UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Low noise amplification with mulitple valves

G8HQP Dave · 25th Nov 2010, 9:47 pm

Normally when connecting audio devices together there is a serious mismatch. The output impedance of a CD player might be 200ohms, but the following amplifier might have an input impedance of 100K. This is as intended, as we have plenty of signal and the aim is to drive any cable capacitance. We don't have to worry about noise as the signal levels are so high.

Impedance matching only occurs in most audio systems at low level inputs such as magnetic cartridges, microphones, tape heads etc. Even then the match would be considered quite poor by an RF engineer! If we have a moving coil cartridge which needs a few hundred ohms input then we can either use a transformer and get some voltage step-up or have parallel or very high gain valves. Most people find the transformer is best - which is what theory says should be the case. Even then most of the signal power ends up being dissipated in the grid resistor. Unfortunately in audio the requirements of low distortion and low noise often pull in opposite directions.

25th Nov 2010, 9:47 pm	#8
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Low noise amplification with mulitple valves Normally when connecting audio devices together there is a serious mismatch. The output impedance of a CD player might be 200ohms, but the following amplifier might have an input impedance of 100K. This is as intended, as we have plenty of signal and the aim is to drive any cable capacitance. We don't have to worry about noise as the signal levels are so high. Impedance matching only occurs in most audio systems at low level inputs such as magnetic cartridges, microphones, tape heads etc. Even then the match would be considered quite poor by an RF engineer! If we have a moving coil cartridge which needs a few hundred ohms input then we can either use a transformer and get some voltage step-up or have parallel or very high gain valves. Most people find the transformer is best - which is what theory says should be the case. Even then most of the signal power ends up being dissipated in the grid resistor. Unfortunately in audio the requirements of low distortion and low noise often pull in opposite directions.