22-02-2017, 11:11 AM
(This post was last modified: 22-02-2017, 11:13 AM by ppppenguin.)
Why is frequency response limited in a power amplifier? No simple answer but here are a few points.
If there is a negative feedback loop then additional low pass or high pass filters within the loop can cause nyquist instability. This is where the phase shift at some frequency approaches 180 degrees while the gain is still greater than unity. Thus turning negative feedback into positive. This is one reason why overall negative feedback in valve amps is limited: there's a huge great high pass filter called the output transformer. Transistor amps don't usually have much by way of high pass filtering, just normal AC coupling. If they are DC coupled then there's none at all. You normally don't want to pass DC through a power amp so there's usually at least 1 coupling cap.
What about the HF end? In transistor amps it's all too easy to have far too much HF bandwidth. This makes the amp vulnerable to RF interference so a low pass filter at the input is a good idea. Valve amps have output transformers that limit the HF response. In a few amplifiers such as the Quad 405 it's essential to limit the input bandwidth as the output stage inherently can't cope with signals that are both high amplitude and high frequency. It won't be damaged but the distortion will get really horrible.
Mark's comment about distortion is worth re-iterating. Distorted sound is subjectively louder than clean sound.
If there is a negative feedback loop then additional low pass or high pass filters within the loop can cause nyquist instability. This is where the phase shift at some frequency approaches 180 degrees while the gain is still greater than unity. Thus turning negative feedback into positive. This is one reason why overall negative feedback in valve amps is limited: there's a huge great high pass filter called the output transformer. Transistor amps don't usually have much by way of high pass filtering, just normal AC coupling. If they are DC coupled then there's none at all. You normally don't want to pass DC through a power amp so there's usually at least 1 coupling cap.
What about the HF end? In transistor amps it's all too easy to have far too much HF bandwidth. This makes the amp vulnerable to RF interference so a low pass filter at the input is a good idea. Valve amps have output transformers that limit the HF response. In a few amplifiers such as the Quad 405 it's essential to limit the input bandwidth as the output stage inherently can't cope with signals that are both high amplitude and high frequency. It won't be damaged but the distortion will get really horrible.
Mark's comment about distortion is worth re-iterating. Distorted sound is subjectively louder than clean sound.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
