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Discussion in 'Amps and Cabs [BG]' started by nunk6, Nov 24, 2000.
what does headroom mean i'v seen it mentioned but i'm not sure i get the concept
It means that if you're playing as loud as you want to play, your rig could still play louder, more or less.
Another view of headroom is the difference between the average level and the peak level of the signal. The "headroom" is the room that the equipment gives you - over and above the average level - for peaks. For example, if you are playing a steady line at a particular dynamic level, which one might measure at 105 dB somewhere, then you wanted to accent a note (or pop a string) - your bass might "want" that transient peak to be 6 dB higher - which would be 111 dB at the measurement point. If we are talking power or sound pressure level, your amp would need at least 6 dB of headroom over the average level (or 4 times the power) to faithfully reproduce the signal without clipping or compression. If you play loud enough so your average level is on the verge of driving the amp to clipping all the time, you basically have no headroom left. This is why people doing lots of slap bass (especially with no or low compression) need lots of headroom for good, undistorted sound: there's simply a wide dynamic range in the signal that needs to be accommodated. I hope this helps.
More less than more, eh?
I thought headroom was never playing with less than 300w,
cheapshot but I took it
If you're playing out, 300 watts doesn't leave much headroom. You want headroom, start at 600 and go up.
I got this amp,a Fedner 135. It sounds great!, I mean really great!- in my living room. I tried playing out once with a couple of cabs to make it most efficient, and next to a 100w Marshall, keys, and a rabid drummer- I got lost. This was of course a trial, and I had my SVT 4-Pro as a backup, which I had to switch back to. BTW thaat amp did not have true power rating. My new Eden 800 had more. Psycho said I would need 2 Eden (Koch) 300 watt tube heads to cut it. He's right. I can't handle the weight, or the $4,000. so the tranny amp has to do. It sounds great also (not as good as a tube amp), and has the headroom to cut through. I use the formula; A 100w tube guitar head needs about 10 times the transistor watts for bass, or a 1,000w head. Dont forget that low line voltages can make your amp sound thin and weak too, so have plenty power.
So you are saying that the WT800 is louder than an Ampeg SVT4-Pro's supposed 1600 watts?
Somehow, that is not surprising.
I know that the Eden sounds better too, IMHO.
[Edited by embellisher on 11-26-2000 at 08:23 PM]
yeah.. the eden is a much better head.. probably is louder too.. ampeg(crate ) may ahve said 1600 watts but meant that as a peak or something.. i personally didnt like the ampeg(crate) svt4 pro .. didnt seem to me liek it pushed 1600...
Embellisher, You got it. Eden watts are more than Ampeg watts! Kind of silly, don't you think? The next guy got it. Ampeg inflates he way watts are measured, and Eden is on the conservative side. Besides, the Eden does not cost much more, and you get better and more sounds.
Interesting. I haven't heard either amp, but I've seen specs on them. Eden (see http://www.eden-electronics.com/) rates the WT800 at 400 watts x 2 into 4 ohms, and claims distortion under 0.1%. Ampeg (see http://express2.expressivetek.com/ampeg/) rates the SVT 4Pro at 490 watts x 2 into the same 4 ohm load. (Comment to the thread: remember, folks, look at the load before jumping to conclusions!) However, Ampeg does not give a distortion figure. The difference in sound level between 400 and 490 watts is only 0.9 dB! If the Ampeg rating is done at a higher distortion level, then the comments in this thread seem appropriate. I don't know if the Eden head is capable of driving both channels bridged into 4 ohms or each channel into 2 ohms, as Ampeg's does. So... it all depends on the impedance (load) you intend to drive with the amp.
