What do the numbers tell you about a driver?

So you want to use a particular driver and you are wondering if it's suitable for your application. If you look at the data sheet, you will see "Thiele Small" parameters. What do the numbers mean? What can you judge about a driver based on these numbers? In this article the focus is on bass drivers.

What you can't tell ...

The parameters don't tell the whole story. They won't show how well it is built or anything about the quality or distortion performance. They also won't show if the driver has a nice flat response or if it has nasty break-up or resonances. What the parameters will do is show you if it's suitable for the box you have in mind and what kind of output and extension you can expect. This is a good starting point.

The main parameters to watch

fs
free air resonance

This indicates the low frequency limit of the driver. In general this becomes the low frequency limit in a vented box, although you can run a driver lower than fs if the box is large. Driver excursion becomes critical as this creates an extra demand. In a sealed box, the bass limit will be one octave above fs. 12db of EQ is required to get extension down to fs however, this will quadruple driver excursion and require more than 10x as much power!

Qts
total Quality factor

This has nothing to do with quality. It is a measure of the damping of a driver. A driver with a low Qts value (<0.3)>0.5) has less damping, often resulting in a peak around fs. Mid Q drivers are a popular choice for subwoofers as they are generally flexible and will suit vented and sealed designs.

High Q drivers are often used in a sealed box use as they tend to need a very big vented box. Some of the new Exodus audio drivers have a relatively high Q. While a fairly large vented box is generally required, it will be more efficient than a lower Q driver in a smaller box.

Low Q drivers allow small boxes, but their efficiency is generally lower so that high power is required. The result is often a sub that is expensive for the performance that is offered. Many commercial subs use low Q drivers to allow small size, but due to the inefficiency, very high powered amplifiers are used. Early versions of the Peerless XLS driver were very low Q and worked in a small box but were not practical for a vented design. A workable vent would not fit in the box so passive radiators were required. The cost jumps up as a result.

VAS
equivalent volume compliance - the volume of air which has the same compliance as the driver's suspension

VAS in conjunction with Qts gives an idea of the size of the box required. Generally, driver with a large VAS need a large box. If the driver has a high Qts, it needs to be even bigger.

SD
effective piston area

This is the total cone surface area that produces output. Generally part of the surround is included as part of the piston area. Drivers with very large surrounds have a lower SD.

xmax
maximum linear excursion (one way)

xmax is typically defined as the point where the force of the motor (BL) drops to 70% and is considered to be the maximum excursion which is useful. This is critical for determining the output capability of the driver, when combined with power handling and SD.

xmech
mechanical excursion limit

This is the limit of cone travel before damage, although some drivers avoid damage when pushed beyond xmech. Ideally you should design a subwoofer so that there is some kind of limiting so that there will never be enough power to push a driver to this point. This can be done by a combination of a few strategies:

• limiting amplifier power
• sealed box where the selected volume will limit excursion to safe levels
• rumble filter in the crossover
• DSP dynamic filtering

Pe
Electrical power handling

This number should always be taken with a pinch of salt. Often an inflated number is given and even when accurately quoted, may not be significant. Car audio subs often have a high power handling and low xmax. This works well in their application, where they will often be placed in a small sealed box in the small space of a car which provides massive acoustic bass boost. In a home theatre subwoofer application, xmax is more critical. Where a home theatre subwoofer is used for music, thermal power handling becomes the limiting factor.

Le
Voice coil inductance

Ideally inductance should be as low as possible. In a crossover, inductors act as a low pass filter. High inductance in a driver has the same effect. If it is very high, it can limit the upper useable bandwidth. It is also said to have a deterimental impact on transient response, although this aspect is the subject of debate.

mms
Driver moving mass

Avoid the temptation to choose a driver with low mms thinking it will sound faster. Mms is chosen to give a desired fs and when considered in isolation does not show how "fast" a driver is.

BL
Force factor

This is an indicator of motor strength. Avoid the temptation to choose a driver because of high BL. Considered in isolation, this means nothing.

Sensitivity

This number is misleading. The design of the box has such a dramatic impact on the actual efficiency, that this number tells us very little. A high sensitivity driver may end up less efficient than a much lower sensitivity driver.

