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Discussion in 'Amps and Cabs [BG]' started by peterboy1968, Jan 6, 2018.
Appreciated, and know I agree the statement is not accurate without context.
Of course it can be bad news.
Earlier it was stated:
"The chances are if your amp produces any D.C. your speakers would have fried right away..."
"any DC" and "fried right away"
That discussion WAS in the context of amplifiers in the post where this was said, so I agree with you. But I am not convinced that "any DC" will fry a speaker "right away."
If this is an absolute truth, then why are some amps allowed to put up to 0.75 volts of DC on their outputs before they go into protection mode? Is 0.75 volts below the threashold of "any"?
This statement carries with it the implication that "any DC" is an immediate death sentence for all speakers under any circumstances.
For many of us here we can put the "any DC" statement in the context of amplifiers or not, because we understand any underlying context that may or may not apply. But those people are in the minority here. This was not intended for that minority. It's not intended for you DUW, nor is it intended for Paulabass.
It is intended for those who are confused when they hear someone say "any DC" will fry a speaker "right away," and then someone else tells them to put a battery across their speaker terminals.
To contest this statement, I intentionally took it out of the context of an amplifier to be able to discuss it without the distraction of amplifiers. I wanted to do this because there is more going on when an amplifier is involved than with just a simple source of DC.
This is a science experiment of sorts, designed not to prove that DC is not bad for speakers, but rather to demonstrate that there are conditions where you can apply a DC voltage to a speaker, even for an extended period of time, without damaging it. It is simply an experiment to challenge the statement that "any DC" will fry a speaker "right away." These are statements made in the form of absolutes. But as so many seem willing to point out in this forum, there are very few real absolutes around here. There are always exceptions.
I posed my challenge to this seemingly absolute wording as a way to let someone know it's OK to question that which has been presented in the form of a universal fact. If I posted the things I did for no other reason than to help one poor conflicted sole who has been asked to do a battery test on a speaker but won't because he is afraid he might damage it, understand that it's OK to do that test, I have succeeded. That is who I posted this for.
Ok then. But I think when an amp goes DC it generally does it with gusto enough that the speakers die.
I concur fully.
Lets stay around with science for a second.
At some conditions drivers can be burdened with DC even if everything is fine with the amplification.
Overdrive can cause an asymmetrical waveform which in total can cause some DC displacement to the driver EVEN if the amplifier output is DC coupled.
No, i don't bring my amp to the shop because of a couple of DC Volts that can be measured at the speaker if the signal shows too much asymmetrical characteristcs.
IIRC Andy stated about 1/8 if the VC was strained with pure DC. For a 200 Watt woofer this would mean about 25 Watt DC power handling. 25 Watt of power equals the generic average power of solder irons we are used to with electronic service work. Quite imaginable a VC might withstand this amount of (thermal) power over time without getting damaged.
The only thing I am arguing is that it is possible to apply a DC voltage to a voice coil if it is properly current limited so as to avoid causing damage to the speaker.
Comparisons with other types of signals are not a part of MY argument.
Amplifiers are not a part of MY argument.
Everyone is making much more of MY argument than what is really there.
Ok, it's "Myth Buster" time
1. A 9V battery will NOT damage the VC of a typical woofer used in bass guitar cabinets. A 9V battery has an internal resistance of typically around 2 ohms (average) and an average capacity of 200mAH at the 1A discharge rate. The typical 8 ohm woofer has a DCR of 5.5 ohms, so doing some rough ohm's law calculations, the total load the battery sees is 5.5 ohms + 2 ohms = 7.5 ohms. Assuming this is close enough to the 1A discharge rate, the following will be true: (9V/7.5 ohms) = 1.2 amps, internal cell voltage = 9V, terminal battery voltage = (9V - 2.4V) = 6.6V Looking at the power dissipated: speaker = 6.6V x 1.2 amps = 7.9 watts and in the battery = 2.4V x 1.2 amps = 2.88 watts. So, the battery is going to get pretty darn warm if anything. Now the other part of this is that the batter has limited energy (power x time), so if you take (200mAH x 9V), you get 1.8 Watt-hours, now divide 1.8 Watt-hours by 7.9 watts and you get about .2 hours or 12 minutes. In actuality it will be less than this because the internal resistance will climb and the cell voltage will fall under these conditions, so maybe 8 minutes might be closer. (Do not attempt this because it's quite possible to have a destructive event with the battery itself.
Voice coils have significant thermal mass, and a rough rule of thumb is that most can take around 1/8-1/4 rated power continuously at DC provided this doesn't cause enough axial displacement to damage the suspension. For an LF driver capable of say 400 watts "RMS", this would equate to between 25 and 50 watts. The reason this power has to be derated from the published "RMS" rating (assuming reasonable published ratings of course) is that significant cooling of the coil occurs from the conduction of heat from the VC to the motor structure. Wire outside the gap models thermally differently than wire inside the gap. Compared with 7.9 watts for 12 minutes or less and it's a total non-event.
In fact, I will test a driver at DC just to evaluate it's suspension symmetry. It SHOULD move forward exactly the same as moving backwards (with plus and minus voltage). Any asymmetry will show up as an audio distortion component, and it may increase at a faster rate near Xmax. This test is a free air test and does not relate to Xmax which should be evaluated under appropriate acoustic loading.
