Sound Quality

[Duane Attaway]

On Sat, 27 Dec 2003, Zscoundrel wrote:

> That 'buzz' is probably a 60hz hum from the power lines.  Many of the
> better turntables used a very sensitive electro-magnetic pickup that put
> out a much lower line-level output than tape decks and stereo systems.  
> You probably need to get a pre-amp or just hook the turntable into a
> stereo that is built to accept turntable level input and puts out
> standard line-level out put.

This is called matching amplifier impedances.  Each amplifier has been
designed with a particular resistance and must be matched with its input
or outputs for maximum efficiency.  If a 50 ohm microphone is connected to
a 1,000 ohm amplifier input, the gain will have to be greatly increased to
overcome massive loss.  The result is maximum noise in the form of static,
hum, the switching of light switches, dimmers, and possibly local radio
stations.

A good amplifier should always state its input and output impedances and
frequency response.  Sometimes the input impedance is measured vaguely in
volts.  A one volt input gives a hint of a higher impedance, say 1,000
ohms.  Our friends at the RIAA were involved in setting these wonderful
standards in the old days...

Speaking of frequency response, pick something that matches your 
application.  An ideal amplifier would cover the entire spectrum, but you 
wouldn't want the real low frequencies including DC voltage...

When designed, an amplifier has a bias circuit that defines its input 
impedance... or its input resistance is infinite in the case of 
differential amplifiers.  The output impedance is usually much lower than 
the input as the final transistors often are driven directly from the 
power supply rails.