Parameters to ignore

Most of the other parameters don't warrant consideration, at least not in a quick analysis of a driver.

Example 1 - Peerless XLS (830500)

fs: 18 Hz
Qts: 0.2
VAS: 139 L
xmax: 12 mm
xmech: 22 mm

This driver works in a very small box. It suits a sealed box or passive radiator box. A 40L box can get decent extension, but there is no way a practical vent will work. Low tuning in a small box doesn't work. You might make it as big as 85L, which is bigger than required but at this size, you can make a workable vented box with a 4" vent. Xmax is moderate at 12mm so this driver will only need 240 - 350w. xmech is quite high at 22mm due to the large rubber surround so there is a fair safety margin. SD is quite low for a 12" driver.

Example 2 - Exodus Tempest-X2

fs: 20 Hz
Qts: 0.42
VAS: 255 L
xmax: 26 mm
PE: 1kw

The parameters show this is a very different driver. It needs a much bigger box as seen in the higher Qts and VAS. It also has more than double the excursion and very high thermal power handling. For home theatre use, a large vented box with a high power amp will yield more extension and output than the modest XLS driver.

The next step

After taking a quick look at the parameters, it's time to simulate the driver. A great deal can be learnt this way. After trying different settings, you will find that you can force fit a driver into different applications.

Open baffle vs Conventional speakers

There is something special about open baffle speakers. They aren't for everyone, but if you are seriously interested in hifi, I suggest you experience them at least once.

Subjectively ...

Compared to conventional box speakers, the sound stage is bigger and creates a more realistic sense of spaciousness. The sound is more immsersive and the sweet spot much bigger. It's much less important to sit in dead centre of the room, and the sound is less inclined to appear as if coming from two speakers. In many cases the need for a centre speaker is completely eliminated. Many describe the imaging as less precise, but I've found it to be overall more realistic and engaging.

The main difference

The most significant difference lies in the way open baffle speakers interact with the room. At high frequencies an open baffle speaker with a single dome tweeter will be the same as a box speaker. Into the midrange a box speaker broadens it's dispersion and quickly radiates a lot of it's energy to the ceiling, floor and side walls, transitioning to omni directional behaviour often below 1kHz. There is generally very little control of the interaction with the room and as a result, it's common to try to fix this with room treatment. Again this tends to deaden the room and take some of the life out of the music.



An open baffle speaker has a more controlled interaction with the room. As you can see above, the polar response of the open baffle speaker has more control over the dispersion. To the sides there is a null - this means less side wall reflections, and less ceiling reflections. The only increase is in the radiation from the rear of the speaker in the midrange. However, if set up correctly, this can add to the ambience and enhance the sense of spatiousness. It's also possible to eliminate completely the first reflection from the front and rear walls, if certain placement guidelines are followed. More >

A more comprehensive discussion can be found at Linkwitz Lab >

Open baffle in a small room

How big does your room need to be for open baffle speakers? My room is 4.7m wide x 3.9m deep and works quite well. I wouldn't want to go much smaller than that, but a little bigger would be better. Here is a room which is about 4.7m in both directions. The following diagram shows a starting point for placing open baffle speakers in a room:

Firstly you'll notice the speakers are toed in so that they point at the listening. Normally this will work best, although in some cases you might toe them in so that their axes cross in front of the listener. It's not likely that it would ever be a good idea to point them straight into the room without any toe in. They should not be parallel to the front wall (behind the speakers).

They should be placed in from the front and side walls, ideally 1m being the minimum at least from the centre if not from the closest edges. More is better as long as they don't sit too close together.

It's no accident that the two dimensions shown on the left side are the same. This results in the first reflection from the front and rear walls being cancelled. The sound wave from the rear and that from the front of the speaker are out of phase but have the same path length to the listener. The result is cancellation.

The dimensions of this room are not ideal, and I would not aim for a room where the length and width are equal - this is shown for the purposes of illustration, to indicate minimum dimensions to allow for. In a very small room it's best to place the speakers so they fire into the room along the longest dimension. This is because open baffle speakers reduce side wall reflections while emphasizing the front and rear radiation.

