Active Crossover Kit for DIY Speaker Builders

Active Crossovers for DIY Speaker Builders

Written and maintained by John Pomann (updated: 6/23/02).

My stable homepage is (as of 12/25/01): http://www.snippets.org/filters/index.htm . Thank you to Bob Stout for supplying this space. My stable Email is: JohnPomann@yahoo.com Please update these listings.

The goal of this project is to contribute to and promote the DIY audio hobby. I do not, and cannot, carry this entire project on my own (it's rather non-profit). Much of this originates from, and will continue to blossom from contributors such as (hopefully) yourself. Please follow this link, written by Bob Stout, to better understand my "buisness model" (hobbiest), and how it fits into the DIY audio community: http://www.snippets.org/ldsg/sect-3.php3#BOUTIQUE. He points out that the hobbiest offerings, "often don't last too long once the owner finds out how much time, expense, and aggravation are involved in running a business." He aptly describes the hobbiest's role as "a labor of love."

Table of Contents for this page:
Updates
The Kit Overview
Details on the Kit
Crossover Frequencies
Power Supply
Other Signal Conditioning (Volume control, DC blocking, Active EQ, Baffle step correction, Delay)
To Order the Kit
Recommended Reading
Electronic Resources

Other Pages of Mine:
The Simple and Flexible Active Sub Kit Proposal

An intro to active filter theory
Frequently asked technical questions (FAQ)
Sub Bass Boost (active EQ) discussion
DIY Op Amp Power Supply Discussion update
Automotive Application of the Active Cross Kit
Who else is using this kit / Testimonials
Loudspeaker Watt Meter (well, not a *true* watt meter...)

Downloads:
Download Excel 5.0 active cross component calculator (versitile spreadsheet for claculating active XO parts values; schematics included)
Download Brian Steele's sealedbox.xls with added active bass-boost and lowpass features (rough prototype)
(You can find Brian's original work at his "Subwoofer DIY Page:" http://www.spiceisle.com/audiodiy/)

You can E-mail me at: JohnPomann@yahoo.com.

Disclaimer: You must have a moderate level of sophistication and training about electricity and electrical safety before building this kit, or using any of the ideas presented here. I am no authority on electronics. All info I have provided about this kit is merely theory as I understand it, and that is all that this kit is based on. There is likely to be some mis-information presented here. You are responsible for any harm that may occur to yourself, or others, or to any property or goods that are used in exploration of any ideas or use of any components supplied via my web page or kit. Be very careful when working with live AC circuits to avoid harm to yourself or your equipment - if you don't know what you're doing, then find someone who does. Note that it is easy to destroy speakers or other audio equipment through the use of this kit (for example, if you hook the tweeter to your woofer amp).

Updates

6/23/02 Starting "real" work for "pay". Will be at it 60-80 hours per week for the next year. Email replies and shipping times will be harder to predict. I will try my best to maintain stock, and ship at least once per month (last weekend of each month). Standard orders are easier than custom. Docs V3.1 have evolved to about 27 pages crammed with many figures, tables, etc. to help you out. Next update in 1 year.

7/28/01 Working hard at school. Delivery times have been poor recently (up to 2 weeks), but I'm currently working to streamline my stock inventory. As usuall, improvents continue to be made to the docs. I started plans for a simple DIY preamp. Let me know what you think.

6/23/00 Since the start, I have been constantly improving my docs based on feeback and new info I find. I recently made a major change to the "Construction" section. Now there are step-by-step "assembly line" instructions and new photos to support building the kit quickly and easily without even needing to rely on the schematic. This should cut the building time WAY down. As always, if you want to do advanced things, then you need to learn a bit more. The docs have lots of "optional reading" sections for those of you who want more advanced applications.
12/30/99 The new docs (V2.0) offer a clear improvement in organization and readability. NEW: The Simple and Flexible Active Sub Kit Proposal NEW: Automotive Application of the Active Cross Kit NEW: Loudspeaker Watt Meter (well, not a *true* watt meter...)
9/25/99 Prebuilt power supply now included, therefore no AC need be in your chassis. I've modified Brian Steele's spreadsheet to include active bassboost and lowpass circuits (see the freq response graphs on the above spreadsheet). I now have my cross component calculator available for downloading (see top of this page).
8/21/99 An EXCEL file has been created to design any odd-order Butterworth filter, and any even order filter (any Q you want). Using this, I now have values for the Shiva PR active EQ discussed in the Shiva White Paper.
6/24/99 Please see 'Other Signal Conditioning' listed above for SHIVA ACTIVE EQ. Info on this subwoofer can be found at Avatar's site: http://www.adireaudio.com/
6/13/99 The Photos - click on the thumbnails to enlarge. The images above show a stereo 3 way, 4th order L-R crossover (about the most complex you can build with this kit) after assembly. Two way designs or lower filter orders are even easier to build. For simplicity, the op amp power connections and supply bypass caps are not shown. In and out lines have also been removed. *** Filter schematic shows possible volume control on the high out (NOT included in standard kit). ***

