classy looking tube amp.  For those that do not know, most amplifiers, such as those powering basically any speaker in any device or set up you own, are known as solid-state; solid-state amplifiers rely on transistors and other semiconductors to amplify the audio signal and power the speakers.  These have come into popularity alongside the transistor in the 80’s and up to today.  Before transistors, people used vacuum tubes, which can be thought of as lightbulbs.  In a vacuum tube a heater (wire that glows red hot) literally boils off electrons which are then attracted to repelled by a charged metal mesh screen.  These electrons then hit a metal plate where it is seen as electricity.  This audio signal is applied to the screen; a small audio signal can have big change to the flow of these boiled off electrons, i.e. amplify.

The tube amp I designed is only 10 watts per channel as compared to most modern amplifiers/receivers which are 50~100 watts per channel.  Even though it is left powerful than it’s solid state counterparts, tube amps are known for their warm rich sound.

I started off my build with a sheet of steel; I haven’t really worked with steel much so the main form of the amplifier is pretty crude.  The only tools I used for this part were shears and a metal brake.  I forgot to take pictures of some areas of the build, such as drilling holes in the metal chassis.  Currently all that remains is for me to solder all the internal electronics, all that is on the amp right now are connectors and large components, such as the tubes and transformers.

 

 

Marked and ready to cut
Marked and ready to cut
Cut up and folded
Cut up and folded
IMG_20130330_125800

I covered the metal with pleather to hide blemishes and added wooden sides

 

Added a bunch of hardware

Added a bunch of hardware

 

Another view

Another view

 

Back view

Back view

Ports/switches

Ports/switches


29
Mar 13

Brands

I’m going to extend my usual speaker design tips and formulas and what not with what may be the biggest impact on both the consumer and the audiophile community: brands.  Brands play a huge impact on how users interpret the sounds of their audio system.  Along side brands comes price; a higher price to the user means a better sound.  This is all has a psychological influence on the consumer.

Let’s start with Beats.  Beats by Dr. Dre is one of the most popular headphones of all time. Just walking around campus shows their popularity.  They were the first popular high end headphone and carried a pretty steep price tag; 300 dollars for the best version.  But do they live up to their price tag?  Despite what people will tell you, the answer is no.  While these headphones are a huge step up from cheap ten dollar headphones or earbuds, they do not hold a candle to other models at the same price.  Too many, Beats will be their first and only “higher-end” headphone, and if they just spent 300 dollars on a pair then they better enjoy it.  In the audiophile community, Beats are associated with garbage.  Their sound quality is not up to par and their bass is incredibly artificial.  These headphones can be beat by many headphones that are less than half its price.  At 80 dollars, these Grado headphones will blow Beats out of the water.

Another brand that is often associated with quality is Bose.  Bose definitely has some nice speakers and some nice headphones, but really they are just coasting on their brand name. Bose is just too business oriented; they cut costs to make speakers and headphones cheaper while still charging a hefty premium simply because the speakers are Bose speakers.  If you want some truly nice sounding speakers–like headphones–you have to goto a smaller lesser known company.  For headphones, companies like AKG, Grado, V-Moda, and Sennheiser all make really good headphones.  For speakers, companies like Klipsch or Paradigm produce much better speakers than Bose.

Simply put, do the research before buying anything audio.  Many brands like Bose were once great but have been producing lacking equipment in recent years.  Just because you pay thousands of dollars on speakers doesn’t mean your sound system sounds better than someone who spent 100 bucks.  In your mind, it must because you’re trying to convince yourself that you did not just waste a lot of money.  In the end, go for quality, not name.


27
Feb 13

More about crossovers

So I just went to a crossover seminar within the past week which was held by the Audio Engineering Society.  The presentation was led by a graduate student who basically states that while what I said in my previous post is a starting point, it is not a mean to designing an entire crossover.  While I don’t know if he was just trying to dance around complicated math for the presentation or this is a 100% legit method, but he mainly suggests experimentation.

