Realistic Minimus-7 Measurements

[beagleman]

The Minimus 7's seem to have a bit of a cult following in the couple of Radio Shack Facebook groups I'm in - however having owned a pair, I never did get what all the fuss was about.

The only thing I did like about them was the aluminium cabinet.

I think simply they are well built, look great, and with the thin metal cabinet appear tiny, yet still can put out some sound, albeit not great, but good for what it was overall.

Not the first nor the best mini speaker, but all the others from that time period were not known by the general public.
Kind of like the "Mach one" speaker and Bose 901 and Klipsch Heresy, not the best in any way, but looks, and so on they just are "Famous"

 

[audio_tony]

I think simply they are well built, look great, and with the thin metal cabinet appear tiny, yet still can put out some sound, albeit not great, but good for what it was overall.

Not the first nor the best mini speaker, but all the others from that time period were not known by the general public.
Kind of like the "Mach one" speaker and Bose 901 and Klipsch Heresy, not the best in any way, but looks, and so on they just are "Famous"

Yes I had a pair of Mach One as well. Only after hearing a friends speakers, did I realise my Mach Ones were not that great. The bass was good but that's where it stopped...

 

[mhardy6647]

Not to get too far off topic, but years ago had a pair of somewhat similar sized mini speakers from Radio Shack, called the Minimus .5 They were honestly a very cheap 4" full range speaker that in some key ways sounded actually better and smoother than the more expensive Minimus 7 model.

It has been far too long to remember exactly how they sounded, but I had a pair of Minimus 7 and when I A/Bed them, found the cheap one to be better without that big midrange peak, but also a smoother rolled off top end, and about similar bass.

Yeah -- off topic... and not off topic. Sort of Schroedingerian.
But, like a moth to flame (or a housefly to... well, you know...), I cannot resist prolonging the agony!

1) The 0.5 has something of a cult following (believe it or not!). Not really sure why, though.
I think that's the one I am thinking of, with a cast aluminum baffle.

2) If one goes back a little further, Radio Shack sold several minispeakers that were naught but a small (sealed) box with one of their OEM Foster (Fostex) FE-103 4" drivers inside. The daddy of 'em all was the Solo-103.
If nothing else, the Fostex FE-103/Radio Shack 40-1197 (which dates back to the 1960s and is still in production and available, in several slightly differing morphs) is a relatively (!) capable 4" "fullrange" driver. It's all relative.

source: https://www.radioshackcatalogs.com/flipbook/1967_radioshack_catalog.html

I'll also note that Radio Shack sold their 40-1197 driver for years bundled with plans to build an enclosure that also included some contouring (via a large, home-made inductor) -- but they wouldn't add one to their own loudspeaker "system" featuring essentially the same driver. Too expensive, I guess.

source: https://www.radioshackcatalogs.com/flipbook/1968_radioshack_catalog.html

source: https://www.worldradiohistory.com/A...lustrated/Electronics-Illustrated-1965-11.pdf

 

[mhardy6647]

Oh. I should mention - The early FE-103 drivers had AlNiCo magnets; later ones, ceramic.
The earlier ones, of course are far pricier on the market. The cult of the arcane, as I like to say.

Transmission Line Speakers

Information on Transmission Line Design for DIY and other audio related information including filter design and bafflestep information

www.t-linespeakers.org

 

[mhardy6647]

I am so embarrassed.

Uh... one more thing... (as Lt. Columbo would have said)

I realized after typing the couple of ephemeral asides above, that I was sitting in our "den" (or TV room or... whatever) listening to Wait, Wait, Don't Tell Me -- via a pair of modern day Fostex "Kanspea" kit speakers using current production FE-103 drivers.
Synchronicity (not the record album -- the thing)!

Fostex P1000E DIY Kanspea 4" Full Range Speaker Kit - PAIR: Madisound Speaker Components, Inc.

Fostex 4" Full Range Speaker Kit Pair. Complete kit with pre-assembled cabinets.

www.madisoundspeakerstore.com

Oh, and in full discosure -- there is a pair of Radio Shack SOLO-103 speakers in the basement someplace. Couldn't pass them up for $5 at some ham radio swapmeet a few years ago.
Yes, I am incorrigible.

 

[restorer-john]

And this is what one of my NOS (December 1988) silver pairs looks like :

Silver got discontinued (not popular but I liked it) and so did the Minimus 7W (wood)- we have a mint pair of those too.

The pair you are testing are from October 1990 (10A0). Revision C.

They are the last series out of Japan, before production went to Korea.

I was working at/running a Tandy Electronics during those years and couldn't resist half of half price, or discontinued ones we 'found' in stocktakes buried in lazy store's backrooms (expired laybys/layaways) which had devalued to a few dollars.

 

[restorer-john]

Tandy/Radio Shack produced this little active (tape loop/pre/power loop) PEQ and subsonic filter specifically for the Minimus 7 range (and other tiny speakers) which really makes them perform much better and, at the same time, has a very effective subsonic/infra filter to prevent overdrive/distortion with vinyl as a source.

I posted some FR curves someplace on ASR for this little boy several years back.

