Kit281 Active Crossovers
            by Michael Price                Updated Nov. 13, 2004

If you have an extra stereo amplifier, the sound quality of Kit281s can be transformed by the addition of an active crossover.  Now that PCBs for the project are readily accessible, it is easy and inexpensive ($150) to build.  The crossover is connected to the output of your source or preamplifier and to the inputs of two stereo amplifiers.  Those amplifiers are in turn connected directly to the drivers inside the speakers, bypassing the original crossovers.  Besides allowing more control over the tone of the speakers, the active crossover seems to improve many aspects of the sound, especially the midrange; making the Kit281 a much more competent speaker for music.

Working final prototype of crossover in my system

Introduction

Adire Audio's speaker kits are a popular choice for DIY enthusiasts because of their simplicity, low cost, and clean output capability. The Kit281, one of Adire's first kit products, is a 2 way MTM vented tower system with a pair of the Adire 8" woofers and a small Audax fabric tweeter (TM025F1). The crossover is 2nd order lowpass and 4th order highpass with some impedance compensation, approximating 6th order acoustic slopes at 2.2KHz. Including all parts, the kit costs $369 per pair. No one complains about the performance for home theater, but the Kit281s performance for 2-channel music has been controversial, especially in comparison to newer kits from GR Research and many others.

Adire Kit281 speaker

With the active crossover project I intend to make it possible for Kit281 owners to improve the performance and flexibility of their speakers without spending lots of money on exotic components or amplifiers. I have designed an external active crossover that costs under $200 to build, and allows obvious improvements in sound quality that would be expensive or even impossible to achieve with a passive system.

A short summary of the concept of active crossovers: In regular speakers (including most of the very expensive ones) the signal from the power amplifier is applied to two electrical networks in parallel, one including the woofers and lowpass circuit, and the other with the tweeter and highpass circuit. In active speakers, an electronic crossover splits the signal into lowpass and highpass sections fed to two separate amplifiers. The power amplifiers are connected directly to their respective drivers. Then the signal degradation due to the high power passive components in a regular crossover is avoided, and we have more flexibility in using complicated filters, equalization, etc. Most people who use active crossovers claim to have gained a clearer, more dynamic sound.

It is easy to buy an active crossover but it is more difficult to transform a passive speaker into a competently performing active one. There a lot of good quality, low cost active crossovers available from Marchand and a lot of pro-audio manufacturers. The problem is that most of them are fixed to a certain slope, use identical frequencies for highpass and lowpass, and generally cannot be customized enough to mimic a good passive crossover. These standard slopes work fine for high quality drivers with flat responses and some overlap, but it would be tough to make them with Kit281s and other carefully designed value speakers. The solution for these speakers is to make an active crossover with custom and/or adjustable filter slopes, Q's and frequencies.

Block diagram of active crossover (from Pass Labs XVR1 manual)

What to change and what to leave alone?

The Kit281 has some easily identifiable characteristics, good and bad, when playing music. In comparison to other kits costing about this much, they probably have the best bass quality and power because of the enclosure size, low tuning frequency, and linearity of the woofers. One would think there is not much to improve in the region below 500Hz, though a couple people have reported a muddy quality to the lowest octave. Above this range the performance goes downhill, with increasing distortion, directivity and frequency response lumps. The main breakup mode is at 2.2KHz, right at the crossover in the kit design. It looks like about a 5 dB peak in Adire's smoothed response graphs, but in my near field measurements it is a very sharp peak about 16 dB over the region below. Also, the directivity of the woofers (radiating size is a bit over 7") becomes significant above 1KHz, leading to a drop in off-axis response around 1.4KHz. Right away this explains the Kit281's subjective midrange quality, which could be critically described as hollow, congested, lacking detail, or slightly harsh. It is nothing out of the ordinary for 8" woofers, and is typical of similar 2-way systems. The Kit281 in its present form is still capable of producing natural sounding voices and instruments.


AV8 frequency response measurement showing breakup peak (6dB/div)


Audioxpress review of Kit281 by Joseph D'Appolito and Peter Millett (PDF)

The tweeters are very good considering the $15 cost, exhibiting a relatively flat on-axis response and clean decay. In the Kit281, compared to much more expensive speakers the treble lacks clarity and, perhaps due to the bump above 7KHz, sounds somewhat grainy. Peter Millett's subjective evaluation for Audioxpress gave the speakers high marks for imaging focus and soundstage size. I found the treble quality similar to that of the GR AV-2 kit, though a bit brighter, in a brief comparison at the 2003 DC area DIY meet.