This is an example of how one can be misled by power ratings. If you can, find out what load is being driven, what the distortion level is at the rated power and load, what frequency is being used to rate the amp's power, whether the power is RMS or peak, and whether it is a continuous or momentary (time-limited) rating. Some manufacturers don't publish this information readily, but once you have it, you can really compare amps more accurately. Most of the reputable mfr's use RMS continuous ratings - so you could assume that if not explicitly mentioned. (Be careful, however, with this last issue when you get into amps well into the 'teen-hundreds, because a standard 120-volt, 15-amp circuit cannot deliver more than 1800 watts total on a continuous basis.)
Mike, I can dig the tech stuff, but I'm not even talking that. Eden rates watts root mean square (RMS), while Ampeg talks "Continuous" or "Program" watts, Whatever that is supposed to mean.
Suprisingly, Ampeg will take a 2ohm load stereo or bridged.
Actually, we *are* talking tech stuff when we are talking about watts. RMS power is a way to express the average power delivered by a sine wave (e.g., a simple fundamental note without any harmonics) over a full cycle. "Continuous" means that the power level can be maintained indefinitely, rather than just for a short burst (for example, during a transient). "Program" means a typical steady (music) signal that the amp is likely to see. Manufacturers often equate this to RMS sine wave power; sometimes they use white noise to approximate a music signal. There are complicated aspects of all this, which I'm not going to get into here - in each case the waveform is different, so the ability of the amp to reproduce the waveform is subject to it. Anyway, both Eden and Ampeg rate their amps using RMS (rather than peak) power ratings, and I believe both are talking about continuous power (which means indefinitely pumping out power at the claimed RMS value).
Relative to your second comment, Ampeg's SVT 4Pro is rated as follows (taken directly from their web site):
2 x 600 Watts @ 2 ohms
2 x 490 w/1200 Watts bridged @ 4 0hms
2 x 300 w/900 Watts Bridged @ 8 0hms
So it appears the amp will *not* drive a 2-ohm load bridged, unless explicitly mentioned elsewhere. In order to do so, the amp would have to be capable of driving two 1-ohm loads in stereo, which is doubtful. They mention 1600 watts bridged mono, but did not explain what load is being driven, nor for how long, etc. (Referring to my previous post, I'm very doubtful that the Ampeg can maintain a full 1600-watt output continuously/indefinitely, given that the outlet can only provide 1800 watts RMS.)
I can't say I'm very impressed with Ampeg's performance documentation (from either their web site or their product catalog). On the other hand, I'm very unimpressed with Eden's web site - they hardly ever update it.
I know what you mean.
Dave from Eden told me,
" Our web guy has been way behind schedule."
and that it's going to be updated this week.
The only point I'd like to add to the other comments on headroom, is that sometimes you don't want too much. Some of the "classic tones" of older amps are the sounds of them being overdriven and creating distorsion well in excess of the 1% rating level.
Obviously, the best tone in the world doesn't mean a thing if they can't hear you swing. So in a live setting more is usually better. But when you are recording sometimes less is more.
On the watts is watts issue - I wish BP magazine put their amps through a tech test and published it. Stereophile does a great job of combining the subjective review with objective testing (and the technical reviewer is a bass player). I realize that the sound of an amp is a subjective thing, and that's what I want to read in a review. But, things like output and distorsion can be measured and help those of us without the EE degrees.
[Edited by Skip on 11-27-2000 at 11:45 AM]
My first experience with the wonders of headroom was many years ago when I was still gigging with a 75 watt combo amp.
I sat in on a rig consisting of an Ampeg B15 cabinet driven by an SVT head (300 watts). What I noticed immediately was that when I dug into the strings, the volume increased effortlessly and the notes sounded cleanly rather than just farting out. I went out the next week and bought a 400 watt amp
Anyway, the thing I appeciate about having more power and more headrom is the ability to play with a wide dynamic range and get the response I want from the amp instead of unwanted compression. If you tend to play with lots of compression already, headroom aint that important.
I have found a tilt steering wheel always helps, but that it a dangerous way to drive. Did anyone see 'The World According to Garp'?