[Kit Details][Frequencies][Power Supply][Discussion][Order Kit] [Recommended Reading][Electronic Resources ]

THE PHOTOS:

An Assembled Kit


The Board

Closeup1

Closeup2

Inside View

THE SCHEMATICS:

Filter

Shiva Active EQ

...more
to come...

The Kit Overview

When I look at where DIY audio is heading, I fear that the entrance investments and hurdles are blocking more and more potential hobbiests and contributors. I believe that this is primarily due to the increasing cost of sophisticated passive crossover design software and measurement tools (not a bad thing in and of itself). I feel that many of these hurdles can be circumvented by increased access to active crossover designs, which are much simpler and more ideal. This lessens the requirement for sophisticated design software and measuring tools. The active crossover will never replace these tools, but at the very least, can allow reasonable confidence in novel designs using good components. For those of you interested in advanced modeling of complete loudspeaker designs (including the behavior of the driver) using computer aided design and optimization of active crossovers, consider the LspCAD software, distributed by: PhilsAudio at http://philsaudio.stryke.com/. My kit fully supports most functions that this software models, except those involving inductors. The only simple change is my use of the 'resistor flexible' 2cd order ecSK filter as opposed to the 'capacitor dependant' ugSK filters used in this software. The conversion between these two types of filters is straightforward, and supported by my docs and ActiveCrossCompCalc.xls at the top of my page.

     When building your own speakers, it would be very handy to have a 'universal' crossover to test your design. The active crossover kit described here brings this ideal closer to reality. When building my active crossovers, I've found that it’s a real hassle to buy all the appropriate parts individually, and furthermore, rather expensive if you’re just buying a couple of resistors and caps of many different values. This problem is compounded when you have to pay shipping to several suppliers. Thus, 'for fun' I've put together a useful tool that I hope others can use. This kit comes with a very large metal-film R kit to maximize the flexibility of your filters without needing to order more parts. I did not intend this to be a for-profit venture. If you consider all that's included, and the time and labor put into the docs and sorting / packing / shipping the parts, you will see that this kit is essentially 'below cost'. Furthermore, the parts supplied are amoung the best known to man. This is a true audiophile's crossover.
     There are many advantages to bi- or tri - amping that I won’t go into (see Rane's website). However, the really cool idea about this kit is that in just a few min, you could alter the Q, filter freq, or order of attenuation of your crossover, and compare the change to the other speaker to see which you like better. This is accomplished with a very large selection of metal film resistors included in the kit. Since the active kit is built on a solderless breadboard, little soldering is required. Once you’ve found the filter function that sounds best to you, you can solder it onto a board if you like. Using the pictures included with my docs, you should be able to build many different types of crossovers, even if you have little experience in electronics - as long as you are patient and follow the docs closely.
     Wouldn’t it be nice to pick up ANY two drivers you have laying around (don't need to worry about incompatible efficiencies), and see what they sound like together after just a few minutes of altering your universal crossover? How about that subwoofer, which you just KNOW is in need of an active crossover (and maybe a little active EQ)? What about that awesome project you built, but still have doubts about the crossover? 1^st order Butterworth, up to 4^th order 3-way L-R, build the function you want, and see how it sounds NOW! (That's the cheesy plug to make your mouth water).
     Cost: intro $75 US for a complete kit with more than enough to build active crossovers for two speakers, and MANY frequencies to choose from. Please email comments, questions, and suggestions to JohnPomann@yahoo.com.

[Updates][Overview][Frequencies][Power Supply][Discussion][Order Kit] [Recommended Reading][Electronic Resources ]

Details on the Basic 3-way Kit

Filter Parts
Due to the demand for absolute quality, this is what I'm giving:
Op amps: 10 X NE5532: a classic and proven dual audio op-amp (20 op amps total). The OPA2134, Burr Brown's very newest high-performance dual audio op amp may be purchased instead as an optional $10 upgrade.
Caps: 40 X Panasonic P-series 2% polypropylene caps; a couple of mylar caps for optional bass boost.
Resistors: 340 (or more) X 1% metal film, 50 ppm.

