How would I raise or lower the frequency output ofan amp?

[Stratlingun]

Hi all. Back with another question. After changing the speaker in my little Marshall from 8" to 12" and loving the result, I was wondering how would I raise or lower the frequency output of an amp(disreguarding the speakers range)? I remember having some "A.R." stereo speakers that bosted a 20-20,000H frequency response. What determines frequency output.

Thanks

[J M Fahey]

On Hi Fi amps you aim for 20Hz/20KHz frequency response because that's the conventional human hearing range.
Guitars' lowest frequency is around 83 Hz and highest harmonics around 5/6 KHz at most, so anything outside those limits is practically useless, so don't worry.

[Steve Conner]

It is mostly the speaker that limits the frequency range of a guitar amp. "Disregarding the speaker" is throwing the baby out with the bathwater.

In a tube amp, the output transformer limits the frequency range, but it's still not as important as the speaker.

Some classic amps had a very small coupling capacitor that tilted the whole frequency response, filtering out bass and emphasising upper mids and treble.

[loudthud]

A 20W guitar amp will burn up a 100W HiFi speaker system in no time flat. Don't try it.

[Austin]

A 20W guitar amp will burn up a 100W HiFi speaker system in no time flat. Don't try it.

That depends on the operator?

Depends on what amp and who is playing it and when and why.

[Stratlingun]

It is mostly the speaker that limits the frequency range of a guitar amp. "Disregarding the speaker" is throwing the baby out with the bathwater.

In a tube amp, the output transformer limits the frequency range, but it's still not as important as the speaker.

Some classic amps had a very small coupling capacitor that tilted the whole frequency response, filtering out bass and emphasising upper mids and treble.

The reason I said "disreguard speaker" is because I already understood that I would need a speaker capable of reproducing the output of the amp. again my question is how would I change the amp itself. I just like to experiment. Lets say my amp has a high end of 2kH, how would I take that up to 10kH in a S.S. amp. I would imagine going lower would be similar but in the other direction. Amp in question is a Marshall MG15DFX.(this is not my main amp so i'm not afraid of hurting it)

[Enzo]

But have you actually determined the frequency response of the stock amp?

DOn't confuse the speaker and the amp. In your first post you ask how to raise the frequency output of the AMP then compare it to an AR SPEAKER. Stereo speakers have tweeters and are designed to reproduce up to 20kHz. Guitar speakers roll off after 3kHz-5kHz or so, so what the amp does above that doesn;t matter to the speaker.

Your amp may well have tons of response in the high freq range, you need to determine that before thinking about modifying it.

Guitar amps do not have tweeters, play your guitar through that AR stereo speaker. Now disconnect the tweeter on it and see how it sounds.

Your MG15DFX is what we call a practice amp, and as such it includes a CD input jack on the front, so you can use the same amp you guitar through to hear to play a CD through while learning a song. So play a CD through that input. How does that sound? Now connect the amp to a full range speaker, like your stereo speaker, now how does the music sound. You are now bypassing the preamp. The preamp is where most of the tone in a guitar amp comes from - other than the giant contribution from the speaker.

[Steve Conner]

Yes, that was exactly my point. I bet that if you connected a hi-fi speaker to your MG15 thing, you would get the full frequency range through the CD input jack.

Solid-state amps naturally have a very wide frequency range, and adding components to limit it costs money.

[J M Fahey]

Agree and add: getting back to the main point, that little Marshall sounds *very good* (I have tried them) but is limited by the relatively cheap and light factory speaker .
If you plug that same "head" into a good 2x12" (or 4x12") cabinet it will blow your head, no kidding.
I mean *guitar* speakers, of course.
If you plug it into a Hi Fi speaker, the sound will be horrible, buzzy, "angry bees in a tin can", "icepick in your ears" type.
Of course, in less than 5 minutes that will change, because you will blow the tweeter and the sound will become quite dull.
Not forgetting that it's easier to burn the Hi Fi speaker because , being very inefficient, it does not *appear* to be receiving much power.

[Austin]

My guitar amp is also my hifi amp. I have a converted Magnavox console amp and at least to my ears sounds great through full range speakers or guitar cabs. I have a older pair of Jbl stereo speakers that I use with it. Sounds fine.

[jmaf]

If I understand the OP's question correctly, he's basically asking how to change the tone of an amp without varying the speaker.

Anything you do to an amp will change its frequency response. From the first gain stage on the preamp, to the tone stack, to the power amp and output transformers, and then on to the speakers....the whole system has a certain character and that character can change significantly with very small changes in some components - the negative feedback loop for example.

But all components have to "agree". No use filtering out lower notes and boosting higher frequencies if the speaker is unable to reproduce the higher frequencies you just boosted.

[Steve Conner]

But all components have to "agree". No use filtering out lower notes and boosting higher frequencies if the speaker is unable to reproduce the higher frequencies you just boosted.

So, you deny the existence of bright caps on Fenders, the Vox Top Boost, and the Dallas Rangemaster?

[jmaf]

So, you deny the existence of bright caps on Fenders, the Vox Top Boost, and the Dallas Rangemaster?

"I don't believe in ghosts, but that they exist, they exist!"
- Someone

But seriously. As Enzo put it, they roll off somewhere near 5K? Those old Jensens, Oxfords all "agree" with the high end boost

[J M Fahey]

To see what are we talking about, the frequency response of a typical guitar speaker, the Eminence Legend 125.
See the *huge* presence peak, +12dB from 2000 to 3500Hz, and the abrupt fall after that, some 24dB/octave.
Not flat *at all*, but that's what defines "guitar sound":

[R.G.]

I just like to experiment.

This is a good and laudable goal. My experience with experimenting is that I usually need to be prepared to either put it back the way it was before the experiment or junk the remains if things don't go so well. As long as you're willing to do this, you're fine.

Lets say my amp has a high end of 2kH, how would I take that up to 10kH in a S.S. amp. I would imagine going lower would be similar but in the other direction. Amp in question is a Marshall MG15DFX.(this is not my main amp so i'm not afraid of hurting it)

SS amps are in general very wide response already unless the maker has taken pains to limit them somehow. Most SS amps already have a response down to DC that is disabled by inserting a capacitor in front of them, and the high end response is usually at least 20kHz, often much more, unless some effort is made to dumb them down.

This is one of those questions that seem like it should have a few simple answers, but actually expose layer after layer of complexity. Similar questions include "How can I change the top speed of my car?" and "Why is the sun so bright?".

The answer to your proposed question (i.e. increase a high end of 2kHz up to 10kHz in a SS amp) is really best answered as "find the method that the original designer used to limit the high end down to 2kHz, and rework it so it only limits down to 10kHz."

All this is very different from tube amps, where the need for an output transformer is a huge limitation on both bass and treble response.

The real answer to manipulating the frequency response of amplifiers is "go learn electronics theory about amplifiers and filters". It's not a few simple tricks, like some things are.