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My Magnepan 1.6QR DIY Upgrade III: Going PLLXO!!

Posted by dumpingground (A) on January 29, 2013 at 20:26:35

First of all a huge thank you to Davey and Neolith, both who put up with my many email's and supplied a tremendous amount of help during this DIY project. Neolith in particular supplied an updated version of his Magnepan Crossover calculator spreadsheet.

Interested readers can review my prior journeys thru Passive Bi-amping Crossover (speaker level) and Active Bi-Amping Line Level Crossover (ALLXO) using a minidsp.

http://db.audioasylum.com/mhtml/m.html?forum=mug&n=171078
http://db.audioasylum.com/mhtml/m.html?forum=mug&n=184970

First of all what is a PLLXO aka Passive Line Level Cross Over? A PLLXO is a crossover that is installed BEFORE your power amps and performs the crossover functions to separate the high and low frequency signals BEFORE they go into the power amps. To use a PLLXO you need to have a preamp or use preout's on your AVR. The PLLXO sits between the preamp/AVR and the power amps. You will also need (for the 2 way Magnepan 1.6) a total of four channels of power amps (left tweeter/left woofer/right tweeter/right woofer).

You of course must also do some surgery on your precious Magnepan speakers to bypass the internal factory crossover since the PLLXO will be performing that function instead. Check out the first link above if you wish to see that part of the story.

So why would I want a PLLXO? In general a Line Level Crossover (active or passive) has the distinct advantage of doing the crossover function before the power amps. There are several advantages over a traditional (AFTER power amp) passive speaker level crossover. Since the high and low frequency signal is split (cross'd over) before the power amp, each individual power amp only has to amplify the high OR low portion of the frequency. This yields greater efficiency and a better separation of the spectrum to be amplified. Also, because the signal is BEFORE the power amp, you tend to use lower voltage/current rated capacitors and resistors which are MUCH cheaper then the upgrade resistors/capacitors that tend to be used in upgraded passive speaker level (AFTER power amp) crossovers. One can also choose to use different power amplifiers for the tweeter and woofer portion of the circuit. For instance for the lower power demanding tweeter circuit a lower wattage tube base Class A amplifier becomes an easier option. In my case I am using Parasound HCA-1000A's on all of the channels. Specifically for the PLLXO, since it is passive, you don't need to worry about a power supply, which greatly simplifies the design.

Now why do I need a PLLXO if I already have a ALLXO (with my minidsp)? Well here the answer is a combination of I am a audio geek wanting to try something new and a desire to see if the extra A/D D/A step of the minidsp is actually holding back the quality of my existing setup.
A couple of PLLXO negatives to highlight. A PLLXO (vs a ALLXO) will be system dependent. For different preamps/AVR and power amps you will need different component values in the circuit. A PLLXO will cause several db (1-8db depending on design) of loss. So you will need to turn up your volume to compensate and hopefully have enough gain. Cables also must be kept short since your preamp/AVR is now driving the sound signal thru not only 2 sets of RCA-RCA interconnects but also thru the PLLXO circuit itself.

Ok, with the preliminaries out of the way, how did I implement my PLLXO. Here are the basic steps I followed:

1. Model different values of resistor/capacitor components to optimize PLLXO for a given Zout/Zin.

See Neolith's wonderful spreadsheet here:

http://home.comcast.net/~neolith/Stereo/PLLXO_16.ods

For a PLLXO the output impedance of your preamp or AVR as well as the input impedance of your power amps will cause the PLLXO circuit to be different for each setup. In my case my Parasound 2100 Preamp has an Zout of 60 ohm and my Parasound HCA-1000A has a Zin of 33k Ohms.

So in Neolith's spreadsheet make the following changes:
Zpre Cell L9 => 60
Zamp Cell L10 => 33000

Then start playing with cells P14 and P16 for the tweeter circuit and cells L21, L22, L23, and L24 for the woofer circuit. Look at the resulting graph and try and line up the solid black and red curves with the dashed black and red curves.

Some ground rules to keep in mind. Any changes to the values will affect the speaker XO (cell L29), fc of the tweeter and woofer (cell L25 and P17), and filter Q (cell L27). Diverging from the OEM values will change the sound of your speaker to some extent. Also, keep an eye on the insertion loss (cell L26 and P18). If the tweeter and woofer insertion loss values are different you will end up with some level issues in your PLLXO between the woofer and tweeter response. And finally, the Zs (cell L30) or impedance of the PLLXO itself should be kept to no larger then 10% of the Zamp (Cell L10).

