Minimus 7 Loudspeakers - Measurements

[Zilch]

Don't know of a good sub for these, will have to start looking around for one.

I posted earlier about having used them with a JBL Control SB-5, which show up on eBay as low as $50 from time to time. They're two-channel, acoustically summed, and the built-in passive crossover includes highpass at 125 Hz, as I recall, for satellites.

They also made a smaller one, Control SB-1, for use with Control 1, a product very much like Minimus 7, which is still made today, and virtually a standard among personal monitors.. We're not talking "rattle the windows" bass here, rather, LF augmentation in a secondary system, and these small passive subs are respectable performers for that application.

If somebody wants to DIY something, a DVC woofer would be a good place to start, combined with one of the JBL crossover circuits.... :thmbsp:

 

[Pete B]

I'll take a look at those JBL subs.

 

[Zilch]

I'll take a look at those JBL subs.

Links here, Pete:

http://audiokarma.org/forums/showthread.php?p=2340459#post2340459

Triple-chamber isobaric bandpass. Fun stuff....

Edit: Brochures, also:

http://www.jblpro.com/pub/obsolete/Control_SB_Micro.pdf

http://www.jblpro.com/pub/obsolete/Control_sb-1.pdf

http://www.jblpro.com/pub/obsolete/Control_sb-5.pdf

O.K., how 'bout a cool tower sub or two to set Minimus 7s on:

http://manuals.harman.com/JBL/HOM/Technical Sheet/S1 ts.pdf

[Same DVC woofer as Control Sub Micro.... :yes: ]

 

[Pete B]

Are people familiar with Apex Jr.? Not sure if this small Vifa driver would fit for those who need replacement woofers. I read somewhere that the Sonance tweeter fits, don't recall if the frame had to be cut:
http://www.apexjr.com/speakerstuff.html
Alesis crossover for $1 might have a bar inductor that could be wound down.

I like stereo subs, thinking of this low cost amp:
http://www.apexjr.com/Dual25FR.html

And a pair of these Polk woofers if the amp can drive 4 ohms:
http://www.madisound.com/catalog/product_info.php?products_id=8349

Or these:
http://www.madisound.com/catalog/product_info.php?products_id=8348

That's a total of about $74 without boxes. Would use passive line level crossovers to the stereo amp.

Ideas for boxes?

 

[Sir.Byrd]

Zilch, are you doing any tests of generation without the ring around the tweeter?

 

[Zilch]

Zilch, are you doing any tests of generation without the ring around the tweeter?

If somebody brings me some, I'll measure them; the stock frequency response would likely indicate whether these mods are applicable.... :yes:

 

[Sir.Byrd]

If somebody brings me some, I'll measure them; the stock frequency response would likely indicate whether these mods are applicable.... :yes:

I'll ship you a pair of 7Ws (same as 7s but with walnut case) without rings on the tweeter for testing if you install the crossover mod for me?
Win-win situation there :yes:

 

[Zilch]

Let's keep that in mind as a future option for now.

I've gotta work up a parts list, retrofit a pair, measure the response with the grilles back on, measure system impedance, etc., i.e., lots more to do before considering taking on anything else relating to this project here.... :yes:

 

[ericj]

I'll ship you a pair of 7Ws (same as 7s but with walnut case) without rings on the tweeter for testing if you install the crossover mod for me?
Win-win situation there :yes:

I have a pair like that as well - my only minimus 7's - and I'm curious too.

So far all I've done was recap with 4.9uf polycarbonates - which made a huge difference to my ears. They may have been stored somewhere warm and dry for a long time, killing the old electrolytics. I bought them at a thrift store for, iirc, $7.

They're currently doing a great job in nearfield use on the computer desk, driven by a Rotel RA-820bx.

Edit: it would also be interesting to know if and how coating the woofer cone might affect the FR.

 

[Zilch]

I gotta get the parts, that must be night and day.

For the venturesome upgrader, ~$30 the pair with 2.0 mH I-Core, looks like.... :thmbsp:

 

[RayW]

Are people familiar with Apex Jr.? Not sure if this small Vifa driver would fit for those who need replacement woofers. I read somewhere that the Sonance tweeter fits, don't recall if the frame had to be cut:
http://www.apexjr.com/speakerstuff.html

I used those drivers in my Boomboxa Rustica. I like them and I've had pretty good comments unanimously from listeners.

I like stereo subs, thinking of this low cost amp:
http://www.apexjr.com/Dual25FR.html

I also have a couple of those laying around somewhere. I'll have to dig them up.

