This is normal for an amplifier with the noise referred back to the input. It just means that (at low gains) the noise is dominated by something further down the chain, e.g. the output amplifier. Output referred noise doesn't change much with the gain.
I'm not sure if this "high" noise is a problem. G = 1 would be used with +10dBu inputs, resulting in an SNR of about 110dB (assumes 20kHz noise bw).
Good find.
Regards, Allan
Mmm, this does look pretty much perfect for my purposes (Though the TL071 has now been superceeded by the TL051). How much current does it draw (typical and maximum, if possible) when in operation? Any idea at all?
Thanks, Jaye.
The four 5k resistors around the output amp together cause less than
20nV/sqrt(Hz) of noise.The two 5k feedback resistors contribute 12.9 nV/sqrt(Hz).
That's 22nV so far.
We need another 45nV (to reach the 50nV/sqrt(Hz) figure in the datasheet)
I guess the input stage current noise multiplied by the 5k feedback resistors makes up the difference. This implies that the current noise from the input transistors is about 6 pA/sqrt(Hz).
(Tj = 300K, which is a probably an optimistic assumption.)
Regards, Allan
I thought an SSM2019 would be better, as it has better performance for similar cost and power consumption (when compared to the above circuit). It's smaller, too.
Neither solution is good for battery power though (~100mW or so). Low noise stuff tends to need a fair amount of current to keep the voltage noise down. Run it from high (+/-15V) rails, and it adds up to a lot of power.
Regards, Allan
Indeed, however, the TL051 (or indeed, the TL054 as I'm wanting a four channel preamp) is easily available locally. Is the SSM2019 pin compatible with the TL051? If so I'll prototype with the TL051 (rather than the TL054) so I can upgrade if need be.
Hmm... perhaps my maths is a little squiffy, but 100mW from a 30V differential voltage comes to 3mA. Even if my power supply is 50% efficient, a 2.4V 2300mAh supply (two good NiMH AA cells) should run four of these preamps for around 7 hours:
100mW * (100/50) * 4 = 800mW (for four preamps from a 50% efficient supply) 800mW / 2.4V = 333mA2300mAh / 333mA = 6.9 hours
That's plenty, especially since I was thinking of using 4 AAs for 4600mAh and 2.4V. Or have I gotten this wrong?
Thanks, Jaye.
Err, I meant that the '2019 would be better than the *entire board*, not just the opamp. The '2019 isn't an opamp; it's an (almost) complete mic preamp.
BTW, I think you can get samples of the '2019 for free from the AD web site.
Regards, Allan
Ahh. I see what you mean. Looks like it's a complete preamp, only needing a bypass capacitor, and a gain resistor. Shall try to get some samples...
Thanks, Jaye.
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