 

[restorer-john]

As I have mentioned before, the Minimus 7 was the world's biggest selling loudspeaker and when the first million were sold, gold plated Minimus 7s were sent to Tandy/Radio Shack head offices around the world with attached plaques. I remember picking our gold one up in our Sydney head office- it was covered in fingerprints!

 

[restorer-john]

@MAB For reference, here's impedance sweeps of the two new old stock speakers from the photo above:

S/N 0492:

S/N 3375:

Pretty good correlation for speakers 2883 units apart and produced 9 months apart. (March 1988 vs December 1988)

 

[MattHooper]

As I have mentioned before, the Minimus 7 was the world's biggest selling loudspeaker and when the first million were sold, gold plated Minimus 7s were sent to Tandy/Radio Shack head offices around the world with attached plaques. I remember picking our gold one up in our Sydney head office- it was covered in fingerprints!

What do you think of how they sound?

I own them for quite a while in the early 80s and I remember being quite amazed at their sound for their size.

 

[restorer-john]

What do you think of how they sound?

I own them for quite a while in the early 80s and I remember being quite amazed at their sound for their size.

In the 1980s, they were incredible little mini-speakers when compared to anything close in size. A huge number were bracket mounted under eaves playing tunes at BBQs, in restaurants, shops, on the parcel shelves of cars and in small "den" systems around the world. They were/are tough and could handle pretty horrible conditions. They came with a bulletproof 5 year warranty and I really don't remember ever needing anything other than a woofer here and there when people turned up the bass and the loudness.

By the early 1990s, they were totally outclassed by myriad other small loudspeakers, particularly ported/reflex designs. Tandy/Radio Shack tried to compete with the Minimus 77 (bigger driver) and it was a dreadful speaker. Also, its foam surrounds rotted out fast. Then in 1996/7/8 came the Pro-X44AV (which is basically a Minimus 7 with a tiny rear mounted port). Here's one below I prepared earlier. LOL.

A pair of Pro-X44AVs I picked up broken for a few dollars. One speaker had been dropped and the woofer basket is/was bent (they have a huge magnet for their 4" size). I just kept them for parts.

Deluxe, multi element crossover...

But in 2024, they are truly a horrible sounding speaker if I am being completely honest. Don't rush out and try to find a pair- it's not worth it. If you spot a pair at a Good Will for a few dollars, by all means grab them and reminisce.

That said, they make good sacrificial speakers when testing/repairing amplifiers or soak testing.

 

[beagleman]

Oh. I should mention - The early FE-103 drivers had AlNiCo magnets; later ones, ceramic.
The earlier ones, of course are far pricier on the market. The cult of the arcane, as I like to say.

Transmission Line Speakers

Information on Transmission Line Design for DIY and other audio related information including filter design and bafflestep information

www.t-linespeakers.org

Yeah -- off topic... and not off topic. Sort of Schroedingerian.
But, like a moth to flame (or a housefly to... well, you know...), I cannot resist prolonging the agony!

1) The 0.5 has something of a cult following (believe it or not!). Not really sure why, though.
I think that's the one I am thinking of, with a cast aluminum baffle.

2) If one goes back a little further, Radio Shack sold several minispeakers that were naught but a small (sealed) box with one of their OEM Foster (Fostex) FE-103 4" drivers inside. The daddy of 'em all was the Solo-103.
If nothing else, the Fostex FE-103/Radio Shack 40-1197 (which dates back to the 1960s and is still in production and available, in several slightly differing morphs) is a relatively (!) capable 4" "fullrange" driver. It's all relative.

View attachment 400057
source: https://www.radioshackcatalogs.com/flipbook/1967_radioshack_catalog.html

I'll also note that Radio Shack sold their 40-1197 driver for years bundled with plans to build an enclosure that also included some contouring (via a large, home-made inductor) -- but they wouldn't add one to their own loudspeaker "system" featuring essentially the same driver. Too expensive, I guess.


source: https://www.radioshackcatalogs.com/flipbook/1968_radioshack_catalog.html



source: https://www.worldradiohistory.com/A...lustrated/Electronics-Illustrated-1965-11.pdf

I also owned another full range speaker for years, that looked very similar to the Solo 103 you posted about. Tall and narrow about a foot high. I would guess 1971-1976 time period maybe?

But it was $19,95 back in the day and called a Miminus 8 (just looked it up) One driver got damaged and when we went to radio shack the guy looked up parts for speakers somehow and said the one we had and the Minimus .5 used the same driver but one just had a bigger box. Anyways we acquired the driver and it was an odd looking 4" full range with some odd looking surround that was gluey and partially see through. Never installed it, but always wondered if it was true?

 

[EJ3]

The Minimus 7's seem to have a bit of a cult following in the couple of Radio Shack Facebook groups I'm in - however having owned a pair, I never did get what all the fuss was about.

The only thing I did like about them was the aluminium cabinet.

I liked the wood cabinet ones better (I had 2 pair of each) for various configurations as a relatively portable speaker (picnics out in the boat and the like).

