>Peter van Hoof wrote:
>
>> An additional tip
>>
>> Noise cancels out (partially) in parallel amps, this works especially
>> well in preamps where you can hang several amps to an input signal and
>> (with series resistors) tie all outputs together
>
>  Sorry, I just can't accept this statement in regard to front end
>noise.  In fact I am sure the opposite is true, as when you parallel
>amplifiers, you necessarily reduce the input signal level to each by the
>split factor, 3dB for two, and make the noise WORSE by that amount.
>
>  Cheers,
>        Paul B.

I think you are assuming that the input signal to each preamp would be
halved because the input impedances of the amps are now in parallel. This
is a faulty analysis, though, and would only be true if the input impedance
of the amplifier was comparable to the source impedance of the transducer.
In fact, the amplifier impedance can be _much_ higher than the transducer
impedance.

For example, a piezo transducer at 40 KHz can have an impedance of about
2000 ohms. But a TL082 op-amp in non-inverting mode has 10**12 ohms input
impedance, essentially infinite compared to that measly 2K ohms; putting
additional op-amps in parallel would have very little effect on the signal
that each amp would see.

--BN


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