Minimus 7 Loudspeakers - Measurements

I had a roommate back in college who swore by his Minimus 7's. They were scratched up and dented, but still put out a very decent sound. Compared to my larger Polk Audio 5's at the time, I thought the smaller M7's were just OK. But later on, I heard them on another person's system and was very impressed.

Over the years since then, I'd hear a little blurb about the Minimus 7's and how great they were in the day (before the downfall in the 1990's). And in the back of my mind, I'd often thought about getting a pair. What held me back is that I just wasn't convinced they were really all that great, and that other speakers of a bit more cost would sound better. That is, until I found Zilch's thread on the crossover upgrade. :D

So... now I'm convinced I should get a set and try out this upgrade. Except for one major problem. My electronics knowledge and soldering skills are really novice. At best, I'm most comfortable with desoldering a component and soldering a new one in its place. Creating a whole circuit from scratch feels rather daunting. I'm wondering if I have all the posted material I'll need to undertake something like this. So, I pieced together all I could find... Please let me know if there's anything I'm missing that might help? Thanks!


PZ 2.1 M7 crossover mod

Parts List
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Hand Drawn Diagram
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PZ 2.1 Circuit Diagram
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Comparison Circuit Diagrams
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Crossover assembled
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Crossover installed
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Crossover Assembly Guide

  1. A cable tie secures the 3-Ohm resistor to the stock aircore inductor.
  2. A short piece of wire (blue) connects IN (-) to the junction of the 40-Ohm resistor and 4.7 uF capacitor, lower right.
  3. All other connections are made using the respective component leads.
  4. The large inductor lead and tweeter wires dress neatly through the center of the stock aircore.
  5. A #4 solder lug installs on the existing plastic boss, upper right.
  6. Drill the rear panel to mount the large inductor between the two bosses on the inside centerline using #4 machine screws from the outside.
  7. Double-stick foam tape secures the two caps and bottom two resistors to the stock input terminal plate/cup. Epoxy and other glues are suitable alternates.

Examples By AK Members
 
myu701: That should be all the info you need, and then some. How you put it together will depend on which era M7 you have; there are a few different setups of the same crossover. Hopefully you have the same style as Zilch, since there's the most pictures and references for it.

As long as you take your time and do your homework, this shouldn't hard at all for you.:)
 
Minimus 7-Silk...

I was about to order the rest of the parts I don't have to do the Zilch mod on my Minimus 7W's and I saw these at P.E. Looking at the spacing & dimensions they might fit the faceplate for the original tweets. I'm thinking adjust the x-over and L-pad values for a 4-ohm tweeter and see what a silk dome tweeter adds to the mix?

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=279-051

Any thoughts or ideas from the group?
 
Did we measure the impedance of the stock tweeters here?

Also, what is the mod's crossover frequency?
 
Thanks for reposting all that data in one place myu701! I was referencing this thread earlier today at a DIY meetup but had to wade through all the posts to get the relevant data.

Hey Zilch, any chance we could get you to repost myu701's Minimus 7 data post into your initial post? I've found the 1st post reference invaluable with the Econowave thread...

You could just quote myu701 and edit for clarity. For instance, he's missing the before and after curves which tell us why we should even bother doing the mod in the first place!
 
Done, Wired guy.

Thanks for the tip! :thmbsp:

Note to readers: Click "First" at the top or bottom right of posts on any page of this thread to access the summary.... :yes:
 
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I looked at the Zilch mod schematic and gave it a go on my M7w's, but I varied it a bit to use what I had on hand. On mine I used some Jantzen inductors (0.91mH) on the woofers. The resistors I had for the tweeter L-pad were close, but not exact. When I fired them up, they worked but the highs sounded flat. On a hunch, I removed the L-pad and the top end returned nice and clear. Blame it on my system, my hearing or whatever, I like 'em better without an L-pad in place.

To bring the tweeter x-o point down a bit, I added a 0.47uF cap to the 2.0uF one already in there. They really sound good now! Only thing is I don't wanna stuff those big indictors inside the cabs. Since this is the bedroom system and the speakers wires are short, I'm gonna use an external x-o box here. I'll also be making a terminal plate to hold dual binding posts on each speaker. My method ended up being a little different than others I've seen here, but I can't argue with the results. The lil' Minimus woodies really sound sweet now.

