[openwheelracing]

hmmm, I need a sub, I don't need a sub, I need a sub.......

I really need a sub Danny, make this one affordable please. 

[rythmik]

if i wanted to build a sub w/a single servo plate amp, i am curious as to whether it would be better to run two 4 ohm subs in series to get an 8 ohm load on the amps, or to run two 8 ohm subs in parallel to get a 4 ohm load on the servo plate amps.  what would the effciency of a dual-driver sub like these be, max output, etc; and if one were to build one, would the cabinet size simply be doubled, vs building a single-driver sub?

thanks,

doug s.

Hi,

This is Brian from Rythmik Audio.  I know there are lots of questions. So please feel free to ask. 

The short answer to your question is that we should use two 8ohms driver in parallel. There are two reasons: 1) the sweet spot of our amps in terms loading is 4ohms. So whatever we do with the driver connection, the amp likes to see 4ohms nominal (or 3 ohms DCR), and 2) there is an additional advantage of parallel connection vs serial connections. Parallel connection provides equal voltage to each driver. So if there is slight thermal imbalance in those two drivers, the amp will actually supply less wattage to the unit with more heat. On the other hand, in serial connection, the amplifier supply equal current to both drivers and the unit with higher voice coil resistance (due to voice coil heat-up) will receive even more output than the other unit which can potentially lead to  "thermal runaway".  We design the dual driver kits so that end user can use either back to back configuration to cancel out the reaction force (same idea as the balance force of ML), or push pull configuration to further reduce even-order harmonic distortions.

We will have two new plate amp models with added functions. The output wattage will remain the same as our A370SE(servo). The first one is A370P with RCA input, one defeatable parametric EQ, and three 12db/24db adjustable settings. The second model is A370E with XLR input, but no PEQ. Other than that, it is same as A370P. These amps will be available by Jan 31.

For dual 8ohm kits, we will come out with a 500-550WRMS@4ohms classH amp. The amp will have exactly the same heatsink and functions as other A370 amps. This will simply the installation and design process.

I would like to make a couple of comments on those questions on OB drivers:
1) I favor parallel connection for the above-mentioned reason. In addition, if one driver is open circuit, the servo will maintain operational by making the other units work harder to achieve the same output.
2) The contraint we have is the excursion limit of each driver (the driver would have 14mm linear xmax and more than 25mm of mechanical xmax. We have designed the driver' parameters such that we can fully extract the excursion capability from the driver. For instance, in OB (or dipole) configuration, this means higher Qts value for the driver. While this higher Qts value can lead to "boomy" sound in conventional subwoofer design, the servo circuit principle can change that. If one ask me what is servo, my answer is virtualization of T/S parameters and it is the most generalized view which enable us to develop vented version of servo sub while others cannot. What is virtualization (a term borrowed from software engineers) of T/S parameters? It is a method of making the system behave in all aspects as if the system possess a different set of T/S parameters, such as frequency response, response to external noise injection (such as the standing wave inside the enclosure, most of us have not realized even with the most sturdy enclosure,  the woofer cone itself is the open gate for us to hear the boxy sound, and that internal standing wave, which is not in the input signal, is an example of external noise) , and all other minor aspects that we may not notice but often manifest as sound quality. Linkwitz Transform circuit does not have the capability of T/S parameter virtualization. Neither does PEQ. Both methods just superficially changes the frequency response. The separation of physical T/S parameters and virtual T/S parameters is what enables us to design the physcal parameters such that the driver will have the best excursion utilization curve and at the same time design the virtual parameters that it will give us the best sound.

[walkern]

Hey Danny,

For those of us with one of your current subs, is it feasible to simply replace the old driver and amp with the new ones and keep the old (2.4 cu ft) enclosure and passive radiator?  I assume this won't be as fast as a sealed box, but I am hoping not to have to start from scratch.

Neil W.

[doug s.]

hi brian,

thanks for your post.  a coupla more questions...     can you use a pair of these drivers in one cabinet, w/both mounted forward?  how about three drivers mounted forward?  (load would be ~3.2 ohms?)  what's gonna be the price of your 500-550w amp?

thanks,

doug s.

