Designing a crossover exposes more faults than solutions

I would like to discuss my results of working with passive/active crossovers that I had while trying to upgrade my studio monitors. Studio monitors are used in the recording studio to provide musicians with a feed of the sound or to amplify an instrument, they are usually not used for mixing or in the control room.


All of that means these are normally a large speaker capable of competing with other instrument amps and loudspeakers so efficiency is important. Mine are a three way design with a 12″ woofer, a sealed mid driver and a large compression horn. They were hand built using commercial sound drivers, the cabinets are thick birch plywood with braces and covered with Ozite and chrome corners with recessed handles and large rubber feet, sturdy and tough but certainly not what you want in your home.


The sub-bass cabinets were added after I built the 3 way cabs when it was found that there was no way I was going to get both a flat response and low enough bass response. These cabs are from the 1950s and originally housed fine German full range high-fi speakers. If you look closely you can see the baffle folds around at the sides and is vented along the entire vertical surface of the cabinet. This design is perfect for my sub cab and was one of my better craigslist finds. I had to recut the speaker opening to fit the beefy die cast basket.

The speakers always sounded good to me but after a session with some talent that continually produced a rather harsh feedback both my horn drivers were blown and I had to open them up. The JBL diaphragms were easily replaceable but I noticed that the tiny crossover looked barely capable of 50 watts and I had the cabinet attached to a 450 watt Citronic/Lantek mosfet amp.


Crossover plot and modified crossover

Years ago I acquired a pair of beefy McIntosh three way crossovers and I used these as a starting place before modifying and upgrading them for my use. Here is the schematic and simulation,

McIntosh crossover schematic

As a control I took two three-way active crossovers (one dbx 223 and an Ashly XR70-12, substituting one for the other) and three amps for the first channel and for the other channel I used the modified McIntosh crossover and a single channel amp.

Much to my dismay, after many hours of substituting and upgrading components I could never get anything that sounded as good as the tri-amp system and the flexibility of having all the multi amp controls sure beat all the soldering and expensive caps and chokes I was going through on the McIntosh crossover. Luckily I was able to get a few caps at a very steep discount and a friend happened to have a few high end chokes I was able to test.

The more I added to the circuit to achieve the response I desired the more the crossover sucked up wattage. Initially I was using a 60 watt amp for my testing but as the crossover grew so did the power requirements and the overload lamp started to light at even low volumes. Now I know why there was such a huge heat sink on that crossover, who wants to pay amp dollars just to heat a resistor?

I finally gave up on modifying the crossover and just built a tri-amp system. My studio monitor system now contains a Rane MA 6S 6×100 watt amp with 4 channels bridged driving the woofers and the 2 remaining channels on the mids. An Ashly SRA with 60 watts is driving the horns and believe it or not I’m using a Behringer CX3400 three-way stereo crossover. Its my first Behringer product and for the money (way too good of a price used at GC) is a screaming deal and has even more control and more modern circuits than the more traditional pro units I was testing with.

Behringer CX3400 Super-X Pro Crossover 3-Way Stereo

Not only does the CX3400 have limiting that will protect those horns that had fried but it also uses the new math theories for crossover design and some advanced features like adjustable time delays for phase alignment and CD horn equalization which I needed but lacked before. According to the reviews I’ve found it offers all the features of units that cost five to ten times as much as I paid for this one.

I was always down on Behringer due to a mixer I used years ago where I leaned over the large console and my elbow left a large dent and some of the faders stopped working, this new piece looks very solid but I’ll keep my eye on it and I’ll let you know how it performs over the long run.

The only real drawback to me is I now have to maintain non-standard speaker cables with 6 leads in 8 pole Speakon connectors that only fit this system. This system has been a real pleasure to use, so much so that I find myself often in front of them for reviewing material and even some musicians who had no idea they were modified commented how much better they sounded.

The rack ready for testing

The rack ready for testing. I’m only one rack space taller than the old setup but overall its lighter and infinitely more flexible.

I realize my system is a bit over the top and may not appeal to audiophiles but I’ve noticed that pretty much all of the control room monitors have gone bi-amp or tri-amp. Even my audiophile friend who has a rare and huge bi-amped Infinity IRS system wanted to borrow my crossover.

UPDATE: 11/7/13: Small rack located for rig

6 way power rack

Just a little wiring job.

Found a nice home for my monitor power rack and wired everything up nicely. I’m still short two Phoenix connectors to bridge the amp to drive the woofers to their potential but that will only take swapping one wire and hitting one switch so it will have to wait for the next Mouser order.

Notice how much hand wiring is required with this gear, the Rane is really designed for installation use so it only has hardwire connectors and the Ashly is only 1 7/8″ tall which reduces the cable choices. I used the terminal strip on the Ashly to avoid the 1/4″ jacks which would have also required a custom cable it’s just less parts with the screw terminals.

A closer look will reveal an extra Ashly crossover tucked in there, it’s not doing anything as it’s sort of semi retired. This crossover was used with a keyboard monitor consisting of a wedge cabinet that held a beautiful Sammi Coaxial Speaker and powered by a BGW 250C. I have them listed on ebay but they really are a beautiful setup for keyboard monitoring and a lot easier to carry than my new setup.

UPDATE: 10/24/16: Finally found photos of completed rig. (they were hiding on an unused USBstick)


Power amp harness

Above is pictured the final wiring harness for the 8-way speakon connectors. The silver XLR connectors are paired with the green Pheonix connectors which supply the inputs from the crossovers to the four channels of the Rane amp. The black XLR connectors are from the outputs of the Lantek, some amps used these on the output and I’m not a fan of using the same connectors for input and output so I tried to install Speakon jacks but there’s not enough room! I did search out connectors with the largest lugs on them but I’m still not happy.


Power amp rack in service

This is how the amp rack looks now after being in service for about two years. Underneath you can see the two 8-way Speakon cables plus two pairs of midi and a silver USB over ethernet cable. All the power cables are routed behind and along the wall far from the audio cables. I’ve spaced out the gear to reduce heat build up which worked as the fan in the bottom amp hardly ever goes on now.

UPDATE: 31/10/2017:

3D Printed Speakon Panel 

Some rearrangement of the studio required the amp rack to be moved more to the rear of the studio which caused the speaker wires to be stretched out fully and still was short a foot or two. The solution was to extend the breakout leads and move the Speakon connectors to the front of the rack.

If you need a handy 2 connector panel that holds 8 conductor Speakon jacks just follow this Thingiverse link.