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Post by gotmojo on Aug 28, 2010 9:46:44 GMT -7
Here's an engineering kinda question! When you use an airbrake, is it actually cutting the wattage that the speaker "sees"? Like if I use the brake with a RxES, with 45 watts of power, does the air brake protect the vintage 30 speaker in a 1-12 cabinet if played say, 2 or 3 clicks back? Anybody know how much power cut there is at each click?
jj
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Post by Jan on Aug 28, 2010 9:57:31 GMT -7
An Airbrake (on a non bypass setting) is a load in series. It does not act exactly like a speaker but it does soak up some current, producing heat instead of sound. Do you need some links to all about Ohm's Law?
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Post by gotmojo on Aug 28, 2010 10:22:28 GMT -7
Um, remember, I have referred to myself as an idiot. So I looked up Ohm's Law and did a quicky refresher course in electrical this and that, but to no avail. Ohm's Law talks of amperes and volts and ohms. But not about electrical power, i.e. watts. So I was hoping someone here with real electrical knowledge can answer this question (like with language understandable to a non-electrical engineer like me, using phrases like "yes it does", or "no it doesn't", maybe with a simple to understand practical description of sorts): does the resistors in a Brake actually decrease the wattage delivered to the speaker? Do you think that 2 clicks back drops wattage by 30 percentage?
Maybe this is a nonsensical question.
jj
this is not new to me, i've asked nonsense before
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Post by Jan on Aug 28, 2010 11:14:21 GMT -7
While reading the Law of Georg Ohm, did you not come across the work of James Watt? Yes, I am a physicist or have been in several incarnations. Watt is, very generalized here, a measure of energy conversion. Remember what happens in our example - the Amplifier pumps current (Amperes ... energy) into the speakers (Resistance). Converting that energy into sound is work which can be measured in Watts. (omitting things like heat for the purpose of this discussion and not getting into the whole Joules per second bits) In terms of units (the bane of all non physics majors) a Watt is the rate of work done when one Ampere of current goes through a potential difference of one Volt. So W = V * A. Back to Ohm's Law ... with a little waving of hands, Watt = A squared R. Umm, what was the question? Sorry, SWMBO is patting her foot in my general direction. Back later.
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Post by gotmojo on Aug 28, 2010 12:38:35 GMT -7
Yes, I did see a reference to James Watt in my quicky review!
a) Dude, you really are a physicist? Very cool. What kinda physics do you do?
b) And my review did take me by the following equation. If Amps squared x R = W, then as you increase R, you necessarily increase W. So as the resistance goes up (using the Brake), the wattage increases! That doesn't sound right, perfessor. What am I missing here? I'm not thinking correctly about this, somehow.
c) Who is SWMBO?
jj
been a little while since i took physics (like 35 years), but i do still have my old college physics textbook, if that means anything (it doesn't) its on the bookshelf over there
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Post by benttop (Steve) on Aug 28, 2010 22:15:11 GMT -7
The layman's answer: yes. Click the Cab Specs link in my signature for the db drop from each click on both the Airbrake and Brake Lite. Now go look up how to figure out how many watts are cut at those db drops... hehehehe.... I'll save you a bit of head scratching - minus 3db is half power. So a 3db drop is the same as using a Maz Jr instead of a Maz Sr (roughly). I'm pretty sure Jan has all the formulas memorized... ;D
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Post by gotmojo on Aug 29, 2010 6:11:47 GMT -7
Yeah Benttop Steve, I had re-reviewed you're chart, but didn't know how to translate to power cut. Dude, a 3db drop is really cutting the wattage output in half? These are real numbers? Real science? jj and you know I trust you like my mama no pseudoscience for me, thank you
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Post by benttop (Steve) on Aug 29, 2010 11:24:30 GMT -7
It's a fact. To get a 3db increase in Sound Pressure Level (SPL) you need to DOUBLE the power. 3db is a LOT.
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Post by gotmojo on Aug 29, 2010 11:56:40 GMT -7
Dudescent, THAT is good news. Cause I'm loving this Vintage 30 convertible cab now with the RxES I just bought from asattwanger!!!!! Was a litlle (not much) worried about blowing the speaker with that head. But at home I'm running on full brake with the MV down just under half. No way I'll run that amp more'n 2 clicks back on the brake, I'm safe.
Right on.
jj
your expertise once again appreciated
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Post by Jan on Aug 29, 2010 15:17:53 GMT -7
For what I jokingly refer to as my rocket scientist phase, I did survivability and hardness analysis for space flight electronic systems. Before that, my business card said, "Laser Physicist" when I did R&D for ophthalmic surgical lasers. A few years have passed since then. Lately I keep my brain involved by helping SWMBO in her cosmic ray particle physics work.
That is She Who Must Be Obeyed. Google will reveal all about that.
Back to that German dude, Georg Ohm. Again, inserting an attenuator into the output signal path is adding a load in series. From Ohm we know that in a series circuit, all loads see the same current, right? Okay, so all else remaining equal, adding a load, a "R" and ...
(what the amp sees) = W = A2 Rtotal = A2 Rairbrake + A2 Rspeaker
But ... you do not hear the A2 Rairbrake contribution unless the speaker is in another room and you hold the Airbrake up to your ear in which case you might hear some high frequency vibrations ... and feel a little heat.
Loads on the output transformer can be thought of as current sinks; they all make therms and some make more sound than heat. An Airbrake is a variable load, silent speaker, sort of. What was the question?
