Tuesday, May 24, 2016

LM-1 Bookshelf Crossover


The schematic along with the driver simulation data is available in the form of an XSim file you can find in this discussion. Feel free to explore this design and it's choices. 

At this point in the exploration, I've completed the final draft of the speaker schematic. Having a little more time and energy I was able to get better measurements and re-think the crossover entirely. The final schematic is very simple and uses only nine crossover components.

In the end I used nearly symmetrical second order filters for both sections. The tweeter's -6 dB filter point is right at 2kHz but the woofer is rolled off a little earlier, around 1.5kHz. 

Let's take a look at the "transfer function" chart, or as XSim calls it, the "electrical response" chart. This plots the difference between the amplifier and each driver and shows us what our filter choices are doing electrically. Remember that these effects are additive to the drivers, they are never independent of the driver response or impedance.

As  you might expect, it's pretty simple. The woofer has no change up until around 700 Hz where the low pass filter begins. The super straight line is partly due to the zobel. Notice however that the tweeter level is significantly lower, about 6 dB below. That's because it's much more efficient, and we needed the R1 to bring it down.  The ripple you see in the tweeter slope is due to the tweeter's own impedance interacting with the high pass filter.

Tweeter Level

In the chart below the blue trace represents using the recommended 4.2 Ohm resistor. The red trace shows the effects of using an 0.5 Ohm resistor. The point of showing both of these lines is that you may substitute any resistor between 0.5 Ohms up to 5 Ohms while still maintaining excellent phase matching, so make yourself happy! 

Zobel Network

C3/C4 and R3 are a Zobel network which of course not only smooths out the frequency response of the woofer but in this case also allows for near perfect phase matching with the tweeter.  In a pinch, any combined value of C3 and C4 between around 8.4 and 9.4 will work, but 9.2uF is the optimal value.

Acoustic Distance

The acoustic center of the woofer and tweeter are offset by only 1.1". The small 5 1/4" driver is shallower than the 6 1/2" equivalent plus we are surface mounting it, pushing the woofer towards the listener. Thanks to this combination we have really struck crossover gold in terms of phase matching. A quarter inch the other way and this simple design would have turned out much more difficult.

Part Selection

For your convenience, a Complete Parts List is in another page, but here we discuss choices for the drivers and crossover components here.


As with Kirk's design, I'll be using Vifa tweeters and Peerless woofers:
  • Vifa XT25BG60-04 1" Dual Ring Radiator Tweeter, $35. Please do not attempt to use the smaller, and only slightly less expensive Vifa XT25TG30-04, it lacks the low-end extension. This driver is also sold for more money under the Scanspeak brand. If you think you spy the tweeter in some megabucks speakers, you aren't wrong. It's the same unit, or often the next model down from this one.
  • Peerless 830991 5-1/4" GFC Cone HDS Woofer $35-$45
The $45 fiberglass driver could be inexactly substituted by the the Peerless 830656 paper cone woofer which cost around $20 each. In combination with the cheapest possible crossover capacitors you will get to around a $400 price point. Of course, the biggest savings is to build the cabinets yourself.

Crossover Components

I present a few different crossover grades below. Regardless of the choice of crossover caps and resistors I always recommend Jantzen air coil as the starting point. L1 should be 18 gauge, L2 should be 15 gauge. Do not use a bigger gauge coil on L2! The temptation is there, but the DCR is part of the design. If you must "mod" the coils, use a small-guage foil coil for L2 such as the Goertz 16 guage 1.2 mH coil available at Madisound or any other coil with a DCR between 0.330 and 0.4 Ohms.

Cheapest Possible

To stick with an absolute bargain build, use with Bennic caps and Dayton audio grade non-inductive resistors. 10W is close enough if they don't have 12W. You could save a few more bucks ($10 total) by using bi-polar electrolytic capacitors in the woofer, but please don't.

Frugal Freddie's Compromises

If you want to spend just a little more, I suggest Mundorf MKP ($8) for the 8.2uF cap in the tweeter section. Use Mills resistors. Audyn and Jantzen are also highly thought of.

Balanced Betty

Betty buys parts that are matched by the price and quality of the drivers. She would suggest Clarity ESA cap in the tweeter section of if you want to stay with all Mundorfs, the Mundorf EVO Aluminum in Oil. Either should be under $20.

Stick with Mills resistors everywhere, and Mundorf MKP caps in the woofer section.To keep costs down she might choose Axon caps in the woofer though.

To the left you can see my own build. I ended up using Clarity for the tweeter, along with mostly Axon caps in the woofer section. To make a boring story short, I happened to have 7.5uF Axon's lying around, so I got 1.8uF Mundorfs to make up the Zobel. All resistors are Mills.

The savvy builder will note that the coils are aligned in the same Z axis. Not to worry, the boards themselves will be mounted at 90 degree angles! One on the bottom of the speaker and the other on the side. 

The "OMG Are you nuts?" Build

Use Jupiter copper film caps for the tweeter. Capacitor cost? About $800 per pair of speakers. Hate your kid? Does he/she have an overripe  college fund? Do it!  OK, I'm kidding, it's completely out of balance. Save this kind of money for your $300 or more tweeters.

Please feel free to experiment with parts you like, can afford, and have available. These are just my personal recommendations. If you find caps you think work really well leave me a comment.

The Scientist Build

This speaker lends itself very well to learning about the sound of capacitors. The reason is the very high quality tweeter and that it uses a single capacitor. If you like the idea of experimenting with capacitors yourself, I'd suggest you wire the crossover so that the tweeter cap is external. Add a second set of banana jacks on the rear, spaced about 2-3" apart and connect the tweeter capacitor there. Now you have a very convenient experimentation lab which would allow you to swap capacitors or add small bypass caps instantly.

Driver Phase Matching

"Phase matching" refers to how well two drivers play together across the band in which they both  contribut. I usually use the -20dB level as my cut-off (more or less). You can see in the chart below that this is about 700 Hz to 3kHz. That's actually a pretty broad range brought about by the low crossover slopes. Still, notice that wihin this range the dotted red and green phase lines are so close together.

We can also see that the phase alignment where they cross 180 degrees is perfect. The longer the phase angles match the more the drivers will blend in with each other and disappear. Here's another view of that effect. Let's compare the normal response with an inverted driver (either one). In theory this is the absolute worst possible alignment:

Not only do we have a 20 dB dip at the crossover frequency, but look at how symmetrical and broad it is. Again, that the inverted driver produces this text-book null indicates the LM-1 have excellent phase matching before, during and after the crossover region. This will allow the drivers to blend in, minimize lobing and comb-filtering as the listeners location changes.

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