Minimus 7 Speakers
- Thread starter Johnny Salam
- Start date
Wiredbecker has a pair of Minimus somethings he said he'd donate to the cause; I'll check up on that. I also have a pair of his 7AVs here for comparison. No, I've never measured them.
:yes: Sounds like fun. We can definitely use my stock black Minimus 7s for the project.
Not so sure about the above statement I made previously. I do remember reading somewhere about a mod that involved a coating for the woofer though. I believe it was from a company called SoundAid but is no longer available.
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http://audiokarma.org/forums/showthread.php?t=66858
10 pages, 139 posts, and a thoroughly frustrating read....
10 pages, 139 posts, and a thoroughly frustrating read....
http://audiokarma.org/forums/showthread.php?t=66858
10 pages, 139 posts, and a thoroughly frustrating read....
Tell me what your findings are! I have a pair with the upgrades done.
I guess I can start there. My problem is conflicting info in previous threads here about this. If somebody has the original 1988 Speaker Builder article, that would be helpful, as well; seller says his design is different in some indefinite respect. This needs to be a cooperative effort to get it right.
Wiredbecker has a pair of Minimus somethings he said he'd donate to the cause; I'll check up on that. I also have a pair of his 7AVs here for comparison. No, I've never measured them.
Maybe I'm missing something here but from my limited research into the upgrade, this is what I've found:
The epay upgrade seller states among other things:
"As the SB article points out, the Minimus 7's, with their solid 4 inch midbass speakers and their quality 1 inch dome speakers are jewels in the rough. This kit actually improves on the original upgrade in the article."
<snip>
"The solution is to do what the manufacturer should have done in the first place: put in a quality low pass crossover section on the woofer and replace the electrolytic capacitor on the tweeter circuit with a quality film capacitor.
This kit does just that: It provides a proper low pass crossover for the woofer using a low resistance high quality cored inductor."
<snip>
"These come complete with an impedance compensating Zobel network"
<snip>
Compare to the schematic below from the Speaker Builder article (from the referenced "frustrating" thread)
We also know that the stock M7 runs the woofer wide open and uses a 0.4mH/4.7uF cap as part of a 12dB/oct hipass (note...0.4mH might also be 0.35mH)
My conclusions:
1. The speaker builder mod is self explanatory.
2. The epay mod deviates from the speaker builder mod by maintaining the stock 2nd order hipass config and simply replaces the NP cap with film type of same value: the stock inductor is maintained. It also adds only a cored inductor to the woofer circuit for a 1st order lowpass compared to SB's 2nd order. The value of the epay inductor isn't noted but considering the heritage of the mod, a safe guess is 0.35/0.4mH. Lastly, the epay mod adds a zobel to the speaker. It's not clear if the zobel is across the woofer or the entire speaker but again considering the heritage and the pics the epay mod offers, it's probably across the entire speaker (since wires are color coded in the pics, it should be easy for someone to check out their M7 and verify...I'm not inclined to tear into a pair at the moment). Again, values aren't mentioned but I don't believe it would be too difficult to come up with proper values thru a bit of experimentation.
Make sense or were greater subtleties being sought?
TerryO
Super Member
One big problem with the Minimus 7s is that Rat Shack would change things and there isn't any designation that would indicate which one you have. There were several versions of the woofers for example. I have heard that there are a couple of different versions of the crossover as well.
Mine (2 pairs) were bought in 1978. A few years back, I tried modifying a pair of them a bit. I gave the woofers two or three thinned coats of Latex Wallpaper Border Adhesive (Golden Harvest GH-50 was recommended by Ed Heath of the old Bass List). That helped clean up the mids a bit, as did replacing the electrolytic cap with a polyprop cap of the same value and better, heaver gauge wire...again improvement. I also used some open cell weather stripping around the tweeter filling all the space from cabinet to edge of dome. With the grill removed they sound pretty good like this, replace the grill and the benefit is much less.
