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Beginners Guide to Tape Recorder Basics

**ironbut**:

I had done some searching for how to do this measurement in the past and always given up.

Thanks to Rick Chinn on the Ampex list, finally have the procedure.

Haven't tried it yet but here it is.

Thanks Rick!

The head is an inductor, so the impedance varies with frequency. I think 1k is the usual frequency for a head. If you can find some old Nortronics lit, you'll see what they used.

You can measure impedance using a signal generator and an ac vtvm, like an HP 400EL/FL or one of the older models.

The generator needs to have a known output impedance. 600R works well.

you connect the generator to the ac vtvm, and you adjust the output for a known reading on the vtvm, say like -10dBu. (I wouldn't drive the head too hard, and you definitely don't want to saturate the core).

Then you connect the head across the input of the VTVM and note how much the output drops by. From this, you can calculate the impedance.

Assume 600R generator impedance. If you're working in dB: then Z = Zs * ((10^dB)-1) If you're working in volts,

then Z = Zs * ((Vunloaded/Vloaded)-1) Example:

you set the meter so it reads -10dBu, with the generator at 1khz. then you connect the head, and the meter reading drops 1dB

then, with your handy dandy calculator (or spreadsheet), you find 10 to the 1th power, which is 10, and then subtract 1, leaving 9.

Then you multiply the source impedance of the generator by 9, giving 5k4-ohms.

If you work in volts, then set the meter so it reads 100mv, with the generator at 1khz. then connect the head, and the meter reading drops to 90mv.

then, 100mv/90mv = 9, subtract 1, and get 8. then multiply the source impedance of the generator by this, 600R * 8 = 4k8-ohms. The head impedance is 4800R at 1khz. (I picked numbers out of the air, so I didn't think that the answers from the two examples would agree, and they don't)

Of course, you can sweep the generator frequency to see the change in impedance with frequency. If you can sweep over a far enough range, you'll see the head's self-resonance, although this is influenced by the source impedance of the generator and the capacitance of the connecting cables. To really see this, you need to raise the source impedance of the generator considerably, so the generator impedance has less influence on the resonant circuit formed by the head, the cable capacitance, and the head's stray capacitance.

This method works with other things than heads.

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