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karlfer

Amp and cab thing I don't understand

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So, the bass rotary on the amp is cut/boost, 15Db around 30 hz.

The cab ranges from 42 Hz to 18000.

As, by my dim thoughts, the cab doesn't deal with 30 hz, what is actually happening if I cut or boost the bass rotary on my amp :blink:?

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This shows bass boost centred at 50Hz but the same thing applies. Notice that maximum boost is at 50Hz but a range of frequencies either side are also boosted. The slope/width of the curve depends on the specific design/amp.

[url="http://s1070.photobucket.com/user/dincz/media/opamp15_zps5denf2ei.jpg.html"][/url]

As far as the cab is concerned, it doesn't suddenly cut off at 42Hz but rolls off gradually. It will still work at 30Hz (to some degree depending on the design) although much less efficiently.

Edited by dincz

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Brilliant dincz, thanks very much for that :D.
Great explanation for a dummy like me.
Cheers dude

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Just an aside, Each 3db of bass boost will demand twice the power and makes the speaker travel twice as far. OK if the volume is low if the volume is up then your amp will run out of power and your speaker may break with 15 dB of boost. Best to keep the tone controls between 3 and 8 o'clock as a general rule.

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[quote name='karlfer' timestamp='1428997267' post='2746717']
the cab doesn't deal with 30 hz, what is actually happening if I cut or boost the bass rotary on my amp :blink:?
[/quote]

In many common tone controls the peak or shelving frequency varies a lot depending on how much boost or cut you apply. Here is a set of frequency response curves for various settings of one of my preamps. Many manufacturers would call this a 35Hz +/- 12dB bass control (I didn't show all of the cut side), but as you can see that doesn't tell you the whole story by a long shot.

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Thanks guys.
My Puma 500 goes through 2 x 8ohm Markbass 210 Travelers.
Generally I actually need to cut the bass rotary back to about 11 o'clock the speakers tend to get a little close to farting out.
Sometimes I have to cut the lo mids fractionally as well.
I use a GRAMMA pad, only use passive basses, finger style, but I can get a little heavy handed.

I just got to thinking, the amp's bass rotary is set to 30hz, but the cabs are 40 hz and I didn't know how that would work.
Thanks for all the input guys, appreciated.
Cheers,
Karl.

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the amp and cabs will try to produce frequencies even lower than 30hz that you'll never actually hear too, which wastes a lot of power and gets the speakers moving more than they really need to which might be contributing to the farting out... The SFX Thumpinator is supposed to help with this. I've not got mine yet so I can't comment from experience, but it might be worth trying one!

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[quote name='chrismuzz' timestamp='1429046473' post='2747441']
the amp and cabs will try to produce frequencies even lower than 30hz that you'll never actually hear too, which wastes a lot of power and gets the speakers moving more than they really need to which might be contributing to the farting out... The SFX Thumpinator is supposed to help with this. I've not got mine yet so I can't comment from experience, but it might be worth trying one!
[/quote]

Look forward to comments on the [s]wife[/s] Thumpinator :D

To be honest, the farting out is very rare, just an occasional room, and it's very quickly eq'd out.

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There are a whole collection of factors to take into consideration when considering what goes on at these low frequencies. Individually none of them is difficult to understand but as with any situation with many variables you have a nearly infinate series of possible outcomes.

One of the most significant is the way ported cabs behave. At their tuning frequency the port air acts as a load on the speaker reducing it's movement with the sound coming from the port mainly. Below the tuning frequency (typically around 50Hz) the cone excursion rises dramatically as the cab is effectively an open box with little resistance to the cone movement. Ported cabs are very liable to 'fart' because of this. You shouldn't really get this problem from the low mids though. By the time you get above 100Hz excursion shouldn't be a problem for most speakers. Using a sharp filter below 40hz should stop most of these problems hence the Thumpinator.

Another factor is room resonances, which boost apparent bass and also make everything sound a little woolly. Cutting bass can avoid this and clean up your sound. A 24dB/octave filter may do this better than a more gradual tone control whilst affecting other frequencies less.