[rythmik]

hi brian,

thanks for your post.  a coupla more questions...     can you use a pair of these drivers in one cabinet, w/both mounted forward?  how about three drivers mounted forward?  (load would be ~3.2 ohms?)  what's gonna be the price of your 500-550w amp?

thanks,

doug s.

Doug,

1) You can use them in one cabinet as long as they are in close proximity.  I don't recommend put them in two separate boxes and we cannot use 3 drivers as it will put the DCR to 2ohms.
2) I don't know yet as I am still waiting for that myself.

[TRADERXFAN]

This is exciting stuff...

Does the servo amplifier need to be in close proxity to the woofers? Would it be possible to co-locate the amplifier near the rest of the electronics so you just run the speaker line to the woofer?

Thanks.

[rythmik]

This is exciting stuff...

Does the servo amplifier need to be in close proxity to the woofers? Would it be possible to co-locate the amplifier near the rest of the electronics so you just run the speaker line to the woofer?

Thanks.

The servo amp can be placed as far as 20ft from the driver.  I once thought about standardizing the connection using NL4 connector.  We will see how the market adopting the technology. If there is enough need, it may be worthwhile making some component amplifiers. 

[Jazz and Baroque]

Doug,

1) You can use them in one cabinet as long as they are in close proximity.  I don't recommend put them in two separate boxes and we cannot use 3 drivers as it will put the DCR to 2ohms.

Brian,

if Doug puts 3 of the 16 ohm versions in parallel, then the impedance seen by the amp should be 5.3 ohms.  Right??

Also, when you say that the servo amp can be placed 20 feet from the driver, do you mean that the speaker cable PLUS the internal cabinet hook-up wire should be no more than 20 feet??

Mike

[Danny Richie]

if Doug puts 3 of the 16 ohm versions in parallel, then the impedance seen by the amp should be 5.3 ohms.  Right??

DCR is actually 4.6 ohms. I intend to run three of them off of one amp in a couple of new designs.

[Milehighguy]

Lets see... If three of these woofers in parallel is 4.6 ohms, then the actual impedence of one of them is 13.8 ohms, right? So if a guy was to put four of them in parrallel, the impedence of that group would be 3.45 ohms, right?
If this is correct, then my question is will four of them at 3.45 ohms work with one of the amps? Is it close enough to the 4ohm "sweet spot"?

[Danny Richie]

They are actually 14 ohms (minimum) each so really they are a 4.66666... ohm load if driving 3 of them.

So yes you could run 4 of them off of one amp. It would only be a 3.5 ohm load.

[doug s.]

hi brian,

i see my original calc of 3.2 was wrong, but isn't it 2.667 ohms w/three 8 ohm drivers, instead of 2 ohms.  a 4 ohm amp can't handle this load?

also, re: running two drivers in the same box, just confirming it's ok to have them on the same face, one directly above the other.  i ask this because you said:

"...We design the dual driver kits so that end user can use either back to back configuration to cancel out the reaction force (same idea as the balance force of ML), or push pull configuration to further reduce even-order harmonic distortions..."

i think i understand when you say "back-to-back" - you mean two drivers on opposite ends of a box; when you say they cancel out the reaction force, are these run in phase, ie: both drivers move out (away from each other) and in (towards each other) at the same time?

i don't at all understand what you mean when you refer to a push-pull configuration.

thanks for further clarification,

doug s.

hi brian,

thanks for your post.  a coupla more questions...     can you use a pair of these drivers in one cabinet, w/both mounted forward?  how about three drivers mounted forward?  (load would be ~3.2 ohms?)  what's gonna be the price of your 500-550w amp?

thanks,

doug s.

Doug,

1) You can use them in one cabinet as long as they are in close proximity.  I don't recommend put them in two separate boxes and we cannot use 3 drivers as it will put the DCR to 2ohms.
2) I don't know yet as I am still waiting for that myself.

