Auto Tuba Size and Drivers

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Drake
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Auto Tuba Size and Drivers

#1 Post by Drake » Fri May 21, 2021 8:39 pm

I am wanting to upgrade my sub. My current sub is a single 12" in a sealed box, tuned to a Qtc of around 0.8 with an f3 of 37 Hz. Nice little sub, but it tends to cause my amp to overheat. My amp is a vintage amp from the 90s that has about 200 watts rms / 400 watts peak. So I'm looking at building an Auto Tuba. I can fit a 19 inch wide tuba nicely in the trunk of the car. And I won't have to worry about my wife packing the groceries in and around the sub (my previous sub drivers died an ominous death due to groceries). I am considering the following drivers:

Code: Select all

| Recommended Parameters |      |       |             | 28 - 40 | 0.3 - 0.5 | 15 - 45 | a  8+ |      |        |
|------------------------+------+-------+-------------+---------+-----------+---------+-------+------+--------|
| Driver                 | Size | Power | Sensitivity |      fs |       Qts |     Vas |  Xmax |  Qes |    EBP |
|------------------------+------+-------+-------------+---------+-----------+---------+-------+------+--------|
| Tang Band W8-740P      |    8 |   120 |          84 |      30 |      0.28 |    20.1 |  12.1 | 0.31 |  96.77 |
| MCM 55-2421            |    8 |   120 |        87.2 |    29.1 |      0.22 |    30.4 |     8 | 0.22 | 132.27 |
| Dayton DCS205-4        |    8 |   150 |        88.6 |    32.3 |      0.37 |    26.9 |   8.8 |  0.4 |  80.75 |
| Dayton DCS255-4        |   10 |   200 |          91 |    37.7 |      0.46 |      32 |   8.3 |  0.5 |   75.4 |
| Dayton UM10-22         |   10 |   500 |        84.8 |    26.9 |      0.49 |      24 |    19 | 0.59 |  45.59 |
For a 17 - 19 inch wide tuba, I see that I could run a single 8, a single 10, or dual 8s. What combo of drivers and width will give the best performance? Or is the auto tuba forgiving enough it doesn't really matter?

Seth
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Re: Auto Tuba Size and Drivers

#2 Post by Seth » Fri May 21, 2021 11:50 pm

Drake wrote:
Fri May 21, 2021 8:39 pm
For a 17 - 19 inch wide tuba, I see that I could run a single 8, a single 10, or dual 8s. What combo of drivers and width will give the best performance? Or is the auto tuba forgiving enough it doesn't really matter?
I've built a 20" wide Tall AutoTuba loaded with 2 of the MCM drivers, an 8½" wide TruckTuba also loaded with 2x MCM drivers, and I'm very happy with the drivers in these cabs. I haven't tried any other drivers in them, so I can't say for sure, but I imagine any gains would be marginal. The MCM 8's are plenty. And a great price too.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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J_Dunavin
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Re: Auto Tuba Size and Drivers

#3 Post by J_Dunavin » Sat May 22, 2021 8:23 pm

I have a single 8 MCM in a 14in wide that I’ve been running for the last 10 years or so. Great sub!
I’m not sure two of them would top a single 10 though🤔
I’ve been meaning to build a wider version but haven’t got around to it.
I can say for certain that you won’t draw as much power from your amp. The auto tuba is much more efficient.
What amp are you running? ( love old school car audio)
You may need an external crossover with high pass. An AudioControl 2SX would be a good option
8 - DR200
2 - DR250
9 - T24
6 - T45
1 - Auto Tuba

Drake
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Re: Auto Tuba Size and Drivers

#4 Post by Drake » Sun May 23, 2021 11:26 am

The amp is a California A400XL MOSFET Bridgeable 2 Channel amp with 400 watt peak power. It has a low pass filter but no high pass filter for filtering subsonic noise. I bought it in 1996 and it has served me well for the past 25 years. I have always used it with sealed enclosures. My original subwoofer setup was a plexiglass box with dual 12s. Looking back, the speaker magnets were relatively small and thus it must have been a relatively high Qtc system, which gave a good bass kick, albeit with higher distortion and more muddied output. After I graduated with an engineering degree, I understood the principals behind speaker design a little better, and so when it came time to replace the sub, I spec'd out a single 12 with a lower Qtc of around 0.8. This definitely improved the detail and transients. However, it does seem to be more power hungry and causes my poor amp to overheat during the summer.

