MINIMUS 7 Modification
Daniel Warren
A couple of weeks ago I posted a comment on the Radio Shack
Minimus 7 speaker and mentioned a crossover modification
which had been described in the 1-88 issue of Speaker Builder
magazine. When I got back to the office, I had quite a few
requests for the mod so I am going to post it for all. The article
was written by William Hoffman.
The stock Minimus-7 crossover has only a hi-pass filter for the
tweeter, the woofer is run full range. The drivers are connected
in parallel. The stock tweeter crossover looks like this:
+ in -----)|----------- Tweeter +
C |
)
) L
)
|
-in ----------------- Tweeter -
Where C is a 4.7 mfd nonpolar electrolytic and L is 0.38 mHy.
The new crossover attempts to equalize the drivers' amplitude
response, add low pass filtering for the woofer, and retain the
high pass filtering of the tweeter. The new tweeter filter looks
like this:
+ in ---------)|----- Tweeter +
| C1
\
/ R1
\
/
|
- in --------------- Tweeter -
and the woofer looks like this:
+ in -------????------- Woofer +
L1 |
|
---- C2
----
|
|
- in -------------------- Woofer -
where the component values are C1= 2 mfd, C2=3.9 mfd, L1=3.5 mHy,
and R1=82? 1watt. Capacitors are 10%, 100 volt film types (metalized
polypropylene is best, else mylar). The inductor should be an air core
type with no more than 1.2? of DC resistance.
Since the speaker has limited and critical internal volume, the author
left the stock crossover inside and assembled the the new one externally,
drilling holes in the back of the cabinet to feed the wires to the
tweeter.
Woofer connections were made to the existing terminals, then the internal
stock components cut free, and wire added to make it to the woofer.
RESULTS: I haven't done the mod myself (yet) so I'll quote the author.
"I measured the speakers with the new crossover, placed face up on a
hard floor of a live room 10x15.25 feet in dimension, miked from 12
feet away. Note the big difference [compared to stock crossover curve],
especially below 2 kHz....the response is now much smoother....most of
the remaining response irregularities are due to diffraction effects from
the front baffle edges of the little speaker. Therefore, placing the
speakers
on a bookshelf, with their backs to a wall, will produce a favorable
change
to the audio response; even better when they are positioned with the
fronts
flush with some books or other large objects. This should eliminate
almost
all the major reflection and diffraction effecs and produce the smooth
response [shown in a figure]. In contrast, if the loudspeakers are
placed on
stands, well away from any walls or other reflecting surfaces, the system
has a very disturbing nasal and box-like sound. Flush wall mounting
should
result in a very flat frequency response...the result is and excellent ?4
dB
from about 85 Hz to 17 kHz; impressive performance for so inexpensive a
speaker system". I will note myself that the difference in the before and
after response curves is dramatic.
Well, I hope this information helps. After lifting this information so
blatantly,
the least I can do is highly recommend subscribing to SpeakerBuilder or
any of
the other Audio Amateur publications (Audio Amateur and Glass Audio - I
get
them all). They are the best source of information for DIYers and can be
reached at PO Box 494, Peterborough NH 03458-3494 or by phone at
603-924-9464. Subscription rates are $25 per year (6 issues).
Have fun,
Dan
War...@env05.chinalake.navy.mil
LISTEN UP-The above is presented for your amusement only. At a minimum,
attempting to perform the mod will void your 5 year Radio Shack warranty.
If you drill holes in your drivers, destroy the cabinet, or burn your
fingers
on the soldering iron, don't say I didn't warn you. Everyone has an
opinion,
these are mine--proceed accordingly.
Daniel Warren
War...@env05.chinalake.navy.mil writes:
>and R1=82? 1watt. Capacitors are 10%, 100 volt film types (metalized
Apparantly the mailer didn't like my symbols, that's supposed to be 82
OHMS. Also, the frequency response is plus or minus 4 dB. Hopefully
everyone can make out the ASCII figures...
Dan
Chuck Forsberg WA7KGX
Shouldn't the resistor be across the tweeter instead of the input as
it appears to be drawm?
--
Chuck Forsberg WA7KGX c...@omen.COM 503-621-3406
Author of YMODEM, ZMODEM, Professional-YAM, ZCOMM, and DSZ
Omen Technology Inc "The High Reliability Software"
TeleGodzilla BBS:621-3746 FAX:621-3735 CIS:70007,2304
Dave Dal Farra
>
>RESULTS: I haven't done the mod myself (yet) so I'll quote the author.