Room treatment

Room treatment for an open baffle speaker should be moderate, especially in a small room. It's best to aim for an overall sound that is similar to what you would find in a typical living room with carpet, modern window treatments (not heavy curtains) and a moderate amount of decoration (pictures on walls, shelving, etc). A very formal living room full of furniture, heavy curtains, deep pile carpet and a dead sound will take the life out of the music. A very live room with tiled floor and large open spaces with little to absorb or diffuse the sound, not even blinds on large areas of glass, concrete, tiles - this will create too much of it's own sound.

A little diffusion behind the speakers can be an enhancement, but it's important not to place diffusion too close to the listener. A diffusor placed too close does more harm than good.

On the rear wall behind the listener some absorption may be used. Experiment with how much.

Please note that the areas shown indicate where the different types of treatment may be appropriate, but don't show the extent. It's probably best not to cover the entire areas shown.

Bass test - answers

No cheating! Take the test here:

Take the bass test here >








Answers below











Don't cheat!













1. At 80 Hz bass can be localised

a. By no one
b. By some
c. By everyone

2. At 120 Hz bass can be localised:

a. By no one
b. By some
c. By everyone

Tests to determine this have found that most can't localise bass below 120 Hz, but there are a small number who can with some difficulty. At 80 Hz, no one can.

3. The Shroeder frequency is:

a. The transition point below which bass uniformly pressurises a room
b. The transition point below which room modes are a problem
c. The point where a speaker's bass response is -6db

Typically in most rooms this will be around 200 Hz.

4. The most effective way to get a smooth in-room bass response at different positions is:

a. Place multiple subwoofers so that problem room modes are cancelled
b. Excite as many modes as possible using multiple subwoofers
c. Acoustic treatment (bass traps, tuned absorbers, etc)
d. EQ
e. All of the above except a
f. All of the above except b

The ideal is to do it all, but the goal is not to cancel modes as many would suggest. The idea of cancelling modes suggests misunderstanding the problem. The reason for the problem is that there aren't enough modes in the problem range - they are spaced too far apart. Imagine walking on a bed of nails. If they are spaced far apart then they will pierce your skin, but if they are all densely packed together so that they are all touching, they will form a hard surface you can actually walk on. The same is true for bass - few modes mean big peaks and dips. But as we keep exciting more and more modes, we eventually end up with a very smooth response.

5. Adding a second identical subwoofer will increase the maximum output:

a. By 3db
b. By 6db
c. By 12db
d. Impossible to say - depends on location

If they are placed next to each other, then 6db, but if spaced apart it's impossible to say.

6. Movie sound tracks are set up so that the subwoofer level is

a. The same as the main channels
b. 3 db louder
c. 6 db louder
d. 10 db louder

7. Subwoofer A has 1kw of power handling and 80 db 1w1m. Subwoofer B has 100w power handling and 90 db 1w1m. Both have adequate excursion and thermal handling to handle the full power output of their amplifiers.

a. Identical maximum output
b. Subw A wins due to power handling
c. Sub B due to lower power compression

On paper they are the same, but power compression will prevent the sub that relies on power from achieving it's potential.

8. A rumble filter is

a. A high pass filter which eliminates all bass below a certain point
b. A low cut filter which attenuates very low frequencies
c. A low pass filter which keeps the cone under control
d. Both a and b

High pass and low cut mean the same thing. It is not a brick wall filter, but one that attenuates below the corner frequency.

9. If a low frequencies were filtered with a second order (12db/octave) filter, below this point

a. Excursion will remain constant with lowering frequency
b. Excursion will be reduced but will still increase with lowering frequency
c. Excursion will be decrease with lowering frequency

10. In the bass range, in order to sound twice as loud, a sound must be increased in level by

a. 3 db
b. 6 db
c. 10 db

Surprised? Have a look at the equal loudness contours and you will see that 10 db only applies to the midrange. For bass, the lines are spaced closer together.

