So you can't afford it? Are you sure?

I'm noticing a trend amongst some audio enthusiasts. Some buy in haste, then sell quickly at a loss. Others are not so extreme, but are in a regular cycle of upgrading. I have a few ideas that could put that speaker you lust after within reach, but it could challenge your patterns of purchase. But if it means you can now afford something that was out of reach ...

Lifecycle costing

Over time, the quicker you upgrade, the more the item effectively costs. Ten one grand amps in ten years costs the same as one ten grand amp in that time. This is where some self control brings a return. So it's important to consider the lifecycle of your items.

Digital sources:
Medium term purchase.

Your DVD player is now obsolete and Blu-ray is now affordable. Buy one that is decent quality and run it as a digital transport and you should have no need to upgrade as long as it's working without problems. At least, not until it's lacking support of important features or Blu-ray becomes obsolete.

Preamps:
Long term purchase.

Power amps:
Long term purchase.

Surround processing/AV receivers:
Medium term purchase.

Surround processing is always changing, so a processor will never be long term. If you buy a receiver, the amps will have a longer life cycle but in many cases will be of limited capability. So it makes sense to buy a receiver for it's processing ability and features or buy a dedicated processor.

Speakers:
Long term purchase.

Speakers and power amps go together, and should be considered together. Your speakers should determine the amplifier chosen, not the other way around. Fortunately both are long term items and in many cases will be upgraded simply because you want better. If you have to cut corners, do it elsewhere.

Suggestions:

• Limit spending on items with a shorter lifecycle
• Focus spending on longer term items and if necessary delay purchasing speakers and amps until you can afford what you really want
• Choose speakers and amplifiers at the same time, even if you can't buy them both at the same time

Focus spending where it makes the biggest difference

It's very easy to spend your money in a diffused way so that very quickly it disappears. The result is often a compromise on the items that matter most. If you become seduced by fancy cables and expensive well reviewed digital sources, it becomes difficult to invest seriously in speakers. As a general rule, your speakers should cost more than everything else. The more you spend on your system in total, the bigger the gap. For an entry level system around AU $4k, you should spend about half the total on speakers. For a higher end system over $20k it should be more like three quarters. In reality you can't go by the numbers, but this is a broad suggestion. If you have a $4k system where the cables cost more than the speakers, then sound quality has taken a back row seat.

* to be continued *

Tweeter pad - a simple tweak that might save your speakers

Here is a simple tweak that will make many budget speakers sound more natural (and even some not so budget speakers), as well as increase the power handling and reduce the chances of tweeter failure when pushed hard. It's called an L-pad and it's a very simple mod that anyone who is soldering iron handy can do. You simply need two resistors added to the crossover.

Too late: a TDL speaker that needs a pad

Recently I fixed a TDL RTL3 speaker that needed fixing. The tweeter often goes on this speaker. A late night party was too much for the tweeter, being pushed way too hard. I replaced the tweeter with a new pair and put an L-pad on to make it a bit more natural and also more robust. Does this look like too much power to you?



Read more about it here >

Why do I need one?

Most well designed speakers of conventional design have a tweeter that is more efficient than it needs to be. Often the designer will allow for this and use a pad, but in some cases the tweeter level is left unaltered so that the treble is emphasised. Often in a retail store with a short demo, customers will choose the speaker with slightly more treble or bass. Over time this becomes less appealing. If you have a speaker like this, a pad is a quick and easy fix.

You may prefer less treble than the designer.

You might also listen at higher levels, where it can be better to have reduced treble. This will also increase the power handling. A typical 3db pad will double the power handling of the tweeter.

How to calculate it

You need to know the nominal impedance of the tweeter. Often this will be shown on the tweeter. If you know the tweeter brand and model number then you can perhaps find a datasheet on the internet. You can measure the resistance of the voice coil with a multimeter when the tweeter is disconnected from the crossover. Please note that this is the DC resistance and gives an indication of the impedance which will be higher.

Examples
Vifa D27TG R= 4.6ohm, impedance = 6 ohm
Peerless HDS R= 5.6ohm, impedance = 8 ohm
Peerless 100 HDT R= 6.8ohm, impedance = 8 ohm
Scan Speak Revelator R= 3ohm, impedance = 4 ohm

Use an L-pad calculator to work out the value of the two resistors.

