[futurebasic] Re: [FB] [X-FB] Recording sounds from tape

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Home   : October 1999 : Group Archive : Group : All Groups

From: Peter Bancroft <pbancroft@...>
Date: Tue, 12 Oct 1999 08:52:14 +1000

Here is a simple voltage follower using an OP Amp:

            | \
o-----||----|+ \
            |   \
            |   /--------||-----o
         ---|_ /    |
V in     |  | /     |        V out
         |  |/      |
         |          |


For audio signals (AC):

V out = V in

The size of the capacitors and internal impedances in the Op Amp sets the
frequency response.

If you remove the capacitors then for DC and AC:

V out = V in

The frequency response will be from DC to some cutoff frequency.

Putting capacitors in parallel with the output (ie across V out) would
produce some filtering effect depending on the output impedance of the
amplifier. (Usually very low).

>But, aren't most capacitors hung just off the line to take up
>surges -- connecting between the "hot" line and ground (i.e., parallel)?

You may be thinking about filtering surges from the DC power supply to the
Op Amp. Putting capacitors there also shorts out HF feedback paths through
the power supply and is a good thing to do.

Most audio circuits use AC coupling (capacitors in series as in my diagram)
to avoid damage to the Op Amp due to DC voltages being applied at the
output, and also to ensure that the DC bias conditions on the Op Amp are
maintained. This is important when you connect battery operated equipment
to mains powered equipment. The internal power supplies may be different in
each. eg Battery 0-6V. Mains powered mixer -15 and +15 V.

Peter		mailto:peter@...		http://www.isoma.com