Inductor swaps/upgrades, M7 Series II

littlewoodboats

I am gearing up for a total rebuild of my Monitor 7's. I have followed quite a few threads detailing cap and resistor swaps but see a thread here and there discussing inductors.

Why is one type chosen over another?

westmassguy

The original inductors were mostly hand-wound. Inductors from Jantzen, for example, are machine wound, then baked. They are superior in every respect to the older type.

littlewoodboats

Thanks for the reply. What about the different types? Looking at some of the parts sites I see air core and some wound around plastic bobbins, iron core types and some that looked to be foil.

Looking at speaker threads at another site I have see different types used and was just wondering if there was a reason. Does a different type yield a different sound?

westmassguy

Stick with the baked Air Core. Foil is good, but tends to oxidize over time. Bobbins are air core, and can be hand or machine wound. They're the same as the small shunt Inductors you have on the crossovers now. Iron Core, P-Core, and Laminated Steel types can be good, as long as you don't exceed their power rating, and if they're poorly designed, they can introduce distortion even before you reach their power rating.
You'll need an accurate LCR meter and DMM to measure the existing coils, otherwise you'll throw off the voicing of the woofers and tweeters.

littlewoodboats

Thanks again Dave. Is the DCR spec listed somewhere? I see the mH and wire awg on the drawings but no DCR. I was hoping I could build the new crossovers without pulling the old ones out of service.

I have spare drivers for everything including the RD0198-1 so my plan is to build a second pair. The cabinet shop around the corner has a WartHog cnc so duplicating the cabinets I have now should not present any issues. If I had one true knock againts the speaker it is the poor quality of the particle board cabinets.

westmassguy

DCR is rarely listed. It's best to measure it. I also always measure the Inductance on the new coils, they aren't always what they're supposed to be.

littlewoodboats

Can you suggest an affordable LCR meter I can pick up? Second hand is fine.

westmassguy

Fluke or BK, used would be my recommendation. Even the used ones don't come cheap though. If this is a one time thing, I can measure my Monitor 7's Inductors for you. Which version do you have? Which Tweeter/Woofer combination?

littlewoodboats

For right now this is a one time thing.

My 7's are the series II and have the 6502mw and sl2500 tweeter (changed to the RD0198-1).

L1 is 0.22mH #22awg, L2 is 1.75mH #18awg

https://us.v-cdn.net/5021930/uploads/attachments/4/6/7/2/8/18774.pdf

Thank you for your time and efforts

westmassguy

I have the earlier version SL2500, so they won't match. Get yourself a good DMM and measure the DCR of the Inductors. You can then match the Inductance and DCR with Jantzens list. They will most likely be special order from Parts Express. If you can get the DCR, I can lookup the part numbers.

littlewoodboats

I did not realise there was more than one type with the SL2500. The list of crossover diagrams only shows the one.

What is the difference between the two? My sn# are 9004888 and 9004903.

westmassguy

There are at least 3 versions of the Monitor 7s, plus the late 70s version that we don't have schematics for.
There is only one REV2 with the SL2500, all the others used SL2000, SL1000, or the originals with Peerless tweeters

littlewoodboats

I thought there was only one SL2500 until you wrote you had the early version SL2500.

westmassguy wrote: »

I have the earlier version SL2500, so they won't match.

Threw me for a minute. Looks like I am going to need the LCR meter. I have enough of the correct types of drivers (mw's and PR's) to do several sets of speakers should this project go well. Generating the file for the WartHog is the time consuming part because of the mapping required for the front baffle and I have it for the M7 Series II. I am not sure if prior model M7's would be the same but see no reason for them to be different.

Always room for another piece of test gear.

westmassguy

Sorry, that was a typo. Should be SL2000.
Are you doing a point to point crossover, or using the original?

littlewoodboats

I am going to do point to point on a piece of perf board. These crossovers are simple enough it should not be a problem

For any SDA project (2B tl) I might get to do in the future I would use the boards from Gimpod you know so well.

I used dayton caps with mills resistors in my 5jr+ and noticed a big enough difference it convinced me to go to higher grade parts for the 7's. Clarity caps, mundorf resistors, cardas wire and solder plus better binding posts. Stock drivers with brand new everything else.

westmassguy

Sounds like a good plan

littlewoodboats

How critical is it to be exact on the cap values? I need three caps. A 12uF and 8uF for the high pass with a 27uF on the low side. The 12 and 27 are no problem but the closest I can get to the 8uF is an 8.2 (Clarity caps totaling right at $120.00 for everything)

The Mundorf MOX is available in the correct value which is good as I do not see a 2.7 ohm in the Mills. I take this back. I do see the 2.7 ohm in the 12 watt Vishay Mills

If they were your speakers what would you do? Thanks Dave

How much would it run to have you build a crossover for the center channel from your custom build?

westmassguy

The 8.2uf is fine, it's within 3%. The tolerances listed on the schematic are +/- 10% for Caps and Resistors, and +/- 5% for Inductors.
If you're doing point to point to point on these, the center channel would be a piece of cake as well. It's a modified Monitor 10 crossover.