Crossover question (Monitor 5JR+)

On3s&Z3r0s

I've been doing some XO upgrades (inductors) on a pair of SDA-2B's and since I was ordering parts and was going to have the iron warmed up I figured I'd play with a pair of Monitor 5Jr's I had upgraded a couple of years back. They were the first series 5Jr+ with the PR and the SL2000 tweeter. I had put a Dayton 12uF cap in them and RD0-194's.

For this latest round of changes I decided to try the RD0-198's in them. So, I got two 12uF Sonicaps and two 1.5uF to make the 13.5 (figuring if I didn't stick with this configuration it would be easy to repurpose the 12s). I just reused the stock PCB and paralleled the 12 + 1.5 combination. So, I have two questions:

First, I'm less than 10 hours into burn-in, but the 198's seem significantly louder than the 194's so far. They're definitely also more detailed, but right now it kinda sounds like the MW-6502's are going to have a hard time keeping up. I'll keep burning in and reserve judgment for a few more weeks, but when I burned in the 198's in the 2B's, I don't remember them getting significantly quieter. Very different speaker obviously. If they don't get quieter I was wondering what it would do to increase the capacitance on the small cap to get say 14uF in that spot instead of 13.5? Likely to make any difference? Or is paralleling one large(ish) and one small cap like this just doomed to fail for some reason?

Second, I was noticing that in the Series II Monitor 5Jr schematics the polyswitch has been removed. I checked out a couple of pics recently posted of the stock version of that crossover (http://www.polkaudio.com/forums/showthread.php?158435-Thrift-store-score-M5jr-series-2&p=2017282&viewfull=1#post2017282) and sure enough, no polyswitch I can see. I had put a .5ohm Mills resistor in the spot for the poly in the earlier crossover I have. Based on the location of the polyswitch in that design it seems like it's on the path to ground from the tweeter. So, could it possibly be having any effect on the sound either way, or should I just pull that out and jumper it?

Thanks, as usual, for the expert advice!

westmassguy

You're actually bypassing the large 12uf cap with the smaller 1.5uf. That's bad juju. It's always best practice to use two caps as close as possible in value. The smaller caps react must faster then the larger one. Sonicap will provide the exact 13.5uf cap, for slightly more than a standard 13uf. That could be part of the problem. I would remove the .5 ohm Mills and put in a jumper.

On3s&Z3r0s

Bummer, I was afraid that maybe the small cap might act as a sort of a bypass cap just because it's smaller, but I googled a bunch and convinced myself that bypass caps did what they did because the ESR of the bypass cap vs the cap being bypassed would be significantly different. I was hoping since these were both Sonicap Gen 1 200v caps that the difference between the 1.5uf and 12uf wouldn't be big enough for that to be a problem. But, something does sound a little off. Friggin electrons... why can't they ever just do what you want.

It sounds like two identical valued caps would have the desired effect. I guess the best apples to apples comparison if I just wanted to hear the difference in the tweeters would be to get two Dayton 6.8uF caps and piggyback those instead since it's a Dayton 12uF I was using with the 194s. I'll have to pull everything apart again to take out that .5ohm Mills anyway. If it sounds very different with the right values for the capacitors I could always spring for the custom capacitors from Sonic Craft later.

westmassguy

I was going to suggest the Dayton 5% 6.8uf for experimentation. A lot cheaper than the custom Sonicaps. Clarity PX might come in the same values too

On3s&Z3r0s

I decided to bite the bullet and go with the Sonicap 13.5. Also removed the .5 ohm mills and replaced with a jumper. Definitely an improvement and a good lesson learned not to parallel different value caps in the crossover. Thanks!

westmassguy

Good deal. Let us know how it sounds when the parts come in

On3s&Z3r0s

The new caps actually came in on Friday... Sonic Craft was johnny on the spot getting those out the door after I ordered them. They've got about the same 10 hours of burn-in on them at this point that the last combo of caps had when I decided something was wrong. With the single 13.5 cap they sound much better. The tweeter isn't overpowering the MW anymore, so overall a vast improvement. Thanks again.

westmassguy

Great to hear. Enjoy them