The Helios CM Loudspeaker Kit by Jeff Bagby and Javad Shadzi
The Helios for the “Common Man”
Helios CM stands for “Helios Common Man ”, the idea here was to produce a speaker with the performance and incredible sound of the original Helios speaker, but w/o the extraordinary cost. Though the CM kit isn’t “cheap”, it is a true reference level speaker with a very high-end sound and quite a bargain for the cost that it does take to build. The Helios concept is a speaker with a very flat response, very controlled dispersion with a waveguide tweeter, very good bass performance with a large format mid-woofer, and a 12db roll-off for bass using a large passive radiator instead of a port.
As above, the tweeter is the SB Acoustics, SB29SDAC – a soft dome mounted to a massive, 3 D printed waveguide. The woofer chosen is the SB23NBAC45-4 woofer and low-frequency tuning is handled by an SB Acoustics 10″ passive radiator.
“How about that tweeter? Look at it 45 degrees off-axis at 15 kHz! Get this – it’s a 29mm tweeter!”
It all comes from the geometry of the waveguide and how it loads the acoustic impedance at some frequencies and directs sound into the room at other frequencies. It is designed to yield a more constant directivity over a very wide bandwidth. In doing so, it will load lower frequencies and increase the sensitivity but at higher frequencies, it will spread them more evenly than can be achieved on a flat baffle.
Andrew Jones – “This is a very good example of why the correct way to design speakers is to use large waveguides. In fact, one of the reasons con-centric drivers work so well is not just that they are concentric, but that they have by definition a large waveguide around the tweeter.”
Jeff created a really novel crossover topology to deal with the waveguide-loaded tweeter, and the waveguide allowing a low crossover point to cleanly handoff to the 8” SB woofer. The result is a very natural sound that closely reproduces the original recording, a speaker that can generate high SPL levels for its size, and a full range, broad-spectrum sound from something small enough to fit on a bookshelf or speaker stand.
The acoustic crossover is quite impressive – at 1180 Hz. In phase, with excellent phase tracking from around 100 Hz to about 3 kHz.
The Bass Alignment
“I wanted the speaker to be a stand-mounted model of reasonable size, this presented some problems with the bass alignment, but I resolved those by using a novel bass loading system that I used on the Testarossa speakers several years ago. I can’t say that no one has ever done this before, but I have not seen anyone else do this before, at least not in the way I am. In some ways, it may sound counter-intuitive, but you will see how it works to give us some of the best bass you will hear in a stand-mounted speaker like this. For a speaker just over one cubic foot of volume to reach into the low 30’s with very low distortion at the same time is really quite remarkable”
The cabinet size is a bit smaller than the Helios at 1.15 cu ft. (32.5L)The SB23NBAC bass alignment was a bit easier to implement with the new, lower mass version of the SB29 passive radiator.
How Do They Sound?
One of the bright spots of the aluminum cones mid-woofers is the liquid midrange quality. The SB29SDAC tweeter has been a long-time favorite among designers so high frequencies are smooth and detailed.
Javad has not done any specific power handling tests, but these get very loud for sustained periods of time, distortion looks very good at those levels, the woofer bottoming out is the limit and even that is very well behaved. Jeff patterned this after the original Helios and with it, Javad was hitting 113 dB at two meters and everything was doing fine. When I took distortion measurements it was hard to get it off of the noise floor until I got really loud. But here is part of the key – the crossover transfer function is at -22 dB at the tweeter’s crossover point, so it’s not really getting a lot of power in that range.
“The Helios has more bass, and will get louder with the 9” Satori woofer, but I really like the sound quality of the Helios CM. They sound just sublime with the soft dome tweeter and have plenty of bass even at higher listening levels. The passive radiator keeps the woofer excursion in check, and allows for a much more controlled lower frequency.”
“These match the Helios in value, you don’t give up much except for SPL, the Helios will get about 5db louder across the bass range, but the CM gets very loud. I actually prefer the cloth dome tweeter in the CM as well, but if money is no issue then go for the Helios.”
Rasmus L – “These are incredible speakers. They are my go-to recommendation for an affordable build with unaffordable sound quality.”
When you buy these, here is what you get:
You can purchase it in two forms, depending on your resources.
The Bare Bones Version:
- 2 x SB29SDAC tweeter – 3D printed waveguide not pictured. Optional machined aluminum waveguide.
- 2 x SB23NBAX45-4, woofers
- 2 x SB Acoustics, SB29NRX2-00 Passive Radiators
- 2 x Crossover parts, Standard Capacitors are Audyn Q4 on tweeter and Audyn Q4 and Solen on the woofer. You can upgrade the tweeter Capacitors, Sand-cast resistors are standard, but you can also upgrade resistors.
- 2 x Jeff’s specified air-core inductors featuring 14ga perfect layer air on the woofer circuit.
- Complete plans –
The Full Kit Package:
- 2 x Choice of the Input Terminals
- All mounting hard hardware (screws)
- Internal damping material
- Color-coded hookup wire
- Optional push-on connectors
- Gasket sealing tape
- MDF boards for mounting the crossover parts.
Upgrade Your Series Capacitors?
There are a great number of brands to choose from. Don’t see your favorite, just ask.
Have trouble soldering or reading a crossover diagram? We can assemble them for you at a nominal charge.