2. DC may have value in amplifier output: Actually, there are distortion techniques that intentionally offset one side or another or a variety of modulation techniques to create specific types of distortion. These can be done within a small power amp and then level shifted back to zero with retains the quiescent DC 0 volt conditions while imparting asymmetrical offset (and distortion) to the signal.
3. There may in fact be significant DC with respect to ground at the output of some amp topologies: Ok, this is sure to cause worry and undeserved consternation, but there are some bridged topologies that are built up from a pair of single ended direct coupled amplifier halves. Using an example that I have sitting on my bench right now, the amp has a rail voltage of +100V above ground and each half is biased at 50V DC above ground at its output. If you measure the output with respect to ground you will measure 50V DC. How can t his work without destroying your speaker you might wonder? Because each amp half is referenced to the other, so if you measure from the output terminal of the first amp to the output terminal of the other amp, you will see ZERO volts DC because both are at the same potential (because they are common mode). However, if you look at the signal component, as you feed in a sine wave you will see the first half amp's voltage head positive while simultaneously the second half amp's voltage head negative (with respect to ground and each other). This voltage is differential mode and if you put an AC voltmeter across the terminals you will see an AC voltage (with respect to V+, +50V and ground, the 3 values will be different). This is exactly how many class D power amps operate!
4. All QSC amps do NOT shut down at .75V DC offset: This is quite a myth actually, and inaccurate because there are quite a few models that were grounded bridge with effective AC coupling, and these amps had no DC offset protection and can not shut down under this condition (and in fact it's REALLY rare for these models to exhibit DC offset due to their architecture. In fact, very few amp designers would choose .75V at the shutdown threshold due to the difficulty in detecting this quickly enough, yet not false trigger under high level low frequency operation or accommodate signal asymmetry, The time constants are just too long, the typical value is around 2 to 5 V but could be higher with higher powered models to allow for appropriate detection time constants. There are ways to do this quicker in DSP, but I'm not aware of anything that does this in practice.
5. Vented Gaps are NOT for cooling: This is a bit counter-intuitive perhaps, but the gap venting in the JBL drivers are there mostly to reduce acoustic loading under the dust cap without making the pole piece vent too large (the geometry places limits on this). These vents in the gap reduce the velocity within the gap itself (where a good part of the cooling takes place) and probably reduces the effect a little rather than increases it. It does reduce distortion significantly at high power, low frequency operation however. It also reduces the squeak and whistle noise potential.
6. From Downunder's post (in context of a discussion about amplifiers DC to the speaker needs no qualification, it's bad news.), this does in fact need qualification. See #3 above, where it is not only acceptable, but absolutely necessary for proper operation.
How's this for a start?
This statement is 100% correct.
I have never had any intent to argue with you, only help you see more of the picture. Sorry if I upset you.
The tech notes on VGC (Vented Gap Cooling) describe the use of air currents to efficiently remove excess heat from the voice coil as a primary benefit of the technology. I should mention several companies make similar claims and, of course, each has their on proprietary way of doing it that is "better" than everyone else.
It's a bit disturbing to read this is essentially marketing drivel, and that the technology may actually degrade the drivers heat regulation, rather than enhance it. Surely companies realize that deception degrades their brand's equity.
@Old Garage-Bander I guess I owe you another apology.
No apology is due.
Rather, I owe you a word of thanks for your continued engagement in this discussion.
Pretty much a redundant term IMHO.
Not that marketing is bad, it simply has a specific purpose, to sell product.
The purchaser of that product has a responsibility to themselves, and possibly to others, to sort through the marketing claims and make enlightened decisions. Marketing somewhat depends on us not doing that.
How does a model in a bikini standing next to a tractor, make that tractor better than the other guy's tractor?
It doesn't, yet the marketing firms still hire the models.
There must be some reason that it sells tractors, or they wouldn't do it.
Catch ya later.
If you think about the 3 vents in the gap, they really act like bypass channels. Air gets pulled and pushed more easily through these channels which means less goes through the actual gap.
There are good reasons for doing this IMO, though the reasons are harder to explain and less sexy sounding than increased cooling.
The tighter the gap is the more efficient the motor is, and the more efficient conductive removal of heat is. As the gap gets tighter, the problem of venting the trapped air becomes more challenging. For some pole piece geometries using very magnetic flux, there's a limit to how much center venting can be done and its effectiveness. Taking a few bites out of the gap seems to work well without significant loss of performance.
I wouldn't say that in this case it's marketing drivel. Perhaps the marketing is maybe not as accurate as it could be, but it does contribute valuable things to the driver's performance.
BTW, the 2242 was one of my favorite drivers except for the weight.
I am quite confident other technical advances were implemented with VGC to enhance the series thermal management over previous designs. In effect the company rolled up several technical updates under the name VGC, and in all fairness it seems the new designs did focus on cooling the gap, just not through the use of enhanced air flow. I still find the misappropriation of facts a bit distasteful and inconsistent with ethical marketing practices, at least the way the subject was taught at my school. However, for practical purposes, the inaccuracy is generally harmless except in obscure circumstances like this discussion, so myth busted, dragon slain, and moving on.
Greatly appreciate your time and involvement.
Conduction cooling was improved a bit by the use of a small turn-up at the top of the pole piece, plus the bobbin material changed from Kapton to glass fiber composite.
Regardless, it's a very high performance driver. That said, the differential drive Neo drivers are a huge step forward IMO.