speaker | Speaker Design

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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

: Cut up and folded

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

Added a bunch of hardware

Another view

Back view

Ports/switches

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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

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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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Let’s talk about dBs, loudness, and sensitivity