11. Two subwoofers are placed in different locations in a room, each having peaks and dips in different places from their interaction with different room modes. In one particular seating location, one has a peak where the other has a dip. Assuming there is no phase difference between them, the combined response of the two subs at that point will show:

a. An average of the two outputs where they cancel according to their respective levels
b. The dip will dominate the response and the peak of the other sub won't be able to couteract it
c. The peak will dominate the response and the dip will be eliminated

12. The smoothest in-room bass can be achieved by:

a. Dipoles
b. Monopoles
c. Multiple monopoles if correctly placed

Generally, monopoles and dipoles on average will perform about the same on smoothness. Certain situations may favour one over the other, but the best chance at smooth in-room bass is multiple monopoles.

13. Phase is significant at bass frequencies:

a. Because it has a big impact on spaciousness
b. As it relates to the transient response and thus subjective speed of the bass
c. To the extent that it impacts the steady state response
d. Both a and b

Our ability to perceive time domain behaviour in the bass range is very limited. Often a mis-diagnosis is made and superior performance of one sub over another is wrongly linked to transient response and phase issues. This is a common practice where attention is paid to the wrong thing while the important thing, the steady state in-room response is ignored. Getting the best result becomes a gamble. Measurements are needed or it's like playing pin the tail on the donkey.

14. Using EQ to fix room issues:

a. Works in one position only and will make all other positions worse
b. Is the only thing you need to do for the best bass apart from get a top notch sub
c. Tends to work for part of a room, but usually not the whole room and can make some positions worse

Is isn't the best thing sliced bread, but it does work and often for more than one seat. It won't probably work for a whole room.

15. When using EQ for dealing with room modes:

a. You can fill in dips if you have enough headroom
b. You can't fill in the dips - no matter how much boost you apply, they will remain

If you have a 12 db dip, then you can fill it with a parametric eq filter applying just the right boost. Done right the result wil be perfectly flat. The downside is that you are giving up massive headroom. It takes a lot of power.

16. When using boost:

a. 10 db of boost requires twice as much power
b. 3db of boost requires twice as much power
c. 6db of boost requires twice as much power

17. A subwoofer driver is placed into a sealed box with a Q of 0.707. It has an fs of 20 Hz. If no EQ is used, the anechoic -3db point will be:

a. 20 Hz
b. 40 Hz
c. 50 Hz

The sealed box will generally push the F3 up one octave above fs.

18. The same driver is placed into a vented box tuned at fs. In comparison to the sealed box, we can expect:

a. Lower extension but the same output
b. One octave extra extension and about 5 db extra output
c. Less extension but higher output

19. We now use a high pass filter to keep the cone under control below tuning. In order to ensure that the excursion below tuning will never exceed excursion above, we need:

a. A first order (6db/octave) slope
b. A second order (12db/octave) slope
c. A third order (18db/octave) slope
d. A fourth order (24db/octave) slope
e. None of the above - it will vary with driver parameters

So you might ask why third order filters aren't more common!

20. THD (Total Harmonic Distortion)

a. Is a useful measure which shows how accurate a subwoofer will sound
b. Is useful depending on the level - the ear is not sensitive below a certain threshold
c. Is an important measure - when considering the equal loudness contours they are more important than you might think
d. Is not critical for the bass range as our ears are not sensitive to bass distortion
e. Doesn't relate to perceived accuracy and is meaningless for subwoofers

Can you pass the bass test?

So you think you know bass? Take the bass test. Don't worry, you don't have to write anything. This one is "multiple guess." Answers below.

1. At 80 Hz bass can be localised

a. By no one
b. By some
c. By everyone

2. At 120 Hz bass can be localised:

a. By no one
b. By some
c. By everyone

3. The Shroeder frequency is:

a. The transition point below which bass uniformly pressurises a room
b. The transition point below which room modes are a problem
c. The point where a speaker's bass response is -6db

4. The most effective way to get a smooth in-room bass response at different positions is:

a. Place multiple subwoofers so that problem room modes are cancelled
b. Excite as many modes as possible using multiple subwoofers
c. Acoustic treatment (bass traps, tuned absorbers, etc)
d. EQ
e. All of the above except a
f. All of the above except b