I know that ported subs have a lower cone excursion near the box resonance frequency compared to a sealed box, which lowers distortion and increases power handling. The tradeoff is high excursion at frequencies substantially below the resonance frequency. Thus subsonic noise must be filtered out. I would assume that the same would be true for a folded horn as a horn works as a transformer to match the high impedance of the driver to the low impedance of the air, thus presenting a high pressure load to the driver which will lower the displacement of the cone and thus reduce distortion. I would assume this is why the auto tuba must have an xmax of 8mm or greater, so that the driver excursion required for the low frequencies will not damage the driver. Does the auto tuba need subsonic filtering? I was under the impression this was necessary only if you were listening to music on a record player with a record that had warpage...

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J_Dunavin
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Re: Auto Tuba Size and Drivers

#5 Post by J_Dunavin » Sun May 23, 2021 12:31 pm

Correct, like a ported enclosure drops off in output below the tuning frequency, a folded horn output also drops off below a certain frequency as well.
Bill or someone else would be able to give a much more technical reason why, but that is the reason for a sub sonic filter. It serves two purposes, speaker protection, and there’s no need for the amplifier to try and Out put those frequencies.
8 - DR200
2 - DR250
9 - T24
6 - T45
1 - Auto Tuba

Drake
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Joined: Thu Dec 21, 2017 1:26 pm

Re: Auto Tuba Size and Drivers

#6 Post by Drake » Sun May 23, 2021 1:07 pm

Getting back to driver selection, perhaps the question should be restated a bit to provide guidance about selecting the proper driver. In an oscillating system, Q is a measure of the energy stored vs the energy dissipated. A higher Q system will be able to play at a higher volume, over a narrower frequency bandwidth. Increase Q high enough and it will be come a one note wonder. Not very musical but an excellent way to win a car audio SPL competition. Lower the system Q and the bandwidth increases, allowing the system to provide a more musical response, but at lower sound pressure levels.

Q is related to the damping, zeta, by Q=1/(2*zeta). In an oscillating system, when the damping is 1, the system will have a transient perfect response with no overshooting and no unwanted oscillations, but with reduced response in the lower frequency region. Decrease the damping to 0.707 and and the frequency response curve will have the maximum flat response with minimal overshoot and unwanted oscillations. Decreasing the damping below 0.707 will increase the response in the low frequency region but at increasing levels of overshoot and unwanted oscillations. In a speaker system, the overshoot will cause higher sound pressure levels at lower frequencies, but the oscillations will reduce the clarity of the sound. Thus, speaker design is a balancing act of choosing high enough damping to keep the musical reproduction accuracy, but low enough so that the bass volume doesn't get reduced to much.

A zeta of 1 for a transient perfect response corresponds to a system Q of 0.5, which produces a very tight bass but at reduced sound pressure levels. A zeta of 0.707 for a maximum level bass response and reasonably small overshoot and oscillations corresponds to a system Q of 0.707. Reducing the damping and increasing the Q to 0.8 or even 0.9 increases the low bass response and warmth of the bass. Increase the Q to 1 and beyond and the bass volume will increase but the bass will be mushy.

From my understanding, vented enclosures and folded horns are not really thought of in terms of overall system Q. However, system Q is what I'm most familiar with so I'll keep the discussion in terms Q. The overall system Q, Qtc, should be some function of the cabinet Q, Qc, and the driver Q, Qts. Thus, Qtc = f(Qc, Qts). The driver Qts is the parallel summation of the mecanical Q, Qm, and the electrical Q, Qes, and thus Qts = Qm*Qes/(Qm+Qes). Ported subs typically get good results with a driver Qts between 0.2 to 0.5. The resonance frequency of the driver, fs, divided by the driver electrical Q, Qes, gives a quantity called the Efficiency Bandwidth Product, or EBP. An EBP of 50 or less indicates the driver is more suitable to a sealed enclosure, while an EBP of 100 or more suggests a vented enclosure. An EBP between 50 and 100 indicates the driver could be used in either a sealed enclosure or a vented enclosure. I would assume that a folded horn will be similar to a vented enclosure, thus a folded horn should benefit from higher EBP levels, perhaps in the 80 to 120 range.