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Post by gotmojo on Aug 29, 2010 16:48:58 GMT -7
Spoken like a true physicist! And I love the line of yours: "What was the question?" I'm gonna borrow that one now and then, wif a tip o' the hat to you.
Academically looking at the Ohm equation, it still seems to me like W increases as R increases. But I maybe I'm looking at it backwards as in W being what the amp puts out, rather than W being the work that the amp must do. So then, increased R means the amp works harder. Is this correct rationale or am I totally lost? How does one convert the work an amp is doing into the "watts output" that we are all so familiar with? Perfessor, please, help us here!
jj
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Post by countrybilly83 (Ty) on Aug 29, 2010 16:59:25 GMT -7
Basically yes it does to our ears but in the science no.. It uses the resistors inside to give the amp a " bigger load" and deliver a small amount to the speaker... At least that's how I think about it.. lol
An RxES at 45 watts shouldn't be a prblem for a V30 thats rated for 60...
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Post by gotmojo on Aug 30, 2010 6:12:52 GMT -7
OMG! I thought the 30 in the name Vintage 30 meant rated at 30 watts!!!!! Idiot. Again. Made for a nice scientific discussion at any rate. Like being back in school!
jj
no lack of knowledge is too great
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Post by doctorice on Sept 5, 2010 9:23:11 GMT -7
gotmojo,
Think of the watt ouput as fixed -- even though we know it varies as the input signal fluctuates. We could say that for a brief instant it really is fixed. Now, if watts are fixed and you stick more resistance in the circuit some of them watts get dissipated on that additional load. Fewer watts get to the speaker, and the output volume goes down.
doctorice-is-not-a-physicist
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Post by 909one on Sept 22, 2010 11:46:46 GMT -7
So I didn't really know much about this stuff either.. But maybe this has been answered already... I don't know... When measuring resistance on each click through the Airbrake Lite, it starts at zero, and then go up drastically from there. I don't remember the exact measurements, but I think it was somewhere up at 50ohms on the highest setting. Isn't that bad for the output transformer, since its supposed to 'see' a load close to its rated impedance?
Also, I love my brake lite, but with my amp the first click doesn't attenuate enough and the second click attenuates just a hair too much for the band I play in. Leaving the AirBrake on #2 puts my amp at the max headroom it can achieve for that volume, so turning up the volume only increases distortion, not volume. Some songs its just the right volume, but others I get lost. Clicking back down to #1 puts my amp in the too-loud zone to get that distortion from the amp. Is there a way to modify it to get a setting between the clicks? I noticed that on click # 3 (i think), moving the clamps on the Ohmite resistor effects resistance on where they are placed. In theory i could just find the setting I want by setting #3 to the proper distance that would yield a resistance between #1 and #2. Or I could get the Air Brake with the Variable Resistor.
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Post by GuitarZ on Sept 29, 2010 5:53:29 GMT -7
You can adjust the clamps on the resistor. Instead of moving #3, you'd probably want to slide #2 over a little. You can then slide #3 as well.
You don't get the flexlibility that you have with a volume control, but I definitely don't get anywhere near my tone without the Airbrake.
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Post by asmith9509 on Oct 5, 2010 14:42:47 GMT -7
Short answer- yes, it reduces the effective power (watts) that the speaker sees. It does not reduce the effective power (watts) that the amp delivers. Slightly longer answer- I've been running my Galaxie full-blast in pentode mode (40ish watts) into a G12H-30 for months with no ill effects. This includes some very loud 3 hour sets. I think the Celestion speakers tend to be slightly under-rated, so you should be perfectly fine.
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Post by Kris (aka smalltownsongs) on Nov 2, 2010 10:37:00 GMT -7
Side note: Steve, using your mega-way-too-informatively-awesome links on amps/cabs/B-L & A-B specs, coupled with reading this thread I.... I think... if you listen closely... my brain just got bigger. I'm starting to comprehend. OP: Great question, made me think enough to follow along, and read and learn. Wow.
Good times here at Z-Talk!
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Post by benttop (Steve) on Nov 2, 2010 11:56:43 GMT -7
...and I thought that "pop!" I heard was the leg on my old rocker. Dude, did that hurt?
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Post by Kris (aka smalltownsongs) on Nov 3, 2010 8:24:25 GMT -7
Barely noticed it! I've got a big old melon. Plenty of room to grow amidst that empty space... ;D
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Post by Deleted on Nov 15, 2010 22:25:47 GMT -7
Yes, I did see a reference to James Watt in my quicky review! a) Dude, you really are a physicist? Very cool. What kinda physics do you do? b) And my review did take me by the following equation. If Amps squared x R = W, then as you increase R, you necessarily increase W. So as the resistance goes up (using the Brake), the wattage increases! That doesn't sound right, perfessor. What am I missing here? I'm not thinking correctly about this, somehow. jj As you increase the resistance of the Air Brake you increase the power (watts) dissipated by the Air Brake. If an amp has a 30 watt output and the Air Brake dissipates 20 watts in the current setting then the other 10 watts will go to the speaker. Power in = power out.
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Post by pintail78 on Nov 17, 2010 9:35:19 GMT -7
Yes, it does. Simply put. The amps output is the same, and the brakelite is absorbing some of the watts so the speakers SEES a lower wattage.
Discussion: The problem is that incr the R, you dont incr the W of the amp output, b/c if you go back to ohms law and keep V the same, incr R, you get a lower I or current. So the A squared part go down real fast for the watts (as its squared).
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