[Ian Bell[_2_]]

wavesoft wrote:
Hi

In MIC preamps and PHONO preamps sometimes an input transformer is added
that bridges the mic/phono to the first stage preamp.

But many times it is done wothout any transformer , so what is the rule ?
When input transformer is needed and when not ?

Thanks
ec



Like most things in engineering it is a compromise. Both have their pros
and cons. Talking solely about tubes then I would say:

Transformers:

pros: galvanic isolation (no hum loops), gain at very low noise factor,
the 'iron' sound

cons: limited bandwidth, limited range of source impedances, cost,
distortion at high input levels

NO transformer:

Pros: wide bandwidth, wide range of input source impedance, low distortion

Cons: no galvanic isolation, difficult to get low noise

HTH

Cheers

Ian