Design introduction - The Modern Minimus 7 (MM7)
Hi everyone. I'd like to share a design that I'm really excited about. As a kid I loved the Minimus 7 when used with the matching receiver with EQ. The EQ makes the small speaker sound really good with great bass for such a small speaker. I figured, wouldn't it be fun to do a modern version of the classic speaker and see how much more performance we can squeeze out with DIY?
The original speaker is a 1" dome + 4" midwoofer in a 2L sealed box. Let's copy that form factor but with modern twists. The drivers will showcase the advancement in modern driver design over the past 50 years.
Components:
Originally, the speaker had the same front dimensions as the Minimus 7 wood version, which was bigger than the aluminum version. The reason is to simplify the build. Then I realized, the original Minimus 7 had surface mounted drivers, and if I also do surface mounted drivers, I can truncate the Purifi woofer frame and make the front baffle smaller without making it any harder to build. I can also push the woofer and waveguide right next to each other to save space, something I can't do with a flush mount since they need a small amount of space in between for rigidity. This shaved off 0.7” on both the 5" width and the 8" height, now down to 4.3" wide x 7.3" tall x 6" deep (1" deeper than before to maintain box volume). This doesn’t sound like much, but going from 5” to 4.3” is actually 14% smaller and it looks noticeably smaller. Now the front dimensions are exactly the same as the classic aluminum Minimus 7. I really like small speakers, and I love that I found a way to make it smaller without making it harder to build, which is usually the problem when trying to squeeze every bit of size out of them.
The original speaker had a rectangular faceplate for the tweeter. To match that visual style, I will be using a 4” rectangular waveguide for the tweeter based off of Somasonus’s 4” waveguide. Please excuse the crappy CAD drawing and the green waveguide that isn't edge to edge, I suck at CAD, but you guys get the picture. This brings an improvement in the sensitivity, directivity, and distortion performance of the Bliesma T25B tweeter, while following the visual style of the original speaker. Form and function, win!
While the drivers are no longer flush mounted, and this is not ideal acoustically, I am deciding to go this route in order to pay homage to the look of the original speaker, and because the negative effects of diffraction is minimized due to the use of a waveguide and there’s actually very little surface area left on the baffle that isn’t covered by the woofer or waveguide. It’s a small compromise I’m happy to make. This speaker isn’t meant to be completely acoustically optimal. The priority is to make something fun that pay homage to a classic.
Simulations are very promising. The Purifi 4" is an amazing woofer with such a long Xmax and strong motor for such a small driver, exactly what I need for deep bass output from a small box. With 100W/ch, 2L is all that's needed to get essentially everything out of the woofer. That's reasonable on the amplifier power and on the driver's power handling capabilities. This is possible from the very strong motor on the Purifi woofer giving it great deep bass efficiency in a small box.
The simulation shows about 98dB at 40Hz and 93dB at 32Hz from a pair. Subtract 2dB for losses like power compression and PR losses and there is still over 95dB at 40Hz before room gain and room modes. To get a rough comparison, because the Purifi has more than 2x the clean excursion of a normal 4" woofer, it plays more like a normal 6" woofer. Not bad!
In future posts I will go into much greater detail on the design choices to maximize performance out of a speaker, particularly a small speaker. I look forward to sharing more. I think this speaker will end up being a pleasant surprise to see how far things have come since 1977
Hi everyone. I'd like to share a design that I'm really excited about. As a kid I loved the Minimus 7 when used with the matching receiver with EQ. The EQ makes the small speaker sound really good with great bass for such a small speaker. I figured, wouldn't it be fun to do a modern version of the classic speaker and see how much more performance we can squeeze out with DIY?
The original speaker is a 1" dome + 4" midwoofer in a 2L sealed box. Let's copy that form factor but with modern twists. The drivers will showcase the advancement in modern driver design over the past 50 years.
Components:
- Tweeter: Bliesma T25B, one of the finest small faceplate 1" tweeters available.
- Waveguide: Modified Somasonus 4" waveguide for the Bliesma T25B in a rectangular frame rather than oval
- Woofer: Purifi 4X to get the best midrange and bass performance out of a 4" driver.
