Increasing Q with a 4 Ohm Resistor.

[jagman]

Somebody in the subwoofer biz told me to add a 4 Ohm resister to the positive lead to each of my AA Avalanche 18" subs. He says it effectively increases the Q and essentially dials down output around the Fs. In my case that would mildly decrease the 10-25Hz range and protect the drivers from overexcursion (and amp clipping). Since that's there, I would be able to turn the volume up more and get more upper bass without fear of bottoming the subs. Have you guys heard of this approach? How might this be different than using a high pass crossover?

[brucek]

He says it effectively increases the Q and essentially dials down output around the Fs.

I suppose it would indeed dial down the output at Fs, but it sounds like a terrible idea to me.....

Normally, all the voltage you supply from your amp will be across the driver. The output impedance of the amp is extremely low, so it acts as a voltage source.

When you add a resistance (not an impedance), it becomes a fixed resistive divider set up between the impedance of the driver and the resistor.

Examine what happens when the drivers impedance is at 4 ohms. The drivers impedance of 4 ohms and the 4 ohm resistor are in series, which effectively divides the maximum voltage you can develop across each component in half.

This doesn't unfortunately divide the power dissipated by each by half, but by about a quarter of what the single driver dissipated, because power is the square of the voltage divided by resistance.

Simply put, the volume will be much lower with two devices in series and you'll have to turn it up a lot more to get the same volume you enjoyed without the wasted power you're dumping into a resistor that produces no sound.. and you're worried about clipping....

In addition you are feeding more noise floor.

Anyway, off the top of my head this sounds goofy...

brucek

[jagman]

Huh... very interesting.

The fellow who recommended it designs servo subwoofers for a living. Like I said in the first post, he said it only effects sub 25Hz frequencies and he suggested I wouldn't need more power. He said he thought adding the resistor would keep excursion in the VLFs in check. Since that's the case, the volume could be turned up which would increase mid/upper bass output while decreasing low end excursion thus eliminating the risk of bottoming my low Q subs.

I'm not saying he's right, I'm just repeating what he told me.

Bruce, your point seems valid. Personally, I don't understand how adding a 4 Ohm resistor would only effect the lowest frequencies. I asked him how this differes from a high pass crossover and he said the highpass crossover doesn't protect from overexcursion due to the amp clipping.

[Joe L.]

In addition to all that has been said, if the driver was wired as a 4 ohm load, then one half of the power would be wasted as heat by the resistor.

So, lets assume you had a 400 watt amplifier... (into 4 ohms)
By adding a 4 ohm resistor in series, you now have a 8 ohm load.
That would result in roughly 200 watts power from that same amplifier into an 8 ohm load... or 100 watts of power for the sub, and 100 watts wasted as heat by the resistor.

Thinking further... ever grabbed hold of a 100 watt light bulb after it has been on for a while... or grabbed hold of the hot end of a soldering iron (most of them are 25-35 watts... so think three times as much heat)? In either case, you can get an idea of how hot the resistor will get. You will need a very hefty resistor to handle the heat, if nothing else.

All told, in my opinion, your friend knows very little about basic electronics, since he said "add the resistor in series with the positive lead" Since the resistor is in series (assuming that is what he meant) it does not matter where it is put in series... positive lead, negative lead, close to the driver, or close to the amplifier...

I'm sure you mis-understood him some way. Perhaps he wanted you to put the resistor across one of the dual voice-coils on that AA driver, and feed the other voice coil from the amplifier. That will affect its T/S parameters.

Joe L.

[terry j]

besides all that, seeing as how this is a BFD (or similar) forum, just control the sub how you want it to with eq.

[jagman]

I have a BFD and REW and an IB subwoofer. That's why I posted this in the IB forum (as opposed to the others). It is relevant to decreasing the risk of bottoming low Q drivers and an alternative to using the BFD.

[jagman]

Joe... your advice is appreciated. The gentleman I referenced has a PhD in electrical engineering. He said the resistor isn't the best alternative, but he thinks using it in conjunction with the BFD is better than using the BFD or the resistor individually.

[brucek]

He said the resistor isn't the best alternative, but he thinks using it in conjunction with the BFD is better than using the BFD or the resistor individually.

I don't personally understand the logic in that assessment.

