If electron tubes, transistors, JFETs are flat from 0Hz to a high frequency why do amplifiers have low and high frequency roll off?

This is due to the surrounding components.


For example capacitors in series reduce bass frequencies.


Capacitors in parallel reduce treble or high freiencies. It makes sense since a capacitor tries to keep the voltage across its terminals constant. There may be stray capacitances, for example a cable and the nearby metal chassis making the plates of a capacitor. Or the conductors of a cable etc.


Inductors in parallel with the signal cut bass.


Inductors in series with the signal cut treble.


The cut off frequency of any case is determined by the value of C or L and the resistance next to them.


R and C defines a time constant. A high R in series or high output resistance will charge a large capacitor slower. In fact in physics or nature R x C makes t or time. 1Ωμ x 1 Farad = 1 second.


So a high output impedance driving a high shunt capacitance will make a duller sound as the cutoff is decreased.


These are called RL filters.


A similar interaction exists between R and L. In Physics or rather nature inductance L devided by resistance R make time.


An inductor is like a mass trying to keep it's velocity constant. A coil or inductor tries to keep the current through it constant or reacts to any change like a mass. This is why treble is cut when an inductor is connected in series.


For example a door which has mass can it be moved back and forth to quickly. Frequency response drops the higher the frequency we want to make the door oscillate. We are the driving source with a certain output impedance. The impedance of the door changes we frequency and we can feel it. with frequency.


For a similar reason bass is cut when it is connected in parallel. The lower the frequency the more a short circuit it is as its reactance in ohms goes down.


When an inductor or a transformer winding is connected in parallel with a real voltage source which has a finite output impedance a potential devided is formed. The low impedance of the inductance at LF creates a greater voltage drop at the output of a mic, electron tube etc. the cutoff frequency depends on R, L and can be anything eg. 3Hz, 40Hz, 300Hz. This can be useful as it can remove bass heaviness making a mic sound flat in frequency response from producer's brain to listers brain. See Pleiades filters. See also Lowe, Morgan paper.


The cutoff frequency depends on R and L. The higher the R and the lower the L the more bass is cut or more high is passed ie the cutoff frequency is raised.


References, bibliography:


Electronics, a systems approach - Neil Storey


Applied Electrinics - T.S. Gray - MIT


Flat frequency response from producer's brain to listener's brain, Sound Picture Recording and Reprocucing Characteristics - D. P. Lowe and K. F. Morgan - J.S.M.P.E.