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Radio Shack Minimus 7 Measurements


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I purchased several pairs of these over the years when they would go on sale.

Always thought that I'd install better drivers someday if they burnt out, and

perhaps shoot for performance close if not better than the Visonic Davids. I

heard these many years ago but not recently. Would like to use a SEAS, Peerless

or Vifa woofer replacement but there are not many that will fit. The long throw

Aura's (NS3 note that this smaller and would have to be adapted) look like they

might work well when seeking improved performance.

Anyway, came across a woofer out of one and measured it:

Radio Shack Minimus 7 woofer UNIT SAMPLE: PLB #1 8-8-2008

UNIT MARKINGS: 1218 8 ohms MALAYSIA

UNIT DATE: estimated 1993 not marked

Notes: woofer has a raised back plate

Impedance file: RSM7W1Z1.ZF2

T&S Impedance file: RSM7WTS1.ZF2

Effective cone diameter = 3.25" measured

RDC = 6.60 ohms

Fs 62.5 Hz

Vas 4.35 liters

Qe .47

Qm 2.13

Mms 5.99 grams

no .219 %

SPLref 85.4 dB

Bl 5.92 T-m

Qts .382

Cms 1.08 mm/N

Zpeak 38.7 ohms

Zmin 7.60

Z1K 11.9

Lvc1K 1.08 mH

Z10K 20.6

Lvc10K .51 mH

Delta M 6.3 grams

Fshift -30%

This is an excellent match for the original driver. The mechanical dimensions

should be checked before trying it:

http://www.mcmelectronics.com/product/55-1853

Zaph's tests show good performance and value for the MCM 55-1853:

http://www.zaphaudio.com/smalltest/

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Thanks for the info Pete. Do you (or any other member) have info on the mods that were recommended in Speaker Builder? I know there is a fellow on ebay selling kits, but I'd rather build my own. Looks like it involves some caps and an inductor. One Minimus 7 enthusiast recommended just replacing the 4.7uF cap and adding more stuffing, but I'm curious about the full-blown mod.

Kent

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Hi Kent,

I have the SB issue filed away. I never did anything with these except replace

the woofers with the stock RS units. I couldn't find a better one that would fit,

and didn't have the time to design a mod anyway back 10-15 years ago.

I'm not certain if this is the SB mod, and there are errors in these schematics

the pos input should be on the other side of the 4.7 uF cap in the top and bottom

schematics. Please note that the first schematic looks to be stock, I'm not certain,

and the other two mods do not really pass a sanity check, the woofer inductor is

rather large. The tweeter crossover was 2nd order, it was reduced to first order:

http://www.diyaudio.com/forums/showthread....936#post1280936

Zaph design, should work sealed with less bass extension:

http://www.zaphaudio.com/ZBM4.html

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Thanks Pete

Actually, I did see that thread but did not know if that was the SB mod, and I was wary of the schematics because some contributers had noted errors. Maybe I'll just replace the 4.7uF caps with some 5uF film caps I have on hand.

If you ever dig up that SB article maybe you could post it here.

I had some Mini 7s--used them as rear speakers for years, then sold them on ebay. Just bought another pair, with walnut cabs, so I'd like to play with them a bit.

Kent

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Pulled out the article. Yes top schematic is stock with the Pos terminal

moved to the other side of C1:

http://www.diyaudio.com/forums/showthread....936#post1280936

Second is the SB 1988 modification, note that drivers are wired

in phase. Inductor DCR is 1.2 ohms. The article shows a much

improved system frequency response. I believe that he's added

significant baffle step compensation which is good for realism,

however I expect a significant loss in sensitivity and maxSPL.

He does not pad the tweeter and it looks to be 2 to 3 dB too hot

in his curves. I suggest a small low cost iron core inductor for the

woofer since it should be as small as possible.

The third schematic is from another SB Mod article (4 - 91) however

there are two errors. The Pos input terminal should be on the other

side of the 4.7 uF cap, and the inductor was not 3.5 mH but rather

it was the tweeter inductor reused, which he states as .4 mH. He

also shows a fairly good FR plot but it is half octave smoothed and

does include a peak at about 1.7 kHz. This version I would expect to

not include any BSC.

