Double Bass Specs Part 1: How Much Power Do I Need? (geek/tech thread) ;)

[drurb]

Suppose you have a 100-Watt amp and 1x12 speaker. Now you want to double your acoustic power. You could get a 2x12 cabinet

All other things being equal, driving the 2x12 with the same 100-watt amp will produce no more output than driving the 1x12. The exception would be placing the two speakers in parallel, thus dropping the impedance in half which for SOME solid-state designs would yield 200 watts.

 

[drurb]

There is an old engineering adage that was born of the pioneer hi-fi days. Take these three characteristics:

1) small size

2) high sensitvity

3) extended low-frequency response

You can have only two at a time. Now, if you are willing to have a mammoth cabinet, you can have a sensitive system with extended low-frequency response. Such a cabinet/driver design will require relatively little power. Want a small cabinet with extended low-frequency response? Then it will not be a sensitive system and will require a relatively large amount of power.

No offense, but the question of how much power one needs is ill posed because it does not specify the requirements (e.g., small cabinet or not). It is a given that all of us here want extended low-frequency response. What we also typically want is the ability to produce prodigious sound-pressure levels at low frequencies. How much power is needed is really a function of the driver/cabinet design.

I leave you with this thought. Some of you may be familiar with the famous hi-fi speaker, the Klipschorn (I owned a pair). That speaker is extremely large and, because of its unique horn design produces 106 dB SPL at 1 meter with a 1-watt input.

Now, your normal moderate sized typical hi-fi speaker cabinet will produce roughly 85-90 dB SPL for the same conditions. Let's say it's 90 dB SPL. Then, there's 16 dB between that speaker and the Klipschorn. The output that the Klipschorn produces with say a 10-watt input would require an input of 400 watts for the more typical speaker! Of course it would catch on fire before you could achieve that. Talk about headroom! You can safely push 100 or so watts into a K-horn and it will produce a deafening output that would require 4000 watts into the small speaker (which would be absurd).

What's the point of all this? We should really be asking what acoustic sound pressure levels our systems can produce and what is required.

 

[fdeck]

All other things being equal, driving the 2x12 with the same 100-watt amp will produce no more output than driving the 1x12. The exception would be placing the two speakers in parallel, thus dropping the impedance in half which for SOME solid-state designs would yield 200 watts.

Exactly. But my point was that a 1x12 with 200-Watt amp could be much more portable than a 2x12 with 100-Watt amp. In other words, there is a practical reason for increasing amp power rather than increasing speaker sensitivity.

 

[tombowlus]

I'm a big fan of headroom. I like the sound of big power amps cruising along without breathing hard. High power amps sound richer and less fatiguing to my ear than lower power amps at the same apparent volumes. I'm not a fan of clipping in solid state amps. It sounds bad to me and kills speakers.

I agree. And this leads me to ask, "what is the best measurement of headroom?" I would have thought that the wattage ratings (if done in a non-misleading manner) would be the "best" measure of headroom. Is there another, more appropriate, spec to consider?

That all leads me to think there is a "right rig for the job".

Again, I agree. And this explains, in part, why I have such a massive gear collection!

Later, Tom.

 

[fdeck]

The wattage ratings for contemporary solid state power amps should be sufficient to tell you what you need to know. If you are testing an existing amp, then a peak-reading level meter would be pretty informative.

Let me take a poll of how many TalkBassists would be interested if I designed an easy-to-use gadget specifically for this purpose?

Otherwise, you can figure out a lot if your power amp has a clipping indicator. If it never lights up, then you have sufficient headroom.

 

[tombowlus]

Let me take a poll of how many TalkBassists would be interested if I designed an easy-to-use gadget specifically for this purpose?

I'd be potentially interested in such a gadget.

Tom.

 

[drurb]

Exactly. But my point was that a 1x12 with 200-Watt amp could be much more portable than a 2x12 with 100-Watt amp. In other words, there is a practical reason for increasing amp power rather than increasing speaker sensitivity.

...and you'll find additional reason if you re-read the beginning of my post!

 

[drurb]

The wattage ratings for contemporary solid state power amps should be sufficient to tell you what you need to know. If you are testing an existing amp, then a peak-reading level meter would be pretty informative.

Let me take a poll of how many TalkBassists would be interested if I designed an easy-to-use gadget specifically for this purpose?

Otherwise, you can figure out a lot if your power amp has a clipping indicator. If it never lights up, then you have sufficient headroom.

Sorry, dead wrong! Dynamic headroom is an established spec that can be given along with the rms wattage. It specifies, in dB, the increase in power that can be achieved at the rated level of distortion for a standard time-epoch (I can't recall the number of milliseconds). The standard wattage ratings do not tell you a thing about dynamic headroom. Dynamic headroom depends upon a number of factors such as current capability, power supply reserve, whether the power supply has "hard" or "soft" rails, etc.....

A peak-reading meter will tell you precious little. You will not be able to determine how long the increase over the nominal rms rating can be maintained and, more importantly, what the level of distortion is. The gadget you describe would be of little or no value.