There's no magic in the math to show this isn't a good idea, so I wish you could elaborate on why you feel its the route to go.

You do understand that when your driver isn't at resonance that half the voltage that originally was available to your driver will be dumped across a power resistor (in the form of heat)? This limits the power available to your driver to a quarter of what you're use to. To enjoy the level of power (and resultant SPL) you're use to will require the sub amps volume to be increased, with the chance of clipping.

How is this better than simply using a high pass filter to limit the bottom end excursion?

brucek

[willy-be]

Perhaps someone could invite Brian Ding of Rythmik Audio to post to this thread. That way Brian and Bruce can discuss this topic directly..

Aloha,
WB

[Jman]

The BFD can only be set as low as 20hz so we can't use that to cut the 10-20hz range without also pulling down 20-30hz.

A highpass filter would be more suited. I have a 12db/octave in my art351.

[rythmik]

It is correct that I am the person who made the recommendation. And let me explain my rationale. Before I proceed to explanation, let me show a few plots. This is the FR of AAA 18" in IB with 350WRMS@4ohm



Now let us look at the excursion curve.



There are several things we can discuss in this plot. But in essence, the combination of low Q and high voice coil inductance has made the FR literally a bandpass filter. For a low Q value, you don't get sufficient excursion capability at fs point because of so-called "stall" effect (I can explain if anyone is interested), at the same time, the excursion at below fs is far higher. This imbalance has given rise to a possibility that user can bottom out the driver very easily. Because the output at fs is less, one tends to turn up the volume to get sufficient output, this often has bring the amp to the edge of clipping. Any small additional low frequency component (<5hz) can cause the driver to bottom out, either through clipping (repeated clipping is actually a very low frequency signal), or attenuated low frequency input signal.

So what is the solution? First, subsonic filtering is quite effective. But that is the obvious. The second solution is to decrease the Vas value (which also reduce the compliance), which is impossible (I want to mention this just to make a connection with other ideas). Car subs with low Vas seldom have bottoming issue simply because of this. The third, which has a similar effect as the second is to add a serial resistor. Both the second and third method increase Q value.

After I add a 5-ohm resistor, the new output change the least at fs frequency point, and attenuate more at below and above fs frequencies.



Now let us look at the excursion curve.



To be able to see better, I reduce the power by 4x and we get



By extrapolating, the new excursion is 22mm vs off the chart (which is more like 50mm before) at around 5hz. Adding a resistor is better than decreasing Vas as the latter will change fs.

[Jman]

I have 4 Av18 in an IB. The only movie that gets the woofers moving dangerously is Blackhawk Down, Irene scene. Even then I pushed it hard and when they were using 2" of travel and I was reading about 118db and the amp wasn't clipping at all. If I use my highpass filter that would probably allow much higher SPL for this scene with the 8hz component rolled off. Basically any other movie with 14hz+ stuff I can do 125db with the cones using barely an inch. The last time I ran Pulse it read 126db at the opposite end of the room and the drivers were moving an inch.

[brucek]

Welcome rythmik, and thanks for the thorough explanation. From the outset (as I indicated in my original post), I needed no convincing that the series resistor would accomplish exactly what you and jagman have explained.

The concern I would have with this are the negative effects that result. I don't think they can be ignored.

Basically the series resistor modifies the output impedance of a solid state amplifier and changes its reasonable approximation of an ideal voltage source into a modified current source. This mod (if my memory serves), was actually incorporated into some Sunfire amps years ago by Bob Carver in his attempt to allow switching of his solid state amps into a tube sound. If anyone wants their system to enjoy the same distortion and high output impedance of a single-ended triode, install a one or two ohm power resistor on the output of your amplifier.

Anyway, it's quite simple to excel graph the change in Q of a frequency dependant resistance (speaker coil) using a current source dumping resistance as opposed to a voltage source. It's the side effects that are a concern.

I don't think it's arguable that a large portion of the amplifiers power is lost in the series resistor. This power loss alone would be unacceptable by most people.
The resistor would need to withstand considerable power when the driver was not in its resonance curve. For safety sake, probably a 200% over spec would be prudent. I would think a resistor of this rating would be wire wound and subject to inductance of its own. That wouldn't be good.