Zaph's woofer network looks to be more reasonable:

http://www.zaphaudio.com/ZBM4.html

1.5 mH to provide some BSC and a 1.0 mH option for use in situations

closer to a boundary. The shunt woofer cap is 3.3 uF. The stock Minimus 7

woofers are very similar so it should be reasonable to use his network if

one wanted to experiment.

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My ebay Mini 7s arrived today. Nice walnut veneer and the grilles are intact.

I had a couple of Sprague 5uF film caps from a previous project, so I pulled the xovers, removed the 4.7uF NPEs and installed the Spragues, then shoehorned the things back together. The cap "just" fit thru the hole in the cabinet, then I pushed ti aside to clear the woofer.

These little guys sound good! And for a real treat, I ran them AND my AR2ax's. Nice. The Mini 7s are temporarily on top of a stack of other speakers, so they happen to be at ear level. I think they enhance the ARs.

I may leave them as is or perhaps later use the SB mod.

Kent

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Good to hear that you're enjoying them.

All the talk got me to pull out one of my pairs, mine are all

stock, only replaced a few woofers at one point.

Disconnected the tweeter and measured the woofer in system;

here are the results. Note that it is not the same woofer as

measured above:

Fc = 117.0

Qtc = .74

Qe = .89

Qm =4.5

Interesting that the Qtc is so low, usually a smaller magnet would

be used to raise Qtc and provide a bump in the woofer LF

response. Qm is high because there is not much stuffing in the

system, this means that adding some series resistance will be

effective in raising Qtc. Adding one ohm raises Qtc to .845, two

ohms to .94.

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Pete,

While you had the mini 7 apart, did you happen to measure the physical dimensions of the woof? The MCM 1853 is:

Dimensions:

•Overall frame diameter: 4.13" (pincushion)

•Required cutout: 3.66"

•Mounting depth: 2.80"

Kent

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Just measured Dimensions:

•Overall width available 4.2"

•Hole cutout = 3.67

I removed the crossover and don't have it handy.

There is 3.4" to the XO mounting flange, and the XO

might hit and require parts to be moved to clear.

XO depth is .85 so 3.4 - .85 = 2.55" looks like parts

will have to move.

Relocate the inductor, cap and bend over the terminals

should provide .55" for the XO depth, yielding 2.85"

should probably work like this if all the dimensions are

correct. Dont forget to insultate the terminals if anyone

tries this.

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Thanks once again for all the info Pete. The Aura will fit, but it is 4 ohms while the RS original and the MCM are 8 ohms. The other specs seem to differ also, but I don't know if those differences are important. What's your advice here?

Kent

oops--I see there is an 8 ohm version of the Aura as well. Is that the one ou mean?

http://www.madisound.com/catalog/PDF/ns4-255-8f.pdf

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Thanks once again for all the info Pete. The Aura will fit, but it is 4 ohms while the RS original and the MCM are 8 ohms. The other specs seem to differ also, but I don't know if those differences are important. What's your advice here?

Kent

oops--I see there is an 8 ohm version of the Aura as well. Is that the one ou mean?

http://www.madisound.com/catalog/PDF/ns4-255-8f.pdf

I prefer the 4 ohm because it is more sensitive and the tweeter is already too

hot without the pad. There is no crossover to the woofer in stock form so it

does not hurt to use this 4 ohm driver, of course then the input impedance

would be 4 ohms. The 8 ohm version is less efficient than the original and

therefore the excessive tweeter output would be even worse.

These Aura drivers have about twice the linear throw of the stock and MCM

woofers and therefore will provide more max SPL in the bass. The cone

mass is higher which will result in a slightly lower Fc and slightly more bass

extension. This is a very small box to be working with and the Aura will

work well here keeping that in mind.

Forgot to mention that it looks like this driver will also hit the crossover in

back, might have to move some parts. The large roll outer suspension

might also hit the grille, so try them at your own risk.

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Zaph's woofer network looks to be more reasonable:

http://www.zaphaudio.com/ZBM4.html

1.5 mH to provide some BSC and a 1.0 mH option for use in situations

closer to a boundary. The shunt woofer cap is 3.3 uF. The stock Minimus 7

woofers are very similar so it should be reasonable to use his network if

one wanted to experiment.