This involves some test equipment (which I recently purchased):

calibrated mic

-amp

-a bunch of capacitors and inductors

-various hookup cables

A calibrated mic is a microphone which has a flat as possible response curve; meaning that it reports the exact same volume level regardless of frequency.  The microphone will be placed directly infront of the speaker and, through audio anaylsis software, provide a frequency response of the speaker.  It is important that the microphone is calibrated because the response is really the response of the microphone plus the response of the speaker; but, in a calibrated mic the response is completely flat, so you really only get the frequency response of the speaker.

Calibrated Mic Frequency Response

Example of uncalibrated microphones

Using software such as smaart, we get to see the frequency response of the speaker and can physically change out the crossover components until we achieve as flat of a frequency response as possible.  In addition to a flat frequency response, we also want a fairly constant slope on the phase response, which can be hard to achieve (especially in the higher frequencies).  Lastly, you must make sure that the speakers sound good to the ear; everything can look good in measuring equipment and on paper, but the real test it how the speaker sounds.  All of these graphs and calculations should only serve as validation to a good sounding speaker (and that it cannot sound any more ideal).

Smaart software

Now that I have these test equipments, I can design a proper crossover for my speaker and will post real values and how they differ from my calculated values.


18
Feb 13

The Crossover

An essential part of almost all speaker designs is the cross over.  The crossover essentially only allows the low frequencies (bass) to the larger speakers (woofers) and only higher frequencies to the smaller speakers (tweeters).  This is because only large drivers can accurately reproduce low noises and only small drivers can accurately reproduce high noises.

There are two main types of crossovers: passive and active.

A passive crossover is inside the speaker and consists of capacitors and inductors.  A capacitor is two parallel metal plates that don’t touch but are very close to each other.  An inductor is a coil of wire.  Capacitors allow high frequencies and block low frequencies while inductors allow low frequencies but block high frequencies.  Larger inductors begin blocking at lower frequencies; larger capacitors also begin blocking at lower frequencies.  Together, combinations of these two components create a crossover.

First Order

The above image is an example of a first order crossover; a first order crossover drops the signal by 6dB/octave; meaning every time you double the frequency from the crossover frequency the signal drops by 6dB.  You can increase this drop off by increasing the order (number of poles/number of components):

Finally, there are different ways of calculating the values of the components, each with their own roll off characteristics, I won’t go into details of calculating, but you should know that different methods yield different results:

The two most common types used in audio are Butterworth and Bessel due to their more linear (desirable) characteristics.

The other type of crossovers is active; passive crossovers which we just looked at have been between the amp and the speaker.  Active crossovers are before the amp, the most common use of active crossovers are in subwoofers.  Subwoofers just want the low frequencies, so everything but the low frequencies are filtered out, then amplified and make noise through the subwoofer driver.  This is desirable because it’s much more efficient and much less expensive.  Besides in subwoofers, active crossovers produce higher, more predictable audio quality than passive crossovers.  The downside is that they drastically increase the complexity of the system (you need an amp for each speaker) and likewise cost.

 

I will be talking more about crossovers whenever I update you guys on my current build!


07
Feb 13

Sealed/Ported Cabinet

This week I’ll be talking about the two most common cabinet types: sealed and ported.  Sealed is exactly as it sounds: a sealed box behind the driver.  Ported is essentially a sealed box but with an open tube poking into the side.  Remember, the cabinet behind the woofer only affects frequency response below 1kHz.

The pros of a sealed box is easy calculation and construction as well as delivering clean tight bass.  They, however, don’t boost the sound level of the bass at all and are somewhat primitive, being one of the simplest of all enclosure types.  Ported cabinets, however give a 3dB (essentially double the power) boost at a select frequency determined by the volume of the enclosure as well as the cross sectional area and length of the port.  The port increases the volume at a particular frequency be basically resonating at that frequency in the same way a bottle resonates when you blow over it.  People use this property to extend the bass response as seen in the graph below.  The downsides of having a ported box is that they are physically larger than a sealed box and if designed incorrectly can easily produce a “boomy” rather than a tight bass.

