Do you need a low pass filter in your amp?

[slimjohnny]

First off i'm a guitar (and amp for that matter) noob.

So i'm making a 30 watt solid state combo as my first amp and i've checked a lot of diy schematics and that gave me an idea on how mine is going to look like. All the schematics i saw were similar, preamp with tone controls and the poweramp. But then i started looking into schematics from marshall and the preamp had more two ICs than the other diy circuits so i did some research and those two IC's behave as a low pass filter.

Now that got me thinking: is the low pass filter really necessary when you already have controls over the frequecy(eq)?. Is it normal for amps to have this? And why did none of the diy schematics that i saw have this low pass filter?

[Jazz P Bass]

How about a schematic snip of the circuit?

[slimjohnny]

That's the preamp section of the amp, and to the right are the two ics.

Do all amps have this?, and why?

[mandu]

This is not a standard in most guitar amps including Marshall. The low pass filter is set to pass signals below 20 kHz. The drive circuit produces lots of harmonics and may be well beyond audible range. The filter may be part of circuit to produce that Marshall sound for this model or the designer added this to his taste. For guitar applications, Low pass filters are generally provided before and after digital delays or BBD circuits. Regards.

[slimjohnny]

Yeah that makes sense, thank you for your reply it really cleared up things for me.