Opinions on Shack Minimus 7 rebuild

[WayneN]

Re: Opinions on Shack Minimus 7 rebuild

I was taking a break from the renovation work and I checked out my MCM email flyer and the MCM 55-1840 caught my eye on sale $7.50. I plunked in the web the model and came up with db41 by Dan Carroll http://users.d-web.com/dbrown/db41/db41.htm

I remembered that I had posted and suggested that you look at Paul’s Overnight Sensations, but maybe this might be a more economical solution and I am not sure if this will work for you? I’m sure the more experienced can comment if the parts will work in the small cabinet with modifications.

A friend of mine used to work for Radio Shack and sent me this information.
Not sure if you have this or need this?
Specifications

http://support.radioshack.com/support_audio/doc31/31593.htm

Detailed Parts List

http://support.radioshack.com/support_audio/doc23/23158.htm

WayneN

[DDF]

Re: Opinions on Shack Minimus 7 rebuild

I believe the Minimus 7 drivers changed over the years. If you want to retain the stock woofers and keep the job simple, I recommend my xover and a Sonance tweeter if you can find it.

Using the stock cabs and woofer (with silicone mod):

Here are the system response files, at “on axis” (0deg hor defined here 4 deg below tweeter axis), 30 deg Hor/0 deg vert and 0deg hor/+10 deg vert (this last one is response at standing height in my room). One comment on the low end, the premature roll off is from the windowing, and that 400Hz dip I think is a floor thing, so true quasi anechoic is a bit smoother.

[c2rower]

Re: Opinions on Shack Minimus 7 rebuild

I have two sets of the Minimus 7's from the late 70's and I recently upgraded them using a crossover upgrade kit that includes a Zobel network. They are wonderful little speakers that have stood the test of time.

[Misch]

Re: Opinions on Shack Minimus 7 rebuild

PDF of my CAD files for baffles. There are small variations between different boxes, so there may be some tweaking required. I have three sets that the baffles were tried on.

[rwrankin]

Re: Opinions on Shack Minimus 7 rebuild

Thanks for posting the CAD drawings Misch, very nice and should help a lot.

Cheers!

[DDF]

Re: Opinions on Shack Minimus 7 rebuild

The very best Minimus 7 mod I've seen, was done just recently over on the AudioKarma site. The thread had several crossovers from different mods featured, with comparisons to the final version done by the OP (Zilch). It's 22 pages long, but on the last page you can see the measured FR with the final crossover.

BTW: The mod uses the original drivers! It's quite impressive, IMHO.

Here's a link:

http://audiokarma.org/forums/showthread.php?t=199790

Best Regards,
TerryO

Having modified a few of the Min 7s, I checked out the audio karma site. The first crossover with that +5 db bump at 3kHz was a bit ill advised. These woofers have little to no bass, the last the speaker needs is a rising or bright high end. The last xover looks much better but it doesn't address the real weaknesses of the speaker: the drivers and the box. The baffle is solid enough, its tiny and dominated by the drivers.

The box should be damped internally and heavily stufed at 1.5 to 2 lb/cft.

I think its hard to term a mod "best" until they're tried. I've tried all these approaches.
- modify the stock xovers
- treat the drivers and modify the xovers
- replace the tweeter and redesign the xover.

Its too bad no one tried the sonance tweeter, its far superior (cleaner, better off axis, plays lower) than the stock tweeter (treated or not), and they only costed $5 a pop.

The woofer also benefits from coating to control the modes. Silicone worked for me, GTforme went further here:

http://www.diyaudio.com/forums/showthread.php?postid=1252280#post1252280

I used silicone and it usefully extended the low end compared to the near field plots on the audio karma site.

The real goal with this project is to minimize cost and reuse the cabinet. If you spend more than $50 or so it would probably be wiser to build something else from scratch.

I've broadly tried all the different approaches mentioned (except slapping in a new baffle/full way) and by far the best sound and measured response was obtained with the Sonance+treated woofer.

No contest.

Dave

[Zilch]

Re: Opinions on Shack Minimus 7 rebuild

Having modified a few of the Min 7s, I checked out the audio karma site. The first crossover with that +5 db bump at 3kHz was a bit ill advised. These woofers have little to no bass, the last the speaker needs is a rising or bright high end. The last xover looks much better but it doesn't address the real weaknesses of the speaker: the drivers and the box. The baffle is solid enough, its tiny and dominated by the drivers.

A fun project born of frustration with the plethora of misinformation found on the 'net relating to Minimus 7, it consumed my entire holiday season that year. We tested every crossofer mod we could find, including the popular "kit" sold on eBay, plus took our own shot at it with good outcome.