Other Parts
One 830 contact-point breadboard with wire kit to plug all this stuff into.
Prebuilt, regulated power supply (see below)
Misc parts such as bypass caps.

What you can build with these parts:
1) One stereo 3-way, 4th order L-R filter with stereo bass boost (gasp!).
2) Two stereo 2-way, 4th order L-R filters of differing frequencies (one high, one low).
3) One stereo 2-way 4th order L-R, and one stereo (or mono summing with active EQ) 4th order sub lowpass.
4) One stereo 2-way 4th order L-R, including other functions (Active EQ, Baffle Correction, Phase Shifting, etc.)
Because the kit is completely modular, you can build thousands of different circuits - you just need the schematic. It is especially easy to change filter freq and order (1st, 2cd, 3rd, or 4th). Using ecSK filters, you can build and cascade 1st order filters with 2cd order filters of nearly any freq and Q you want. See 'Other Signal Conditioning' below, as well as my Intro to Active Filter Theory. I will continue to add relavent schematics to my kit as I find the time, or recieve them from YOU (hint, hint). I now have an EXCEL spreadsheet that helps you design even order filters of nearly *any* Q very easily (see top of page to download this).

[Updates][Overview][Kit Details][Power Supply][Discussion][Order Kit] [Recommended Reading][Electronic Resources ]

Crossover frequencies (Hz) that you can realize with parts included in the standard kit:
(exact values may change slightly, depending on the R availability, but you get the idea)

4th order L-R

9,175.6
7,043.6
5,418.1
4,157.7
3,271.6
2,512.9
1,931.3
1,478.3
1,113.9
874.0
670.8
516.1
396.5
304.3

--
--
--
8,315.4
6,543.2
5,025.9
3,862.6
2,956.6
2,227.7
1,748.0
1,341.6
1,032.3
793.0
608.6

634.2
486.8
374.5
287.4
226.1
173.7
133.5
102.2
77.0
60.4
46.4
35.7
27.4
21.0

--
--
--
574.7
452.3
347.4
267.0
204.4
154.0
120.8
92.7
71.3
54.8
42.1

(gasp!)

1st order Butterworth

12,974.2
9,959.6
7,661.3
5,879.0
4,626.1
3,553.3
2,730.9
2,090.3
1,575.0
1,235.9
948.5
729.8
560.6
430.3

--
--
--
11,757.9
9,252.1
7,106.6
5,461.7
4,180.6
3,150.0
2,471.7
1,897.1
1,459.6
1,121.3
860.6

896.7
688.4
529.5
406.3
319.7
245.6
188.8
144.5
108.9
85.4
65.6
50.4
38.8
29.7

--
--
--
812.7
639.5
491.2
377.5
289.0
217.7
170.8
131.1
100.9
77.5
59.5

(whew!)

[Updates][Overview][Kit Details][Frequencies][Discussion][Order Kit] [Recommended Reading][Electronic Resources ]

Prebuilt, Fully-Regulated Power Supply Included

A prebuilt, fully-regulated power supply is provided with the kit. It has a DC out cable that will plug into your chassis. The DC out uses a DIN connector, and a NEW CHASSIS-MOUNT female DIN connector is provided. The supply provides +/- 12V. It also has a 5V output, which a few of you might be interested in. This is a perfect supply for the kit. Connecting a 90 ohm power resistor in series with a .1 uF ceramic cap between the +12V and -12V rails, I see less than 5 mV noise between the rails, and about 2% precision matching between the opposite polarity rails.
Because this unit is external, no AC enters your chassis. This means less potential for humm. Also, because the supply is outside, there are fewer size constraints on selecting your chassis. Finally, the fact that it's prebuilt gets you up-and running in a shorter amount of time.

A link discussing the KIT supply (also in stock).