One of the most egregious errors I see is when people drop any one woofer into any old box and slap it into a car.  While still producing a ton of bass, the system will not perform anywhere close to as it could if everything was optimally designed.  To determine the volume and port length of these enclosures, many online calculators are freely available and easy to use:

http://www.diyaudioandvideo.com/Calculator/Box/

 

Lastly, it’s important to note that not all drivers are suitable for a sealed enclosure and not all drivers are suitable for a ported enclosure.  In order to behave predictably and perform well, they have to have a suitable EBP (see previous post).  An EBP of between 0 and 50 will really only work in a sealed box and an EBP above 90 will really only work in a ported box (between these values can be either).  Here’s an online calculator to figure out your EBP:

http://www.diyaudioandvideo.com/Calculator/SealedPorted/

 

Ported is by far the most common enclosure type on the market today; from cars to computer speakers to hometheatre systems to just about everything.  They are pretty easy to design and they give some of the most satisfying results for effort put in.


28
Jan 13

Thiele/Small Parameters

First off, a little bit of terminology, a speaker what you can buy readily as a whole component.  The driver is the component that goes inside of a cabinet/box.  See the pictures below:

Speakers

Drivers

The Thiele/Small parameters is a set of data that tells you everything about the physical and electrical properties of a driver.  If we were to think of this as food, the Thiele/Small parameters would be the ingredients to a cake (driver).  Using the ingredients, we can calculate how they come together and would taste, or in a driver’s case, sound.  Dropping the analogy, the Thiele/Small parameters give information that is useful for determining frequency response and box size as well as box type (box type will be another post).

 

Thiele/Small parameters were created from the hard work of (wait for it!) A.N. Thiele and Richard Small, who, through experimentation, came up with a method of measuring certain features (parameters) of a driver and how they mathematically influence the design in which the driver is to be used.  Many times in the history of speakers, one or two individuals can be seen as the grandfather of a certain specific field.

Qualitatively, the parameters can be described as follows:

Fs [Hz]: The resonant frequency of the driver (think like a vibrating string).  Typically a lower fs is better; frequencies below the resonant frequency are less efficient and in some cases can cause large excursions (the cone, the part you see moving too far in and out).

Qms [unitless]: The mechanical damping of the driver which is basically frictional and elastic loss.  A higher Qms indicates lower mechanical loss.

Qes [unitless]: The electrical damping of the driver through eddy currents and what not; the electrical equivalent to Qms.

Qts [unitless]: The total damping of the driver; however, it is not simply Qms plus Qes.

Re [ohms]: DC resistance of the driver’s voice coil; see picture below.

Le [henries]: inductance of the voice coil.

Cms [m/N]: The stiffness of the driver

Vas [L]: The volume of air, when compressed to one liter that matches the Cms.

Vd [L]: The volume of air that the cone will move

BL [T-M]: Strength/power of the magnet, the higher the better!

Mms [g]: Mass of all the moving parts (cone, voice coil, etc)

EBP: Efficiency Bandwidth Product, it determines what kind of box necessary, I’ll post next week

Xmax [mm]: how far in/out the cone can move

Sd [m^2]: surface area of cone

Phew!  That was a lot of information without a whole lot of meaning… at least for now!  These parameters are to be used in formulas or calculators (either online or through computer programs like winISD) to aid in the design of the box the driver has to go in to produce optimal sound quality and performance.  So what should you take away from this post?  Aim for drivers with low Fs, as performance typically dwindles below that frequency.    Smaller Vas will require a small speaker enclosure, but efficiency (see previous post) will suffer as this requires a heavier cone and a stiffer assembly in general.

Don’t worry or be intimidated by all of these parameters; most of the time you will just be copying and pasting these numbers into formulas without regards of their actual meaning.

 

As an fyi, here’s a datasheet for a driver I recently purchased, these parameters can be seen on the left side.

Click to access 290-802s.pdf

 


24
Jan 13

Hoffman’s Iron Laws of Speaker Building

1) Bass Extension

2) Efficiency

3) Small Enclosure

The law is that you can only have two of these three attributes in speaker building.  Ideally we would want all three, we want speakers with good bass, can play really loud, and are small.  Unfortunately we cannot have all these.

It usually goes hand in hand that speakers with good bass are fairly large (ie subwoofers etc), and this is because different speakers require different volumes of air behind them which is described by the Thiele-Small parameters (more details in future posts).  The volume of the box behind the speaker effects the way it produces noise below 1kHz, which mainly is composed of drums, guitars, a bit of spoken word, etc.  Imagine blowing air across an empty soda bottle; it produces a low tone.  When the bottle has more liquid in it and has a smaller volume of air, it produces a higher tone.  Above 1kHz the volume is so small that it doesn’t need to be taken into account; a similar principle explains the why behind the box size behind speakers.