>"I measured the speakers with the new crossover, placed face up on a
>hard floor of a live room 10x15.25 feet in dimension, miked from 12
>feet away. Note the big difference [compared to stock crossover curve],
>especially below 2 kHz....the response is now much smoother....most of
>the remaining response irregularities are due to diffraction effects from
>the front baffle edges of the little speaker. Therefore, placing the
>speakers
Smooth compraed to what, an AI? I've measured several sets of Min-7
drivers and systems on MLSSA and I can guarantee you that this little mod
won't produce a flat response. The woofer has a mode around 4 kHz that would
blow your head off and leave a stump. This is due no doubt to the hard
dust cap. I'm doubtfull that the little second order could reduce it
without either leaving a hole in the mid response or sacrificing tweeter
power handling.
To be fair, I'm sure the mods are better than leaving it stock, but don't
trust those "measurements" quoted from Speaker Builder.
The measurement method used by the author raises serious question as
to it's validity. He doesn't appear to be using a MLSSA. At
12 feet away, he's getting an in-room response. Based on th hard
floor part, you can be guaranteed that the results aren't repeatable
elsewhere and are heavily dependant on the listening room.
You want to know how bad the M7's are? Here's my mods:
1) Coat the woofer lightly in silicone, front and back. I didn't measure
the resultant efficiency change but it was slight. I didn't worry about
the cabinet now being mis-tuned because it was off by a country mile to
start with. The silicone hasn't proven to be a problem in making the
structure any less rigid (to the touch) even after being coated 8 or
9 years. The coating smooth the upper mids greatly.
2) ditch the tweeter. It's unsalvageable. It's so bad, that I replaced it
with an old philips soft dome I had laying around that I had modified with
a coating of liquid gasket (!**!) on the rear side (it was used in a car,
and the coating kept it from drying out). The cludged tweeter measured much
better and even sounded better than the stock screacher.
3) the xover involved about 8 components (designed using MLSSA and Calsod).
The final result was actually about plus mius two, and surprisingly didn't
sound disgusting. I use it in a spare room on the walls and can listen
for hours with zero fatigue. Now that's something that can't be said
of the unmoddified beast!
I have one more pair around that I plan on modding a bit more formally
(ya, I'm a masochist):
1) I've lightly coated the woofer with silicone (front and rear: be sure
to apply it evenly or distortion due to rocking modes will develop).
2) Brace the cabinet front to back and side to side with an aluminum dowel
and epoxy. I'm not sure if the low internal damping of a metal rod
will cause a problem, but I don't know if a thin maple dowel would be
strong enough.
3) Stuff the cabinet like an xmas turkey.
4) Replace the tweeter with something cheap. I'm still looking on this
one. I'm avoiding a hard dome as I think a gently rolling and soft
sounding soft dome will sound better with this bass shy puppy.
5) Design an xover on Calsod and MLSSA.
I'll post the design when it's done. No guarantees when that is though.
Busy busy busy.
Dave D.F.
"It's true they say that money talks. When mine spoke it said
'Buy me a Drink!'."
Daniel Warren
c...@omen.COM writes:
>Shouldn't the resistor be across the tweeter instead of the input as
>it appears to be drawm?
No, that's how it's presented in the SB article. Supposed to help
present a more constant load to the amp, according to the author.
Hey Dave DF: Lighten up! People asked me for the info so I posted it.
I think it's clear from my post that I wasn't making any personal claims
about this mod. You're right about the room effects on measurements,
this is discussed slightly in the full text of the article. I'm not sure
CALSOD or MLSSA were available 5 years ago but I would very much like to
hear about your results using these tools. It seems RS has sold a bunch
of these recently, interest is high so anyone with any other ideas should
feel free to jump into this :)
Lon Stowell
>
>To be fair, I'm sure the mods are better than leaving it stock, but don't
>trust those "measurements" quoted from Speaker Builder.
>
>
I have an even better "mod" for the Minimus 7's. Go buy a pair of
Celestion C-1's and pretend you got them at Radio Shack.
But keep at it Dave, after all Colin Chapman managed to make a
decent motor out of the old Escort pig...after spending more money
than one designed from scratch would have cost. >:-)
Dave Dal Farra
>Hey Dave DF: Lighten up! People asked me for the info so I posted it.