5. Adding a second identical subwoofer will increase the maximum output:

a. By 3db
b. By 6db
c. By 12db
d. Impossible to say - depends on location

6. Movie sound tracks are set up so that the subwoofer level is

a. The same as the main channels
b. 3 db louder
c. 6 db louder
d. 10 db louder

7. Subwoofer A has 1kw of power handling and 80 db 1w1m. Subwoofer B has 100w power handling and 90 db 1w1m. Both have adequate excursion and thermal handling to handle the full power output of their amplifiers.

a. Identical maximum output
b. Subw A wins due to power handling
c. Sub B due to lower power compression

8. A rumble filter is

a. A high pass filter which eliminates all bass below a certain point
b. A low cut filter which attenuates very low frequencies
c. A low pass filter which keeps the cone under control
d. Both a and b

9. If a low frequencies were filtered with a second order (12db/octave) filter, below this point

a. Excursion will remain constant with lowering frequency
b. Excursion will be reduced but will still increase with lowering frequency
c. Excursion will be decrease with lowering frequency

10. In the bass range, in order to sound twice as loud, a sound must be increased in level by

a. 3 db
b. 6 db
c. 10 db

11. Two subwoofers are placed in different locations in a room, each having peaks and dips in different places from their interaction with different room modes. In one particular seating location, one has a peak where the other has a dip. Assuming there is no phase difference between them, the combined response of the two subs at that point will show:

a. An average of the two outputs where they cancel according to their respective levels
b. The dip will dominate the response and the peak of the other sub won't be able to couteract it
c. The peak will dominate the response and the dip will be eliminated

12. The smoothest in-room bass can be achieved by:

a. Dipoles
b. Monopoles
c. Multiple monopoles if correctly placed

13. Phase is significant at bass frequencies:

a. Because it has a big impact on spaciousness
b. As it relates to the transient response and thus subjective speed of the bass
c. To the extent that it impacts the steady state response
d. Both a and b

14. Using EQ to fix room issues:

a. Works in one position only and will make all other positions worse
b. Is the only thing you need to do for the best bass apart from get a top notch sub
c. Tends to work for part of a room, but usually not the whole room and can make some positions worse

15. When using EQ for dealing with room modes:

a. You can fill in dips if you have enough headroom
b. You can't fill in the dips - no matter how much boost you apply, they will remain

16. When using boost:

a. 10 db of boost requires twice as much power
b. 3db of boost requires twice as much power
c. 6db of boost requires twice as much power

17. A subwoofer driver is placed into a sealed box with a Q of 0.707. It has an fs of 20 Hz. If no EQ is used, the anechoic -3db point will be:

a. 20 Hz
b. 40 Hz
c. 50 Hz

18. The same driver is placed into a vented box tuned at fs. In comparison to the sealed box, we can expect:

a. Lower extension but the same output
b. One octave extra extension and about 5 db extra output
c. Less extension but higher output

19. We now use a high pass filter to keep the cone under control below tuning. In order to ensure that the excursion below tuning will never exceed excursion above, we need:

a. A first order (6db/octave) slope
b. A second order (12db/octave) slope
c. A third order (18db/octave) slope
d. A fourth order (24db/octave) slope
e. None of the above - it will vary with driver parameters

20. THD (Total Harmonic Distortion)

a. Is a useful measure which shows how accurate a subwoofer will sound
b. Is useful depending on the level - the ear is not sensitive below a certain threshold
c. Is an important measure - when considering the equal loudness contours they are more important than you might think
d. Is not critical for the bass range as our ears are not sensitive to bass distortion
e. Doesn't relate to perceived accuracy and is meaningless for subwoofers

You can find the answers here:

View answers >

A basic guide to bass bliss

One of the most challenging aspects of your sound system to get right is the bass. With most components, all you have to do is choose wisely then plug and play, but this is almost never true with bass. You can't simply plug and play a subwoofer, and you can't rely on high end loudspeakers to give great bass, no matter how good they are. To get accurate bass in a domestic room requires tuning - there is no way around it.

Update

I strongly recommend that you aim to add bass traps to your room. Not convinced it's worth it? Take the free bass trap test.

A few things to consider

Music or home theatre? Either way not having subwoofers is a compromise. While the design of the subs will be different for a music only system, for the most accurate bass you need at least one subwoofer.