L-pad calculator >

How much power?

Obviously you don't want 1/2w resistors. The power required depends on the amplifier used and the proportion of that power that the tweeter will see. Higher crossover points and steeper slopes will result in less power going to the tweeter. Let's work out an example.

100w amplifier
2nd order crossover at 3k

Above 3k, the tweeter will see 15% of the power so in this case 15w is the most that the tweeter will see. In normal use, the continuous power is more likely to be less than this so a pair of 10w resistors will be fine.

Soldering it in

Conect one in series and one in parallel with the tweeter. The pad should be inserted after all components involved in the high pass filtering components.

Correct
Amplifier > tweeter high pass > L-pad > Tweeter

Incorrect
Amplifier > L-pad > tweeter high pass > Tweeter

If done incorrectly, the L-pad will now see a full range signal and thus it's power rating will be inadequate. It will overheat and most likely fail.

Now evaluate by ear and see what you think. You might need to try different values. Most pads will be in the 2 - 4 db range. Less will be too subtle and more will rarely be required.

Ensure the resistors are securely soldered with a strong bond, then fix them to the crossover board with a hot melt glue gun, or with straps.

Conclusion - give it a go

Sometimes this simple tweak can save an upgrade. If you think your speakers are a little too bright, this could be the answer. It's simple and cheap. If your speaker already has a pad, but you want to increase the value, first find out their values, then with a pad calculator such as the one linked above you can work out the attenutation. Then work out the values for more attenutation.

Why you need a rumble filter

In this article I show why high pass filters, also referred to as rumble filters, are important. In the following simulations I'll demonstrate with a high excursion driver, the Exodus Audio Tempest X2. It has a one way xmax of 27mm. In the first example, we'll investigate a sealed box, where rumble filters often aren't considered necessary. 

Sealed box - 200L each with a Linkwitz Transform included, excursion plot:

Yellow - input power limited to avoid over excursion at 5 Hz

Blue - input power limited to avoid over-excursion at 20 Hz

Red - rumble filterinput power limited to avoid over-excursion at 20 Hz 

You can see that at high output (blue), the excursion gets out of control below 20 Hz, even in a sealed box. If we reduce the input power dramatically, we are safe to 5 Hz. This is probably good enough, however, the output as we will see in the next chart is limited. With a rumble filter, we can set up the system to avoid ever exceeding xmax. We can be sure our driver is safe.

SPL for the input levels previously shown

Yellow - input power limited to avoid over excursion at 5 Hz

Blue - input power limited to avoid over-excursion at 20 Hz

Red - rumble filterinput power limited to avoid over-excursion at 20 Hz 

You can see that output is much greater when a rumble filter is used. We've gained around 8 dB of output, which is equivalent to more than doubling up on drivers and box size. 

Now lets consider the same driver in a large low tuned vented box. 

Excursion

Orange - input power low to avoid over-excursion below 20 Hz

Green - no rumble filter

Magenta - rumble filter in place

 

Again we see much higher SPL as a result of using the rumble filter. We also avoid vent chuffing as a result of very high velocity in the vent. 

High end bandpass?

Is there such a thing as a high end bandpass? Bandpass subs get bad press. Mention one on an audio forum and you can expect to hear things like "boom box," "one note bass" and "poor transient response." Bandpass subs are the forgotten gems of subwoofer alignments.

What is a bandpass sub?

A bandpass sub uses a tuned acoustic chamber which provides acoustic filtering. The two most common types are shown below:

A 4th order bandpass (above left) can be thought of as a sealed box with an acoustic filter added. In the same way, a 6th order bandpass (above right) is a vented box with the same chamber added. The low end extension is similar to the sealed and vented equivalents, but the top end id different. An 8th order bandpass adds another acoustic chamber to both vents so that the final output of the sub comes through one vent.

Why bandpass?

The answer is simple - acoustic filtering. Conventionally a sub has a low pass crossover which is an electrical filter. An acoustic filter also filters out distortion products. Distortion makes it easier to localise a sub, so this filtering means that it becomes harder to pick the location of a bandpass sub. This means greater placement flexibility.

Covert sub placements

Sometimes the measured ideal placement for a sub doesn't work because it damages imaging. You might get a nice smooth in-room result with a sub in the rear corner, but you can localise sounds coming from the sub. A bandpass sub can potentially fix this. You might get away with that position - a bandpass sub is a covert one and it's location can be a well kept secret.