Hey Dan. Believe me, I was going easy on it. ;-)
My comments had nothing to do with your post, I was just giving
info on my test results. I hope you didn't take my post
the wrong way. It's a fun speaker to tool around with, but the actual
process of tooling is 99% of the reason to bother. In the end, you
still can't expect much. I'll let you know what I come up with.
Richard D Pierce
>>
>>RESULTS: I haven't done the mod myself (yet) so I'll quote the author.
>>"I measured the speakers with the new crossover, placed face up on a
>>hard floor of a live room 10x15.25 feet in dimension, miked from 12
>>feet away. Note the big difference [compared to stock crossover curve],
>>especially below 2 kHz....the response is now much smoother....most of
>>the remaining response irregularities are due to diffraction effects from
>>the front baffle edges of the little speaker. Therefore, placing the
>>speakers
>
>Smooth compraed to what, an AI? I've measured several sets of Min-7
>drivers and systems on MLSSA and I can guarantee you that this little mod
>won't produce a flat response. The woofer has a mode around 4 kHz that would
>blow your head off and leave a stump. This is due no doubt to the hard
>dust cap. I'm doubtfull that the little second order could reduce it
>without either leaving a hole in the mid response or sacrificing tweeter
>power handling.
The best part about a Minimus-7 is the cabinet, since I agree that the
drivers are for shit, and no crossover can help the woofer. The woofer
mounting frame, though, is reasonably standard and most drivers that come
in the standard Far-Eastern 100 mm basket fits the cabinet nicely. Try a
Verstatronics DOB100R which is a polypropylene coned/rubber surround
driver. It's not the best but orders of magnitude greater. Also, the
Audax TW01 series tweeters can be shoe-horned in without great
difficulty. These and a reasonable crossover can make a halfway decent
speaker out what is clear intended for the pigs ear, to mix a metaphor or
two.
> < many words about extensive mods that lead to an "acceptable" result
deleted>
But my suggestions and Dave's ideas and all th others beg the question:
why bother? If you have a pair and are willing to buy into the concept of
sunk costs becuase you were desparate when you bought them 5 years ago
and you have lots of time and some money on your hands then maybe the
ends can arguably justify the means, but to pay any amount of money just
to get what are otherwise small mediocre die-cast cabinets and a pair of
driver sets that are less than useless does not make a lot of sense to me.
The best you can do, as Dave has suggested, with the stock components and
a lot of work is laudible, but starting from scratch, it's not hard to do
better.
--
| Dick Pierce |
| Loudspeaker and Software Consulting |
| 17 Sartelle Street Pepperell, MA 01463 |
| (508) 433-9183 (Voice and FAX) |
Richard D Pierce
>In article <1993Nov30.1...@omen.COM> Chuck Forsberg WA7KGX,
>c...@omen.COM writes:
>>Shouldn't the resistor be across the tweeter instead of the input as
>>it appears to be drawm?
>
>No, that's how it's presented in the SB article. Supposed to help
>present a more constant load to the amp, according to the author.
Unfortunately, all it does is present a lower total impedance, constamt
or otherwise. The result is a load that many small amplifiers and
reciever find worse than the original
>Hey Dave DF: Lighten up! People asked me for the info so I posted it.
>I think it's clear from my post that I wasn't making any personal claims
>about this mod. You're right about the room effects on measurements,
>this is discussed slightly in the full text of the article. I'm not sure
>CALSOD or MLSSA were available 5 years ago but I would very much like to
>hear about your results using these tools.
My MLSSA system is over 4 years old, I believe they've been avialable for
nearly 6 years. But there was plenty of stuff around that would have
worked better than what was used, and would have revealed the fact that
the mods made little substantive improvements
>It seems RS has sold a bunch
>of these recently, interest is high so anyone with any other ideas should
>feel free to jump into this :)
Sorry, but Radio Shack ALWAYS sells a bunch of these things. I have seen
sales figures of hundreds of thousands of pairs per year when they are
not on sale, and similar numbers during the month they are. The sales
numbers is, I assure you, completely unaffected by hobbyist interest in
making mods on them. If that number totalled 5,000 woldwide per year, I
would be surprised. And even if true, such a "huge bunch" would be
unnoticed in RS's total sales of these things.
Chuck Forsberg WA7KGX
small size makes it ideal for use with shortwave sets.
Recently I replaced it with a $49 Venturi because its magnetic field was
causing problems. Sound reproduction seems more natural with the Venturi.