[rythmik]

i don't at all understand what you mean when you refer to a push-pull configuration.

thanks for further clarification,

doug s.

The term push-pull is borrowed from amplifier design, in particular, the output stage design. Even order harmonic distortion is a result of asymmetry in  the transfer function curve between input and output (after taking out DC offset).  In other words, if the transfer function curve is symmetric, the even order harmonic distortions (2nd order, 4th order, ...) will be completely absent.  It is very easy to to make it symmetric though. Mount one of the drivers inside out, and also inverse the phase of that driver so that both drivers move in and out at the same time. So if we see it from outside, we have one mounted with cone facing out, and one with cone facing in. The same thing is when you look at both drivers from inside the enclosure. That is symmetry.  I guess push is the opposite of pull, the configuration is such that no matter how you look at it, there is always one driver doing the "push" work and the other doing the "pull" work and that is how they achieve symmetry.

Brian

[rythmik]

They are actually 14 ohms (minimum) each so really they are a 4.66666... ohm load if driving 3 of them.

So yes you could run 4 of them off of one amp. It would only be a 3.5 ohm load.

Also those 14ohms drivers are specifically designed for OB, Dipole, and IB. The Qts is around 0.8 with no enclosure. With enclosure, the Qts will be even higher and that is not good.  For the same reason, the 3ohms and 6ohms drivers cannot be used for OB as the Qts is too low and the back EMF will crimp the power flowing into the driver, and as a result, the output will be limited.   Some people have incorrectly used drivers with low Qts for dipole and OB application.  Replacing those drivers with Danny's 14ohms drivers, one can hear clear improvement in the passband output. Again, using servo we can bring the system's equivalent Qts down to 0.3 again so we have clean sound again. That is really the best of both worlds: higher passband output (with higher physical Qts), and clean sound (with the low equivalent Qts from servo system). In addition, these OB drivers have very low moving mass (around 100g-110g). The reaction force is also lower than other drivers (with the same cone movement). 

   

[doug s.]

thanks, brian.  now if you could answer my other questions...   

1) i see my original calc of 3.2 was wrong, but isn't it 2.667 ohms w/three 8 ohm drivers, instead of 2 ohms.  a 4 ohm amp can't handle this load?

2) also, re: running two drivers in the same box, just confirming it's ok to have them on the same face, one directly above the other.  i ask this because you said:

"...We design the dual driver kits so that end user can use either back to back configuration to cancel out the reaction force (same idea as the balance force of ML), or push pull configuration to further reduce even-order harmonic distortions..."

3) i think i understand when you say "back-to-back" - you mean two drivers on opposite ends of a box; when you say they cancel out the reaction force, are these run in phase, ie: both drivers move out (away from each other) and in (towards each other) at the same time?

and, last, but not least, if i do not run these push-pull, will they still be good if two are run on the same face, mounted conwentionally, facing outward on the baffle, in phase, in a sealed cabinet?

thanks,

doug s.

i don't at all understand what you mean when you refer to a push-pull configuration.

thanks for further clarification,

doug s.

The term push-pull is borrowed from amplifier design, in particular, the output stage design. Even order harmonic distortion is a result of asymmetry in  the transfer function curve between input and output (after taking out DC offset).  In other words, if the transfer function curve is symmetric, the even order harmonic distortions (2nd order, 4th order, ...) will be completely absent.  It is very easy to to make it symmetric though. Mount one of the drivers inside out, and also inverse the phase of that driver so that both drivers move in and out at the same time. So if we see it from outside, we have one mounted with cone facing out, and one with cone facing in. The same thing is when you look at both drivers from inside the enclosure. That is symmetry.  I guess push is the opposite of pull, the configuration is such that no matter how you look at it, there is always one driver doing the "push" work and the other doing the "pull" work and that is how they achieve symmetry.

Brian

[rythmik]

thanks, brian.  now if you could answer my other questions...   

1) i see my original calc of 3.2 was wrong, but isn't it 2.667 ohms w/three 8 ohm drivers, instead of 2 ohms.  a 4 ohm amp can't handle this load?