Here is an example of a sealed sub compared to a 4th order bandpass with the Peerless XLS 12:



The response is very similar, but the sealed box uses a 2nd order low pass crossover at 47 Hz while the bandpass has no electrical filter. Both have EQ for extension. You can see why below, where the raw response with no filtering is shown:



Where high output is desired over a narrow bandwidth, a bandpass can increase efficiency with a bigger box and give you more output than you could achieve with a vented box.

In part two I discuss 6th order bandpass subs.

Flareit - quiet vent design

In my previous post I introduced vent design to reduce compression - vents that won't chuff. Now I'll show you how to use a clever application I mentioned in this post.

Flareit.exe
Download here >

There is a simple process:

1. Start with a decent and sane design in WinISD where vent velocity isn't crazy

Non crazy starting points:

100mm vent with 20m/s
150mm vent with 25m/s

2. Enter the required vent velocity into the table and port diameter

Shown below under "Required velocities"

3. Click on calculate



The red line shows the absolute limit of the vent walls. It's generally best to aim for a lower velocity than shown here. When the velocity reaches this point the entire vent will experience compression and it's effectiveness breaks down.

In this example, I've let Flareit calculate the flare radius that is needed to meet the velocity requirement.

The blue line shows the limit based on the flare selected. The limiting factor is the lowest level, which could be either the red or blue line. If the blue line is above the red, then the radius is more than required - the vent walls are the bottleneck. If the blue line is below the red, then the flares are the bottleneck.

In this example, the port requires a flare on the ends with a 35.5mm radius.



If we manually enter a flare of 10mm with everything else the same, as shown above, you can see the result. The flare isn't enough and below 25 Hz the vent will chuff.

4. Choose your compromises

It's a juggling challenge. You will need to keep changing tuning, box size, vent diameter and flare radius until you get the best compromise. If you have a very large box as in the previous post, then compromise isn't necessary. If you use a driver like older versions of Peerless XLS, even though it has less than half the excursion, the challenge is far greater due to a small box. You may need to make the box bigger to get a vented design to work.

WinISD - how to design a vent that won't chuff

Vent design is one aspect that most don't handle very well. It's common for vented subwoofers to perform poorly when pushed. There are two problems:

1. Port compression - this is a reduction in output from the vent due to poor aerodynamics where high velocity compresses the output and reduces dynamics. As a result, the sub doesn't live up to its potential.

2. Port chuffing - an audible and distracting sound as vent velocity becomes extreme. It can be louder than the bass itself. These two problems are in fact one problem to a different extent.

Typically, many will follow a basic rule of thumb, such as using a 4" vent for a 12" driver, 5" for a 15" and 6" for an 18" driver. Others go by the velocity, aiming for arbitrary numbers like 34 m/s. This is a very hit and miss approach and I don't recommend it for anyone serious enough to design and build their own subwoofer. Just a little extra effort and you can design a high performance vented subwoofer. If this seems like too much effort, then I suggest a sealed design.

A few basics

The performance of the vent is determined by the velocity and the aerodynamics of the vent itself.

Port velocity is increased by:

• larger drivers
• high excursion
• reducing the diameter of the vent

Our first challenge is to keep the velocity as low as possible. Once we have chosen our driver, when using all of its available displacement (excursion x piston area) we want to make sure the velocity in the vent is workable. We can reduce the velocity by:

• increasing the vent diameter
• using a rumble filter below tuning
• lowering tuning

Normally we would choose the box size and tuning based on other factors. We should also consider a rumble filter necessary, otherwise we will often have an unworkable vent.

Note: commercial subs often use high excursion subs in a sealed box or lower excursion drivers with a vented design which tends to avoid making the vent design critical. It's not common to see too many designs with very high excursion drivers vented. Typically there are too many compromises involved. Also keep in mind that many subs claiming to use high excursion drivers have an xmax of around 12mm. Frankly, subwoofers have progressed a long way since 12mm xmax was impressive.

Increasing vent diameter. Sounds like a great solution, but this makes our vent longer. As soon as you make the vent big enough for your 25mm xmax driver, you have something that is huge. Oops! Now this is where we want to start getting away with higher velocity. Now we work on aerodynamics.