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These brass posts are merely temp hook-ups for the woofers.
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more data

Nice thread, although I did not wade through the whole thing.

I've had a pair of M7 with blown tweeters for a while and have been wanting to use the woofers in another project, seeing as how they are quite nice little units. Finaly ran to tests and measured the following T/S parameters, average of two:

Re: 6.3 ohm
Fs: 48.5 Hz
Qms: 1.99
Qes: 0.375
Qts: 0.316
Vas: 6.81 liters
Sd: 57 sq.cm
BL: 6.1 N/A (quite a strong motor for a 4")
SPL: ~85dB

To calculate Vas, I used the sealed box method, using the M7 enclosure itself! I took out the tweeter, crossover and the fiberglass stuffing, blocked the holes with small plastic plates and ductseal and ended-up with a total volume of 1.8 liters, including the driver's cone volume when installed magnet-outside. This volume is pretty close to what can be assumed inside a complete, stock, functioning M7 enclosure, taking into account the fiberglass stuffing that make the box seem bigger. WinISD simulation demonstrates this:

M7 winISD sim.gif

As we can see, the Qtc and rolloff point are on par with what I've often seen described/measured.

I'll probably build transmission lines for my pair of 1218 woofers, as it would be a fun and not too expensive experiment. A 55Hz quarter-wave tuning in a straight (no taper) line will result in a pipe lenght of 155cm (61") and a cross-sectional area of 150 sq.cm (23.25 sq.inches). I'll match this to one of many pairs of tweeter I have at home. As I understand, the 1218 woofer starts rolling-off around 1.6kHz on-axis. I'll first try it as used in the M7, with no Xover on the woofer and 12dB/oct for the tweeter and adjust as necessary. I just don't like going that low with a typical 1" soft dome tweeter. I might try the 1.5" Foster soft domes from Sears Professional cabs; I have 4 of these and they have their resonance around 750Hz. I just don't know if they will go high enough for my tastes.

az
 
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I have always loved these speakers. I bought my first pair a long time, over 35 years ago, back. I recently replaced the caps in two pair of mine with KK (USSR Military Surplus {Kommie Kaps}) Paper in Oil 2.2uF (two in parallel for 4.4uF) caps. The sound improved but now I want to try the AK improved crossover.

And thank you for the circuit diagram, that makes it perfectly clear.
 
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Okay, salivating at the pics and stories...just wondering...

are there any ways/plans to do this with the 77's ??

I am listening to a pair I finished refoaming a few days ago, and I'm just stunned at the detail. Heard mouth clicks and rhythm "stuff" on Bobby McFerrin's novelty (but still great) song "Don't Worry, Be Happy" that I'd never heard before.

Just wondering if it could apply at all. Thanks for being so vigilant about these speakers. :thmbsp:
 
Another M7W pair upgraded

I bought the parts ages ago but finally got around to upgrading crossovers tonight.

The version of M7W I have here is different than the ones I've seen in this thread. Perhaps i missed a post with a similar version, but there sure is some variety.

The version I've got has a rectangular cutout on the back and the same push-button terminals as the aluminum version. Those terminals are integral to a big chunk of black plastic that in some ways mimics what you get in the metal one with a couple differences:

1: The inductor is wound on a 2"x2" bobbin of heavier gauge wire. It's a much bigger bobbin than it needs to be, and it is heat-fused to the upper right-hand corner of the plastic panel in a position that prevents easy mounting of the big 1.0mH choke.

2: The nut for the center screw is inexplicably raised about 3/4" off the panel by a riser integral to the panel. Nothing is attached to it internally so i have no idea why they did this. It just gets in the way.

I tried to think up a way to adapt the plastic panel to the new crossover easily and ultimately gave up.

Instead i bought a few feet of 4"x1/4" wood and cut off two 6" panels, then glued 1mm-thick closed-cell foam to one side of each panel to provide a gasket.

