Minimus 7 Loudspeakers - Measurements
- Thread starter Zilch
- Start date
Both have die-cast aluminum boxes; the 77 cabinets are larger than the 7 cabinets.
The 77 uses a larger woofer than the 7.
They use rather similar tweeters, but I believe the tweeter mounting plates are different.
Both were manufactured with the same (very simple) crossover circuit, and both can use the same PZ-2.1 crossover circuit (although the physical design of the crossover and its mounting to the box may be different, depending on the speaker model and version).
I believe that both were probably painted with a primer and an enamel paint (white or black). If you want to repaint, you might simply sand the existing cost of paint lightly to roughen it and then spray on several light coats of enamel. Or, if the existing cost of paint is in bad condition, you might want to strip it off, down to bare metal, then same and prime and paint. You would want to use a good primer and enamel that are made for painting metal.
Nice to see your measurements confirming that the mod also works well for the 77.
How did you take that measurement, speaker location, mic distance, measurement tool?
lukiedog
Super Member
I'll be taking pictures of my setup and documenting results from various tweaks at some point. In the meantime, here are some additional measurements someone took for the Minimus 7 with a replacement woofer.
http://www.lostinsound.space/minimus-7-upgrade/
http://www.lostinsound.space/minimus-7-upgrade/
Mazza
Active Member
Fantastic write up Lukiedog. Thanks for going to the trouble for us all. I notice that your inductors are right besides each other and have a similar orientation. I've read that inductors need to be set at different orientations so as to not influence each other. Regardless, your 77's measure great.
I'm curious as to what you meant by this comment?
"All of the specified resistor values are ridiculous! I used a standard low tolerance 3R3 for R1, a 39R for R3 and would have used a 2R2 for R2, but 2Ω was actually obtainable. I just don’t know what some people are thinking sometimes. Anyway …"
I too have 77's in my shed which I updated a while back.
This weekend I did one of my 7's. Your write up is just what I need to press on with the second. I think I spent about $5 on resistors and caps. The extra inductor I scrounged (and partially unwound) from my horde of surplus crossovers....
I finished some other projects and when I had all my spare parts out I said, "What the heck" and found all the parts for the 2.1 crossover. Built it up, put it in the box and WHOA!, this is not your father's Minimus 7. VERY nice. I only have one, so the search begins to make it a pair. It took me forever to get to this, but well worth it.
Pairs of these pop up on BT frequently. I did the mod and share your opinion. I have another pair and parts, time to get er done, again!
RTally
Speaker addict
I have the parts to do two pairs. I just haven't had the time to do them yet. I did the ebay upgrade for a third pair of Minimus 7 and it was a big improvement. I use that third pair for my computer system. It will be interesting to A/B the 2.1 crossover with the ebay upgrade.
RTally
Speaker addict
js22
Active Member
Whew! I read the whole thread in one go last night. I think building the XO will be easier than that. Thanks to Pete and Zilch (and everyone else) for keeping this thing moving along. It looks like fun.
I inadvertently stumbled into the party yesterday. I made my second stop of the day at the thrift store (it's very close to home) and found a pair of Minimus 7s. I'm a little embarassed to have paid $19 for them, but the ones I got (white) are pre-A (i.e., no suffix) and in pretty nice shape cosmetically and sound-wise. I was impresed at how good they sound for such a small speaker. But, as evidence that I'm taking baby steps on the path of audiophilia, I did in fact find them fatiguing after a while, even with the bass turned up. I was glad to switch back to my $15 Nova-8s, which I kinda enjoy more than my $500-ish DIY SR71s (Zaph Audio design). The rest of the family preferrs the bottom of the stack, thank goodness.
I put in an order to Madisound for caps and resistors. Even though PE has the 1.0 mH inductor on sale for like $3.85 right now, I'm nothing if not a CSOB. So I'm going to try a little something different.
I tend to keep a lot of stuff "just lying around". Years ago, I tore apart a microwave oven transformer and kept the core. Here's the top part (the I's) with 50-ish turns of thin magnet wire.
That's 6.48 mH, measured at a frequency of 1 kHz. The plan is to split the top part into halves or thirds. It's a stack of laminations, so pretty easy to split. Then I'll wrap the cores with some 14 gauge enameled wire I have (laying around...) One nice thing about this particular transformer core is that there are holes through all the layers (barely visible in the lower part of the "E's" in the background.) So it should be easy to rig up some kind of mounting scheme.
I should say that I have *some* knowledge of magnets and transformers, but I'm no expert, especially where "practicalities" come into play. There could well be some pesky detail that makes this whole idea fall aprt. (Any ideas?) But I thought it would be fun to try, and if it saves me $8 plus shipping, there's that.