• How low do you want to go?
• How loud do you want to go?
• Are you happy with just one sweet spot?

Aren't high end floorstanders enough?

There is very little chance that you will get the best bass with placing speakers in the room without tuning the system. From a bass point of view there are four types of spaces, and a domestic room is usually the most difficult one to get right:

1. Very small room (car) - uniform bass
2. Small room - typical - room mode problems
3. Very large room (commercial cinema) - no room modes problems
4. Outdoors (half space) - no bass

Room modes are the problem with bass in a room. Let me illustrate. This is a subwoofer measured so that you don't see the impact of the room:



It's called a nearfield measurement. The mic is so close that it only "hears" the driver.

This is the same subwoofer measured as you would hear it in a normal room. This room is my room, and it's better than most.



The grey band shows the +/- 3 db range the speakers are designed to work within. You can see what the room does to it!

Along a sound wave there are successive nodes and antinodes - peaks and troughs in the response. In the midrange these are so densely spaced that they are not a problem, but in a domestic room, which can be considered as acoustically small, there is a point where they become space far enough apart to create a problem. The result is peaks and dips in the response. To make matters worse, they are different as you move around the room. EQ can be used to compensate for one position, but the result may be that other seats are still a problem, and they may even be made worse. There comes a point where the wavelengths become so large that they are not able to cause modes and thus as they approach the dimensions of the room the bass becomes uniform. In a very small room such as a car interior, the dimensions are so small that room modes don't create this problem. Outdoors we have a different problem - lack of room gain, so we simply need more output to compensate. In a very large room such as a commercial cinema, we still have room gain but room modes are less of an issue. So in a domestic audio room we have the most difficult of all possible spaces to get right.

We can change the bass response by physically moving the bass source and the listening position. As the listening positions will be fixed, what we need is the flexibility to choose the best locations for a bass source. This is why floorstanders alone will rarely be enough. They should be placed for optimum midrange performance and for best imaging - away from walls with an appropriate toe in.

7 steps to bass bliss

1. Place the main speakers for best imaging and midrange performance

Ideally they should be set up so that they overlap the subs. (Yes, I realise this is very different to what you have been told).

Placement should look something like this diagram above. It was created with open baffle in mind, but applies equally to most speakers.

2. Find a position for your first subwoofer

You'll need to place the subwoofer in the listening position, then place the mic in every position that you could consider using. Measure each one with a calibrated mic and then choose the position which gives the most low frequency extension and output. Ideally you should repeat this process for each listening position. This subwoofer should be the most powerful and be responsible for giving you all the extension and output that you desire as the other subs won't add much output.

3. Find a position for your second subwoofer

The main function of this sub is to smooth out the response. Place this sub in the position that will compensate for the in-room response problems of the first sub. Keep in mind how the subs will sum together as it doesn't happen as you expect. The curves do not simply add or subtract. Where one has a peak and the other has a dip, they don't average out as flat. The summed response will be that of the peak and the dip won't subtract. Where one is flat and the other has a dip, the dip will be removed. As a result, give priority to removing dips. The only exception to these rules is where phase shift occurs.

After choosing a position, place the sub there and measure both together, experimenting with different phase settings until you get the smoothest result in all positions.

4. Repeat the process for the third subwoofer

Do you really need 3 subwoofers? There is no one size fits all solution, but typically 3 is a good number. Normally with two subs the room won't be fully optimised, a third will yield an improvement but going beyond that won't yield much improvement.

5. Review bass filtering on the main speakers

The main speakers will add two bass sources and it's often best not to set them to small and high pass filter them so that they provide little or no bass. The bass should overlap. This way the room will benefit from five discrete bass sources. Review the measurements and they will tell you where the mains provide a benefit. In my room I find that they help remove dips down to 60 Hz. Below that point there is no need for bass output from them.

Note: I know what you are thinking!
This goes against what all the experts have told you. They will probably say that you want to bi-amp with the subs at say 80 Hz so the mains are "relieved" of bass duties. This does have some sense, but bass quality will suffer. A better option exists and the main reason the experts have not suggested it is that a) they didn't think of it and b) most speakers are lacking in oomph so the standard advice gives them a hand. But if you are dedicated enough to have a dedicated room, then a better solution is to use speakers with more capability overall that don't need to be relieved. The mini monitors have to go!

