>We made this mistake with magnetic heads for credit cards too.
>The voltage mode circuit is hideous, and has to be tweaked to
>a range of frequencies.

I've thought of using a current mode circuit for audio tapes--at least
until I read a Winfield Hill post on the 'net.  He was building a digital
tape drive (probably the same one in the AoE), and they tried using the
current from the head.

The only problem was the shorted turn they created from the tape reader
head slowly erased the tapes, high frequencies first!

Did you have any similiar problems?


later,
newell

ps.
Found the post of DejaNews, in case you're interested:
>I can relate a relevant story.
>
> With a tape head, the voltage output is the derivative of the field
> on the tape, dB/dt, but the current output (voltage across a small
> load resistor or better into an opamp S.J. as you suggested) doesn't
> suffer this "disadvantage."  I became somewhat enamored of the slightly
> unconventional "measure into a short" approach some years ago, while
> designing a digital data recording system.   However I discovered there
> may be a downside.  It appeared that the taped signal became partially
> erased, especially at high frequencies, by repeated reading with a tape
> head used in this fashion.
>
> To complete my design, I decided instead to use an "advantage" of the
> traditional dB/dt signals, namely, the peaks of the signal correspond
> to the point of the original tape flux-changes.  The zero-crossing of
> the derivative of the dB/dt signal nicely identifies this point.
>
>--
>Winfield Hill    hill@rowland.org
>The Rowland Institute for Science
>Cambridge, MA USA 02142-1297