If we build a lazy vent, with no flares on the ends, it will chuff even with low velocity ie 7m/s. Let's say we have a 100mm vent. Add a flare with an 18mm radius and we can now live with around 10m/s. If we increase that radius to around 50mm then we can now get up to 20m/s. At that point, we can't improve further because the vent reaches its "core limit" at which the vent walls will start to cause problems. To go to the next level, we need a bigger vent.

Stepping it up to 150mm, the core limit now goes up to 25m/s. If we start getting really fanatical, we might go for 200mm, just so we can say "my vent is bigger than yours!" At this point, we can deal with 34m/s.

So where am I plucking all these numbers from?
I have to give some credit where it's due. Bill Collison (Collo) did some experiments on vents to determine where chuffing becomes audible. He then used the results to create an application called Flareit. The numbers here all come from that application. If designing a vented subwoofer, I suggest you open up Flareit along with WinISD. It's an essential tool.

Download Flareit here >

Worked example

Driver: Exodus Audio Tempest X 15"
Box: 400L vented tuned to 16 Hz



With the right settings, we have a textbook result with -3 db points at 20 and 80 Hz. With around 800w we use up all of the xmax capability.



A rumble filter keeps excursion under control below tuning. The cone will be moving up to 54mm from peak to peak.

If we use a 100mm vent, we have a problem:



Vent velocity goes off the chart! 54 m/s. At that velocity your subwoofer will work quite well as a fan to keep you cool in summer, and it will be noisy and chuff like mad. Things change as we use a bigger vent.



Now we have something workable. What happens if we turn off the rumble filter?



Again we have a problem. Very low bass that is present in movies will cause chuffing.

Given that our box is very large, we can live with a bigger vent. This vent is only 400mm long, so we can easily go up to the next size and have very low compression.



Now we have very low velocity, and that means we don't have to pay as much attention to aerodynamics and more difficult to construct vents with large flares.

Active options

Considering the many challenges of passive crossover design, many DIY enthusiasts are running their speakers actively. The advantages are compelling:

• ease of tweaking and making changes
• far more powerful and advanced filtering and EQ options to precisely control the response
• freedom from impedance and tolerance issues
• much greater efficiency in using power
• freedom to use low power amps such as valves and class A
• cleaner sound at high output

Update: How do they sound?

In a recent series of listening tests, I had a chance to compare four different active options in two systems. Some of the results were surprising.


Active crossover listening tests >

Analogue vs digital

Siegfried Linkwitz uses analogue active filters in his speakers. The advantage is that no digital to analogue conversion is needed, although as the complexity of the crossover increases, any sonic intrusions are cumulative. To work in analogue, the crossover must be designed and soldered and this is quite involved. It's a custom solution just like a passive crossover and can't quickly be changed. Working in the digital domain, once the digital to analogue conversion is made, as many filters as the chip allows can be made, including time domain and phase related corrections. Digital crossovers and DSP are far more powerful, and can be quickly and instantly changed without making physical changes to the boards. For this reason DSP is the way of the future. It is possible to construct an entire system where there is only one digital to analogue conversion, in which case DSP becomes even more attractive. It's important to keep in mind that while in the digital domain, changes can occur that impact on sound quality.

DCX: a bells and whistles crossover

The first choice for many is Behringer. Ultradrive DCX2496 and Ultracurve DEQ2496.

These are low cost and powerful options. Ultradrive has 6 channels, allowing stereo 3 way or 2 way for mains and centre in a surround application. Each channel has many different EQ options. Ultracurve is a dedicated EQ unit with much more powerful EQ options. It can be used for response shaping and is powerful enough for EQ that deals with room modes. Ultradrive doesn't have enough EQ processing for difficult room modes.

Ultradrive is used by Emerald Physics in their highly regarded speakers using waveguides and pro drivers. BESL use Ultradrive in their speaker development process, and supply it for use with their speakers.

One of the very attractive features of DCX is digital delays. Each channel can be time-aligned. There is an auto-align feature that calculates the required delay. The delays assigned are not neceesarily what you might expect, and might not correspond to physical driver offsets. If you sit closer to one speaker, you can also correct for this. This feature can have a dramatic impact on imaging.