 

[EJ3]

Tandy/Radio Shack produced this little active (tape loop/pre/power loop) PEQ and subsonic filter specifically for the Minimus 7 range (and other tiny speakers) which really makes them perform much better and, at the same time, has a very effective subsonic/infra filter to prevent overdrive/distortion with vinyl as a source.

View attachment 400178

I posted some FR curves someplace on ASR for this little boy several years back.

I have one of those. Bought new at my favorite Radio Shack about 5 miles from my home.
I wonder if it could be improved by going through it and replacing it with modern caps (perhaps the same values, just better quality)?

 

[restorer-john]

I have one of those. Bought new at my favorite Radio Shack about 5 miles from my home.
I wonder if it could be improved by going through it and replacing it with modern caps (perhaps the same values, just better quality)?

The issues with the unit are residual noise and hum in the two units I have. That could definitely be improved with a bit of effort.

 

[EJ3]

The issues with the unit are residual noise and hum in the two units I have. That could definitely be improved with a bit of effort.

The unit seemed to do what it was supposed to do.
I had no hum but could tell that it added noise when I had it in a preamp out, a tape loop or a processor loop.
I know someone that could likely make it quieter (if he would be willing to work on it).
After the 1st of the year, I hope to be scheduling with him for about a $1000 (or more) worth of work on my 2 Apt/Holman Pre's & my 6 NAD 2200's (it's been more than between 5-7 years [and some more things have been discovered that can improve both the NAD 2200's & the APT/Holman preamps] {without changing the circuit topology, which I won't allow}), maybe I can sneak this thing in too, as a "bonus" challenge.

 

[AnalogSteph]

Here's a schematic of this guy:

Aussie Models were IEC Class I, so I'm not overly surprised that @restorer-john had hum issues with his, you'll easily catch a ground loop like that. Electrolytics should probably be replaced at this age (I can't imagine they used anything particularly fancy); I would recommend 35V types for C19/20 and 25V types for C17/18, and if you can fit 1000µ/25V all the better. You can get films to replace C13-C16 these days, and I hope C1-4 aren't ceramic or else those should go as well. PSRR on the negative rail in particular is a bit critical because of the input follower, which couldn't have any more than about 54 dB by itself.

One obvious noise source I can see would be R3/R4, though the bulk may be coming from the gyrator (250k pot should be a hint). One would have to throw the whole thing into a simulation and see whether it's possible to reshuffle the impedances if you're willing to accept larger values for C9-C12 and assuming you can find a compatible pot of say 2x 22k (I think A = linear here, right?).

 

[restorer-john]

Electrolytics should probably be replaced at this age

No. Not unless they test out of spec. That's just plain bad advice. Shotgun cap replacement is a fool's game and you know it. Who tests resistors? Transistors for noise/gain? Test. Repair/improve. Test again. That's the rules.

Look at the C-R-C power supply. Come on, we can do better and the RC-4558? Let's use a 5532 and power the whole thing via an external +/-15V supply.

The basic design is good, although they cleverly cheapened as much as they could without badly compromising the performance. Not bad for the late 1970s/ early 1980s.

 

[AnalogSteph]

Look at the C-R-C power supply. Come on, we can do better and the RC-4558? Let's use a 5532 and power the whole thing via an external +/-15V supply.

Sure you could do that, but I'm not convinced it would do much if anything to actually address the noise issue. An NJM4558 isn't that bad either, certainly not by line level standards, and it's much less picky on decoupling than a 5532 would be. (I would investigate in LTspice but really don't have any time for this right now.) And I wouldn't be knocking the CRC filtering too much, its ripple rejection at 100/120 Hz would seem to be fairly decent and it's not a wideband noise source like early 3-terminal regs at least (Sansui got caught out by that in the AU-5900 as their preamp circuitry had very low inherent PSRR). If anything, I would aim to replace R9/R10 with current sources and get both better PSRR and distortion at the same time.

 

[restorer-john]

Sure you could do that, but I'm not convinced it would do much if anything to actually address the noise issue. An NJM4558 isn't that bad either, certainly not by line level standards, and it's much less picky on decoupling than a 5532 would be. (I would investigate in LTspice but really don't have any time for this right now.) And I wouldn't be knocking the CRC filtering too much, its ripple rejection at 100/120 Hz would seem to be fairly decent and it's not a wideband noise source like early 3-terminal regs at least (Sansui got caught out by that in the AU-5900 as their preamp circuitry had very low inherent PSRR). If anything, I would aim to replace R9/R10 with current sources and get both better PSRR and distortion at the same time.

You're most likely right on the 4558 vs 5532.

The thing is sitting there, staring at me, so I might have a bit of poke around and see what I can do. I've got a second unit down at my father's place. I might test them both to get baseline numbers and see if my recollections are valid in 2024. I'm pretty sure I reverted one of them to double insulated decades ago due to hum. It's been a long time...

Plenty of RS (Tandy's) equalizers had the same problem- they earthed them poorly (random screw to the chassis) for Australia and created hum loop monsters from designs that were not made with a star ground/earth design or even a consideration of issues that would crop up when the device was actually inserted in a tape/pre in/out loop with other earthed gear.