Dan Wiggins measured them with Praxis against a stock 7 and the FR was much smoother and a bit flatter. More work, of course, needed to be done on the crossover, but at this point I moved on to other projects.
I've heard that the Sonance tweeter from Apex jr. works well in these, but I haven't tried it myself, so take it for what it's worth. The interior of the cast aluminum cabinet can be damped with a layer of Oil Based Modeling Clay, rolled out with your wife's favorite rolling pin and then cut to fit. It's then pressed into place with your fingers. It must be oil based modeling clay, as this stuff never hardens and can be reused over and over again if desired. This was another of Ed Heath's suggested techniques that work in all kinds of cabinets. I suppose if you're really the adventurous type you could make some phase plugs for the mid/woofer which might help the sound, but it's a gamble as I don't know if it'll work to improve the sound in this particular application.
Some of the other tricks that have worked for me on other drivers includes gluing felt strips to the inside of the spokes of the basket, applying modeling clay to the outside of the spokes, up to the edge of the magnet. In the past, I've actually filleted the area between the frame and the magnet with my special epoxy/pulverized Lime putty (the lime is used for marking ball fields and costs about $3-$4 for 40 lb). I let it set up over night before applying the clay to the spokes. Finally, you can fashion a felt "hat" to fit over the back and sides of the entire magnet, glued with the speaker modifier's favorite glue: Aleene's Original Tacky Glue, a soft glue that never seems to actually get hard, but holds well and probably provides a bit of damping by itself. After applying a good layer of Aleene's, followed by the precut felt "Hat," it's held in place with an audiophile approved rubber band until the Aleene's sets up.
These speakers are a lot of fun to work on and it doesn't take much money to make them sound better. Now that I'm thinking about it, perhaps I'll take mine to the next level :^)
Best Regards,
TerryO
Mine (2 pairs) were bought in 1978. A few years back, I tried modifying a pair of them a bit. I gave the woofers two or three thinned coats of Latex Wallpaper Border Adhesive (Golden Harvest GH-50 was recommended by Ed Heath of the old Bass List). That helped clean up the mids a bit, as did replacing the electrolytic cap with a polyprop cap of the same value and better, heaver gauge wire...again improvement. I also used some open cell weather stripping around the tweeter filling all the space from cabinet to edge of dome. With the grill removed they sound pretty good like this, replace the grill and the benefit is much less.
Dan Wiggins measured them with Praxis against a stock 7 and the FR was much smoother and a bit flatter. More work, of course, needed to be done on the crossover, but at this point I moved on to other projects.
I've heard that the Sonance tweeter from Apex jr. works well in these, but I haven't tried it myself, so take it for what it's worth. The interior of the cast aluminum cabinet can be damped with a layer of Oil Based Modeling Clay, rolled out with your wife's favorite rolling pin and then cut to fit. It's then pressed into place with your fingers. It must be oil based modeling clay, as this stuff never hardens and can be reused over and over again if desired. This was another of Ed Heath's suggested techniques that work in all kinds of cabinets. I suppose if you're really the adventurous type you could make some phase plugs for the mid/woofer which might help the sound, but it's a gamble as I don't know if it'll work to improve the sound in this particular application.
Some of the other tricks that have worked for me on other drivers includes gluing felt strips to the inside of the spokes of the basket, applying modeling clay to the outside of the spokes, up to the edge of the magnet. In the past, I've actually filleted the area between the frame and the magnet with my special epoxy/pulverized Lime putty (the lime is used for marking ball fields and costs about $3-$4 for 40 lb). I let it set up over night before applying the clay to the spokes. Finally, you can fashion a felt "hat" to fit over the back and sides of the entire magnet, glued with the speaker modifier's favorite glue: Aleene's Original Tacky Glue, a soft glue that never seems to actually get hard, but holds well and probably provides a bit of damping by itself. After applying a good layer of Aleene's, followed by the precut felt "Hat," it's held in place with an audiophile approved rubber band until the Aleene's sets up.
