+1 I like that design with quite a few SS bass heads where it sounds completely the same through its entire volume range, and then just gently compresses and stops getting louder when you reach its absolute volume/power limit.
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GBE1200 Power Ratings...
- Thread starter JGR
- Start date
+1 I like that design with quite a few SS bass heads where it sounds completely the same through its entire volume range, and then just gently compresses and stops getting louder when you reach its absolute volume/power limit.
IMHO - I don't think you can say "even-order harmonics". GB didn't.
Did you measure them?
Will you measure them?
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Sorry, I am going by previous correspondence with Andy. Looking back, it does not look like he mentioned if they were even or odd. It was my understanding that they were even.
We did not measure for this speficially, other than measuring total harmonic distortion, in general.
Tom.
Usually "tube-like" = even-order harmonics. But he didn't say...Andy are ya out there?
alexclaber
Commercial User
It's not often my QSC PLX runs out of power but it seems to have particularly nice limiters compared to the Mackie M1400 I had before, though some of the improvement is due no doubt to the limiter having less limiting to do (3dB more headroom).
Alex
Will you measure them?
It seems like total THD is not as important as the amp staying musical at its power extremes - i.e 10% THD.
And I'd say the odd harmonic needs some revisiting, it doesn't appear to be in line with what is often repeated. Measurements would sure help to clear it up.
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Well, yes, but the primary plan is to do this with our ears.
Seriously, there really is only so much room for measurements/specs and graphs!
That being said, the THD topic is an important one. Any of the other heads we tested would have put out more power at 10% THD, but whether or not this would be 'musical' is a valid question (and yes, we could try and measure this via an analysis of odd/even order harmonics, if we only had the time, space, and reader understanding required to make it worth doing).
Tom.
Usually "tube-like" = even-order harmonics. But he didn't say...Andy are ya out there?
It looks like it won't be measured, but I have found plenty of evidence that supports this often repeated claim is not supported.
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Usually "tube-like" = even-order harmonics. But he didn't say...Andy are ya out there?
Depends. A simple Class-A tube gain stage, and a Class-AB tube power amp, should produce completely different harmonic structures.
Here is from the Wikipedia article on even and odd functions (my bold):
Note that both tube and solid state power amps use symmetric push pull designs.
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The limiting function occurs about 1.5-2dB before power amp clipping and it takes an additional 7.5-8dB of drive to get the amp to hard clip. So the amount of "compression" that is available between the clip point and the limit point (less the slope) is ~(8dB-2dB)=6dB.
Harmonics are not just even, it's a mix of even and odd that vary somewhat in ratios with respect to frequency and amount of overdrive. There's a lot going on that occur as a function of 3 or 4 variables.
Asymmetrical signal means the limiter gets to work asymmetrically and this adds a whole layer of harmonic distribution as well. Another reason why limiting must by itself be symmetrical, to PRESERVE the asymmetry of the original signal and the harmonics it causes.
My opinion is that the lab measurements for general readership be limited to things that make a big difference in actual use or that comparisons are based on identical conditions.
6 more days on the road and then I'll have more opportunity to pop in.
Harmonics are not just even, it's a mix of even and odd that vary somewhat in ratios with respect to frequency and amount of overdrive. There's a lot going on that occur as a function of 3 or 4 variables.
Asymmetrical signal means the limiter gets to work asymmetrically and this adds a whole layer of harmonic distribution as well. Another reason why limiting must by itself be symmetrical, to PRESERVE the asymmetry of the original signal and the harmonics it causes.
My opinion is that the lab measurements for general readership be limited to things that make a big difference in actual use or that comparisons are based on identical conditions.
6 more days on the road and then I'll have more opportunity to pop in.
It's good to know deep analysis and engineering went into this limiter. I wish more amp manufacturers did as much. The behavior at extremes of the amps seems important to musicians and a good limiter is essential to staying musical. Typical limiters squash the dynamic musical performance in their attempt to keep THD down. It's a novel idea to add an appropriate mixer of even and odd harmonics to enhance the extreme performance. Good work!