True enough. With my Carvin rig, I have the best of both worlds: there is a gain stage just after the input which allows me to overdrive the preamp (and blend in some tube distortion if I want), yet I can keep the gain on the power amp low. If I don't overdrive the preamp stage, then I can get a lot of headroom by virtue of the high output of the power amps, if I crank their gain instead.
Amen, brother. I've been having the very same thought myself lately. I used to get Audio magazine, and every month they would have a rather in-depth dissection of an amp or speaker or whatever, with lots of measurements, including frequency response, slew rate, THD, IM distortion, polar directivity vs. frequency, efficiency, impedance vs. frequency, etc. It was more than most non-engineers could digest. Audio magazine presented not only the measurements, but also did subjective listening tests as well, with commentary. I agree - wouldn't it be nice if BP or a similar publication commissioned such lab tests on bass amps and speakers? That would be ultra cool!
It might be counterproductive. It works in hi-fi audio publications because in general you're looking for transparency in the product.
If BP tested an SVT and a Walter Woods, the SVT might look pathetic spec-wise compared to the Woods and the specs don't help explain what the amp SOUNDS like very well.
On the other hand, I wish there was SOME testing, like does the unit actually meet the manufacturer's published specs (GP used to do these sort of tests on amps up till the late 80s).
One good example of total confusion (for me) is the claims made by BP about Acme speaker cabs, namely they need "gobs of power". In recent reviews they found a 60 watt (!!) Allesandro tube amp (not a cheap box, but decidedly LOW power) drove one to "acceptable levels" and they said the same about the 120 watt Acoustic Image Contra amp. So what's with these amps that's different than other amps with less than 1000 (!!!) watts that supposedly can't drive those cabinets? Or is it just that the reviewer's idea of "acceptable levels" is poorly defined? I would sure as hell like to know!!!
There are marked sonic differences between different amps with the same power ratings. Part of it is due to different test criteria as has already been pointed out. I'd like to see standardized test conditions for all amp manufacturers under ISO.
I think there are two main things to this. First, tube amps vs solid state amps: a tube amp will invariably be able to go louder than a solid state amp of the same power rating. I'm not trying to say that tube watts are better than solid state watts, a watt is a watt is a watt. But, tube amps can be driven to much higher levels of THD before they start sounding bad. This is because when tubes overdrive they tend to produce even order harmonics, which sound warm and pleasent to our ears. Transistors tend to produce odd order harmonics when overdriven, which sound harsh and ugly. There's also the bit of black magic involved in tube amp design! don't believe me? Try playing an SVT against any other 300w amp (or practically ANY solid state bass amp). In terms of sheer volume, the SVT will win every time!
The second issue is solid state design. Some designs work better than others. The biggest issue here is probably conventional power supply vs switching power supply. An amp with a conventional power supply will sound louder than an amp with a switching supply, especially in low end response. I'm not exactly sure why this is. Maybe some of the EE's could explain this. The ability of an amp to deal with the reactive loads presented by speakers is tied in with this. An amp rated with a fixed load resistance will definately behave differently with a speaker which has a very complex, frequency dependent impedence, and also produces currents in the opposite direction to the current from the amp (coil moving in a magnetic field). The production company I worked for before I started my engineering degree was using Crown Macrotech series amps. When Crown came out with the k2, the boss decided he'd demo them because they were abot 50lbs per unit lighter than Macrotech 2400s (lightness is the big advantage of switching power supplies). Both amps are rated very similarly. When we set up the AB comparison, the macrotechs produced considerably more SPL than the K2s at the onset of clipping. This was especially apparent below 100 Hz where the Macro was a full 10dB louder than the K2!! So we checked the K2 against a macro 1200, which was slightly less than half the power. On the midrange and high bandpasses they were comparable, when we put them on subs, the macro 1200 blew the K2 away! So, different amp designs perform differently even if their power specs are similar. Jeez, this was a long post, sorry!