Ray

 

[ericj]

I like stereo subs, thinking of this low cost amp:
http://www.apexjr.com/Dual25FR.html

And a pair of these Polk woofers if the amp can drive 4 ohms:
http://www.madisound.com/catalog/product_info.php?products_id=8349

The datasheet for the STK module it's based on seems to imply that 4-ohm will work.

I've got a passive sub that contains what looks like a pretty reasonable dual voicecoil woofer inside a thoroughly inappropriate 4th order enclosure (which renders it curiously bass-lite). I frequently consider using a tablesaw to free it from it's prison so i can couple it with that amp in some more realistic box.

 

[Pete B]

Thanks for checking those modules.

I don't care much for bandpass boxes so I suppose that I'd agree with you.

 

[Zilch]

Issues and Solutions

Widely spreading the filter frequencies as we have done here results in a broad section of the spectrum in which, since the load is diminished, the impedance goes inordinately high. Perhaps mitigating this was the purpose of the seemingly gratuitous 82-Ohm shunt resistor in the Speaker Builder mod. I have addressed it here with an additional resistor comprising a fixed L-Pad in an updated PZ-2 circuit, substantially reducing the variability and absolute magnitude of the system impedance peaks:

Compare to stock system impedance here:

http://audiokarma.org/forums/showthread.php?p=2350372#post2350372


After several days' listening, now, I have come to agree with Pete's suggestion that so much HF attenuation may not be desireable, concluding, in fact, that it renders Minimus 7 somewhat boomy and resonant in the 200 Hz region. Thus, I have reduced L1 to 1.0 mH and improved the balance of the sound:

Here, then, is the measured (and somewhat further improved) performance of PZ-2:

And finally, a summary of that of the primary five Minimus 7 mods evaluated in this thread:

HAPPY NEW YEAR to all Minimus 7 enthusiasts! :thmbsp:

 

[Robie]

HAPPY NEW YEAR to all Minimus 7 enthusiasts! :thmbsp:

Back at ya, Zilch!

I hope to be able to try modding one of my pairs once it appears y'all have optimized the mod.

 

[Zilch]

I hope to be able to try modding one of my pairs once it appears y'all have optimized the mod.

We're in the "Voicing" phase now.

Unless someone comes up with a better means of dealing with the impedance issue, I'll next be working up an installation strategy for the PZ-2 components.... :yes:

 

[ericj]

Would it be horribly uncouth of me to use a ferrite bobbin inductor like one of these?

http://search.ebay.com/370129689864

I know air-core are preferred, but these are so much smaller. Can anyone quantify how much SQ is impacted by something like this? I ask because I have ADS and Boston speakers with ferrite bobbin inductors that sound just fine, to me.

(Yes, I know the part is SMT, but i figure it could be glued face-down and wires tacked to the pads anyway. Generous hot-glue or RTV strain relief on the wires after assembly would probably do plenty to keep things together.)

 

[Zilch]

I'm sure not liking the 1.3-Ohm DCR there.

JBL uses ferrites in some of its cheapest mass-produced products.

Though I don't know the details, they are apparently "bottom tier."

 

[Steve O]

I'm sure not liking the 1.3-Ohm DCR there.

JBL uses ferrites in some of its cheapest mass-produced products.

Though I don't know the details, they are apparently "bottom tier."

There's more to not like than 1.3 ohms. These are inductors meant for switch-mode power supplies. As such, no doubt they're designed to be used at very high frequencies: probably +++20KHz. I think you'll run into saturation issues at audio woofer frequencies. I'm sure there's a reasonable solution to the inductance vs physical size dilemma in this app...just don't know what it is at the moment.

 

[Pete B]

I would not use those, the saturation point is a function of the core size and they look small. 1.3 ohms is rather high.

Forgot to mention these 20 ga air cores from Zalytron, they're quite small and I think perfect for this application:
http://www.zalytron.com/erse.htm

I would go with 1.5 mH for users who do not need high SPL feel free to experiment. One could add a 1.0 tap to a 1.5 mH.

1.0 mH is probably better for those planning to use a subwoofer where some overlap can be provided to help with baffle step compensation.

All ferrites are not alike, there are higher quality types with better saturation characteristics. I dislike them and try to avoid them in my best designs. The primary issue is with saturation at high current and given that this is a low power 8 ohm design it is much less of a concern. Some have pointed out low signal level hysteresis as an issue with many ferrites. Here is an overview of some popular core types:
http://www.fair-rite.com/newfair/materials.htm

Don't like the dip to 6 ohms with the 30 ohm resistor, I'd probably make it 50 ohms or so, allowing the max to go as high as 16 ohms, then adjust the 1.5 ohm to maintain the correct tweeter level.