These speakers are a lot of fun to work on and it doesn't take much money to make them sound better. Now that I'm thinking about it, perhaps I'll take mine to the next level :^)
Best Regards,
TerryO
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Thanks, Steve! I'll read through it again.
Here's the Cliff Notes version:
#3 - "Crossover: 0.4mh coil for the woofer, 4.7@50 mf bi-polar cap for the tweeter." Values are correct, but use is wrong. It's a two-pole highpass. W=1218 - 8 Ohms, T=SD-60 - 8 Ohms, Model ?
#5 - "I used some formula-2001 in the surrounds. This is similar to armaroll (sp?) but it is better suited for rubber parts." GC Service Cement fills cracks in surrounds.
#10 - Black craft foam for baffle face.
#17 - "One thing that can happen on these speakers is the flexible wires that go to the voice coil can become intermittant or break. You can patch them or replace them with solder wick desoldering braid."
#20 - "Let me suggest using some blobs of E6000 glue to put the grills back on."
#23 - "The System Seven Receiver had a EQ circuit tuned for the 7s that made them sound much bigger than they were at the flip of a switch." Which did what, I wonder? :scratch2:
#29 - "Armor-All is great on surrounds. IIRC, the M7 surrounds have a fair amount of vinyl in them and they stiffen up quite a bit with age."
#30 - "The early version used the 40-1022 woofer."
#31 - "Another mod for the Minimus 7 is to replace the funky spring wire connectors with dual 5-way binding posts." Model 40-2030C.
#32 - "PE #290-372 this partsexpress woofer is a spot on replacement for the RS unit."
#36 - "Krylon Semi Flat Black is a near perfect match to the original black used on these speakers." Also, suggested driver replacements.
#50 - 4-Ohm PE #290-370 woofer and crossover mod. Retains 4.7 uf HP.
#58 - An alternate woofer.
#60 - Naval Jelly.
#65 - "Although I do have the schematic for the upgraded crossover (and I'll try to get it posted later for those of you that are interested)...." Split the 2-Pole HP and make 1-Pole HP and LP. "Moving the coil to the woofer and replacing the cap on the tweeter with a Dayton metalized film cap or similar will probably get you about 90% of the improvements that the upgrade crossover from the guy on eBay will, and you can do it for about $3."
#67 - "I found some discussion on AA, what was recommended was to replace the 4.7uF stock with as close to 7.75uF (let's say a 7.5 Dayton poly and a .22 film) and put a .4mH in series with the woofer. This lowers the crossover point from about 4500 Hz to 2700 Hz, for a first order crossover...." Link to AA discussion. "The inductor in series with the driver adds some resistance and you can put a 150 ohm resistor in parallel with the woofer to bring it back to 7.6 ohm (or whatever you measure on yours)."
#73 - "I finally got around to doing this project, put a 3uF dayton poly in parallel with the existing 4.7uF to get 7.7uF and re-wired the inductor to be in series with the woofer. Results? A bust. The stock arrangement was far superior...." [Forget the alternative AA mod?]
#75 - "...where on earth do I get replacement drivers?"
#81 - "Are replacements available?"
#86 - "Is there anyway to put a 5-way binding post on these things?"
#91 - "Are there replacement speakers available???"
#123 - "I have found some confusion in the cross over parts from the Speaker Builder article for mods on the Minimus 7. The original had 3.5 mH in series with the woofer but one of the post on this thread had .35 mH for the inductor. Which is correct?"
#125 - "0.35 mH"
#130 - "I researched the SB article on the Minimus 7 and part of the findings (by Hoffman) were to cross over the woofer and tweeter at different points. So that very large inductor value (3.5mH) is correct. The first mod was mentioned in a later SB article back in 1990 which stacked two of the mini speakers with a modified crossover. By doing this (and using two woofers) the value of the inductor could be reduced to 1.7 mH."