Sometimes people seem to be talking from the speaker sits; I don't recall
making this mistake with the Minimus-7.
I don't know how well either speaker would do connected to a wide range system.
Dan Kerl
closed out. They've got them marked down to $29.95.
Dan Kerl
dlk...@cmack.b11.ingr.com
Lon Stowell
>The reason these are selling so well right now is that the model is being
>closed out. They've got them marked down to $29.95.
>
Roughly once or twice a year, the Minimus 7 sells for 29.95. Even
at that price it is more useful as a toy for modification
enthusiasts than as a speaker for all but the very lowest lowest
budgets.
Every now and then the folks at Service Merchandise, Best, etc.
put the Koss M/85 on sale. To my ears that speaker sounds
considerably better than the M7 as a budget speaker.
Daniel Warren
lsto...@pyrnova.mis.pyramid.com writes:
> Roughly once or twice a year, the Minimus 7 sells for 29.95. Even
> at that price it is more useful as a toy for modification
I think we should remember our (at least my) priorities here. For the
regular price of $100/pr, the M7 ain't worth it. At $60 a pair, they
offer a hell of a lot better sound that similarly priced contenders. I'm
not going to use these as a reference, they're mounted almost 20 ft high
in the room corners as surrounds in a pro-logic system. The nice white
cabinet blends well with my decor. I'm no Polk fan, but a couple of
years ago I bought a pair of Polk in-wall speakers for this purpose.
They cost over $200.00 and sounded like crap. Both the "absolute
maginitude" and the "sound per dollar" values of the M7 are better than
my in-walls.
Regarding modifications, Dick Pierce asks "why bother?". Certainly I
would agree with him that it would not be the brightest idea to purchase
these speakers with the intent of modifying. To really get good sound
out of them would be quite a job...witness Dave Del Farra's efforts.
Yes, I also agree that I could do better from scratch, but knowing myself
I'd spend a lot more than $60 doing it, and take a year to do it too.
(that damn SWAN project :-) )
Now however, I've got a pair of them hanging on my wall, and to top it
off I can't resist tweaking. I don't think it is too heretical to
investigate the possibility of improving their sound a little. Isn't a
tweak supposed to be a low cost/high return mod? In this regard, playing
with the crossover seems like the best thing one could do. I myself
would start to see diminishing returns when it came to changing drivers,
but hey, that's me. My comment on the high interest in these speakers
was not based on any huge RS sale, but on the number of e-mails I had
waiting for me after I mentioned the SpeakerBuilder mod.
Anyway, some Saturday afternoon when I'm bored I'll gamble 10 bucks and
try the SB mod and if it sounds better I'll smile and if not I'll change
it back. It's supposed to be fun, folks.
BTW, Dick, I liked your piece in the Polydax flyer, and the drubbing you
gave that goon at acoustic impact.
Andrew Rogers
>> Roughly once or twice a year, the Minimus 7 sells for 29.95. Even
As several others have pointed out here, the M7's are being closed out and
are available yet again for $30. Their replacement is the "Optimus PRO 7",
at $60. Has anyone actually heard these? I keep meaning to drop by RS for
an A-B test against the M7's, but still haven't gotten around to it...
>I think we should remember our (at least my) priorities here. For the
>regular price of $100/pr, the M7 ain't worth it.
Not when you can mail-order the Pinnacle PN-2+ from J&R for $100! I admit
that I haven't heard the PRO 7 yet, but they'd have to be a major league
improvement over the M7's to be worth $120, especially since I picked up my
Paradigm Atoms for only $140 and J&R has the Wharfedale Diamond III's for
$150.
>At $60 a pair, they offer a hell of a lot better sound that similarly priced
>contenders.
I agree - my M7's are unquestionably the finest speakers I have ever traded
an old refrigerator for! :-)
>Regarding modifications, Dick Pierce asks "why bother?". Certainly I
>would agree with him that it would not be the brightest idea to purchase
>these speakers with the intent of modifying. To really get good sound
>out of them would be quite a job...witness Dave Del Farra's efforts.
Well, since I already own a pair I may try out the modifications (one
speaker at a time, so I can A-B the stock and tweaked versions) that
were posted here. What the hell, even stock they definitely sound better
than the old refrigerator ever did...
Incidentally, I once read an article that suggested coating the woofers
with polyvinyl acetate to stiffen them and help dampen some of the weird
resonant peaks. The only thing was, this article gave no hints at all
about where to get polyvinyl acetate... anyone else ever heard of it?
