MTG Designs - Volt-12

Volt-12

12" Coaxial

-Volt-12 Coaxial-

An offshoot of the Volt line carried by DIY Sound Group.

I had initially done some work on this design around the same time I was working on the Vortex-12 for DIYSG utilizing the same test enclosure I had built for that driver. Like the rest of the Volts this one uses one of the Eminence Beta Coaxials (Beta-12CX to be specific). I had previously tested the Celestion CDX1-1010 and the custom 1446 Erich had made in this coaxial but wan't too happy with the crossover point that was achievable with those drivers (~1800hz). The crossover point higher then I wanted lead to pretty strong beaming from the woofer and therefor a notch in the midrange as you moved off axis. So it sat for over a year.

Rather recently I picked up one of the new Eminence ring radiator compression drivers (F110M-8) to test for reviving another old design using the Dayton H6512 waveguide. In the end it didn't work out in that design due to the fairly strong dip that the F110M-8 has near 13k, that dip was just too deep in the Dayton waveguide to flatten out passively without using more parts then I wanted for the budget design. However I noted the driver did have good low frequency extension and distortion was lower then I was expecting too. Remembering the Volt-12 design that I had shelved I decided to pull out the Beta-12CX and try the F110M-8 in it. To my surprise it worked quite well and the dip which proved unworkable in the Dayton waveguide was not an issue when mounted in the Coaxial driver.

So How Does It Sound?

Quite good. After a few days of listening and tuning the crossover to get the right balance in the mid-range and high frequencies I am quite pleased with the sound.

Bass in this tower enclosure quite good though most would likely still want a sub woofer for extra kick. I wouldn't call it fullrange as the f3 is around 50hz and it's a shallow slope to that f3 from 80hz, that said the large tower does give extra kick and weight to the bass that you just don't normally get fro pro woofers crammed in a smaller box. Though I would think these speakers would certainly be acceptable on it's own if you are running an older receiver that has a bass knob or loudness button which would give them just enough bass to be satisfying for most music.

The mid-range is where I think this shines, the coaxial being a point source you have totally coherent dispersion from the driver and I voiced it with a very slight forward sound to the mid-range. Vocals both male and female are natural, it's well detailed and has a solid foundation towards the bottom and no harshness at the top end.

I'll admit like the other Volts designs the Eminence Beta Coaxials won't be winning any competitions for best high frequency reproduction due largely to design limitations of putting a small horn inside the 2" voice coil, the transition of which to the cone of the woofer imparts several artifacts to the frequency response and dispersion pattern that cannot be overcome with crossover design. That said I don't find the sound objectionable. It took a little tuning to get the right balance of detail without being overbearing or harsh but in the end I'm happy with the sound for what the design is, an inexpensive high output coaxial that could work well for home theater or light PA which can be fully built for <$200.

Specs

Sensitivity of the design is ~96dB.

In the enclosure I have it in you can't really push more then 100w into it running full-range without exceeding xmax with heavy bass content but that does get it quite loud. With an 80hz crossover in a HT setting you can likely get away with a 200-300w amp without much trouble from the woofer.

Impedance is 8 Ohm nominal dipping to a minimum of 4.7 ohms at ~12-13 kHz. Besides that small dip at the high frequencies it should be very easy to drive as it's >8 ohms from 200 Hz to 7 kHz.

Crossover Design

The overall topology of this crossover is quite similar to what is done on the other Volts. The woofer gets a 2nd order lowpass with a small cap across the inductor acting as a HF notch to add a little bit extra slope to the filter. The tweeter is also 2nd order with an inductor wired in parallel with the series padding resistor to provide some contouring of the frequency response. The crossover frequency is ~1350 Hz, the F110 seems to handle it well even though it's far below spec but it's also about low as the crossover could easily be pushed with the simple passive network. Ideally it could be slightly lower to make for a more uniform directivity transition but I feel it's decent enough in its current state as it does prevent any kind of major hole in the mid-range as you move off axis.

There is an unfortunate dip at 1k caused in part due to the woofer having a natural hole there and some interaction with the HF driver but due to the the narrow width it's hardly noticeable .

I have found after several iterations of the other Volt speakers that the most natural sound is achieved when aiming for a generally smooth power response rather then the flattest on axis response. If the on axis response is overly flattened you end up with a large peak in the 6-8kHz range and the top octave also tanks too quickly as you begin to move off axis.

Speaking of which here is the Frequency Response from 0-90 degrees in 10 degree increments.

-measurement is gated and resolution below 400 Hz is limited.

Crossover Schematic

Optional Crossover PCB for the Volt-12

Bill of Materials for the design

Drivers/Crossover Components

Enclosure Design

I mentioned above that the enclosure I used for this design was originally built for the Vortex-12 coaxial but it works quite well for the Beta-12CX too. That enclosure is 40" high x 14" wide x 15.5" deep, ~3.5cuft net and tuned to 45hz using three 3" diameter x 4" long ports on the rear of the enclosure.

Besides normal lining of the walls I had to strategically place some extra fill in this enclosure in order to tame a vertical standing wave which caused a notch in the upper bass. The bottom 8-10" of the enclosure is filled with damping material as is the the top back corner of the enclosure from about halfway back and midway at the driver upwards.

Now the enclosure does not have to conform to the exact size/volume I used but I do recommend trying to keep the baffle width similar.

Staying ported it could be shrunk to about 2 cuft tuned to 40-50hz which would give an f3 in the mid-low 60's. This would be something like 22" tall x 14" wide x 16" deep.

Going sealed it could be made as small as 1.5cuft which would give an f3 right around 80hz. This could be 22" tall x 14" wide x 12" deep.

Optional Crossover Modifications

I voiced the stock crossover for general use but if you are going to be placing the speakers in a baffle wall or mounting in/on wall or in columns I had also designed a crossover with less BSC. This can also be used if you wanted a more forward presentation from the speaker. The changes for this configuration.

L1 should be 3.0mH,

L2 should be 1.5mH.

R1&R2 should be 25 Ohm.

*Note if you are using a Class-D amplifier to drive the speakers it may be best to wire a 5-10 watt 10-20 Ohm resistor (lets call it R3) in series with the 0.47uF (C3) capacitor. C3 connected to one side of L1, R3 connected to other side and the remaining leads on C3 and R3 are wired together. This prevents a capacitive load in the ultrasonic range which some Class-D amps have trouble with. This modification does not alter the frequency response in any appreciable way and could be used with any amplifier topology not just Class-D.

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