audio transformer ?

Discussion in 'General Electronics Chat' started by Mathematics!, Nov 20, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    OK , I have just finished doing amplifier circuits with the LM386 chip.

    What I am curious about is I bought also radioshacks audio output transformer part 273-1380. What is this used for???

    I would think since it's a transformer it would either step up or step down voltage/current.

    But most transformers I have worked with give you the input/output
    like 120 vac 60hz to 24 vac 60hz (like the transformer used in the furnace of a house)

    When I look on the back of the package I see nothing identifying input/output voltage/current in the way I use to see transformer info.

    What would be the ac input for this transformer 120 vac 60 hz I don't know about that?
    Where and when would you use this ???

    I am shakey on what this input , output , dc resistance (output/input) , and insulation resistance means in terms of voltages/current rating ,...etc

    Maybe I am totally lost any help would be great
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    In the days when audio amps were tube or transistor, designers took the output from the plate or collector. They needed to maximise the plate / collector impedance to maximise output. However loudspeakers tend to have low impedance.
    Audio transformers are generally used as impedance matching devices to provide good power transfer. So they are designed on the relationship between turns ratio and impedance ratio, not as step up or down devices.
    In fact they were also sometimes used for interstage coupling for the same impedance matching reasons.
  3. Audioguru

    New Member

    Dec 20, 2007
    It is an audio transformer, not a 60Hz power transformer.
    As usual, Radio Shack provides no spec's on the web except for its weight. Radio Shack does not know anything about electronic parts.
    Did it come with spec's?

    Its impedances, power rating and frequency response should be spec'd or it is useless. I found a little sentence at the end of its web page that says 1k to 8 ohms. Maybe it was used as the output transformer in a little tubes amplifier in 1955. A 70V PA speaker would have a max power of 5W if the transformer can handle 5W.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    To go a bit beyond studiot's post, an audio transformer will often have the primary and secondary resistance listed. Something like 8,000 ohms and 300 ohms, for example. A power audio transformer will have the power handling listed, and then something like 1000 ohms and 4/8/16 ohms, meaning the secondary is tapped to match the speaker.
  5. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    well the back of the package gives
    impedance 1kHz , 1V, 0mADC
    input 1k ohm +- 10%
    output 8ohms (loaded)
    dc resistance input 70 ohms +- 20%
    dc resistance output 0.62 ohms +- 20%
    frequency response 300 - 1000Hz +- 3db
    insulation resistance more than 100M ohms @ 250VDC

    Ok , so the output could directly connect to a speaker.
    What would the typical input be could it be a mic???

    What is this dc resistance stuff thought all transformers worked on ac?
  6. ftsolutions

    Active Member

    Nov 21, 2009
    I use this very transformer, along with a couple smallish series resistors in the input 'legs' (~470 ohms) so that my wife (physics teacher) can plug in a small 8-ohm speaker into a laboratory signal generator. This Signal generator needs a load of at least 1K to be able to drive it. It isn't an audiophile quality transformer, but it does the job well enough.

    One needs the DC resistance figures to calculate the current for a given input/output voltage.
  7. SgtWookie


    Jul 17, 2007
    A small speaker, perhaps. It's a signal-level transformer, not a power transformer.
    It's an impedance matching transformer. It takes a higher voltage lower current input, and gives a lower voltage higher current output. But if you try to run more than a few milliamperes through it, you will destroy it.

    Read the package again.
    Top line reads:
    Impedance 1kHz , 1V, 0mADC
    That means the impedance was measured in Ohms using a 1kHz 1v input signal.
  8. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok , just want to make sure I understand this.

    So they measured the transformers input resistance to have 1kohm resistance by generating a 1kHz wave with Rms = 1 volt

    I am not really getting what the impedance is for and the input is for?
    If the impedance is just the value they measured what happened to the input/output of the transformer at that value 1volt 1khz then ok I get it.
    But if not I am confused.

    Also what is this insulation resistance for?