Switching to an active crossover allows us to use steeper crossover slopes (approaching 8th order acoustic) which help to reduce the effect of the woofer breakup and directivity, which is doubtlessly the worst characteristic of the Kit281 system. I started with a crossover topology similar to the present kit design, then created a steeper version with 4th order LP and 6th order HP slopes at a slightly lower frequency (1.9KHz) to further improve the midrange.  The ideal operation of the steeper filters requires staggered Q and frequencies for each 2nd order filter blocks, which is impossible in any commercially available crossover I know of.

Design and construction details

The electronics themselves are simple, buffered 2nd order Sallen Key filter blocks. Firstly, an RC shunt circuit compensates for the baffle step shelving effect centered around 500Hz. With a potentiometer the amount of compensation can be adjusted, though the best amount seems to be 4 dB or slightly more. The lowpass is close to a 4th order Q=0.5 filter at 1KHz and the highpass is a 6th order Q=0.8 filter at 1.9KHz. 

Schematic of one channel and power supply, using 8th order filters (click for full image).

In most active crossovers, op-amps are used as buffers to present the output of each filter section with a high impedance load, so the sections do not interact undesirably. Using IC op-amps in this case is not really necessary, and could potentially degrade sound quality. I chose to use a simple JFET buffer suggested by Erno Borbely in his June, 1999 article in Audio Electronics magazine (available online here). Each uses a JFET source follower loaded by a constant current source, consisting of two 2SK170 FETs and two 22 ohm resistors. These take up somewhat more space than a normal DIP op-amp, but cost about the same: $3 for a pair.

One channel of the prototype, which used a more complicated (unnecessary) JFET buffer

I built a prototype on perforated board using IC sockets for the filter resistors and capacitors, so they can be easily switched out. A separate board has an LM317/337 regulator providing +/-24 volts DC to the JFET buffers. That supply is connected to a small 44VCT toroidal transformer, which I bought on Ebay. All of the components are typical and cheap; the filters use standard metal film resistors and polypropylene capacitors. This crossover has regular unbalanced (RCA type) connections.

Speaker modification and system setup

The first step to implementing an active crossover is to bypass the passive crossovers. I disconnected all the wires from the inside of my Kit281s and left them taped to the walls of the enclosure. Then I connected 16 gauge wires directly from the binding posts to the drivers - the top posts connect to the tweeter and the bottom posts connect to the woofers in parallel. The jumpers between those posts need to be removed, and the tweeter connected with inverted phase relative to the original. Then, the output from the CD player or preamplifier connects to the input of the crossover, and the two outputs go to power amplifiers. Each amplifier output is connected to the corresponding binding posts on the speaker. If you find that your treble amplifier has a significant DC offset (more than 50 mV) yo go to power amplifiers. Each amplifier output is connected to the corresponding binding posts on the speaker. If you find that your treble amplifier has a significant DC offset (more than 50 mV) you might want to use film capacitors of 20uf/250V or greater to protect the tweeters.

The prototype board and power supply set up behind my CD player

It's a good idea to take advantage of the differing power requirements for the drivers. The tweeters have an efficiency around 91dB/W and because there is less musical power in the range above 2KHz, it's excessive to use an amplifier of more than 30 to 50 watts. Depending on tastes you could even use a small tubed amplifier (which is generally terrible for driving the Kit281s full range). I have a Nelson Pass designed Zen type amplifier for this purpose, which is capable of about 20 watts. I replaced the electrolytic output capacitors with 20uf Solens.

In contrast, the low efficiency and high power handling of the woofers dictates that for best results, you need a big solid state amplifier. I would suggest between 200 and 500 watts at 4 ohms, to get the best dynamics out of them. I'm using typical big (400+ watts) Class AB monoblock amps.