What's NOT provided / What you'll need
      I don't supply RCA connectors with the standard kit for the following reasons: 1) I once preferred to use RCA cables in half and connected directly to the board. 2) Radio Shack sells gold plated RCA connectors at a very reasonable price. 3) Some people prefer different connectors. If you want balanced I/O connectors (professional equipment), you'll need to get them. An example of a balanced input schematic is shown on Luc Henderieckx's page: Power Supply and Balanced Input jpg. From time to time, I may have gold-plated RCA connectors in stock at special request ($1.50 each).
    There are no custom printed circuit boards. They are not necessary to make typical applications of the kit work. I DO supply a generic PCB for those of you who want "The Works" kit or a kit power supply. The board I'm referring to looks like a solderless breadboard, to make the transfer easier. You can find a similar one at Radio Shack for about $3. You can also use these boards to solder the filters onto, once you've found the magical circuit.
      There is no cable / wire included. I suggest Radio Shack's 3 roll (red, black, green) 22AWG solid hook-up wire (#278-1221, $4.49, 90 ft total) to make custom length's of interconnect wire on the breadboard. Twisted pairs, or single strands of this can connect to and from the RCA jacks. The #64-2129, $2.99 wire stripper is also handy. Spring-loaded, small diagonal cutters and needle-nose pliers are a great help. A multimeter may help in testing.
     There is no chassis (housing, case, etc.) provided. This is a classic opportunity for the DIY'er. You may choose to make a functional box out of tupperware, or purchase or build a sleek-looking case.
     You should have a soldering iron and solder for making a few connections (RCA jacks or cables).
     Finally, you'll need an extra power amplifier for every 'cross' that you do with the active filters. I tend to use garage sale amps, at least for testing. Some amps sound slightly better on my tweeters, some on the woofers. Note that you only need lower wattage amps when you bi or tri-amp (relative to the passive crossover requirements). These amps tend to be cheaper. If you are looking to buy an amp for this project, you might consider looking at some 'home theater' amps made by Onkyo, etc. Look for one that has '5 channel direct input'. This means you can run a sub and 2 two-way satellites directly from the crossover. Maybe you will still have the volume control on the amp. Alternatively, you might consider purchasing 2 low-wattage NAD amps for your mid and tweeter, and use your big amp bridged to drive your sub.
     I may throw in some wire or other useful hardware as I find it, or if I have excess. No promises. Experimentors may want to find some pots to make continuosly-variable circuits (attenuators, etc.). Suggested Radio Shack parts are given in kit docs.

Left-over Parts / Add-on's
For many of you, this kit is more than enough to satisfy two projects. However, depending on your 2cd project, you may find yourself in need of a few extra parts. I'll see where the interest is, and provide cheap 'add-ons' (power supplies, resistors, caps, op amps) to those who have purchased the basic kit. This way, hopefully you could leave your basic kit 'intact' in order to have the flexibility to test new designs. Alternatively, you may use the excess parts to upgrade other equipment you have laying around. The 340+ metal film resistor kit is wonderfull to have for any future DIY audio projects.

[Updates][Overview][Kit Details][Frequencies][Power Supply][Order Kit] [Recommended Reading][Electronic Resources ]

The Beauty of Designing with Op Amps

Crossover design is one of the most important aspects of a good loudspeaker design (IMHO disclaimer now: note again that many of the ideas presented here come from others... sorry no referances... this is just my understanding..... feedback is appreciated on points you may disagree with). Cookbooks exist to assist you in designing appropriate passive crossovers. Typically, you'd start by determining your cross freq and order of attenuation based on your driver's specs. Then you need Zobel circuits to help keep the driver's impedance more constant. You need attenuation for your tweeter. You need asymmetrical order in your crossover to account for driver roll-off. You need a notch filter for that kevlar ringing. You need freq-dependant attenuation for your baffle step. You may also try to correct for horizontal driver offset with delay. Individually, any one of these problems may be easy to solve. There are relatively simple equations to find the appropriate R's, C's, and L's to correct for any one of these. However, when you try to solve all of them simultaneously, you run into a few problems. The first problem is that each piece of this pile of passive components is interacting with the rest. There are ways to minimize this interaction - up to a point. Computer Aided Engineering (CAE) can help predict and solve for this - if you want to invest enough to trust the output of your software. The beauty of the active design is that each function is isolated from the other by op amps. These functions are completely modular, and relative to passive designs, very easy to understand and calculate. The three circuits that allow you to accomplish nearly any of the above functions: the R-C circuit (filter), the voltage divider (2 R's), and op-amp gain (2 R's and op amp). Each of these has relatively simple equations. Just stack the functions together to get your desired output.

