Input Impedence and Transformers

[kt88pppamp]

Hi Guys,

Impedance in electrical engineering terms refers to reactance, or the opposition to AC signals seen due to inductors (inhibit AC currents) and capacitors (inhibit AC voltages). The term impedance I think is used too casually. If there is just a resistor (what sets the input impedance of most amplifiers) and no reactance so to speak of, why is it called input impedance? Input impedance would be zero and resistance (not reactance) would be whatever that resistor is.

I start to become confused when transformers are introduced into the picture.

- DC resistance of transformer coils is very low so input impedance is clearly not DC resistance.

- Frequency response is flat from 20 to 20k and beyond in a good quality transformer so reactive impedance would be constant so to speak in the audible range.

- 600 ohms seems low for an impedance number, as most preamps have input impedances ranging from 20k and upwards.

A standard impedance ratio is 1:2. Lets say it is 600 ohms to 2400 ohms. How would you determine the input impedance to your amplifier if a transformer with a 600 ohm primary and 2400 ohm secondary is used? Would it be just 600 ohms?

[battradio]

Its 600 ohms AC impeadance , and a bout 60 ohms DC or less . I'm in the process of building a pair of mono amps with UTC LS-12 input transformers ,which are 600 ohm to 120K CT impeadance . The first amp ended up with a 350 mv AC RMS input sensitivity for full power out , but the driving device has to have the current drive ability to drive it . A cd player output IC's have the current capability to drive it , a transistorized preamp would most likely drive it . A tube preamp would need a cathode follower or an output transformer to drive it . I will build a tube preamp line stage with Stancor A-36 output transformers or UTC A-24 output transformers .

Mark

[kt88pppamp]

I am putting the transformers on inputs to a line stage for ground loop/EMI isolation. Do consumer devices such as tape players/CD players etc have enough drive for only 600 ohms?

[battradio]

A lot of the newer ones do , but if you know the IC it uses look it up
http://www.datasheetcatalog.org/datashe ... Xrtrsq.pdf
This IC is speced into a 600 ohm load