 

Assuming two different speakers are of equal quality, the more sensitive speaker will require a larger volume of air than the less sensitive speaker (just trust me at this point).  An anecdotal example: I built some bookshelf speakers with a 3.5″ driver that requires only about half a cubic foot of air in the enclosure; it had a fairly low sensitivity of 83.5dB.  I am currently designing another pair of speakers, which has about the same amount of bass, but requires five cubic feet of air in the enclosure, ten times the other pair!  It should, however, be noted that these new speakers will have an insanely high sensitivity of 98dB.  Using some math from my previous post, if my new pair is playing at 4w of power, my old speakers will have to be using about 64 watts to play at the same volume!  By the time these new larger speakers reach 64 watts of power (upper limit of clean power from most receivers), the old speakers would require over 4000 watts and would have burned out long ago!

So, what should you take away from this post? Typically, if you find small speakers with good bass, they will require a lot of power or won’t get very loud.  If you find big speakers with good bass, they won’t require much power.  If you find small speakers with no bass, they will not require a lot of power and will get decently loud.  All off these are vague statements and basically amount to use common sense; you won’t get a tiny subwoofer unless it has some insane power requirements and you won’t have tiny speakers that rattle the house.  Build and shop realistically!


14
Jan 13

Let’s talk about dBs, loudness, and sensitivity

First off, welcome to my blog!  This blog will be primarily talking about how to interpret speakers’ specs and how to interpret them both when designing speakers and purchasing ready built solutions.

The main job of speakers is to reliably and accurately reproduce audio waves.  What people primarily care about are two things: loudness and sound quality.  This week we will be focusing on the former.

Sound pressure levels (i.e. loudness) are measured in decibels (dB).  dB is a logarithmic scale, unlike most linear scales we are use to.  I’ll explain with example first; here are some examples of everyday noises and their loudnesses in dB:

30dB – Quiet rural area

40dB – Quiet living room

60dB – Normal conversation

70dB – Car

100dB – Jackhammer

110dB – concert

120dB – Jet taking off

The scale is logarithmic because otherwise the numbers would be huge; a jet engine is millions of times louder than a quiet rural area, but on this scale is just scores 90dB higher. 1dB is about the smallest amount that a human can differentiate, and we hear logarithmically; we can differentiate well at low volumes but poorly at high volumes.

So, going back to speakers, one of the given specs is sensitivity.  This spec is usually given at 1w/1m, meaning that 1 watt of power (a pretty low amount) produces the given amount of sound 1 meter away.  A typical value is around 87dB.  To get a louder volume, you increase the volume on your amplifier, thus increasing watts.  Every time you double the amount of power, you increase the volume by 3dB (woah!  logarithmic!!!!).

So in this example we have the following volumes at different power levels:

watts     dB

1           87

2           90

4           93

8           97

16         100

32         103

64          106

128         109

Another reference, an increase in 10dB is an apparent doubling of loudness.

so here it would take 128 watts to reach a volume of 109dB at 1 meter away.

If we had higher sensitivity speakers, such as 97dB 1w/1m, we could reach this volume at just 8 watts, or about 6% of the power.  Even further still, we could increase the number of speakers; everytime you double the amount of (identical) speakers in a system, you increase the overall volume by 3dB for the same amount of power.  This rule falls apart when you get to large numbers of speakers, though; as you get to large amounts of speakers (like over 10) the benefits of additional speakers roll off to eventually nothing.  Sensitivity is crucial when designing high volume applications.  So why don’t we always buy the most sensitive speakers?  I’ll get into the downfalls in future posts.

Other important things related to dBs to think about when designing speakers for a room:

Every time you double the distance you are away from the speakers, you decrease the dB by 6.  Ever notice how the sound is louder in the corners or edges of a room?  Corners get a 9dB boost and edges get a 6dB boost from sound bouncing off walls.

 

Speakers are a somewhat complicated topic, so hopefully this blog will make more sense as it progresses, but hopefully from this post you will have a grasp on volume and how you can control it.

 


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