>BTW, Dick, I liked your piece in the Polydax flyer, and the drubbing you
>gave that goon at acoustic impact.
Yeah, I heard once that the M7's were the largest-selling speaker in the
world... I'm surprised that Acoustic Impact isn't cloning them in addition
to the Bose 901's!
Andrew
Dave Dal Farra
>
> But keep at it Dave, after all Colin Chapman managed to make a
> decent motor out of the old Escort pig...after spending more money
> than one designed from scratch would have cost. >:-)
Does he need a pair of mini-monitors for the hatch-back
I'm sure he'll find the price more than reasonable.
(grin grin grin: it's Friday!!!)
Anyways, I can relate to his messed up "vision". I'm in the midst
of modifying my GPz750 motorcycle into a hybrid 60's cafe racer/
early 80's AMA superbike. Of course, it'll be slower
than todays 600's, but that's not the point. Come to think of
it, I don't know what the point is. Who cares! It's FUN!
....DD
Lon Stowell
>
>Does he need a pair of mini-monitors for the hatch-back
>I'm sure he'll find the price more than reasonable.
>(grin grin grin: it's Friday!!!)
>
Our local Pyramid-scheme Audio Appreciation Society has suggested
that you could take 32 M-7's and wire them in parallel sets of
16 for subwoofers or something.
I still think the best way to upgrade an M-7 is to remove that
"Realistic" logo from the M-7 and glue it to the front of a
Wharfedale Diamond, Celestion C-1, Metronome-7 and pretend.
richard.b.dell
>lsto...@pyrnova.mis.pyramid.com writes:
>> Roughly once or twice a year, the Minimus 7 sells for 29.95. Even
>> at that price it is more useful as a toy for modification
>
>I think we should remember our (at least my) priorities here. For the
>regular price of $100/pr, the M7 ain't worth it. At $60 a pair, they
>offer a hell of a lot better sound that similarly priced contenders. I'm
>not going to use these as a reference, they're mounted almost 20 ft high
>in the room corners as surrounds in a pro-logic system. The nice white
>cabinet blends well with my decor. I'm no Polk fan, but a couple of
I looked at the Minimus 7's last night ... I couldn't remember which
ones they were. I had considered them some years ago, as they have frequently
gone on sale for about $30 each, but I decided on an alternative ... which
may be of slight interest her.
What I bought might be considered a _white van_ equivalent of the
Minimus 7's, bought at a local surplus merchandise outlet. They
seem to always have these, in the section with car stereo speakers
and such, sold in pairs for $30, $40, and $50 (as I recall .. I haven't
bought any in several years). I tend to go with the $50 pair, which
are slightly larger cases than the Minimus 7's, and forwhatever its
worth have three drivers (I know, I would rather have two slightly
better drivers too). I wouldn't use these in the listening room, but
for extensions in the kitchen. They come with swival brackets as well,
which allows me to mount them at the ceiling, angled down towards the
normal listening position (in my case, roughly at the stove, just
past the table). I also have a pair at my desk, mounted on the top shelf
angled down, but have not hooked them up (no room for the tuner/amp
as yet here). SInce the desk is where I would be more critical, I
won't comment too much on the smoothness. As I recal, they seemed
a bit strong in the treble, or lacking in the ower mids.
So for extensions in strange places (I am thinking of another pair
for the cellar) look around at surplus/junk outlets. You might find
some junk that is priced fairly.
--
Richard Dell
Paul Rubin
>closed out. They've got them marked down to $29.95.
Minimus 7's go on sale for $29.95 a couple of times a year.
This has been going on for as long as I can remember.
Dwight M Evers
Robert Majors
"...the M7's are being closed out and
are available yet again for $30. Their replacement is the "Optimus PRO 7",
at $60. Has anyone actually heard these?"
I did an A/B test of the two (after forcing the Radio Shack salesperson
to put all EQ to flat--he wanted to tweak his graphic EQ), and
with considerations that the amp was a low cost RS hi-fi amp, and that
neither were in a shelf enclosure or room corner (rather just on a flat
surface), the M7's actually sounded more transparent, or like listening
over decent headphones--the PRO's sounded less clear or definitive, and I
did NOT notice any improvement in the bass, despite the PRO's being
ported for better bass response. Of course, you'll want to try both out
yourself.