I would also have a concern over the drivers damping. A standard solid state amplifier with a typical output impedance of 0.01 ohms driving a 4 ohm load would create a damping factor of ~400 (ignoring wire impedance). The series resistor (of 4 ohms, for example) would create a modified output impedance of slightly greater than ~4 ohms, and would create a damping factor of slightly less than 1. What would stop the cone movement? It has no low impedance path to ground. I can imagine the bass would sound muddy as a result.

So what is the solution? First, subsonic filtering is quite effective. But that is the obvious.

hehe, yeah, exactly. As you say, it seem like an obvious answer. It's so simple and effective to add a high pass filter. I can't 100% agree with your excursion extrapolation (extension) without introducing the notion that most electronics adds its own inherent first order high pass filter response. This protection would often be enough, but that can be easily tested with a program such as REW. Either way, a subsonic filter adds much less problems than the dumping resistor, I would think. But I've been wrong before...

brucek

[rythmik]

brucek wrote:

Basically the series resistor modifies the output impedance of a solid state amplifier and changes its reasonable approximation of an ideal voltage source into a modified current source. This mod (if my memory serves), was actually incorporated into some Sunfire amps years ago by Bob Carver in his attempt to allow switching of his solid state amps into a tube sound. If anyone wants their system to enjoy the same distortion and high output impedance of a single-ended triode, install a one or two ohm power resistor on the output of your amplifier.

Since you know the transfer function experiments, it is very easy to communicate the ideas with you as you understand more than just basic circuit theory. Most of stuff I will say next has been posted at the other forum.

When I first got into this hobby, I had thought low Qts, that is, higher (BL)^2/Re is a universally better driver, even though there are conventional wisdom everywhere that would tell you a low Qts drivers are better for vents and Mid Qts drivers are better for sealed. My rationale of favoring a low Qts driver is simple (probably very similar to your concern right now), that is the speaker is basically an ideal motor with a serial voice coil resistor. When everything connected together, we essentially see a lumped resistance of amp resistance, speaker cable resistance, plus the voice coil resistance, driving a back EMF which equals to B*l*v, where B is the average magnetic strength, l is the voice coil wire length, and v is the cone velocity. This lumped resistor is the barrier for us to completely control this back EMF, which completely control the cone velocity. It is like flying a kite. If the lumped resistance goes to superconductor, the amp output directly drives the back EMF which means we would have completely control of the cone. Therefore my rationale is low Qts driver (which means BL is very high) is a better driver (which is in line this quest of always higher (BL)^2/Re). But when I tested actual subs, I also observed one thing, at resonance frequency of a sealed box, a low Qts driver actually produces less output. This can be explained by B*l*v because at the resonance peak, almost all voltage from amp output goes to back EMF, or B*l*v. However, higher B*l leads to lower v, this is what people refer to as stall effect. And that has completely changed my mind about low Qts. Even though low Qts gives us lower distortion, but the price to pay is very high. A system with Q=0.4 actually needs 4 times of power (2x the voltage) to drive the driver to the same output of a Q=0.8. Let me also give you another actual example that NCA told me. One time their customer used a very large motor to design a driver, after it is done, it was found the Qts is too low, not good for sealed box sub. So this customer widened the gap to weaken the motor. Poor customers !!! They could have paid less if the solution had been keeping the gap the same, but used a smaller magnet. But if the magnet becomes smaller, they may not able to sell as many of them because everyone wants to see big motors.. Should the customers to be blamed for this insanity? Now this story also indicates that, unlike most other engineering optimization problem, there is no so called optimal solution for Qts selection. It all depends on application. For the same reason, there is no optimal (BL)^2/Re. The added resistor has better tempco than copper and that means lower thermal compression.

I don't think it's arguable that a large portion of the amplifiers power is lost in the series resistor. This power loss alone would be unacceptable by most people.
The resistor would need to withstand considerable power when the driver was not in its resonance curve. For safety sake, probably a 200% over spec would be prudent. I would think a resistor of this rating would be wire wound and subject to inductance of its own. That wouldn't be good.

Just think of this resistor as part of the voice coil resistance. Actually it may be better because the heat can be dissipated in open air, instead of in that small gap.

