> This is certainly not 150
> ohms, so I wonder what the actual real-world noise performance of this op=
amp
> will be. The data sheet states "near theoretical", but stops at giving re=
al
> figures. So maybe it is good, but maybe not as good as they would like on=
e to
> believe.
Hi Greg,
Many years ago I built a home-made preamp using the INA163 for using with m=
y ancient SONY TCD-D3 recorder. What I can say is that it performed very we=
ll, at least good enough for the microphones that I used (Sennheiser MKH 60=
).
> In any case, I doubt that it will be showing up in devices such as an
> LS-xx recorder because of the supply rail requirements.
Yes, this is of course true.
> Transformer coupling at the input of an op amp can be beneficial for a wh=
ole
> host of reasons.
Well, this seems to be indeed a controversial issue ;-)
> By picking a good, low-noise op amp and then picking the
> transformer to match the=C3=82=C2=A0source (mic)=C3=82=C2=A0impedance pro=
perly to the op amp's Ein/Iin
> figure, one can obtain an excellent mic preamp. I have done this in the p=
ast
> with very good results.
Do you have any references on transformer-based mic preamps that provide an=
EIN better than -130 dBu(A) at 150 ohm termination (this is what can be ac=
complished today with transformer-less designs)?
An interesting an often overllooked detail of transformer-less preamps is t=
he fact that the two opamps that make up the differential input stage actua=
lly improve the overall noise performance by 3 dB compared to a single opam=
p (transformer-based preamps use a single opamp only).
That way, each of the two opamps amplifies one half (u/2 and -u/2) of the m=
icrophone output signal. Those two components are finally subtracted from e=
ach other within the A/D converter (u =3D u/2 - - u/2), which corresponds t=
o an increase of the final signal level by 6 dB. This also means that the d=
ifference of the two uncorrelated EIN components (which add geometrically) =
will increase only by 3 dB.
That is one of the reasons why recorders with balanced mic inputs are usual=
ly less noisy than those with unbalanced 3.5 mm TRS inputs.
> The subtle nonlinearities of transformers are also
> considered to have beneficial effects to the sound.
Yes, this is indeed a real difference between transformer-less and transfor=
mer-based preamps and it is obvioulsly a matter of taste.
> One other issue that I see with the INA163 is the slew rate of 15 V/uS. T=
hat is
> not terribly good for a preamp that may be called on to amplify frequenci=
es
> above 20 KHz.
Agreed - this was just an example.
Best,
Raimund
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