I inadvertently stumbled into the party yesterday. I made my second stop of the day at the thrift store (it's very close to home) and found a pair of Minimus 7s. I'm a little embarassed to have paid $19 for them, but the ones I got (white) are pre-A (i.e., no suffix) and in pretty nice shape cosmetically and sound-wise. I was impresed at how good they sound for such a small speaker. But, as evidence that I'm taking baby steps on the path of audiophilia, I did in fact find them fatiguing after a while, even with the bass turned up. I was glad to switch back to my $15 Nova-8s, which I kinda enjoy more than my $500-ish DIY SR71s (Zaph Audio design). The rest of the family preferrs the bottom of the stack, thank goodness.
I put in an order to Madisound for caps and resistors. Even though PE has the 1.0 mH inductor on sale for like $3.85 right now, I'm nothing if not a CSOB. So I'm going to try a little something different.
I tend to keep a lot of stuff "just lying around". Years ago, I tore apart a microwave oven transformer and kept the core. Here's the top part (the I's) with 50-ish turns of thin magnet wire.
That's 6.48 mH, measured at a frequency of 1 kHz. The plan is to split the top part into halves or thirds. It's a stack of laminations, so pretty easy to split. Then I'll wrap the cores with some 14 gauge enameled wire I have (laying around...) One nice thing about this particular transformer core is that there are holes through all the layers (barely visible in the lower part of the "E's" in the background.) So it should be easy to rig up some kind of mounting scheme.
I should say that I have *some* knowledge of magnets and transformers, but I'm no expert, especially where "practicalities" come into play. There could well be some pesky detail that makes this whole idea fall aprt. (Any ideas?) But I thought it would be fun to try, and if it saves me $8 plus shipping, there's that.
js22
Active Member
I finished the inductors. Spliting the material from the transformer core is very easy. The old glue/shellac is all that holds the layers together. I did a first cut with a cold chisel while clamping the core in a vise. Then I removed them and popped off a couple of layers one at a time with a wood chisel and light hand pressure : the single layers just lift off. For no particular reason, I ended up with cores 18x20 mm, and 105 mm long.
I 3-d printed some coil forms. The "sleeve" part is in several 10 mm sections. Here is the result:
To wind the cores, I chucked them up in a lathe, but did not use the motor. Just rotating it by hand was easy and gave me plenty of control.
After adjusting the inductance (I erred on the high side!) I slathered on plenty of hot glue. Here's the happy result:
Oops! The price tag of my Pioneer SX-780 accidently snuck into the frame.
Here's some tests. Both coils had a DC resistance of 0.136 ohms.
Coil A, 0.998 mH:
And Coil B, 1.003 mH:
The Eqivalent Series Resistance (ESR) of about 0.4 ohms includes the 0.136 ohms DC resistance, plus all other AC effects which dissipate energy, namely, heating of the iron core. I'm guessing that an ESR of 0.4 ohms is small enough compared to 8 ohms to not matter. None of the 1 mH inductors at PE list the ESR; I'd be curious what they are like.
The next challenge : figuring out how to fit these inside the Minimus 7 case!
I 3-d printed some coil forms. The "sleeve" part is in several 10 mm sections. Here is the result:
To wind the cores, I chucked them up in a lathe, but did not use the motor. Just rotating it by hand was easy and gave me plenty of control.
After adjusting the inductance (I erred on the high side!) I slathered on plenty of hot glue. Here's the happy result:
Oops! The price tag of my Pioneer SX-780 accidently snuck into the frame.
Here's some tests. Both coils had a DC resistance of 0.136 ohms.
Coil A, 0.998 mH:
And Coil B, 1.003 mH:
The Eqivalent Series Resistance (ESR) of about 0.4 ohms includes the 0.136 ohms DC resistance, plus all other AC effects which dissipate energy, namely, heating of the iron core. I'm guessing that an ESR of 0.4 ohms is small enough compared to 8 ohms to not matter. None of the 1 mH inductors at PE list the ESR; I'd be curious what they are like.
The next challenge : figuring out how to fit these inside the Minimus 7 case!
RTally
Speaker addict
Good work!
Mounting a small enclosure for the crossover outside the cabinet is always an option. Make it easier to assemble and secure. Trying to secure the crossover to the inside is a pain.
js22
Active Member
Slight case of tool envy after reading your post,lathe, 3D printer, etc!
Helps to have friends in the right places.
I love rolling my own. Inductors are simply, simple.
js22
Active Member
Do you wind iron/ferrite/powedered-iron inductors ? Where do you get the core material? I used pieces of an old microwave oven transformer just sort of guessing that the same proerties that work for one application would be "good enough" for another. So far, it seems to work. I'd be curious what other cores people have used.
