Some bass-ic head questions (sorry)

[SamIAm]

If it takes 10 men, 43 hours to dig a ditch, how long ... ooops, wrong question!

 

One

If a head is rated as follows

and the speakon connector sayas 4 ohm min load.

 

Is it fair to think it will put out half of these figures into an 8 ohm load?

 

Two

I'm using a multi f/x (MOD Dwarf) that has amp modelling capabilities.  If I want to get an approximation of FRFR, and use a cab that gives low colouration ... can I use a head that has an F/X loop and use the multi f/x into the FX return to bypass any EQ in the head?

 

Three

If the answer to Two is yes, would it be reasonable to expect the head to not add colour when used in this way or would this require a 'special' head or simply selecting the 'right' head?

 

 

S'manth x

[Bill Fitzmaurice]

13 minutes ago, Smanth said:

and the speakon connector sayas 4 ohm min load.

 

 

Speakons are rated for amperage, not impedance. Amps are rated for impedance loads.

Quote

would it be reasonable to expect the head to not add colour when used in this way

Any amp will put out a flat signal if the applied EQ gives a flat signal, be that the internal or external EQ or a combination of the two. But unless you have a method of measuring it you can't know.

[nekomatic]

I assume @Smanth means that the label next to the Speakon specifies the amp’s maximum* load as 4 ohms. In which case you might get a bit more than half of the power rating in 8 ohms, as the 4 ohm spec might be limited by the thermal design and/or the current capability as well as the voltage capability. For example the Warwick Gnome is rated at 200 W into 4 ohms, but 130 W into 8. 

* maximum load as in fewest ohms

Edited November 21, 2023 by nekomatic
fewer ohms considered loadier

[rmorris]

Wrt One: you just have to try it basically. Speaker load impedances are complex as it's a reactive load and actual power delivery will depend on the details. In particular any Zobel network in the amp or can. Then add on the acoustic efficiency of the cab across the frequency range. And fwiw a basic doubling of power into a speaker results in only a minor increase in loudness as heard due to the quasi logarithmic nature if hearing response.

[Phil Starr]

Hi Smanth 

 

Power is all about the transfer of energy the watt is a transfer at a rate of one joule per second. In electrical terms it's calculated by multiplying the voltage by the current. So for 100W you could have a power supply of 100V at 1A. For a 10v supply you'd need to drive 10A and for 1V supply you would need 100A.

 

Your amplifier will be built with a power supply which has a fixed voltage which it simply can't go above and it will also have a current limit where it starts to overheat, the voltage will drop and eventually it will burn out (OK it won't as it will probably blow a fuse or trip something, but lets keep it simple)

 

So the current is reduced as the resistance increases. If your amp is a 50V amp and your speaker is 8ohms you'll get 50/8=6.25 amps. 50Vx6.25A=312W If you use a 4ohm speaker you'll get 12.5A which would give you 624W and a 2ohm speaker 1248W and so on, but of course you won't get unlimited current so in the end the amps power supply will overheat and break down in some way (usually controlled  ) This is why you see so many 300W into 8ohms and 500 into 4ohm put a  amplifiers. It makes no economic sense to put a bigger power supply into a medium power amp when you could up the voltage a little and deliver that power to an eight or four ohm speaker.

 

So the instantaneous peak is just advertising nonsense. You can measure the power supply voltage and you can measure the maximum current and that gives the power. The instantaneous power is just the real power multiplied by 2. Sometimes they give it away with an asterisk* sometimes they don't and even don't tell you they haven't ever measured it, because you can't measure a ghost!

 

*calculated (as if multiplying a number by two is much of a calculation) There is a fiddle here which lets the advertisers get away with it which I'm happy to explain but it's to do with RMS voltages and I'll only answer if you ask

 

So minimum load is 4ohms because any lower would draw too much current and overheat everything if you turn the volume up too high and the Speakon is only marked as an extra warning, it's written on the amp and in the manual too so you can't say you weren't warned.

[Downunderwonder]

3 hours ago, Smanth said:

 

If the answer to Two is yes, would it be reasonable to expect the head to not add colour when used in this way or would this require a 'special' head or simply selecting the 'right' head?

You are on the right track.