We really don't hear deep bass very well at all and 'bassiness' is a very subjective thing, just cutting treble and boosting the volume so the subjective sound level is the same sounds bassier for example. Very little truly deep bass is present from the pickups and most of what we hear as bass is a rich harmonic content rather than deep fundamental tones.

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[quote name='karlfer' timestamp='1429082921' post='2747696']
Look forward to comments on the [s]wife[/s] Thumpinator :D
[/quote]

the missus hasn't thumped me for a while, I must be doing something right :D

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[quote name='Phil Starr' timestamp='1429088953' post='2747777']
There are a whole collection of factors to take into consideration when considering what goes on at these low frequencies. Individually none of them is difficult to understand but as with any situation with many variables you have a nearly infinate series of possible outcomes.

One of the most significant is the way ported cabs behave. At their tuning frequency the port air acts as a load on the speaker reducing it's movement with the sound coming from the port mainly. Below the tuning frequency (typically around 50Hz) the cone excursion rises dramatically as the cab is effectively an open box with little resistance to the cone movement. Ported cabs are very liable to 'fart' because of this. You shouldn't really get this problem from the low mids though. By the time you get above 100Hz excursion shouldn't be a problem for most speakers. Using a sharp filter below 40hz should stop most of these problems hence the Thumpinator.

Another factor is room resonances, which boost apparent bass and also make everything sound a little woolly. Cutting bass can avoid this and clean up your sound. A 24dB/octave filter may do this better than a more gradual tone control whilst affecting other frequencies less.

We really don't hear deep bass very well at all and 'bassiness' is a very subjective thing, just cutting treble and boosting the volume so the subjective sound level is the same sounds bassier for example. Very little truly deep bass is present from the pickups and most of what we hear as bass is a rich harmonic content rather than deep fundamental tones.
[/quote]

That's a great summary. I have a hard time convincing other band members etc that the really low frequencies are barely audible and often muddy sound, for guitars or my bass through the PA. The 'bass guitar' sound that they like is more of a low-mids emphasis IME.

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I had it in rehearsal the other day. The rehearsal rig was farting out as our bassist had raised the lowest slider on the graphic EQ to the max. I asked him to cut it to minimum, and the only difference was that everything was immediately clearer, and the amp/cab stopped distorting. Despite this, everyone else in the band agreed that it was no longer bassy enough, and that the slider (30Hz I think) had to go back to the top. I guess this is how rehearsal rigs get knackered.

Edited by cheddatom

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Just a thought; if the amp is sending more lower frequencies than the speaker/s can 'efficiently' handle - isn't that likely to cause 'farting' or at least tend to overwork the drivers?

Generally speaking - no specific amp or drivers or cab design.

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[quote name='cheddatom' timestamp='1429092412' post='2747824']I had it in rehearsal the other day. The rehearsal rig was farting out as our bassist had raised the lowest slider on the graphic EQ to the max. I asked him to cut it to minimum, and the only difference was that everything was immediately clearer, and the amp/cab stopped distorting. Despite this, everyone else in the band agreed that it was no longer bassy enough, and that the slider (30Hz I think) had to go back to the top. I guess this is how rehearsal rigs get knackered.[/quote]

If you have a large number of bands on the graphic (like 12) then the 2nd/3rd sliders are usually the ones to use for boosting lows. Even with a 7 band graphic the lowest band is often a bit too low to boost much without cabs farting and the next one up is a good bet. Depending on the Q of the bands you can boost two adjacent bands equally and end up with the average frequency of the two actually being the centre of the boost, so that's another option.

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[quote name='cheddatom' timestamp='1429092412' post='2747824']
I had it in rehearsal the other day. The rehearsal rig was farting out as our bassist had raised the lowest slider on the graphic EQ to the max. I asked him to cut it to minimum, and the only difference was that everything was immediately clearer, and the amp/cab stopped distorting. Despite this, everyone else in the band agreed that it was no longer bassy enough, and that the slider (30Hz I think) had to go back to the top. I guess this is how rehearsal rigs get knackered.
[/quote]

Sounds like another case of people wanting distortion but not realising there are pedals for that!

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