Complicating the mathematics of the passive design are 2 other factors: the passive XO does not work into a constant load, and the tolerance or qulaity of the passive XO parts is poor (or the parts are very expensive). Take a look at any CAE 4th order 3-way passive design and consider the DIY'er who didn't measure the specs of his particular driver, and went with the published data instead. Even with 5% C's and R's, can you look at the schematic and really predict what's going on with any accuracy (V vs T transient response across any two points)? I can't (but, I'm no expert). Wouldn't a nice buffer between the driver and crossover help, along with tight-tolerance components and buffers between each filter function help sort out the mess? The active design can afford to employ small 2% polypro caps and 1% metal film R's throughout. These tight-tolerance components work into a constant load (as opposed to the driver) to assure that the mathematics are as straightforward as possible, and that the output is close to theory.

Next, consider what's between your super high-end amp and the driver. You just spent $50 + on some sweet cable to reduce resistance (etc) between the amp and the driver. However, inside your passive crossover are some giant resistors (etc) to attenuate your midrange relative to your woofer, not to mention the giant inductors in your woofer cross. That awesome damping factor of your amp goes swirling down the drain. I believe that an active crossover combined with mid-grade amps and zip cord hooked *directly* to each driver will perform better. This way, your amp has intimate control over the drivers. It can absorb the back EMF, and damp the driver.

Finally, I suppose I should mention the reason that some high-end speakers use active crossovers: reduced IM distortion. This is really the big deal that most discussions of active crossovers talk about, so I'm not going there.

These are the reasons that I think active crossovers are the future for low-tech DIY audio hobbiests: 1) Only a few simple circuits need to be understood to build most functions. 2) You can look at someone else's schematic, and actually understand what's going on, and then tweak for your brand of driver 3) Even if the electrical theory of an individual active circuit is not understood, it can still be incorporated into your design 'cook-book-like' because each function is isolated from the next by an op amp (as long as you understand the filter-theory). 4) Function-isolation, and tight-tolerance components are used to help assure predicted (theoretical) output, reducing the requirement for sophisticated software and measurement tools. 5) One tool (similar to my kit) can be used to accomplish nearly any desired function - you don't need to orders new components every time you want to try a change or new function (*many* R's provided allows you to do this). 6) Using a solderless breadboard, you can tweak with ease using ear or test equipment.

Without the active crossover tool, many hobbiests will be forced to purchase ready-made complete speaker kits rather than designing for themselves. This is not a bad thing for many - I applaud the companies that have put out good DIY kits. But the pioneers will be few. GO DIY!

Some people ask, "What is the virtue of an active crossover, if you use only a 'textbook XO function'; driver integration is never 'textbook'! " Although you may realize many different functions with this active cross kit, I want to put in my plug supporting the 'textbook active 4th order L-R active cross' as an excellent starting point. Here is my answer (see also Frequently asked technical questions (FAQ)):

Tweeter to mid cross:

There are generally many advantages to using a high-order crossovers. Lower tweeter cross frequecies are achiveable using higher order filters, both of which result in reduced sensitivity to both horizontal and vertical displacement - factors which are paramount amoung the advantages of these filters (see figures 7.61 and 7.62 in Vance Dickason's Loudspeaker Design Cookbook, 5th ed.). When implementing high-order crosses, I propose that active crossovers are much more ideal - i.e., it is easier to be assured that you are actually achieving the function you desire when you use tight-tolerant components which operate into a constant load. This is easily achieved using components such as mine (2% tolerance caps, and 1% tolerance R's) in an active crossover (constant load), while it can be very difficult to achieve using passive conponents (5% to 10% tolerance) into a varying load (the driver itself). High-order filters (4th order) are much more sensitive to low-tolerance components than lower-order filters; therefore, even a computer-generated passive crossover response is dependant on the tolerance of all calculated componets, including the complex load that is the driver itself.

Finally, prescion higher order filters are less sensitive to individual driver roll-off characteristis. This means, using the same crossover (at about 3kHz), I can pair different tweeters with different midbasses without changing the crossover at all. I like to put my tweeter in a separate enclosure sitting on top of my mid-bass, as this allows me to just "mix and match" any drivers I want. This makes it very easy to upgrade your system when a new tweeter or mid-bass comes out, often without making one change to your crossover (at least as a starting point for evaluation). Are there some dB humps in the response? Most likely! But, if you are simply a hobbiest, I feel confident that you'll be happy with a stock L-R cross in many typical tweeter to mid-bass crossover applications. This is most true if you choose drivers that have a good overlap (tweeter plays flat down to 1 octave below cross point; mid plays flat up to 1 octave above cross point). Of course, as you become more comfortable with active XO's and filter theory, you may tailor the slopes to your desire with surprisingly small modifications to the breadboard, and also add other functions such as phase shifting. These topics and more are now discussed in my docs.