6. EQ

Now that you have achieved a reasonably flat response in-room, it's likely you'll benefit from a little EQ. Aim if possible to remove the peaks first. You may still need to boost some dips, but you should do this carefully. Quickly the headroom in your system will disappear so there is a trade-off between headroom and flatness to consider. Every 3db of boost will double the power required at that frequency. Every 10db will multiply it by 10. Effectively a 1kw sub will act like a 100w sub.

7. Measure

Measure it all again to be sure you've got it right. You might need to tweak a little and keep measuring. Once you get it all flat, keep in mind that the result will be underwhelming if totally flat. While peaks and dips should be avoided, the level of the bass should be higher than the midrange. Movies are mixed to add 10 db of extra gain to the subs, but music tends to benefit from boosting the bass level. This is a matter of personal taste.

Conclusion

The most common mistake audio enthusiasts make is to think that it's all about getting the best speakers or the best subwoofer. That's a good start, but nothing more. It's highly unlikely that you will find out what your speakers and sub can really do unless you have gone through this kind of process. It will take quite a lot of time but there is no way around that. The good news is that with moderately priced subs done right you can embarrass much more expensive subs, which are rarely fully optimised.

Before you replace your big sub with three small ones, it's important to realise that the first sub will determine the output level. If you add a second identical sub stacked right on top you will get 6db extra output, but no improvoment of smoothness. In a multisub arrangement the additional output isn't anything to get excited about. You might not gain any extra output. You might get 3db more than just one sub, you might get 1.5 db! In the rare instance where your room measures perfectly flat, you only need one sub, or a number of subs clustered together for higher output. The only way to know is to measure. Fortunately for those who have never measured anything audio before, it's simple and low cost equipment will do just fine. Purchase a Behringer ECM8000 mic and a Behringer Xenyx502 mixer and some cable, download the Room EQ Wizard (REW) which is a free download and visit the Home Theatre Shack where you will get all the help you need on how to use it.

This article is based on a combination of online forum posts by Dr Earl Geddes and my own experience and opinions. I've also learnt from other enthusiasts in online discussions.

DSE A2760 amplifier modified

Recently I purchased a Dick Smith 2 channel integrated amplifier via the StereoNET classifieds. For those outside Australia, Dick Smith is an electronics retailer. I bought this amp for use in my next speaker project - active speakers. It's a discontinued amp, but they still go on ebay from time to time for around $80.


How good can a cheap amp be?

General consensus seems to be that it's good for the price. It's hard when listening to such an amp to not have a tarnished impression simply by a little expectation bias. In my first listening session with it, I found the sound wasn't appealing. I believe this is due to removing EQ settings I normally use, resulting in sound that isn't quite right. Since then I've had a chance to spend some time with it and I haven't found anything to complain about. It seems to be quite transparent and has potential to work in a high end system. Of course, that comes with a word of warning - nothing suits every system.

In a blind shootout, this amp came second among a handful of more expensive amps.

What makes this amp worth using?

This amp is ideally suited to a simple light duty mod project. Apparently the preamp is the weakest link. A few minutes under the hood and you can bypass the preamp and have a dedicated power amp. All you have to do is switch the jumpers internally. They are labelled on the PCB.

Mod list

The idea is to modify the amp for improved sonic performance and give it a facelift.

• new binding posts
  
• new RCA inputs - input selection removed
  
• re-painted enclosure
• minimalist face plate - just one knob
• remove preamp
  
• a minor tweak to lower crossover distortion
• reducing input gain
  

More mods are possible and have been documented by others. To date I've done all of the above except improve crossover distortion.

1. Power amp conversion

This is easy to do by simply re-routing the jumpers - no soldering necessary. I went to the next step and hard wired the connections with light duty hookup wire in twisted pairs.

Preamp board removed - it gets in the way and I may find another use for it. DC input to the preamp disconnected. This is also simple as the jumpers can be removed, again without any soldering.