The auto tuba listed range for driver Qts is 0.3 to 0.5. I assume the overall Qtc is a directly proportional function of Qts and Qc. Thus, increasing the driver Qts should increase the overall Qtc. I would expect a Qts of 0.3 to produce more detailed bass, while a Qts of 0.5 would sacrifice some clarity for more volume in the low frequency range. Therefore, a Qts below 0.3 will tend to reduce the low frequency volume due to higher damping. Therefore, I don't really see any benefit to going below 0.3 for Qts. Is my reasoning correct? Will a driver Qts of 0.3 produce more detail while a driver Qts of 0.5 produce more volume? Will drivers such as the MCM 55-2421 with a Qts of 0.22 produce excessively damped bass with lower volume in the low frequency region? Or is my reasoning to simplistic for the complex behavior of folded horns?

Drake
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Re: Auto Tuba Size and Drivers

#7 Post by Drake » Sun May 23, 2021 1:19 pm

Following similar reasoning, the lower f3 frequency of the system should be a function of the fc frequency of the cabinet and the fs frequency of the speaker. Obviously the fc of the horn is a function of the geometry, the horn opening diameter, and is fixed for the auto tuba. The listed frequency range for the driver is 28 to 40 Hz. I would expect a wider cabinet to benefit from a driver in the lower frequency range. Is there any negative results from picking a driver fs below 28 Hz? Say in the 25 Hz range? Will it reduce the volume of the bass, or the clarity? Or does it not matter as long as the EBP is above say 75?

Drake
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Re: Auto Tuba Size and Drivers

#8 Post by Drake » Sun May 23, 2021 1:24 pm

And finally, the Vas of the driver. The listed Vas range for the driver is 15 to 45 liters. I would expect narrow auto tubas would benefit from a lower Vas, while wider tubas would benefit from a larger Vas. It would also go to reason that using 2 drivers would double the Vas, and therefore need a wider cabinet. So a single 10" driver with a Vas of 40 would have the same width requirements of two 8" drivers with a Vas of 20 each. Therefore, if I want to build a narrow auto tuba, I should use a driver with a Vas on the low side, and if I want to build a wider auto tuba, I should use a driver with a Vas on the high side.

Drake
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Re: Auto Tuba Size and Drivers

#9 Post by Drake » Sun May 23, 2021 1:36 pm

If the above reasoning is correct, then I can choose the various parameters to produce an auto tuba with the desired characteristics. For example, if it's desired to have an auto tuba that is low and loud, then choose a driver with a Qts of 0.5, a Vas of 45, a really large Xmax, a low fs, and build it wide. If it's desired to have an auto tuba that produces tight, detailed bass, then choose a driver with a Qts of 0.3 or lower. If it's desired to have a narrow auto tuba, then choose a driver with a Vas of 15 liters and build it narrow. If I want a compromise between detailed bass and low and loud, then pick a driver with a Qts of 0.4, a Vas of 40 liters, a low fs, and build it wide. Does this logic hold up?

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Bill Fitzmaurice
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Re: Auto Tuba Size and Drivers

#10 Post by Bill Fitzmaurice » Sun May 23, 2021 4:36 pm

Forget all that, it doesn't apply to horns. What does apply is that you can get maximum output with maximum driver displacement, T/S parameter Vd, which is xmax times Sd. Most AutoTuba owners will tell you that you need not worry about it, any of the recommended drivers will work far better than what you have now, but if you must have maximum output that's the limiting factor. However, with 200 watts more than 8mm xmax won't be reached, so there's no point in spending big bucks on a driver with 14mm xmax. You should have a high pass filter, set at 40Hz.

Seth
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Re: Auto Tuba Size and Drivers

#11 Post by Seth » Sun May 23, 2021 5:06 pm

I'd venture to say, you won't be able to tell the difference in sound quality with any driver that falls within the specs, in any specific width cab. Especially without something to compare it to. I'd go the route of the MCM 8", for a measly $35 each. Then you have a baseline to compare other drivers against. However, you'll really like the MCM and probably wouldn't ever get another driver to compare to it.