#138 - "The kit provides a 1st order low pass crossover on the woofer at around 3000hz. There's none in the stock M7. The kit also includes a Zobel. This tames the spikes in frequency at the crossover point."
johnda
Addicted Member
I have the Optimus X7's which are a family member and ran them with the RS 12" subwoofer. They were better than the Bose Optimass (is that the term?). My grandson now has the subwoofer and the X7's are now my computer speakers, being driven by the Accurian amplifier. They are good, but need a sub if you are going to use them in a full stereo system.
Thanks, Pete!
They certainly do NOT pass the "sanity test" as drawn. :tongue:
I'll do them both, as you corrected them, plus the eBay kit, and we'll see what's up.
My guess is that Murphy's stock FR curve is correct; he seems to get it right.
Planet10 shows the stock impedance curve:
http://www.planet10-hifi.com/minimus7/
I can't guess why the DIY guys weren't all over those schematics. That seems to be the source of some of the erroneous info in the "frustrating" thread....
They certainly do NOT pass the "sanity test" as drawn. :tongue:
I'll do them both, as you corrected them, plus the eBay kit, and we'll see what's up.
My guess is that Murphy's stock FR curve is correct; he seems to get it right.
Planet10 shows the stock impedance curve:
http://www.planet10-hifi.com/minimus7/
I can't guess why the DIY guys weren't all over those schematics. That seems to be the source of some of the erroneous info in the "frustrating" thread....
Holy cow there Mr. Z, apparently I misunderstood the nature of your frustrations. I thought you were looking for the xover mods and not all this other stuff. I'm a bit confused now as to what the real objective here is: A crossover mod? (seems to be more up your alley) or a total makeover? In light of what the M7 is and the extended discussion of the epay mod, I was under the impression it was the former...not so?
Personally, to my way of thinking most of the above is extraneous and/or misguided. The xover is the real meat of the project. I'd be really wary of applying anything to the woofer cone. And I can assure you from experience that PE PN 290-372/370 IS NOT a replacement for the woofer apart from the fact that it physically fits. As for application of damping materials to the enclosure and speaker basket, I have to admit that I never considered them excessively resonant so the thought never crossed my mind. I'd also be concerned about reduction of box volume that damping materials would cause. That concern also appliess to added volume of large film caps and additional inductors.
FWIW, we do know now that the zobel of the epay mod is applied across the woofer and not the whole speaker.
Yes, the focus is the crossover mods, but I included the other stuff as gestalt, since I was going through it again anyway. Much of the crossover info is extraneous and misguided as well, and I finished up just as conflicted on second reading.
Pete has brought us considerably closer to the facts here. I have the eBay kit on order, and we'll get the answers regarding that soon, too, plus comparative data regarding how all of them perform. Murphy's curve is an eye-opener; these clearly need help:
http://murphyblaster.com/content.php?f=minimus.html
I am willing to do it, but less inclined to start from scratch, which is why I am trying to find out the details of the existing upgrade for comparison.
I don't want to put in a bunch of effort to another chorus of yawns here and elsewhere. The task is NOT trivial....
Looks like the hook was appropriately set. :thmbsp:
Zilch, thanks for taking this on. Also, thanks to Shrinkboy for "dangling that hook." Glad to see others jump in so quickly too.
Paratima (and Archie2), I can't read the values on the handdrawn diagram. Also, is the first set of info (before the "and...") from the Speaker Builder article?
I'll be lurking.
Wiredbecker has a pair of Minimus somethings he said he'd donate to the cause; I'll check up on that. I also have a pair of his 7AVs here for comparison. No, I've never measured them.
Zilch:
Allow me to muddy the waters here: I have a pair of 7Ws with an updated crossover done by another AK Member; I'd be happy to bring them to Zilch Labs for comparison purposes.
Let me know if you are interested...
Looking at Murphy's plot, I wonder if a crossover mod can fix what I see, esp if the one wants everything to fit into the existing box. Are driver changes anticipated?