Mid (or sat) to woofer cross:

Here's my simplistic recommendation: If you want a cross point above 120Hz, then you can seal your mid-bass, making sure that the F3 of the sealed system is 1/2 the cross point or less, and use a symmetrical (standard) 4th L-R cross. For example, with a 5.25" mid sealed into a Butterworth enclosure of 80Hz Fc, you should cross at 160Hz or more if you'd like to use a symmetrical 4th order active XO. In this case, the active XO will greatly increase the displacement-limited power handling of the driver.

If you want a low cross point for your mid-bass (or passive satellite), then you'll need to consider the rolloff of the driver-cabinet system combined with the rolloff of the active filter. For greastest simplicity, I prefer the sealed mid-bass enclosure, with a box volume such that the Qtc = .707. This enclosure acts on your mid-bass as a passive Butterworth 2nd order highpass filter. If you add an active Butterworth 2nd order highpass filter set to the same F3 as your sealed mid, you get a 4th order L-R overall (electrical+mechanical) highpass. This will mate nicely to a 4th order L-R active sub lowpass. This means that your 80Hz (or so) active highpass filter should be 2nd order instead of 4th order, and set to the same freq as you sealed box F3. With my layout, it is very easy to switch between 2cd and 4th order attenuation (1 wire location changed).

Vented satellites have a 4th order highpass function inherent to their design.. There are 2 different approaches to crossing a vented sat to your (sub)woofer. (1) You might choose to set the cross freq one octave or more above the F3 of your vented sat. This way, the rolloff of the active filter will not be significantly 'contaminated' by the rolloff of your sat. This allows for a pure 'electronic cross'.

Alternatively, you might want to choose a 4th order lowpass for your sub, leaving your vented sats "full range". I have seen very few commerial home subs allowing a 4th order lowpass to cross to the sats. This is another (relaively) unique application of my kit. The Q and F3 of the sub filter should theoretically depend upon that of your sats. This can be a grey area because of the varying vented alignments in existance, and the difficult-to-predict actual in-room response of your sub and sats. Success can be maximized only if you use filters as flexible as these. I assure you better flexibility using my kit than the commercial apps. In this sort of arrangement, you might also choose to add an active "very-low-freq blocker" prior to the vented sats' amp. You might choose a freq one octave below your decided cross point so that the active filter rolloff will not significantly increase the 4th order rolloff of your sats. This will provide some protection to the vented sats at high volume levels, esp when playing bass-rich music.

Subwoofer applications (active EQ):

Please see immediately above for info regarding the mid to woofer cross. In terms of sub-woofer bass boost (active EQ, please see: Shiva Active EQ for a complete discussion on sub boost applications. This is another area where my kit(s) really excel.

Other Signal Conditioning

There are many other functions you may wish to build with this kit. Some possibilities are listed below. Many others may be found all over the web. See http://www.t-linespeakers.org /xolinks.html on the Transmission Line Speakers page for a great list of related links. Most of these other functions can be built with a few extra parts from Radio Shack. These part #'s are given in the docs with the kit. Detailed discussion of filter theory is beyond the scope of this page or my docs (entire texts are written on this). The docs DO provide an intro though....

Output level control

If the efficiency of your drivers are not identical, e.g. the tweeter is more efficient than the woofer, then you will probably want to attenuate one relative to the other. My own amps tend to be integrated amps, which means each one has a volume control. I set these to compensate for the differing efficiencies ONCE, then use the preamp BEFORE the active crossover to control the 'master volume'. However, many amps are fixed-gain, meaning they don't have a volume control. They are expecting the preamp to take care of that. I think few people wish to buy a seperate preamp to put AFTER the crossover for each pair of drivers in the system, in addition to the one needed for the main volume BEFORE the active cross (this would be a sight to see!). Therefore, for fixed-gain amp applications, it would be useful to have an output attenuator (volume control). This can be done using one pot between the output of the acitive cross and your amp (see 'high out' on Filter , above). Connect the wiper (center tab) of your pot to the RCA cable going to the amp. The other two tabs of the pot should be connected between the crossover output and ground. I recommend a 5K audio taper pot. I think you can get these at Radio Shack (no pots provided with the basic kit). This is the simplest way, but it might not work well with every single make and model of amp out there. This is because I don't know what the input circuitry of your particular amp looks like, and this passive pot may interact with your main amp's passive input components. It also might add noise, esp if you make a long cable run after the pot. If you add another op amp between this pot and the main amp (i.e. buffer it), then it will definitly work fine (see the volume control on the Shiva schematic). Alternatively, you can get the attenuation by using different combinations of resistors in the kit to make a voltage divider (this takes only 2 resitors). Obviously, this attenuator is not adjustable unless you pull a resistor out, and replace it with a different value. If you know the efficiencies of your drivers, you can calculate what % of the voltage you want to attenuate, and put in the appropriate resistors. Note that these are PASSIVE attenuators if you do not add the op amp buffer at the output. This type of arrangement may encourage noise pick-up in your RCA cable (esp if it's long). See Paul's site: http://www.linkline.com/personal/phorn/audio/rss_schematics.html for a schematic of an active attenuator - superior to the passive one in many ways. Recently, I decided to add an active volume control option:

The 6-channel volume control option ($22) comes with the following:
Generic PCB (soldering required), 3 extra dual op amps (or maybe a quad and 1 dual), 6 cermet pots (made log by the addition of 1 (included) resistor / ch). See also the Frequently asked technical questions (FAQ) on the volume control pots topic.

Alternatively, if you are looking to buy an amplifier for this project, you might consider looking at some 'home theater' amps such as those made by Onkyo. Look for one that has '5 channel direct input'. This means you can run a sub and 2 two-way satellites directly from the crossover through the amp. Maybe you will still have the volume control on the amp. Alternatively, you might consider purchasing 2 low-wattage NAD integrated amps for your mid and tweeter, and use your currently-owned big amp bridged to drive your sub. This solutions allow the volume control to remain on the amp. Recently, I bought an Adcom GFA-2535 amp. This amp has four channels, and a variable stereo attenuator on the input of both channels.

DC Blocking

Some people have asked me about employing a sub-sonic filter with the kit parts. If the application is for your subwoofer, remember that you can get away with using mylar caps (which are now included in the standard kit) because dissipation factor of these are OK at low freq's. With these larger value caps, the resistors in my kit allow a big range of very low frequecies. Bypass these each mylar cap with one of my polypro caps if the filter is intended for your enitre system or tweeters. This will allow the high freq's to pass to your tweeter flawlessly.

Other DC blocking issues are discussed in the Kit docs. For the ultimate in tweeter protection, you might consider adding a passive large-value polypropylene cap in series with your tweeter. Most people agree that these are not necessary, unless you are concerned about the stability of you amps, or other potential faults/miswiring.

Active EQ

One easy and popular circuit can be used with a sealed sub to extend its low end by one octave (takes extra power from your amp though). Essentially, this is a high-Q highpass filter (can do with 1 or 2 op amps). The large bump at the filter freq pulls up your low end. I now have on-line schematics for the Shiva sealed sub active EQ (Shiva Active EQ) discussed in the Shiva White Paper Rev 0.99, page 26. This can be built with my standard kit, and allows a 25.5 L sealed box to perform "essentially flat from 14 Hz to 70 Hz" (certain assumptions apply). General woofer considerations are also discussed. I also have a spreadsheet that will plot the freq-resp curves for any sealed bass-boost alignment you wish to try. Finally, I can provide the parts for any vented Shiva White Paper active bass boost alignments. Just tell me the Fc and Q of the filter you want.

Delay Circuits

I now have a few analog phase circuits shown in the docs. These would typically be used to delay the tweeter relative to the mid in order to realign the voice coils (which are otherwise typically offset when both are mounted on the same flat baffle). My current recommended ciruit uses 1 dual op amp and 2 small polypro caps per tweeter channel. Let me know if you want these extras (special request).

Baffle Step Correction

Others have done this work - see John Murphy's article: http://www.trueaudio.com/st_diff1.htm or Jason Neal's discussion: http://www.t-linespeakers.org /bafflestep/index.html for the basic intro, and to calc the freq's that are attenuated. For the active correction circuit, see Alex Megann's work (http://www.soton.ac.uk/~apm3/diyaudio/Diffraction.html) on the Transmission Line Speaker Page. Really, this is just a bass boost filter that boosts freq's that radiate into full space rather than 1/2 space. Which freq do which? Depends on baffle size. See the above pages. 1) Find the midpoint of your Freq dip using John's or Jason's equations. 2) Use Alex's schematic and formula (R1 = 1.19/(6.28 X Freq X C X 0.414)) ; R2 = 1.19/(6.28 X Freq X C)) to build your active baffle step correction. Now solve the equations using the cap values provided in the kit. Many thanks to these individuals for doing this work, and providing it for all of us. I recently added these calculations, along with the schematic into my CrossCompCalc.xls downloadable from the top of this page.