Personally, in a car, I wouldn't bother going out of my way for a high pass/subsonic filter unless I was using a high dollar driver and pushing it's limits. In car, the AutoTuba plays cleanly and fairly flat all the way down to and below 20Hz. A decent portion of my listening material has content in the 20-30Hz range and I personally appreciate, desire, and enjoy that extension. It's a powerful body shake for sure. I have overpowered them before and experienced coil slap on a few occasions. I bought more MCM drivers than I needed in anticipation of abusing them and needing to replace damaged drivers. But, haven't damaged one yet. On the rare occasion it slaps the coil, I just turn it down a click or two. If I had an amp with an adjustable subsonic filter, I'd probably set it at 20Hz. But, there's no science or practical calculations behind that choice. I assume Bill's 40Hz recommendation is based on keeping the excursion less than Xmax. And really, nearly all mainstream music would be well represented with a 40Hz high pass filter. But, again, I've beaten the piss out of mine with 16-40Hz content, well over the rated power handling, and no high pass... and they still live.

Since you guys are into vintage gear... I powered my 20" wide 2x MCM (parallel) AutoTuba with 2 (of 4) channels bridged of a late '90s HiFonics Andromeda 125XIV (125w x4, or 500rms x2 @ 4 ohm bridged). It was stable down to 1ohm and never shut down no matter how hard I beat on it in this configuration. But, it did kill 3-4 alternators. Class A/B... sucked more juice than the poor stock replacement AutoZone alternators could bear. Gotta love their lifetime warrantee on the alternators. I got the amp in trade from a guy that worked for/with HiFonics, so I don't know what they actually retailed for, but the MSRP was $4999.99 :shock: Big bucks in todays money, let alone in the '90s!
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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Tom Smit
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Re: Auto Tuba Size and Drivers

#12 Post by Tom Smit » Sun May 23, 2021 11:26 pm

Seth wrote:
Sun May 23, 2021 5:06 pm
I got the amp in trade from a guy that worked for/with HiFonics, so I don't know what they actually retailed for, but the MSRP was $4999.99 :shock: Big bucks in todays money, let alone in the '90s!
:shock: :shock: :shock:
TomS

Seth
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Re: Auto Tuba Size and Drivers

#13 Post by Seth » Sun May 23, 2021 11:49 pm

Tom Smit wrote:
Sun May 23, 2021 11:26 pm
Seth wrote:
Sun May 23, 2021 5:06 pm
I got the amp in trade from a guy that worked for/with HiFonics, so I don't know what they actually retailed for, but the MSRP was $4999.99 :shock: Big bucks in todays money, let alone in the '90s!
:shock: :shock: :shock:
It had some pretty neat features for that time era. 70amp MOSFET's, military spec gold plated PCB, it did have a subsonic/high pass (very rare back then) adjustable from 10Hz-4kHz (still rare), and a couple interesting features... balanced and unbalanced inputs, two fully variable and independent 0º-180º phase shift control for left and right channels, and built in x-over which wasn't too common back then but this one had separate adjustable cutoff frequencies which is still uncommon. That MSRP's gotta be inflated (as they tend to be) but it was a pretty bad ass amplifier. I sold it a year or so ago for $100. Loved it, but after going through so many alternators, the quality of class D amps now-a-days, and how affordable DSP is... I just decided I wasn't going to ever use it again.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

Drake
Posts: 24
Joined: Thu Dec 21, 2017 1:26 pm

Re: Auto Tuba Size and Drivers

#14 Post by Drake » Mon May 24, 2021 12:01 am

Wow that's some amp! Thing about vintage gear is they do make great heaters. However, they seem to last forever! Sounds like my little 200 watt RMS amp will not overdrive dual MCMs. And $35 each is down right affordable! If by chance they do get overdriven, I can always upgrade to the Tang Band W8-740P. For the ultimate in Vd, it looks like the Dayton UM10-22 would be something else! I'm assuming that the UM10-22 would work since the fs is 27 Hz, the Qts is 0.49, and the Vas is 24 liters. The Vd is 6228 cm^3, which is about double the Vd of dual MCMs. It would take a new amp to drive it though... for those who just have to have obscene amounts of volume!

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