The audible differences between amplifiers seem to be reduced by active crossovers, because the range in which each amplifier plays is reduced. The treble amplifier does not have to deal with the high current draw of the woofers at low frequencies. I found that my old 100 watt Audiosource amp sounded very clean (nearly as good as the Zen) whereas it was dry and flat sounding when used full range, or as the woofer amp. Don't spend too much on fancy amplifiers. Pro audio amps would probably work well for these woofers; if you're selecting a digital amp, pay attention to the damping factor.

The radiation pattern and bass level with these speakers is about the same as the original Kit281, so placement changes should not be necessary. They are still relatively forgiving to room placement in the mid and upper frequencies.  This version of the crossover has a bit less power in the high treble frequencies so you might want to reduce the toe-in slightly.

Results

The active system has a frequency response similar to that of the Kit281, except it is flatter through the crossover region.  The influence of the AV8 breakup peak is now seen as a blip in the already declining woofer response, and it barely shows up in the overall response..  The amount of baffle step and both output levels can be adjusted in the crossover with potentiometers, allowing pretty much any balance of bass, midrange and treble. The upper and lower rolloffs are the same as before.

My best guess at the frequency response of the active system, based on new AV8 measurements.

In my system, I heard markedly improved musical quality compared to the passive Kit281. For a while I used one speaker active and one speaker passive. The frequency responses were similar enough for good soundstaging, but the active speaker sounded much clearer and more dynamic. The bass impact is much greater, due to the increase in electrical damping (the passive Kit281 has about 0.4 ohms in front of the woofers). The speakers present low notes more forcibly. While it would be tough to point out this weakness by listening to the passive system alone, the active version can reproduce kick drums more strongly whereas the passive Kit281s sounded relatively muffled. As with regular Kit281s, the clean output at 40Hz and above can be ridiculous.

Each range of sounds is improved in some way by switching to the active crossover. The tweeter in particular seems cleaner and less restrained, presenting a more natural ringing sound with less fuzziness. The midrange has become less grainy sounding, or smoother and more open.  I'm at a loss to explain some of these differences given the low distortion of amplifiers and passive components.  The benefit of the active crossover is far larger than that of switching to a different amplifier or CD player, so it's worthwhile for anyone trying to improve the performance of a Kit281 music system. 

Building this crossover

I think it's a good idea to start with a PCB. My hand wired prototype was very complicated and difficult to debug. I designed this board, which includes 2 channels of filters (baffle step, 4th order lowpass, 6th order highpass) and a regulated power supply. It is about 6.3 by 3.9 inches. The 3-pin regulators need small heatsinks lying down on the board. When less steep slopes are needed, you can bypass the unnecessary filter sections with wires. Besides the completed PCB, all you need is a transformer and an enclosure. All of the electronic parts and connectors besides the transformer should cost about $75 including shipping from Digikey and MCM/Newark.  You can see a picture of the completed PCB at the top of the page.

PCB layout (click for larger image)

Parts list for crossover PCB

I should note that this PCB and circuit topology is not limited to the Kit281. It can be used with any 2-way speakers if you change the resistor and capacitor values in the filters. This might be a viable option for a high quality active system in general, since it has discrete buffers and every component can be customized. Multiple boards would be needed for 3-way or 4-way systems. There are more economical ways to implement the crossover between a subwoofer and main speakers adequately; I wouldn't suggest using this board.

I had my first board manufactured in Bulgaria by Olimex Ltd. and it was excellent quality.  I'll be able to order more boards from them at a cost of $34 each including postage.  If more than one person at a time wants to build the crossover, that price could go down significantly.

An appropriate surplus transformer usually costs between $25 and $40. A very appropriate one is available from Apex Jr. for only $12.50.  You could use a pre-made enclosure from Hammond, which are sold by Parts Express and Digikey; or build one from aluminum, plastic, wood, etc. 

In conclusion

I'm really excited about this crossover because it seems like an easy way for a lot of people to get more enjoyable sound from a Kit281 system.  The difference is especially obvious in the reproduction of voices, and busy, complex or loud music.  None of this stems from any particular difference in design; in fact, the topology here is a logical extension of Adire's design principles that would be too expensive or impractical to implement passively.  The drivers used in the Kit281 are not competitively high-end, but it's still possible to get really satisfying sound out of them with this careful implementation.

If you have any questions or you're thinking of trying the Kit281 active crossover, just send me an e-mail.