2) also, re: running two drivers in the same box, just confirming it's ok to have them on the same face, one directly above the other.  i ask this because you said:

"...We design the dual driver kits so that end user can use either back to back configuration to cancel out the reaction force (same idea as the balance force of ML), or push pull configuration to further reduce even-order harmonic distortions..."

3) i think i understand when you say "back-to-back" - you mean two drivers on opposite ends of a box; when you say they cancel out the reaction force, are these run in phase, ie: both drivers move out (away from each other) and in (towards each other) at the same time?

and, last, but not least, if i do not run these push-pull, will they still be good if two are run on the same face, mounted conwentionally, facing outward on the baffle, in phase, in a sealed cabinet?

thanks,

doug s.

1) You can use 3 drivers, but it is not recommended. Previously, I have a customer used a A350-basic to driver two 4ohms speakers in parallel. He needed to use a fan to keep the amp cool because he was getting 40% more power than a 4ohm load (even though he had a lot of fun with the setup). Power leads to heat dissipation. More power means more heat dissipation. I cannot recommend that to my customers.  Actually, he knew I would not warrant the amp based on how he had used the amp. We have our reasons. 
2) and 3) You can definitely put two drivers on one wall.  Instead of using two 12" drivers, one can also use a single 15" driver to get the same "cone area" (acoustic output is cone area times excursion).   We offer both of these to meet different goals. In the previous posts, I merely emphasized more on what we can do with two drivers that cannot be done with one driver.

[mfsoa]

Hi Brian,
Can you comment on the use of RDES or other types of EQ with your servo subs?

Pros/cons/no-nos etc?

And as far as driver size goes, is the primary advantage of the 15" max output or does it also go substantially lower than the 12". My room is smallish (13 x 18 x 8', 2 door-sized openings) so I know a pr of 12" will be plenty of output but will a pr of 15" go noticably lower?
(music only, not head-bashing volume, looking for quality and extension over output)

Thanks in advance, and I must say you have a very nice website with gobs of information.

-Mike

[rythmik]

Hi Brian,
Can you comment on the use of RDES or other types of EQ with your servo subs?

Pros/cons/no-nos etc?

And as far as driver size goes, is the primary advantage of the 15" max output or does it also go substantially lower than the 12". My room is smallish (13 x 18 x 8', 2 door-sized openings) so I know a pr of 12" will be plenty of output but will a pr of 15" go noticably lower?
(music only, not head-bashing volume, looking for quality and extension over output)

Thanks in advance, and I must say you have a very nice website with gobs of information.

-Mike

First, thanks for the compliment.  I am not against EQ. The upcoming model has a "defeatable" PEQ.  I like the EQ to be applied strictly for room modes, not for correcting the deficiency in frequency response. For frequency response correction,  LT or servo can do a better job. These two issues (driver and room) are very different. What is good for one may not be good for the other.

The 15" driver will have the same frequency response as the 12".  Because we use servo circuit, we can make all of our sealed sub models match frequency response within +/-1db between 5hz to 80hz.  That is why we have: 1) adjustable extension filter to customize for playback level and number of units, and 2) we can do an apple to apple comparison among our models. If the frequency response is same, time domain response is the same, and yet two speakers still sound differently, something is wrong beyong the realm of FR and TDR. That is how we can ensure consistent sound quality, and that had also helped us to select the correct components to make good drivers.

[doug s.]

brian, when will you have cabinet plans for dual forward-facing 12" drivers?  and, dual forward-facing 15" drivers?   

best,

doug s.

[JeffB]

Rythmik,

I have a Visaton B200 wide range 8" driver that I am using on an open baffle.
In some ways this driver is very good, but it has a rising frequency response that most people feel needs some kind of filter on it.
I don't have a filter on mine, simply because I have not got around to implementing one.  I definitely think it needs one though.
However, I feel this is the wrong approach, and what I really would like is a driver like the B200 but that does not require a filter.

Have you given any consideration to building a driver for open baffle use using this servo technology that works from say 150Hz on up?