 

Some heads indeed do colour things in the output section. Not many though and not any that I am aware of in the 250w arena. So, any number of 'right' heads up for selection.

 

130w @8ohm per 200w @4ohm head seems common. I wouldn't get too hung up on a few tens of watts. Speaker sensitivity is far more important when power is in limited supply.

 

So long as your preamp gizmo isn't overdriving the ever loving pish out of the fx return you should get the very same signal upped in voltage out of the speakon. It would behoove you to include some high pass filtering in your preamp in case you have short circuited the one in the amp.

Edited November 21, 2023 by Downunderwonder

[SamIAm]

So in simple terms @Phil Starr, if the amp is quoting 250W into 4Ω and I hook it up an 8Ω cab, it will be pushing out circa 125W, maybe a tad more?

 

For a 100W rated cab, one would need to avoid going full volume, otherwise the cab might get pushed too hard, will sound bad and perhaps even be damaged?  The amp will not being running at its max and distortion will likely be less?

 

For a 300W rated cab even at full volume the amp cannot overload the cab (Tho will not sound as loud as it could)?  Presumably in the real world, driving an amp at full power will introduce more distortion?

 

Would it be fair to assume that IF these two cabs had the same sensitivity (Which I don't really grok) that the sound perceived would be in the same ballpark or would it be quieter on the 300W cab?

 

In tangible terms, in this demo, the amp can't blow the cab as the BP-800 has Power Output (RMS) of 800w/4Ω, 400w/8Ω and the C4 is listed as Handling 400 watts (RMS) and is 8 Ω? (Please note I do not ask about RMS lol)

 

S'manth x

Did someone mention a C4!?  

[Downunderwonder]

40 minutes ago, Smanth said:

For a 300W rated cab even at full volume the amp cannot overload the cab (

That's a dangerous assumption. Firstly we have no way of knowing how much low end the cab can really take.

 

My 250w 4 ohm Trace amp could smash my 300w 8 ohm Trace cab no problem if too many lows were invoked. The manual even said so.

[SamIAm]

15 minutes ago, Downunderwonder said:

That's a dangerous assumption. Firstly we have no way of knowing how much low end the cab can really take.

 

My 250w 4 ohm Trace amp could smash my 300w 8 ohm Trace cab no problem if too many lows were invoked. The manual even said so.

🤯 There I was thinking I was beginning to understand! lol

S'manth x

Edited November 22, 2023 by Smanth

[Downunderwonder]

I only watched the first 5 or so minutes of the demo vid. They like to dime the low bass knob but they are only using a fraction of the available output. If they cranked up the volume it would likely turn to fart city unless there is an aggressive highpass filter in there.

 

The Trace manual advises that the 30hz slider is red for a reason and should only be used for cutting not boosting.

 

There's a lot of good info in the high pass filter thread in the effects forum.

 

Generally cabs are rated only for their thermal capacity with a standard 1khz signal. Bass lows cause a different criterion to apply and it's caveat emptor all the way. Low frequency makes the speaker move very much further with each oscillation to get enough energy into it that it can be heard, as compared mids and treble. The speaker tends to break from over excursion.

 

It is entirely logical, the speaker has to keep going for a longer time before turning around. If it wasn't moving with comparable speed it would hardly be as loud as the skinny string folk. Quid pro quo it has to travel further.

[Downunderwonder]

1 hour ago, Smanth said:

250W into 4Ω and I hook it up an 8Ω cab, it will be pushing out circa 125W, maybe a tad more?

I believe a good tad more is usual with class D. Double was the rule for older solid state but this newfangled stuff is more like 66% @8 to 100% @4 so 160w thereabouts for your 250w amp.

 

The other point that needs stressing is that we're talking about the clean power rating. Depending on the quality of the amp it may put out a clean enough thump if you send it. That thump could be most of the 500w briefly, and briefly is all it takes to shred a driver.

[Bill Fitzmaurice]

1 hour ago, Downunderwonder said:

 Double was the rule for older solid state

There are two considerations. First is the small signal response. In that case the halving of impedance results in twice the power delivery. But at full power you don't get twice the power delivery, as power supply sag at full power prevents that. This applies to almost all SS amps, no matter the class. Not that it matters all that much. As already touched on perceived loudness isn't linear with respect to power.

