Introducing the LM-1C

The LM-1C speakers are part of the Leach Memorial speaker project, started here.

The 1C's are tonally balanced to be placed as shown, on an entertainment center. On or in an entertainment center however is never ideal so despite my best efforts, modest equalization will be required for them to sound their best. If stand mounting, EQ is strongly suggested to make up for the minimal baffle-step compensation built in.
  
Originally I was going to make the LM-1 and then add a center/D'Appolito-like speaker for the center or for those who wanted more output and clarity, but I made it first because I really needed a center channel more than I needed main or surround speakers. However, the qualities of a good center channel like this is exactly the same as for any other speaker in a home theater or stereo system. You may use the LM-1C as a center channel with four LM-1's or you can make all of your home theater speakers LM-1C.  The choice is yours.  There's nothing particularly "center" or "home theater" about the LM-1C.

The LM-1C is MUCH better free standing than the LM-1 however. The LM-1 does better in a bookshelf or on top of a desk or entertainment center. So this is one reason for choosing the LM-1C over the LM-1. 

Now, I did promise to post the design and details of the LM-1, but the 1C is a separate project.  It doesn't mean I won't post the schematic.  Just I won't be as diligent. The picture is the tweeter crossover prototype which I have built from parts I already had lying around. You'll need another small board for the woofer.

Instead of wire ties and anchors you can just hot glue instead. Wire ties are best for tweaking and experimenting.

As you can see, the LM-1C has the same driver complement as the LM-1, just doubled up on the woofers, and the ports are not the same either. One thing I've noticed so far is that the speaker needs to be tilted up to give the correct tonal balance. 

Here is the schematic.  I was using actual measured values for the coils, based on the parts I had on hand.

I used the  Dayton 0.75 cubic foot MTM in Cherry along with a pair of 1.5" ports cut to 2 7/8".

 

Costs

Here we go, the "money shot." Prices are for the individual speaker

• Cabinet: $149
• Drivers: $95
• Crossover: About $75 and up. 
• Miscellaneous: Speaker connectors, 2x 1 1/2" ports

Approximately $350 assembled.

Measurements

Update March, 2015:
I took measurements on top of a sub instead of on an entertainment center. Results were MUCH better than below, so I strongly encourage you to try this speaker out in free space on a stand if possible. Of course, if you can't, your receiver/processor's normal room EQ should handle the changes needed automatically.

Original article:
After assembling a speaker it's always a good idea to compare the finished impedance plot to the predicted plot.  This is a quick way to tell if you messed something up.

Here you can see I got it pretty much spot on.  The minimum impedance is around 4.3 Ohms at 195 Hz with an electrical phase near zero (not shown). If you are using any sort of solid state amplifier this will be a very easy load.  Some tube amplifiers may not be happy with the impedance peak though, so consider adding a 100 Ohm resistor across the speaker inputs if you are using a tube amp.  Also System7 over at DIYaudio likes to do this just to prevent the chances of ultrasonic oscillations.  May be worth trying.

 

Of course, what everyone wants to look at is the FR.  Sadly I only have the predicted so far.  I just don't have the time / energy to measure it again, especially since I'm still moving it around.  But for now, here is the FR as predicted by XSim.  As before, this should be far-field responses on the cabinet, as opposed to quasi-anechoic.  Scale starts at 20Hz.

Cheating with DSP

So this is all well and good, but putting a speaker on top of or in a TV stand is rarely the best sounding location even if it is the most convenient in the case of the center channel. At the end of the day I cheated a little by adding parametric filtering via miniDSP:

Center Channel EQ

Sadly I have lost the "final" results, but as you can see the EQ required was pretty modest, and mostly involved broad cuts between 70 and 250 Hz with a couple of boosts. I deliberately limited the EQ to be between 30 Hz and 500 Hz to minimize stress on what really is a small speaker, and to avoid over-EQing what is otherwise a pretty well balanced design.

The bosts at 62  and 300 Hz are 2 and 3 dB respectively, while the 3 deepest cuts average around 6 dB. Overall a good power balance, especially with the lowest change being a significant cut around 40 Hz. This should not stress the amplifier or speakers extraordinarily, and indeed when playing Pirates of the Caribbean and Battle Los Angeles at theater levels from 9' away there was no audible strain at all. In addition to this EQ, I relied on the Oppo 103's integrated bass management to provide high pass filtering at 80 Hz.

Of course, this EQ is very much situation dependent! I measured the speaker after I had placed the TV behind it, and then used OmniMic to create my filters, so I would not encourage anyone to take this chart and attempt to apply it to their own build.

What I mean to show is that a little judicious EQ can help improve the LM-1C or really any center channel speaker. Subjectively the thing that improved most was reducing boominess in the dialogue, which is usually in the 100-200 Hz range. Reducing that made dialogue much clearer and the center much more transparent.