I would also have a concern over the drivers damping. A standard solid state amplifier with a typical output impedance of 0.01 ohms driving a 4 ohm load would create a damping factor of ~400 (ignoring wire impedance). The series resistor (of 4 ohms, for example) would create a modified output impedance of slightly greater than ~4 ohms, and would create a damping factor of slightly less than 1. What would stop the cone movement? It has no low impedance path to ground. I can imagine the bass would sound muddy as a result.

Damping factor is a notion of the past. If you look at the lumped resistance I mentioned above, which one is the major contributor? It is not the amplifier output resistance. It is the voice coil resistance itself which has a temp. coefficient of 0.3%/Celsius. 20 degrees C will give you a variation of say 3ohms x 0.3% x 20=0.18ohms. As you can imagine that temperature can get a lot higher. So in the big pic, the 0.1 ohms amp output resistance is not significant at all. If we look further, there are production unit to unit variations, a 10% reduction of BL value is equivalent to 19% increase of Re value. That would have even more impact. Not to mention we would have learnt from history that exact ideal transfer function does not give us the best sound either. I am wondering if you still remember shortly after Bob Carver's transfer function work, there were a frenzy of ideas derived from that, such as Hafler which has a tweaking control to exactly match the amp out to amp input under load. That idea does not go anywhere. The transfer function is useful only to "copy" the sound. It tells nothing how to come up with the first "best" sound. So we still have no clue which contributes good sound or not. Also Bob Carver's transfer function will not work for speakers either. It is funny because speaker has the least number of components, but each one of them is so difficult to model.

BTW, I still like the low Qts and that is how we implement our servo subwoofer. We use a driver with proper Q that will give us best physical output (without stalling), which normally means the driver that will achieve Q=0.8 when not equalized. And then use servo to get us the lowest Qts value we can possibly achieve so-called "fastest" sound possible. That is how we get the best of both worlds without being pull on both ends.


Jman,

BTW, the exact cause of bottoming is not as simple as what I have described. A lot of that also has to do with the spider roll thickness and surround width. In other words, compliance of a speaker is one of the least linear components and linear compliance normally work against us when it comes to bottoming. It is hard to say you didn't get bottoming because the spider system prevent you from getting it.

[brucek]

I also observed one thing, at resonance frequency of a sealed box, a low Qts driver actually produces less output. This can be explained by B*l*v because at the resonance peak, almost all voltage from amp output goes to back EMF, or B*l*v. However, higher B*l leads to lower v, this is what people refer to as stall effect. And that has completely changed my mind about low Qt. Even though low Qt gives us lower distortion, but the price to pay is very high. A system with Q=0.4 actually needs 4 times of power (X the voltage) to drive the driver to the same output of a Q=0.8.

Interesting - that changes things. Appreciate that my electronics background (combined unfortunately with a total lack of knowledge about speaker design) is what drives my treating a speaker as a simple coil and magnet device. hehehe, it's why I usually avoid this area of the forum. Seriously though, interesting stuff.

Just think of this resistor as part of the voice coil resistance.

Well OK, but that part offers its power output in heat and not sound. But in light of the quote above, it does change my tune somewhat.

Damping factor is a notion of the past. If you look at the lumped resistance I mentioned above, which one is the major contributor?

Yeah, and I agree even simple cable impedance can overwhelm the damping formula (with regard to amplifier o/p Z). I have read several articles that test various damping factors (derived from progressively higher lumped Rs values) with the resulting increase in Qes and Qts....... But, when you start adding five ohm blocks of resistance, with system damping less than one, I start to question whether the decaying signal will muddy the sound or not. You say no, OK, I have to believe you, since I have no experience in the field of speaker/driver design. I acquiesce on that point.

Anyway Brian, we appreciate your posting here, and hope you will do so in the future.

brucek

[rythmik]

brucek wrote:

But, when you start adding five ohm blocks of resistance, with system damping less than one, I start to question whether the decaying signal will muddy the sound or not. You say no, OK, I have to believe you, since I have no experience in the field of speaker/driver design. I acquiesce on that point.
brucek

It will add to the signal decaying which will "muddy" the sound. That is for sure and I cannot dispute that. It is more of a trade-off between the good and bad of low Qts vs reasonable Qts.