And here is what the circuit looks like:

2. Prototype the PLLXO using a solderless breadboard.

I used this to test out my configuration with a varity of low cost capacitors and resistors sourced from digikey:

http://www.amazon.com/BB830-Solderless-Plug-BreadBoard-tie-points/dp/B0040Z4QN8/ref=cm_cr_pr_product_top

Be careful that the leads do not come loose during your testing or you could damage your speakers.

Keep in mind that one of the main shortcomings of a PLLXO is that it is impacted by cable length, capacitance/resistance of cables, input/output impedances from your preamp, AVR and power amps. In some cases you may have spec values for all of those items, but the specs may not be accurate or have some frequency dependences that are not easily understood. Prototype and measurement is recommended.

Just in case, I initially hooked up my solderless breadboard XO and poweramp mess to some old 2 way speakers I had around. The XO was completely wrong of course for this random speaker, but my purpose was just to make sure that the speaker did not blow up from a short in the XO or some other issue. I then moved on to hooking up the XO to my magnepan speakers.

3. Measure with REW

I highly recommend that you measure the before and after response of your PLLXO vs the stock factory crossover. In my case my actual acoustic performance had a 2.5 db drop off on frequencies above 2500hz. The spreadsheet model did not show this. But my REW measurements clearly showed the problem.

You can use REW or your favorite acoustical measurement package.
http://www.hometheatershack.com/roomeq/

I was never able to determine what caused the 2.5 db rolloff. I swapped out different cables and switched between my preamp and an AVR preout signal to feed the PLLXO. So I went back into step 1 and changed the spreadsheet to compensate for the rolloff.

Here is my initial test using the following values:
Woofer circuit: R1=2200 R2=3300 C1=100 and C2=120
tweeter circuit: R3= 3650 C3=2.92

Note the incorrect 2.5 db drop off measured above 2500hz. Purple line is the PLLXO.

I then adjusted the model with the following values:
Woofer circuit: R1=2700 R2=7500 C1=68 and C2=100
tweeter circuit: R3= deleted C3=3

Note that this does not model at all close to the factory XO however it measures nicely with + or - about 1 db to my minidsp (which implements the factory XO curves).

Not shown, but I had previously measured my minidsp response vs the original factory curve and they both agreed nicely. Also not shown is the phase thru the crossover region, but I measured that as well and the PLLXO matched up nicely to the stock values.

All measurements were made at about 1 foot from the speaker mic pointed at the tweeter panel. Because magnepan's are planar speakers, they do measure somewhat strangely. I also did some measurements at the listening position. (not shown) At the listening position the general downward dip measured with the PLLXO and minidsp above 800hz levels out somewhat.

4. Solder a final board

Ok, now that I have a good circuit it is time to get out the soldering iron. Lets talk a little about component selection. You can go crazy with exotic resistors and capacitors. That is up to you. Keep in mind since you are buying devices for a PLLXO, they will be placed in the signal path BEFORE the power amps, so much lower value and rated devices are ok (and significantly cheaper).

For capacitors, I recommend that you avoid polyester technology caps and stick to metalized polypropylene film caps. Try and find some with 5% tolerance or less. Some good ones to consider are Vishay MKP-1837/MKP-1839 (160V), Vishay 416 series, Rel-Cap RTE, Panasonic ECW-F and ECW-H.

For resistors, I recommend metal film because the have good thermal noise characteristics for signal path audio. Look for 1% tolerance and low 50 ppm/c and .25 or .5 watt. Some nice ones to consider, PRP, Yageo MFP, Vishay CMF55 series.

I used PRP resistors thruout. Rel-Cap RTE in the tweeter circuit and Vishay MKP-1837 in the woofer circuit. Plus the cost of RCA jacks and terminal blocks on the PCB, Cost in parts was under $50. Significantly cheaper then my previous passive speaker level crossover and also minidsp ALLXO.

Parts were sourced from digikey, soniccraft and partsconnexion.

I initially built my PLLXO and placed it in a plastic project box. However initial testing revealed that it was highly sensitive to signal and power cable noise from my adjacent equipment. There was a noticeable hum from my speakers.

I had a electrical 2 gang outlet box and I rehosted the PLLXO which provided shielding and all but eliminated the hum issues. Note that the shield/ground of the RCA jacks are all grounded to the metal box as well.

5. Enjoy your sound.

I will update this thread with my listening impressions as I enjoy my new PLLXO. But so far I am very impressed with my experiment. The PLLXO sounds quite a bit more "airy", "3D", "clear" and engaging compared to my minidsp which itself was pretty good.

Edits: 01/29/13 01/29/13 01/30/13 01/30/13 01/30/13 01/30/13 01/30/13

This post is made possible by the generous support of people like you and our sponsors:

Parts Connexion

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