Amp EQ question.

[fretmeister]

What happens to the frequencies far outside the EQ control freq centres and Q?

For example, my Mark Bass Marcus model has high at 3.8khz. So what happens at say 5Khz? Does the control actually cut everything above a certain freq? Or is there a LPF stripping the highs out at some point irrespective of the High control setting?

I’m in a curious mood!

[itu]

First of all, bass is not able to produce very high frequencies with coil-magnet pickups.

Tone can be wide or narrow (and adjustable), or shelving.

[PaulWarning]

I've pondered this question as well, I assume the 'Q' setting depends on how many EQ controls there are, for instance my Trace Elliot GP12 must have a narrow Q range but a simple Bass Mid and Treble amp must have a wider one, of course I could be talking bollocks

[nilebodgers]

Attached pic is a simulation of the freq curves for the high mid control of a Little Mark II. Total range is +/-15dB at approx 800Hz and the different colours correspond to control rotation in 11 equal steps (flat centre is 12 o'clock).

You can see the effect at other frequencies either side of the centre freq. Q is about 1.2 for the 2 mid controls.

[Greg Edwards69]

Many typical amp’s treble and bass eq controls are shelving type. Thus, the treble control will affect everything above the eq centre equally, and likewise, the bass control will affect everything below the eq centre. Each will also affect the frequencies on the other side of the eq centre to a lesser degree. Imagine a slope followed by a plateau on an eq graph. 

Markbass used to publish eq curves in the owner’s manual, it might be worth looking up. However, they are not in the pdf manual for the Little Marcus head on their website, but I’ve just looked up the manual for the LM2 I used to use, and the treble and bass controls appears to be more like a “bell” type as opposed to shelving. Usually, this means the eq centre will be affected the most, and frequencies either side will be affected equally to a lesser degree - imagine a bump or trough on a flat eq graph. Markbass appear to have designed a more specific curve for the bass and treble controls on this particular circuit. 


 

You should also take into account the type of speaker you’re using. A tweeter-less cab doesn’t usually put out much above 5khz, and most typical bass cabs drop off below 50hz

This is a generalisation, there are other eq circuits that behave in different ways. 

Edited July 4, 2020 by Greg Edwards69