Effects of Corner Loading / Room Gain

This should be considered in your overall design. Placement of your speakers in your room can have a big effect on their sound. If you design a sub that is anechoically flat down to 25Hz, and then stick it in the corner of your room, lookout! It will probably be too strong down in the very-low freq's. One design I was investigating long ago involved using my attic as an infinte baffle, with the sub (I think it was a Shiva) mounted in the ceiling. I decided that spacing the center of the driver .2 meters from one wall, and .7 meters from the other produced a reasonably flat response from 0 to 100 Hz in a very large room. I used Richard Howe's (basstalk@richardhowe.demon.co.uk) spreadsheet to decide this. Discussions on Corner Loading / Room Gain may be found at:
http://www.trueaudio.com/st_spcs1.htm
http://www.silcom.com/~aludwig/Physics/Main/Cavity_modes.html

Ordering Details
Crossover Kit and Power Supply $75.00
The kits are often ready to ship, but my schedule is highly variable. The docs include schematics and layout details. The kit(s) will be shipped via U.S. Priority Mail ($7 shipping / handling charge to be added to the $75.00 kit cost), or another carrier as I see fit (unless you specify). See the disclaimer above. I try to provide support for applications and construction of this kit as my schedule allows. I prefer money orders from the USA post office, but will accept personal checks (with some delay between reciept and shipping). PayPal can be used if you want to purchase via credit card, but I must charge an extra $4 for the cost of this service.
For PayPal service, please add $4 to your order.

Other Info:
Basic 3-way Kit with prebuilt power supply: $75
Bass Boost option: Now free with every kit (mylar caps); might need an extra breadboard or PCB to make it fit though.
Shipping/H: $7 (US orders by check or MO); $11 (US orders by Paypal); $12 (Most foriegn orders by bank check, or post-office (government) check in USD).
Upgrade to BB2134 op amps: $10
6-channel volume control for kit: $22 With 3 stereo panel-mount pots instead of 6 PCB mono pots: $25. I have a limited qty of panel-mount stereo Noble pots for sale.
Stereo Tweeter Phase Shifter / Delay option (no pots): $18.
Generic PCB's: $2.50 each (I recommend you get 2 for a Basic 3-way kit if you want to eventually solder it)
THE WORKS: All the above options, 3 PCB's, and 1 breadboard. $129 US shipping included. Add $8 for 3 stereo Noble pots if desired.
Extra BreadBoard: $10; upgrade to beutiful double-size BB with aluminum back: $8
New: The Simple and Flexible Active Sub Kit - $47 intro price (US shipping included)
A-La-Carte / Excess inventory / Surplus / Sale page.
Foreign Orders: Typically, I'll provide a "Kit Supply", but no transformer is included (trans is heavy to ship, and probably won't work for you anyways). You need to find a bank check, traveler's check, etc. made out to me in US funds linked to a US bank. I hear that most foriegn post offices sell international money orders. Shipping cost is variable. Often it is about $12. Email me about special concerns.
Please e-mail to me your name, address, and the following information:
1. What do you plan to build (in case I decide to throw in some extra parts).
2. Tell me if you have any special requests (e.g. specific extra parts wanted, alternate shipping methods, etc.). If we had a long discussion, you must copy and paste together a summary list of what we discussed, and what you expect to receive at the time you make payment. Otherwise, you might end up with a basic untailored kit (I do my best - but this would really help me when I go to pack your kit).
3. Consider payment by US postal money order (preferred; email me for address) or credit card using PayPal, which now accepts foriegn transactions:
<--- Click There to Buy, but please add $4 if you want to use Paypal (their fees are getting out of control).
4. Dont forget to send your address and an email where I can reach you! Include your phone number only if you wish.
5. Please follow this link, written by Bob Stout, to better understand my "buisness model" (hobbiest), and how it fits into the DIY audio community: http://www.snippets.org/ldsg/sect-3.php3#BOUTIQUE. He points out that the hobbiest offerings, "often don't last too long once the owner finds out how much time, expense, and aggravation are involved in running a business." He aptly describes the hobbiest's role as "a labor of love." With this in mind, I appreciate any contribution of time you may have to offer to developing the content of this site.

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