Even if BGM updates their testing process (which I still hope they will), I don't think the numbers for the GBE 1200 will improve much over the initial test, because of the type of limiter Genz Benz uses with these amps (that 5% distortion limit on the BGM tests).
Sounds like a very cool design that would take a lot of good engineering to do correctly.
At least now we have a better idea of why they work (and test) the way they do.
I'd still love to gig with one (1200 or 750) a few times, I bet they would be right up my alley.
That depends on what they update their tests to include or allow for. Using a test that simulates a real playing load is what really needs to happen, as continous power tests don't mimic that at all. As for harmonic content, that's a tough one since the end result in tone is so subjective. So again, comparisons between heads are difficult with one set of test parameters. And if the test isn't designed to allow an amplifier to do what it is DESIGNED to be good at, then the test is pretty useless (edit: for comparisons b/t heads). What good is an output power test that stops at 1% THD on an old SVT? Pretty worthless.
Edit: So in a way, each test should really test the head to it's strengths. Some will say this makes the results from the heads uncomparable. This is true, but they are already uncomparable using a standardized test.
Chef
In Memoriam
I disagree with that last one Eublet.
Standardised tests lead to results that can be compared across many amps/speakers.
I think that's the end goal here.
The test procedures may not be everyones idea of ideal, but they're repeatable, and will compile useful info over time.
Think about this: you don't hear anyone griping about the LMII test numbers, or the Carbine test numbers. If the GBE product hadn't been tested til 4 issues from now; and no one else had griped about test procedures...would current concerns be the same?
What I'd prefer not to happen is for testing procedures to be changed because one particular product fell short of current users expectations for how it would test.
If all three amps tested in this issue had failed in everyones eyes, I'd probably have different ideas about this than I currently do.
Standardised tests lead to results that can be compared across many amps/speakers.
I think that's the end goal here.
The test procedures may not be everyones idea of ideal, but they're repeatable, and will compile useful info over time.
Think about this: you don't hear anyone griping about the LMII test numbers, or the Carbine test numbers. If the GBE product hadn't been tested til 4 issues from now; and no one else had griped about test procedures...would current concerns be the same?
What I'd prefer not to happen is for testing procedures to be changed because one particular product fell short of current users expectations for how it would test.
If all three amps tested in this issue had failed in everyones eyes, I'd probably have different ideas about this than I currently do.
You're missing the point. Testing an amp until it hits a certain %THD is faulty because some amps are designed to breakup when driven. The classic SVT is a big example. If you stopped pushing it at 1% THD, the head would seem woefully wimpy, which anyone who's played one knows isn't the case. The heads are designed to breakup when pushed. If you certify numbers based on 1%THD, then that test is useless when making power comparisons between heads.
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This is why we give both 1% and 5% THD numbers. And rest assured, all of the amps would have put out more power if we upped THD to 10%, but as this gets to be more and more subjective with regard to how 'good' or 'bad' a given amp sounds at 10% THD, we decided not to use this as part of the empirical bench tests.
Tom.
Agreed, Tom. My last two posts were a bit ambiguous, so I've edited them. Any way you slice it, a standardized test alone, no matter how good it is, can't be used to make head-to-head comparisons. Too many subjective factors come into play. I still don't know how anyone could certify output power and have the results be comparable across a lot of heads. It really does boil down to using the heads and knowing if they work for you or not. Still, there has to be some way to "allow" for extra THD when testing the power of heads that are designed in such a way. I of course have no way of even beginning to know what that is.
My guess is that all high power amps with pre-amps designed to roll the lows off will do poorly if test procedures remain unchanged.
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I wouldn't use the word 'poorly', but I would say that any head - high-power or low - would perform similarly on the '5-string power output test' if their preamp was designed to roll off the lows in similar fashion to the GBE 1200.
IME/IMHO, this rolloff is not necessarily a bad thing. In fact, it is an intentional result of the design. And the 5-string test does illustrate function of this design goal. There's less output at 31Hz because there is supposed to be.
Tom.