I tried this network, and I was mistaken this is not good as is for the

Minimus 7 woofer. I found a significant peak in the electrical response,

3-5 dB due to resonance of the 3.3 uF with the voice coil inductance.

I am surprised since the MCM woofer looks to have similar inductance,

however I did not check closely.

I don't like any of the modifications that I mentioned above for various

reasons.

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  • 4 months later...

Measured a 1992 SD-60 Minimus 7 Tweeter:

Radio Shack Minimus 7 1" Dome Tweeter PLB#1 1/3/2009

UNIT MARKINGS: SD-60 MALAYSIA

UNIT DATE: 13 NOV 1992

Rvc = 7.14 ohms

Fs = 2049 Hz

Qt of 2.58

Qm = 3.82

Qe = 7.92

Zmax = 10.9 ohms

Zmin = 7.98 ohms

Z10K = 9.44 ohms

Lvc10K = 52.9 uH

Qtc is rather high but is probably due to the high Fs, and somewhat weak motor.

Removed the front plate and dome assembly to have a look. The edge suspension

is simply a flat continuation of the dome fabric without any roll. Linearity with

excursion might be a concern, however the high Fs of 2049 Hz should help reduce

the max excursion.

Lead in wires are tinsel, this is good, and the voice coil former is paper would rather

see aluminum or aluminized paper for better heat dissipation. There is no ferro-fluid

in the gap.

There is a soft iron lip that extends from the dome assembly into the gap that is not

usually seen in modern tweeters and I would expect that there is some loss in

efficiency due to the larger gap.

There is a felt pad on top of the center pole, however this one was placed slightly off

center. I was concerned that it might rub, and I trimmed a small amount off to

avoid any issues.

This appears to be a low cost copy of the classic 1" dome tweeter.

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  • 2 months later...

Tried the Aura 4" and it fits, have not yet worked out a crossover:

http://baselaudiolabs.googlepages.com/RS-M7-AURA1.jpg

This driver has some impressive Xmax and looks like it will perform well.

I do notice that it has a Kapton former and I'm concerned about the driver's thermal capacity.

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  • 2 weeks later...
Tried the Aura 4" and it fits, have not yet worked out a crossover:

http://baselaudiolabs.googlepages.com/RS-M7-AURA1.jpg

This driver has some impressive Xmax and looks like it will perform well.

I do notice that it has a Kapton former and I'm concerned about the driver's thermal capacity.

The Aura 4" driver measured in a stock Minimus 7, with stock fiberglass and crossover:

Fc = 125 Hz

Qtc = .89

Qe = 1.1

Qm = 4.9

Fs is slightly higher than the original woofer, however it is fairly insignificant on a

percentage basis. The higher Qtc provides more peaking around and above Fc

as compared to the original which should sound better and more than make up for

the higher Fc.

These are the figures I provided in an earlier post for the original woofer:

Fc = 117.0

Qtc = .74

Qe = .89

Qm =4.5

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  • 5 years later...

I never took the Aura woofer any further since it was discontinued, but

it looks like the Dayton ND-105 is a near clone of it and will also drop in

so I'll probably purchase a pair. Quick look at the parameters suggests

that it will have better bass (lower Fc) than the Aura:

http://www.parts-express.com/dayton-audio-nd105-4-4-aluminum-cone-midbass-driver-4-ohm--290-212

The Minimus 7 internal volume is .0592 cu ft or 1.76 liters.

ND-105 Vas is 4.5 in liters.

Here is how the Aura 4" fit:

http://baselaudiolab.com/RS-M7-AURA1.jpg

RS-M7-AURA1.jpg

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  • 2 weeks later...

How's the Dayton project coming along? Looks like the surround protrudes quite far. Does it hit the stock grille?

What about xo? Does the PZ-2.1 xo design work?

I have a pair of Mini 7s with one bad woofer. Had bought a pair of the MCM 55-1856 as replacements but it's a very tight fit.
Kent

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  • 2 years later...

Designed a PC board for the Minimus 7 mod and finally finished mine.  Details here, pic below:

http://audiokarma.org/forums/index.php?threads/minimus-7-pz-2-2-mod-diy-circuit-board-by-pete-basel.750334/#post-10182495

M7-DONE-FRONT.jpg

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  • 2 years later...

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