[jagman]

Jman wrote:

I have 4 Av18 in an IB. The only movie that gets the woofers moving dangerously is Blackhawk Down, Irene scene. Even then I pushed it hard and when they were using 2" of travel and I was reading about 118db and the amp wasn't clipping at all. If I use my highpass filter that would probably allow much higher SPL for this scene with the 8hz component rolled off. Basically any other movie with 14hz+ stuff I can do 125db with the cones using barely an inch. The last time I ran Pulse it read 126db at the opposite end of the room and the drivers were moving an inch.

I've played that scene turned up pretty high and the subs were probably moving a good 2" but showed no signs of strain. I assume you're talking about the part with the chopper blades going and everything in the HT starts shaking . I bottomed mine on the scene were the alien/robot thing comes out of the ground in War of the Worlds. It's been a while, but I believe it was the part when the pavement cracked. At any rate, since that happened I've been a little gun shy with the volume. I don't want to destroy my Avalanches.

[jagman]

I decided to give the resistors a shot and see how they affect things. Brian's plots accurately reflect how these affect output (above and below Fs). Output in the VLFs is reduced (sub 15Hz). Theoretically jumping from a of Q4 to 9 limits excursion to 22mm. Since the Xmax is 27mm, I figure I'm pretty safe. That's music to my ears since I've soft bottomed them twice and worry about doing it again. I'll post some plots after listening to the new setup for a while. I haven't had a chance to listen to it much, but I can say it has changed the sound. It definitely sounds more muddied with music than before. I'm sure it will offer loads of piece of mind while watching movies... though I'm not sure if it's worth the price of increased distortion. I'll have to listen more before making up my mind.

[Jman]

My IB is pretty much bulletproof now. Some things I ran into along the way.

1) not enough power, I used a Samson amp first. It's rated for 1000 watts but I doubt it puts that much out. It caused some really funky, ugly clipping sounds. The driver is basically sent rubbish and you get back some scary ugly sounds, like clunking, vibrating wildly, etc. The Behringer ep2500 fixed this; I run it 2 ohm stereo and each pair of Av18's get about 1000 watts each.

2) clipping the BFD. This is a tricky one as you want to send it enough signal to "use" lots of bits (too low, less bits utilized). I thought it was fine but then some really hot movies made it flash red and it created ugly harmonics that sound like grunge/hash. The master and commander DTS or WOTW DTS tracks seem to be the hottest signals so I use those as the ceiling for setting my receiver LFE.

3) I recently put my ART351 eq back in the loop. It has a 12db/octave highpass filter. I played a 16hz tone with REW and then turned the highpass until it just began to affect my SPL meter and then backed it off slightly. That gives me a 12db rolloff by the time it reaches about 8hz. Then 4hz is further reduced by 12db. That allows the 12hz+ stuff to play pretty much as is while protecting against some of the almost suspect "bass" that is slipping into soundtracks these days.

All that said, can you add another pair of drivers? The Fi 18's are close enough that they could probably work with the Avalanche 18's. That gives you 6db more headroom, which is probably what you need. The highpass is another good option, just make sure it's adjustable. The resistor idea just feels counterintuitive and if you can clearly hear an effect on the sound quality then it's doing something bad.

[jagman]

Jesse,

Thanks for sharing your experiences. I haven't run into the difficulties you mentioned (I haven't clipped my amp nor the Behringer). That doesn't mean more power wouldn't help, I just think I probably don't drive my IB as hard as you. The ART351 EQ sounds interesting. I'll see how I like the resistor, but I'll keep your suggestion in the back of my mind. Unfortunately I can't add more drivers as my two 18s are opposed, and a large water pipe runs along one of the remaining sides of the manifold. The other issue is the opening in the manifold is really only big enough for two 18s.

Thanks again,

Krister

[Jman]

2 manifolds?

The highpass makes a huge difference. For the BHD scene, setting it to ~16hz slashes the excursion in half. You lose the 8hz fundamental but you can go for the 12hz+ instead.

[jagman]

Two manifolds?

Maybe one front and one rear a la the Harman white paper. That would probabaly also smooth out the curve. Of course it wouldn't do much for coupling the drivers... then again I wouldn't have to cut as much. Hmm. This just keeps getting more and more expensive .