Helium - a true micromonitor

[CZ Eddie]

I never got around to posting up my DIY build. Mostly because while it's completed, I do have an issue.
First, I have no A/V receiver to power my build, so it's untested in that regard.
All I have is just the aptX Bluetooth Amplifier I installed, which is rated for 50W x 2 @ 4 ohms and is currently taking the stereo Bluetooth signal and summing it to one channel (mod done by the manufacturer). I'd guess it's something like 100W+ to the crossover.
I tried both low gain and high gain Bluetooth settings and have same issue with both.

The speaker gets fairly loud.
But bass sounds bad at high volumes.
Lots of distortion/breakup/etc below maybe 100hz, I'm not sure.

What filter/capacitor inline with the ND91-4 would be recommended to remove bass below maybe 80hz?

I am using the suggested internal volume and the port is tuned as suggested.

Finished product gif:



Cutting the wood discs to build the enclosure.



Ghost eating a throwaway wood disc.



Clamping & gluing the cutout discs together.



Four different trial enclosures.
The one on the far left is the final and made from real cherry wood (see my original post here at labs).



Speakers wired up without any crossover or extra wires showing yet.



Interior view showing speakers, crossover and that white tube is the vent.



A look without the vent installed.



Bottom/removable plate with power & etc. wiring.

[Millstonemike]

All I have is just the aptX Bluetooth Amplifier I installed, which is rated for 50W x 2 @ 4 ohms and is currently taking the stereo Bluetooth signal and summing it to one channel (mod done by the manufacturer). I'd guess it's something like 100W+ to the crossover.
I tried both low gain and high gain Bluetooth settings and have same issue with both.

The speaker gets fairly loud.
But bass sounds bad at high volumes.
Lots of distortion/breakup/etc below maybe 100hz, I'm not sure.

What filter/capacitor inline with the ND91-4 would be recommended to remove bass below maybe 80hz?
I am using the suggested internal volume and the port is tuned as suggested.

Normally, a 500 uf NPE cap in line with the driver would create a 1st order high pass at 60 Hz. But the ND91-4's impedance spikes at 74 Hz so it may not be well behaved w/o a zobel on the driver.

What make and model amp are you using? It is probably possible to create a 1st order HP on the amp's inputs (PLXXO).

[jwjarch]

Got back on track with the Heliums over the last week or so after getting my chip amp up and running. I got one assembled for testing tonight. It took some work but I got my oversized XO to fit really well. After playing a few Marian Hill songs through the one, all I can say is this is the best speaker I've ever heard for its size. I wanted to keep finding different tracks to play. I brought it in from the garage to audition for my wife and she said, "Wow, that sounds AMAZING!" WAF earned! I think the centered tweeter is a working well as far as I can tell. I'll have to get to work on the walnut baffles and start the finish on the cabs. Thanks again Scott!

[joeybutts]

Really simple yet effective port integration. How about a finish? Whatcha thinking?

[jwjarch]

Thanks Joey. No port noise either. Although I couldn't turn them up too far with the kid asleep. Haha.

I'm thinking white lacquer with the walnut baffles will look clean, modern and have a high WAF. I just need it to warm up a little so the paint will cure.

[joeybutts]

Ugh... beautiful concept. Can't wait to see it

[Chris Roemer]

The Helium DOES have a Zobel on it, kinda (well, a shunt RC anyhow). They only control a driver's Z-climb above Z-min.
They don't affect a driver's Fs peak, which (in this vented box) is actually a double peak near 40 and again at 120Hz.
Near 70 is close to the box's Fb, and where the cone moves the least. Doubtful that THAT's your problem.
Even the peak near 40 (which is more likely your issue) isn't touched by a 500uF cap.

You'd have to drop it down to around 80uF to net -6dB @ 40Hz, and that really screws up the rest of the FR, losing almost -15dB @ 70Hz, and about -4dB around 200.
A passive filter in the XO net is not really viable here.
I've got to wonder if overexcursion is YOUR problem though. Are you sure everything's sealed well? Did you possibly warp the woofer frame? Is your vent maybe chuffing? Is your port's inside end right behind the ND91?

[Navy Guy]

Got a chance to hear these at InDIYana. They sound much bigger than their size suggests. These are a great choice for anyone looking to build a small speaker.

[silverneedles]

PE doesnt have this 0.20mH 22gauge inductor... what can i use instead ?

[Chris Roemer]

#257-024 would be ideal.

[silverneedles]

if i want to use 5-6 of these as surround on an '8ohm' receiver "100wpc"... should i make the 8ohm version ?

[scottsehlin]

It really depends on the receiver. Some can take the lower impedance just fine, and some will go into protect at higher levels. The Helium's impedance curve isn't as bad as many "4 ohm" speakers and I haven't really heard of anyone having an issue with their receiver driving these.

The 8 ohm version is an option, but the design was based on a simulation, so I haven't really heard them. When I did the soundbar project, I came up with a slightly different crossover.

[Jake]

I decided to build the helium using the nd90-4 in a .144cf with a "sprite" size port tuned box. I used a dayton poly and 40ohm resistor on the tweet, and a mundorf e-cap 3.9uf on the woofer. Initially the increased bass from the bigger box and rear port created a tonal imbalance, but as things settled in, the balance shifted towards neutral. These have very satisfying bass and a wonderful midrange. They remind me of the Paradigm atom v5. Right now I would not change a thing. Huge thanks to Scott for this great design. I will be building more in the future.

[Chris Roemer]

Hey Jake, welcome.

You did NOT build Scott's Helium. What you did do was to use the same tweeter along w/his XO, but with a different woofer and box.
Also, adding the 39ohm shunt (parallel) resistor (to gnd) actually makes the highs a little more laid back, not more prominent.

[Jake]

Yes chris, not exactly the Helium. A helium variation I credit to scott. Very happy with the reults