2. Binding posts upgrade

Existing damaged binding posts were replaced with new binding posts.



The damaged posts are shown above. I had to add a sheet of black acrylic to mount the new binding posts.

Above: new binding posts labelled inside




Completed rear view.

3. RCA inputs

I didn't like the look of the cheap RCA inputs and there is no need for multiple inputs. The existing rotary switch can create a nasty sound when switching, but I would never use it for a power amp. So this was a good chance to dress it up with some better quality RCA gold plated inputs. I had to attack the chasis to allow the new inputs to be spaced differently, then I placed some acrylic sheet over the previous inputs. I was lucky to have some on hand, and after cutting with a hacksaw, I filed then sanded it smooth, followed by cut and polish to get a nice finish.

4. Volume pot

A volume control isn't necessary in a power amp, but I prefer to have a passive attenuator for this application. As a power amp driving a tweeter directly in an active setup, it's useful to be able to turn down the input. This helps with determining if there is a problem with turn on/off thumps without full volume. It also enables level matching.

Inputs now are fed directly to the pot, then outputs from the pot feed the power amp. Elliot Sound Products have some good information on pots. If you are trying to get your head around how to wire it up, look no further. You'll also find information on how to improve a standard pot. If you find a better diy electronics resource online, please tell me about it. I haven't found anything that comes close.

5. Reducing gain

The gain of this amp is insane. I've never experienced an amp that tries to murder your speakers in the 9 o'clock position.

This is easily fixed by replacing R230 & R253 - the 1k is replaced with 1k8. Yes, that means 1.8k ohms.

The gain of this amplifier is determined by resistors:
R230, R231
R253, R281

Before:

Gain = 1 + R231/R230

Gain = 1 + 47k/1k

Gain = 48

After:

Gain = 1 + R231/R230

Gain = 1 + 47k/1k8

Gain = 27

6. Reducing crossover distortion

Crossover distortion is one of the amplifier specs that actually matter. It is a form of distortion that occurs in conventional solid state class AB amplifiers when switching between class A and B operation. The amp starts in class A at low power then switches to class B as the power increases. We can reduce crossover distortion at the cost of running the amp hotter. For my application the extra heat is a non issue - an active speaker driving a compression driver with 108 db sensitivity doesn't need much power. The full power output of this amp here is enough for a commercial cinema!

There are two options for reducing crossover distortion. One involves replacing R209 and R257 with 160 ohms. This is the simple solution. I chose the better and slightly more difficult option - using 1k trim pots. The value is set by adjusting the trim pot until the voltage is 55mV while the amp is on and at normal operating temperature.

Please note: working on an amp while it is powered up is dangerous. There is a risk of electric shock. If you choose to do it, then it is at your own risk. Please read the disclaimer at the bottom of this post.

7. Modified face plate

I wanted a more high end feel for the amp as well as a customised version. I decided to strip the face plate with the belt sander, then with some sanding by hand. It had some damage, so I filed and sanded the edges to smooth this out. All text was stripped back and give the amp a more minimalist look. I placed black acrylic behind the holes that are no longer used, attached with contact adhesive.

Above: Face plate stripped and holes filled in with acrylic sheet

8. Modified case

Due to scratches, I decided to sand the case back and repaint, as seen in the first photo. I prefer matt black to the stock silver case.

Do you have one of these amps?

I'd like to hear from anyone who has a DSE A2760 - feel free to leave comments below on your experiences with this amp. You might also like to submit a link to your modified version so that this can be a good resource for modders.

Disclaimer:

There are all kinds of dangers associated with taking on a project like this. If you aren't sure of your capabilities, then don't try. Even when disconnected from the mains, power supply caps can retain enough voltage to give you a shock if you touch certain parts inside. Nothing written in this post should be considered as advice that anyone should undertake a project like this. It is at your own risk. There may be other risks but don't ask me, I'm no expert!

More info

The details of this mod are based on instructions here:
Overclockers Amp mod wiki
Greg Erskine >

Technical Questions

Before asking techy questions, keep in mind I'm not an electronics geek. If you want to discuss the circuit design of this amp then I'm happy for you! You should make new friends at diyaudio.com >