Downsides

These units use servo balancing which is necessary for professional use but not for home use. As a result, in the home noise is introduced into the signal chain and it would be better to have unbalanced lines. The power supply is also noisy switchmode. Some have argued that the DAC isn't the best quality. As a result of these downsides, many different upgrades are available, many of them costing more than buying the unit itself. The ability to upgrade does make it more desirable.

Having used both units I can say that you can get a very good result with them and that they can improve most systems. However, for a refined and high end system correctly set up in a dedicated room, I'd suggest something better is warranted.

DEQX: an advanced and complete option

For a serious high end system, the premium choice appears to be DEQX. The cost is much higher than Behringer, but it is more powerful and has been designed for high end systems, taking an input from a digital transport (CD/DVD/Blu-ray digital output) and also including a high end DAC and multi channel preamp. It has a remote unlike the Behringer units and very steep slopes as well as phase correction so that no time domain abberations are introduced.

DEQX is used by Legend Speakers in their top of the range active speaker Legend Tikandi. They also use it in their Big Red (Kumbar Wirri) speaker, which is available active and passive and the difference is described by Ed Kramer in a 6 Moons review, on the second page.

Hypex: a built-in system

Hypex have created a built in system that uses a plate amp module for each speaker and digital active crossover which is set up by PC. Simply run an output from your preamp into the plate amplifier built in to the speaker. This eliminates amplifiers in your AV rack and gives a nice neat solution.

Hypex AS2.100 >

This is similar to the solution used by Australian custom speaker manufacturer Aslan Acoustics in their R1 and R3 Reference speakers. Combined with their music server, you have a complete system with just two components. For surround sound you just need more power amps and surround speakers.

DIY analogue options

For those inclined to get into DIY, a good analogue solution is found at Elliot Sound Products. A PCB is supplied with instructions for a 3 way active analogue crossover. It's important to keep in mind, however, that this type of active crossover simply uses textbook LR filters. My experience has been that a well designed passive crossover is better because more sophisticated filtering is needed. The other options are more powerful in that regard. I currently have a similar active crossover that I don't use as it wasn't an improvement over the passive crossover. The slopes and crossover points are all fixed by the components and multiple changes damage the PCB tracks.

MiniDSP: low cost and flexible

A very appealing new option is now available - MiniDSP. They supply fully assembled DSP modules that can act as an active crossover and EQ unit at a very low price. You simply need to put together a power supply, which can be as simple as a plugpack, then mount it in a case. Digital I/O modules are available and each one has 4 channels to allow for stereo 2 way or 4 way for one channel. Essentially this has the same functions as the Behringer units at a much lower cost without the issues related to balanced lines and the power supply. The settings are configured with a PC.


UPDATE:
MiniDSP now provide a plug and play solution and the cost increase is minor. In fact if you purchased a box and connectors it would probably cost you more.





One of the key features of MiniDSP is it's flexibility. Do you want to use your own DAC? The board is designed to allow this and many other options you would often not have.

Allocator: a PC based powerful crossover

Another powerful solution is PC software based DSP and crossovers. One good option is the Frequency Allocator at Thuneau.com.



It is an 8 channel option that requires a good quality 8 channel sound card. The software itself is cheap, and the solution is made more affordable if using an existing PC, but the cost of the sound card should also be considered. In order to get the most out of it, most users will need to upgrade their sound card.

How does it compare to the other options? In terms of features and performance, it seems to be comparable to DEQX. It allows up to 8 channels and so it can be used for stereo 4 way and it also can perform phase correction unlike MiniDSP and Behringer DCX. It can be used also in a surround system, although this means using the Lite version and running multiple instances. Phase correct isn't included in that case. For those who want to also use their system for 2 channel, it might make sense to run the full version for the mains and use MiniDSP for the surrounds.

There are many other benefits in using a PC in the sound system. It can perform the functions of a PVR and stream downloaded audio and video content. A PC can be a good platform for storing and playback. In many cases the cost of a PC solution is far less than stand alone components. For many, however it comes down to the question of whether you want a PC in your sound system and if you can get past the idea that surely the sound quality from a PC can't be "high end."

Other options

CAD Audio active modules

This option was pointed out in my other active options page.

CAD Audio DSP modules >

I won't comment just yet as I haven't had a chance to investigate this option, but it does appear promising.

Next: which active option is right for me? >



Active crossover listening tests >