 

[tombowlus]

Dynamic headroom is an established spec that can be given along with the rms wattage. It specifies, in dB, the increase in power that can be achieved at the rated level of distortion for a standard time-epoch (I can't recall the number of milliseconds). The standard wattage ratings do not tell you a thing about dynamic headroom. Dynamic headroom depends upon a number of factors such as current capability, power supply reserve, whether the power supply has "hard" or "soft" rails, etc.....

Good points. So, do any bass/PA amp manufacturer's list this spec? It's not one I am used to seeing. Any easy way to measure dynamic headroom, cumulatively?

Tom.

 

[wneff]

IMHO, I'd put that in my number 3 point above. However, the folks at EA each forget more physics and engineering in any 60 second period than I've ever learned.

Gary used to work in plasma physics (like me), and as far as I know John teaches at Princeton. So, I strongly doubt that that statement is correct.

As far as the difference between the 200 and the 500 W amp goes, I would expect the 500 W to be clearly louder but ultimately limited by the power handling of the speaker. If you connect a passive Whizzy to both ampa I would expect the difference to be clearly audible (about 4 dB, and remember that a difference of 1 dB is a clearly audible difference. 1 dB is the difference between 100 and 126 W. )

But, the only way to tell (and believe) if the 500 W is a lot louder is to try them.

BTW, the decible rating does not tell you how loud something appears. This is the phon scale. A 50 W Guitar has typically more phone than a 200 W bass amp. That is because the ear is more sensitive in the upper mids than at the bass frequencies.

 

[drurb]

Good points. So, do any bass/PA amp manufacturer's list this spec? It's not one I am used to seeing. Any easy way to measure dynamic headroom, cumulatively?

Tom.

Well, the truth is I don't know if any do and there is no easy way to measure dynamic headroom without equipment that would cost more than the usual bass amp.

Without such a specification, the conservative approach would be to assume little or no headroom and buy based on rms wattage, assuming a distortion spec is given. Unlike hi-fi power amps, musical instrument amplifier manufacturers don't always give distortion specs. When they don't this means that the validity and usefulness of their wattage ratings rises and falls with the integrity of the manufacturer! Is the wattage rating continuous rms? All of this was standardized in the hi-fi industry in the 70's in order to protect the consumer against inflated and non-standardized amplifier ratings.

I reiterate that there is no straightforward answer to how much power one needs. In order to answer such a question, at least two requirements must be known. What sound-pressure level do you want to produce and how sensitive/efficient is the speaker/cabinet system.

 

[wneff]

Here is my thought on this topic:

a) I use specs only as a guideline.

b) Use common sense: Is a 100 W bass amp (independent of design,
speaker etc. ) loud enough to play in church with a piano? Yes. Loud enough when you play with a medium volume rock/folk band with PA and a good monitor (where they can give you bass monitor?) probably. Loud enough for a heavy metal band with two guitar players? Probably not. (unless you have a vintage Marshall bass amp with 100 W tube power amp and 2 4x12" cabinets, that should work)

c) The only way you can tell is to try. Either an amp/system does the job or it doesn't, no matter what the numbers say. Every situation is different and peoples tastes in sounds are different. The only way to tell is to go to a store, pick a system where you like the sound, are able to transport and has a chance to be loud enough (by turning it up in the store, not by looking at the numbers) and ask if you can bring it to the next band rehearsal. Set it up and check if you hear yourself and if you like the soud. Also check if there is any room left to go higher than you absolutely need.
If all of that is OK, it does not matter if the amp sais "10W" or "1200 W". If it works for you it works.

d) On bass there are many ways to trick your system to get louder. If you cut your bass you can play significantly louder withe the same equipment. When I find I didn't bring enough amp I always do that.


The smallest system I have where I know 100% I'll not have trouble with volume is my EVM 15L Pro line cabinet paired with one CXL112L from Euphonic Audio driven with an iAMP800. This speakers also works with my 400 W Craaft amp.
In most situations (where I don't have two heavy metal guitar players on stage) I get away with 2x CXL112 and the iAMP (which delivers more focussed sound than the 15" EV and is much easier to carry).
In many situations (Cafe House gig, radio, TV, brass band ) my GK 200 MB is sufficient, but for sound reasons I use iAMP + 1x CXl112.



Wolfram

 

[fdeck]

Well, the truth is I don't know if any do and there is no easy way to measure dynamic headroom without equipment that would cost more than the usual bass amp.

Without such a specification, the conservative approach would be to assume little or no headroom and buy based on rms wattage, assuming a distortion spec is given. Unlike hi-fi power amps, musical instrument amplifier manufacturers don't always give distortion specs. When they don't this means that the validity and usefulness of their wattage ratings rises and falls with the integrity of the manufacturer! Is the wattage rating continuous rms? All of this was standardized in the hi-fi industry in the 70's in order to protect the consumer against inflated and non-standardized amplifier ratings.

I reiterate that there is no straightforward answer to how much power one needs. In order to answer such a question, at least two requirements must be known. What sound-pressure level do you want to produce and how sensitive/efficient is the speaker/cabinet system.

