Specs on crossover for Polk MM5251

jtorre4272

Well, I am here because I did not get an answer from Polk tech support that provides me with any info I can use. Their response was to refer to the owners manual, however, aside from sensitivity and frequency range, none of the good info I need is in there.

I don't think asking for a schematic is a big deal simply because the layout is simple enough that anyone with an engineering background could trace the circuits out. Guess Polk considers it a trade secret. My opinion is that this type of stuff is not a trade secret but I will not challenge their position.

I can reverse engineer the crossover. It will just take time. It uses what appears to be a second order network (12dB/octave) of coils and caps for the tweeter and woofer, and also an L-pad attenuator circuit (two voltage divider resistors) for the tweeter to achieve the -3dB switch setting. I also found it uses a simple resettable fuse (The brownish looking disk). I also think there is a dB adjustment network in there as well since the woofer and tweeter likely have different sensitivities. I think it may also use an attenuation circuit to get rid of the peak at the crossover region (If a Butterworth implementation). I will know more tonight.

12dB/octave seems low for a tweeter. I was thinking more like 18 or 24 dB, but when I count up the components, the tweeter may be on a 3rd order filter.


Below is my email

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Dear Polk Support,

Hopefully you can forward to the appropriate people in engineering as my topic is somewhat complex. I'll preface by saying I am an electrical engineer by trade and not some hack looking to waste anyone's time.

I am trying to build a 3-way component crossover network using your MM5251 2-way system. I just purchased it along with an MM6501 system.

I have added a DXI350 3.5" coaxial speaker to the setup where I carefully removed the tweeter and what appears to be a 3.3 microFarad capacitor in series (surprisingly high 6dB/octave HPF @ 12kHz? Perhaps a way to get away with a 1st order HPF due to the more gradual roll-off?) and plan to use the woofer as a mid/high component of the MM5251 system with a band pass filter between 500Hz and 5kHz. Not sure yet whether sensitivity will require me to add a dB resistor to have the response match the other speakers.

The supplied capacitor included with the DXI350 woofer is 2 microFarads, and has too low of a cutoff frequency (200 hZ) for what I need. I can't use it.

I am writing because I would like to integrate a 2nd order bandpass network with the crossover that came with the MM5251 speakers. However, I have no idea if it needs to be a 6dB/octave, 12dB, 18dB, etc. It all depends on how the 2-way crossover is configured, I guess. Plus this will tell me how to set the polarity on the speaker.

The vehicle I am installing this system in originally had a factory 3-way component system in each front door, and I am hoping to keep it that way.

Is it possible for you to share a schematic or at least some detail on the crossover setup I currently have?

I could reverse engineer the circuits if necessary, but that would be very time consuming and I figured I would ask you first for the technical info I need.

Thank You

MacLeod

24 dB more than likely is what you're going to want at cutoffs of 315 and 3.5 to 4 KHz then let your tweeters take over from there.

jtorre4272

Well, as mentioned, I spent a couple hours tracing out the crossover network. If anyone is interested, I can provide a schematic.

highlights:

1. Surprisingly the woofer circuit (2.2 ohms) uses only a single inductor for 6 db/octave roll off at 1.1kHz per my calculations.
2. The woofer likely has inherent inductive instability thus they decided to implement a Zobel network to the woofer circuit. It is a good solution as it maintains a stable load on the amp and keeps the passive inductor in the LPF working to a constant impedance at all frequency ranges beyond the resonant peak. What I don't understand is why 1.5 ohms was selected for the zobel resistor. It should be at least as high as the woofer, which is at 2.2 ohms.
3. The tweeter circuit appears to not use an L-Pad attenuator like I thought. But rather a resistor in series with the load. 0dB is 4 ohms. -3dB is 6.2 ohms.
4. The tweeter uses a 2nd order filter that I believe has a 12 dB/octave roll off at 3.375kHz. I had to back calculate the cap value based on the inductor. I couldn't see the value on the cap as it was buried under glue.
4. There also appears to be a resetting fuse in the tweeter circuit. Not surprising as the tweeter is sensitive to spikes.

Based on all this, I am implementing my 3rd mid-high component with erse components for the crossover, and using a 2nd order band pass at 600 to 6kHz in conjunction with a Zobel circuit. Problem is I can't seem to find the inductance of the DXI350. I sent an email to Polk but no reply. Will post a request separately.

I did my best to understand the circuit so hopefully I have not screwed anything up.

pentoncm

Why not just make things simple and go active?

jtorre4272

I guess I like the challenge of building my own circuits. Plus its pretty cheap to do it all myself as the hardware isnt all that expensive. I am an electrical engineer.