[Today, I'd go after it with PCD.... ]

[DDF]

Re: Opinions on Shack Minimus 7 rebuild

A fun project born of frustration with the plethora of misinformation found on the 'net relating to Minimus 7, it consumed my entire holiday season that year. We tested every crossofer mod we could find, including the popular "kit" sold on eBay, plus took our own shot at it with good outcome.

[Today, I'd go after it with PCD.... ]

They're addictive aren't they?

Its the DIY speaker equivalent of playing the nickel slots. You figure "nothing in, nothing to lose" until you realize how much time you've burned.

Still, I can say this is the only speaker I've ever made that I nailed the crossover on the first try, and didn't even change a resistor.

The low end overloads with only 10 watts or so but they're perfect for my own near field low level use.

I made a 6.5" wide by 10" high bookshelf to replace them and they were like VMPS towers next to the little cabinet of the M7. I still can't find a combo in this size that I could replace them with. When they're that small, cabinet wall thickness becomes a major issue.

[johnnail]

Re: Opinions on Shack Minimus 7 rebuild

I recently came across an old pair of Rat Shack Minimus 7 speakers that I bought back in the late 70's and because of their age they need a serious redo. I received some advice from one of the techs @ PE and he suggested these drivers:

Tweeters - Pioneer 3/4" Polymer Dome Tweeter

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=270-058

Mid/Woofer - Peerless 3.5"

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=299-256

Suggested crossover point was suggested @ 3,500 Hz

see attached for XO topology.

The PE Tech also mentioned that an L-pad would be needed and suggested a variable one. I am thinking that I would like a fixed value L-Pad and need a little help on that one. I have never been that good at crossover design, so please don't hold that against me! Once completed, they will be mated with a small sub that has yet to be designed and built.

What do you think? Any suggestions?

Thanks a ton!

I am not familiar with the Minimus 7, but if you can make it sealed, then I would take a look at the sale driver at PE Part number 299-148...it is a 4" pincushion style woofer, poly cone, rubber surround...Atlantic Technology driver....good for a sealed box of about .1 cu.ft....with an f3 of 110hz...perfect for mating with a subwoofer. Should be a good driver for what you are looking to do. As for the tweeter......is there anything wrong with the tweeter that is already in there? Or is it damaged? Maybe it would be just as easy to leave the stock tweeter alone and just replace the midbass unit. That type of retro-fit has been done before with Minimum/Optimus speakers to good success.

John

[depthsounder]

Re: Opinions on Shack Minimus 7 rebuild

gtforme00 == depthsounder

The tweeter in these things isn't that bad really, it should be very workable, even with the stock woofer (I agree that with DDF that it could be improved ultimately). The woofer in my Pro X7's had a pronounced breakup mode at 5kHz that caused them to be unpleasantly harsh in stock form.

I've been revisiting the design as promised in that thread, and am trying to get my measurement system working again to get the driver measurement files. My Wallin Jig has a mysterious bug in it (probably the cheap switches I used), so I'm going to make a more simple jig this weekend to get the impedance plots.

Listening to my modified PRO X7's vs. my ZMV5's

The PRO X7's have a fantastic midrange and very good imaging, though I'm aware that this may be a personal preference for a tilted down top end response.

The ZMV5's have more articulate high's (though a little bright for my taste in stock form) and obviously cleaner midbass, the mid-bass on the PRO X7's is very "poppy" and probably needs to be crossed over above 120Hz with the port sealed and cabinet stuffed.

Higher volumes is no comparison, the ZMV5's win hands down.

The woofer mod in my build was a bit extreme, and in hindsight I probably should have tried a notch before cutting up the woofers which were in very nice shape.

-David

[Chil]

Re: Opinions on Shack Minimus 7 rebuild

I have a set of Minimus 7's that I bought around '81 and have been thinking of doing a rebuild as well.

I've been looking around at a lot of 4" woofers and spotted these Daytons below. Any comments on how they might work?

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-214

And as for that Pioneer tweeter, I think it may be a little below the quality of the original soft dome that was used.



Chil

[1MOR]

Re: Opinions on Shack Minimus 7 rebuild

The very best Minimus 7 mod I've seen, was done just recently over on the AudioKarma site. The thread had several crossovers from different mods featured, with comparisons to the final version done by the OP (Zilch). It's 22 pages long, but on the last page you can see the measured FR with the final crossover.

BTW: The mod uses the original drivers! It's quite impressive, IMHO.

Here's a link:

http://audiokarma.org/forums/showthread.php?t=199790

Best Regards,
TerryO

I did the XO rebuild and yes, it is quite impressive!