Thanks for educating me. I agree with your last statement. Also, I was unaware that power amps with modest ratings were designed with significant headroom above the RMS rating, so I did not think about it very much.

Do you think I could make up my own headroom test by simply creating an arbitrary waveform with a big transient peak? I am set up to do this without too much trouble. It will be interesting to test my DIY switching power amp when I get it built.

The clipping indicator that I added to my GK combo actually detects when the output has stopped tracking the input waveform, rather than simply comparing to a fixed threshold. Thus, it does indicate when I have run out of headroom, loosely defined by the onset of clipping distortion.

I would very much like to know if bass amp manufacturers are regulated by the same FTC rules that govern hi-fi power amp makers, in terms of power ratings.

One eye-popper was seeing some pictures on the Web, of the innards of 1500-Watt PA power amps, and guessing the power transformer rating to be in the ballpark of 500 Watts. This was discussed at length on the rec.audio.tech newsgroup. The only way to make sense of the rating is if the manufacturer assumes some sort of "typical" music signal. It's also the case that a power amp rated at 2500 W, and not close to 100% efficient, cannot operate continuously on a standard US wall outlet.

 

[Tbeers]

With the live gear I've used, I find that most often the answer is to keep the gain knobs below 12 o'clock on the preamp section, which usually produces the most "natural" or "relaxed" sound. The exception seems to be the "input" section of the iAmp 800, which works differently for some reason. But beyond that, I think the most valuable aspect of a powerful amp is all of that power that stays in reserve. If someone could explain why that is, I'd be interested to listen.

I've read your post a couple times now and it just keeps bothering me, Chris. I have been told many times to let the preamp section do the work, and keep the master knob down. I usually set the gain on my schertler stat-pre to 12, set the master on my AI Clarus to around 10 o clock, and then put the preamp section level knob wherever it needs to be.

Can any of the technically knowledgeable people here shed some light on what preamp/master volume settings will usually produce the best sound?

 

[anonymous0726]

I think that this is all in the preamp design. With Trace Elliott amps, of which I've had a couple, you have to generously load the preamp or the sound will be cold and brittle. With my GK 200MB, none of the knobs did very much anyhow, and the pre/post balance wasn' really noticable. With the Walter Woods you want to run the pre kinda low for a nice airy, transparent sound (nice in a dry room) as the higher you push the input the more direct and compressed the sound becomes (helpful in a boomy room).

 

[Chris Fitzgerald]

I've read your post a couple times now and it just keeps bothering me, Chris. I have been told many times to let the preamp section do the work, and keep the master knob down. I usually set the gain on my schertler stat-pre to 12, set the master on my AI Clarus to around 10 o clock, and then put the preamp section level knob wherever it needs to be.

I'm no techie, but my experience is exactly the opposite in that I hear the "color" as coming from the preamp section, and the power from the master amp. When I want a transparent sound, I'll set the preamps low and the master at about 12; then, when more volume is needed, turn up the master. Later, if even more volume is needed, turn up the preamps, but IME this is where the "forced" sound comes from. YMMV.

 

[Tbeers]

Well, I'll give some different settings a try. Again, I'm far from being an expert. I have had my Clarus for a year and a half, and I still have no idea what the "Brite" knob does!

 

[macmrkt]

Gary used to work in plasma physics (like me), and as far as I know John teaches at Princeton. So, I strongly doubt that that statement is correct.

What was written was supposed to be a huge compliment to their abilities, but I guess it was too convoluted?!

 

[macmrkt]

I would very much like to know if bass amp manufacturers are regulated by the same FTC rules that govern hi-fi power amp makers, in terms of power ratings.

Those up on the regulations may know more, but I think the FTC made standardized guidelines for reporting so consumers could make fair comparisons of the advertised specs. But that's all they did - kind of like the MPAA rating system for movies. But there were and are a slew of ways to follow the reporting guidelines, show stellar 'numbers' and produce a low-fi sounding product as I'm sure DRURB can attest (big solid state stereo receivers from the 70's and 80's come to mind). Back to my main reason for this thread, and as many have chimed in as well - use your ears, not the specs.

 

[drurb]

Thanks for educating me. I agree with your last statement. Also, I was unaware that power amps with modest ratings were designed with significant headroom above the RMS rating, so I did not think about it very much.

I believe you cannot count on there being much real headroom above the rms spec.

Do you think I could make up my own headroom test by simply creating an arbitrary waveform with a big transient peak? I am set up to do this without too much trouble. It will be interesting to test my DIY switching power amp when I get it built.

As I recall, the dynamic headroom test is done with 1-kHz tone bursts. I do not recall their duration. You really don't need a waveform with a big transient. You just push the tone burst and make the measures.

The clipping indicator that I added to my GK combo actually detects when the output has stopped tracking the input waveform, rather than simply comparing to a fixed threshold. Thus, it does indicate when I have run out of headroom, loosely defined by the onset of clipping distortion.

Wow. How did you do this?

I would very much like to know if bass amp manufacturers are regulated by the same FTC rules that govern hi-fi power amp makers, in terms of power ratings.

Well, from the specs I've seen they do not seem to be. I believe that's the difficulty.