- Passive radiator: Purifi 4" passive radiator on the back tuned to 33Hz to help get ~5-6dB more bass at the 30-50Hz range.
- DSP and amplifier: Ashly FX125.4. 4x125W FIR DSP amplifier. The speaker will be active with DSP, and the aim is to get a flat response to an unusually low 32Hz in order to cover 99% of musical bass. DSP is a wonderful modern technology to allow for much deeper than normally possible bass out of small speakers.
Originally, the speaker had the same front dimensions as the Minimus 7 wood version, which was bigger than the aluminum version. The reason is to simplify the build. Then I realized, the original Minimus 7 had surface mounted drivers, and if I also do surface mounted drivers, I can truncate the Purifi woofer frame and make the front baffle smaller without making it any harder to build. I can also push the woofer and waveguide right next to each other to save space, something I can't do with a flush mount since they need a small amount of space in between for rigidity. This shaved off 0.7” on both the 5" width and the 8" height, now down to 4.3" wide x 7.3" tall x 6" deep (1" deeper than before to maintain box volume). This doesn’t sound like much, but going from 5” to 4.3” is actually 14% smaller and it looks noticeably smaller. Now the front dimensions are exactly the same as the classic aluminum Minimus 7. I really like small speakers, and I love that I found a way to make it smaller without making it harder to build, which is usually the problem when trying to squeeze every bit of size out of them.
The original speaker had a rectangular faceplate for the tweeter. To match that visual style, I will be using a 4” rectangular waveguide for the tweeter based off of Somasonus’s 4” waveguide. Please excuse the crappy CAD drawing and the green waveguide that isn't edge to edge, I suck at CAD, but you guys get the picture. This brings an improvement in the sensitivity, directivity, and distortion performance of the Bliesma T25B tweeter, while following the visual style of the original speaker. Form and function, win!
While the drivers are no longer flush mounted, and this is not ideal acoustically, I am deciding to go this route in order to pay homage to the look of the original speaker, and because the negative effects of diffraction is minimized due to the use of a waveguide and there’s actually very little surface area left on the baffle that isn’t covered by the woofer or waveguide. It’s a small compromise I’m happy to make. This speaker isn’t meant to be completely acoustically optimal. The priority is to make something fun that pay homage to a classic.
Simulations are very promising. The Purifi 4" is an amazing woofer with such a long Xmax and strong motor for such a small driver, exactly what I need for deep bass output from a small box. With 100W/ch, 2L is all that's needed to get essentially everything out of the woofer. That's reasonable on the amplifier power and on the driver's power handling capabilities. This is possible from the very strong motor on the Purifi woofer giving it great deep bass efficiency in a small box.
The simulation shows about 98dB at 40Hz and 93dB at 32Hz from a pair. Subtract 2dB for losses like power compression and PR losses and there is still over 95dB at 40Hz before room gain and room modes. To get a rough comparison, because the Purifi has more than 2x the clean excursion of a normal 4" woofer, it plays more like a normal 6" woofer. Not bad!
In future posts I will go into much greater detail on the design choices to maximize performance out of a speaker, particularly a small speaker. I look forward to sharing more. I think this speaker will end up being a pleasant surprise to see how far things have come since 1977
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Just spitballing here but there was an Australian only small speaker [ whose model number I forget] that used a 4" woofer plus a co-axial tweeter on a 4" midrange. I'd try to emulate that if better bass in a tiny box was the aim
I did a revamp a few years back called the Synchaetas. Used a PRV 4MR60-4 for the woofer and an ND25 tweeter. I got an F3 sealed at about 100Hz and an F10 in the 50Hz range, and a worthwhile sensitivity of 86dB.
I need to get the info posted in the InDIYana armada pages.
I need to get the info posted in the InDIYana armada pages.
A small comment on this draft:
In my opinion, a passive radiator with a Vd equal to that of the active bass driver unnecessarily restricts the level stability. It would be better to have twice as much Vd compared to the active bass driver.
In my opinion, a passive radiator with a Vd equal to that of the active bass driver unnecessarily restricts the level stability. It would be better to have twice as much Vd compared to the active bass driver.