Quote

250W into 4Ω and I hook it up an 8Ω cab, it will be pushing out circa 125W, maybe a tad more?

Quite a bit more, perhaps 150W. But that's at full power, which we very seldom use. Why? Because of another subject not yet touched on, the actual power handling capacity of the speakers. That's hardly ever what's listed, which is the thermal capacity. What really matters is the mechanical capacity, which tends to be half the thermal capacity or less. Power, whether it be that of the amp output or the speaker thermal limit, is insignificant. What we engineers concern ourselves with is the amp voltage swing and current capacity and the speaker excursion capacity.

[Skybone]

Two: 

Yes, if you plug your pre-amp into the FX Return of the amp, it will bypass the amps pre-amp and go straight into the power amp section. I've been using this for quite a while with an Ashdown combo. I like the results, but its not everyones cup of tea.

 

Three: 

It's not the bass amp that will colour the sound, it's the cab. The colouration of the sound will depend on the cab you're using, but it will give you a good approximation of the sound you're after. You can always tweak the settings on the pre-amp to try and work around this.

[Bill Fitzmaurice]

Many amps do color the sound, via built in non-defeatable EQ known as pre-shaping. That was usually the case with valve amps, which could not be set to flat response. It's less common with SS, but even those in many cases have pre-shape, even if just by dint of high passing the signal at 30Hz or so. Speaker coloration is almost always present. The main effect of speaker modeling tools is to use EQ to emulate the inherent coloration of various speakers for playback through speakers that actually have flat response, Hi-Fi and high end PA.

Between amp and speaker coloration the practice of setting all the EQ knobs at noon to give flat response doesn't work. Not that you'd want it to, as flat response is as desirable as flat beer. At the very least boost in the low frequencies is a must, necessary to compensate for the response of the instrument, which isn't flat either.

Edited November 22, 2023 by Bill Fitzmaurice

[tauzero]

14 hours ago, Smanth said:

Would it be fair to assume that IF these two cabs had the same sensitivity (Which I don't really grok) that the sound perceived would be in the same ballpark or would it be quieter on the 300W cab?

 

To aid your grokking of sensitivity, it's a measure of how much sound you get out for a given amount of power in. The figure quoted is in decibels for 1 watt input, measured 1 metre from the speaker. So if they have the same sensitivity, you should get the same level of sound from each for the same input level.

 

Further to this, doubling the input power to the speaker will increase the volume by 3dB. Quadrupling the power raises the SPL by 6dB. Doubling the perceived volume needs about an extra 10dB, which requires 10 times the amplifier power. Then you have to start considering your hearing.

 

[Phil Starr]

On 22/11/2023 at 00:13, Smanth said:

So in simple terms @Phil Starr, if the amp is quoting 250W into 4Ω and I hook it up an 8Ω cab, it will be pushing out circa 125W, maybe a tad more?

 

For a 100W rated cab, one would need to avoid going full volume, otherwise the cab might get pushed too hard, will sound bad and perhaps even be damaged?  The amp will not being running at its max and distortion will likely be less?

 

For a 300W rated cab even at full volume the amp cannot overload the cab (Tho will not sound as loud as it could)?  Presumably in the real world, driving an amp at full power will introduce more distortion?

Hi again

 

You've kind of asked several questions here, I don't have a lot of time so I'll just answer the one in this bit of your post. So first of all you are spot on, basically you get twice the power through 4ohms and half the power moving back up to eight ohms . But that's theoretical, the theory works when the amp is running with the volume down but when you turn up full practical limits come in and it's almost always slightly less than double. 125W is as good a guesstimate as any.

 

The second bit is about how much power the speaker is safe with. Matching the amp to the cab (there is a third question about matching two speakers but I'll ignore that for now) It's slightly complex because the power rating of a speaker is measured very differently to the way an amps power is measured. For the amp you are just measuring it's electrical output. If you know the Volts and Ohms you know the theoretical power and you just need to check that the amp can provide the current.

