Amplifier Bandwidth

Rock_Zorro

I sometimes read about “dark sounding” amplifiers but wouldn’t the bandwidth of most guitar amplifier circuits - solid state and valve - generally pass 10kHz? Unless deliberately designed not to.

Meaning the bandwidth limiting component is always going to be the speaker, typically having a falling response from 5kHz, and the idea of a dark sounding amplifier a bit of a myth?

I’m trying to get a better understanding of where the limitations of any set-up might be

stratman3142

Guitar amplifiers aren't designed to have a flat frequency response and the emphasis or removal of certain frequencies is part of what gives an amplifier its character, together with other factors such as break-up/distortion.

ICBM

stratman3142 said:

Guitar amplifiers aren't designed to have a flat frequency response and the emphasis or removal of certain frequencies is part of what gives an amplifier its character, together with other factors such as break-up/distortion.

This.

A lot of guitar amps have high frequencies purposely removed at one or more places in the circuit, this is either to prevent instability, reduce valve microphonics, or stop the amp sounding too fizzy or buzzy which a lot of highs can.

Sometimes the designers go too far and you end up with an overly dark or dull-sounding amp.

Modulus_Amps

Anything with modern gain will likely have a roll off from 3kHz and upwards, this is normally set in the pre amp circuit, though I know Diesel design there OT's to roll off the high end too which means even the clean sounds are not full bandwidth, allot of guys who build vintage amps like 18watts use filters on the output transformer output to tame the fizz too as some those transformers can produce to 20kHz as they are based on old RS hifi transformers.
Early JTM45 used a hifi spec output transformer too (not the re-issues) the negative feedback can help stop the amp sounding too harsh  (i.e. keep the presence down), adjusting the negative feedback can make the amp seem more lively too or very dull, Dull is good for high gain, lively is good for jangly stuff

it is easy to make an amp sound dark, and fairly easy to make one sound bright, how bright is the question as the thing I struggle with is the level of brightness vs harshness

Rock_Zorro

Cheers. I already understood the frequency response of guitar amplifiers isn’t normally flat because of the interaction of the tone stack. I didn’t know designers like to use low-pass filtering too, which could, subjectively, make for a dark sounding amplifier.

ICBM

It's a fine balancing act because historically, guitar amps were often heavily scooped - that's why plugging values into the 'tone stack calculator' makes people think that you need to turn up the mids and down the treble and bass to get a 'flat' response. In fact, that's because the designers *were* aiming for a fairly flat response - when going through typical guitar speakers and cabs, which are of course all midrange, so they needed to cut that and increase bass and treble.

The problem comes when you increase the gain and make those same traditionally-voiced circuits distort - distorted sounds contain far more high frequency content than clean ones (because the sharp-edged waveforms result in more high harmonics, Fourier analysis shows why), so the result is then harsh and buzzy. You need to somehow take that off, without making the amp then sound too dull at lower gain settings.

ecc83

No names no pack drill of course but sometimes a mnfctr has to "match" an amplifier to a less than stellar speaker to hit a certain price point in the market. Fit a better, smoother speaker and such an amp can sound "dark"/ "lifeless"/"flat" pick your adjective.

Low wattage single ended valve stages produce a lot of odd harmonics if driven hard and these do not sound good. A CR series network across the traff primary is a dodge pinched from the early days of radio and TV sound stages.

Dave.

ICBM

ecc83 said:

No names no pack drill of course but sometimes a mnfctr has to "match" an amplifier to a less than stellar speaker to hit a certain price point in the market. Fit a better, smoother speaker and such an amp can sound "dark"/ "lifeless"/"flat" pick your adjective.

Or they make an overly dark, lifeless, flat amp which you fit with a better, brighter and more lively speaker and it comes to life!