I drilled the new wood panel to match the mounting holes in the plastic one and drilled two 1/4" holes for banana sockets.

Also drilled both panel and the back of the cabinet for #10 screws to bolt down the big choke.

The caps i'm using are donors pulled off of some A/D/S/ car audio crossovers that i picked up for something like $2/ea from Goldmine Electronics years ago. Thus, they have short leads - but they seem like good caps.

The big sandstone 40ohm resistor was out of stock everywhere i was buying parts for this, so i used a 3w 1% flameproof non-inductive wirewound Dale resistor from Mouser. I don't imagine that this will be a problem. Somebody let me know if it will be.

I broke the factory choke off of the plastic panel, coated it liberally with hot glue to keep it from unwinding on itself (as it looks like it was wound by hand by chinese prison labor) and used a crescent wrench to break off the huge flanges on it's bobbin, to allow mounting closer to the woofer.

Board is held in by 4-40 1" screws w/ washers on the outside, lock washers on the inside. I'll probably dab them with some nail polish before closing up the cabinets for good.

Below is the layout i ended up using. The big choke looks askew because it isn't bolted down yet - i can't find my long 10-24 screws and the hardware store is closed.

M7W-new-crossover.jpg


The position of the 2uf cap is Not Ideal. If it had normal length leads it would have been mounted on the center of the board or something.

Come to think of it, I could have swung the 40ohm resistor between the terminals and placed it in the center anyway. Huh. Live and learn.

The lead from the big choke to the + terminal isn't quite long enough so i bridged the gap with about 1/2" of some tin-plated brass strip that was on the workbench. I doubt that this will have a noticeable impact on acoustics.

If i had to do this again, I would ignore the hole in the cabinet for the center screw and not drill a corresponding hole in the wood board. This would allow me to mount the chokes and the 3ohm resistor a half inch closer to the bottom of the board.
 
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Helps if you have smallish hands. Every time I stuck my hand in the hole, I wondered if it was coming back out! :D:D
 
We wanna know how well the mod worked, too, of course.

[Don't be pullin' a Zilchster "I don't do 'How's it sound'" on us here, now... :tongue: ]
 
I have large hands, scraped them up a bit going in and out. You got to see the 2nd crossover i built. The 1st one had some trial and error with the positioning of components vs. the back of the magnet.

I just wish i'd had the self control to go looking for a pair of nitrile gloves before wrangling with the fiberglass.

Anyhow, I think they sound much more clear and neutral than they used to. less 'fun', more 'engaging'.

My set are still hooked up in parallel with the Cambridge Ensemble II passive sub, powered off an old Rotel RA-820BX, and tucked between shelves on my computer desk. The sub is under the desk, and it appears that, at least in this configuration (where the sub is in some sense inside a leaky box with one side open and one side half open), the sub is now outstripping the efficiency of the M7W speakers by a few db.

I need to experiment with the positioning of the sub to see if i can mitigate that. It's not boomy or unpleasant, just feels like sort of a lot of bass for a desktop rig. Right now the sub is against the back of the desk with the ports facing the side of the desk. Perhaps if i move it to the side of the desk away from the wall with the ports facing out the open front.
 
Finished mine today

My wife's new computer speakers were just finished today. The Minimus 7's that were modified are the 40-2030A's, which were built in Japan. As stock the rising treble response is very "in your face." The Zilch inspired PZ 2.1 crossover tames them quite nicely.

I laid out a simple PC board to replace the plastic mount used in the cabinet. The input terminals were cut out of the plastic mount, and held in by the circuit board which has a large cutout to clear the terminals. Both circuit boards were made from a single 4" X 6" board. I used 5 Watt resistors, since the Minimus 7 power capability is very limited. The two FR's are the before and after FR's.

The pics tell the story. Very easy fix, and well worth the doing. They will be powered by a 20 W per channel Sansui, which should provide all the volume the wife needs.

PS: I sure miss talking to Zilch.
 

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Very nice jackgiff! I think that is the neatest and cleanest version of the mod yet.:thmbsp:

Enjoy your speaks, Zilch would be proud.:yes:
 
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