 

Speakers are more complex, you can destroy them in two ways, pass too much power and the coil gets hotter and hotter and eventually blows. (heat causes other problems too but let's keep it simple). The second thing that can destroy a speaker is uncontrolled movement beyond the design limits of the speaker. The more power you apply from the amp the further the speaker cone and couil move, but this is also affected by frequency. The lower the note the more the speaker moves and a combination of excessive bass and power is what is more likely to cause failure by over-excursion than heat alone. It's that frequency element that makes it complex because: a.) nobody knows how you are going to eq or play your bass and b.) The cab plays a big part in controlling excursion so the same speaker may handle 300W in one cab and only 150W in a different cab. 

 

So, at the factory where they design and build drive units they test speakers by putting them in a standard box and by passing a standard signal through the amp for several hours until eventually they know how much power they can apply before the speaker fails. The standard AES test is for two hours and the standard signal is white noise (all the frequencies) but filtered to mimic the mix of frequencies in 'typical' music. The AES test says the speaker should be able to handle the power for two hours without failing. It's a good fair test and easy to do so you can compare speakers. Engineers  and speaker designers can be pretty confident with this information but they knows that this is only half the story.  Once the drive units leave the factory they cannot know what cabinet they are going into and what instrument they will be used for or how the musician will play, there are so many unknowns that no attempt is made to rate the speakers for use.

 

So when you buy a 2x10 fitted with two Celestion speakers rated at 200W ea you will be told its a 400W cab. and samples of the speakers have been through the severest test possible of running for two hours at full power without harm. The reality though is that your 500W amp isn't going to overheat them. Music isn't continuous noise it has loud bits, quiet bits and times when you stop playing. Have a look on the back of any modern amp and it will give a figure for power consumption which is way below the maximum power of the amp. I've got the 200W Gnome in front of me and it says 60W power consumption. That breaks down to 10W of energy wasted heating the amp and 25% of the 200W going to the speakers on average when the amp is running flat out. If you keep the bass eq down and play 'normally' you can safely use a 1000W amp without your 400W speaker over-heating and some people actually do that. The problem is that you might like a bit of bass boost or you may add in all sorts of pedals and you still have the excursion limits of the speaker to worry about. There might be a frequency where each of those Celestions can only handle 25W each and your 400w cab is only a 50W cab at that point.

 

So going back to your question, you could almost certainly use your 100W speaker with your 125W amp. 99% of the time if you use a 300W amp with a 300W speaker you'll never have a failure, you can probably risk going a lot further and use an amp that is double the power. People rarely turn their amps up full anyway and in reality your average power use will never reach 25% of full power. You don't need to be over protective but you do need to be aware of the over-excursion issue. If you are using a of of bass boost, slapping your bass without compression or increasing the frequencies below 100Hz in an unusual way and you hear unusual noises from your speakers at high volumes then you just might have a problem. I suppose I'm saying just be aware you are comparing apples with pears when looking at amp watts and speaker watts, six apples don't necessarily weigh the same as six pears

[Dan Dare]

Is the amp in question a TC? If so, reduce claimed wattage figures by around one third...

[itu]

On 22/11/2023 at 16:35, tauzero said:

I do not know where did you find this, but acoustics lecturers told me 120 dB is the threshold of pain. I have seen something like this in some American materials. Scale is based on 5 dB steps which is awkward. 3 dB is based on hearing.

 

0 dB is approximately the threshold of hearing, but some people can hear -3 dB levels.

[Bill Fitzmaurice]

0dB is the approximate threshold of hearing as was defined by Bell Labs when they invented the audio decibel, which is a tenth of a bel. Care to guess where bel came from? At the same time they quantified 1dB as the minimum difference that the average person could hear, but that varies from person to person and frequency to frequency. The threshold of pain also varies from person to person and frequency to frequency. Our hearing is most sensitive in the vicinity of 500 to 1kHz, both with respect to intensity and pain. In the midrange we might feel pain at only 115dB, in the bass it might be 140dB. Where hearing safety standards are concerned 'A' weighted metering is used, as it's most sensitive in the same region as our hearing.

[Chienmortbb]

On 21/11/2023 at 21:02, nekomatic said:

For example the Warwick Gnome is rated at 200 W into 4 ohms, but 130 W into 8. 

That is an example of current limiting.