Basic questions about microphones and mic preamp design...

[John Woodgate]

I read in sci.electronics.design that martin griffith
[email protected]>) about 'Basic questions about microphones and mic preamp
design...', on Mon, 13 Dec 2004:

Sorry Win
I gave up my AES membership years ago, Icouldnt keep up with the maths
Maybe Woodgate would have something more up to date.

There is a huge amount of stuff on this, about 80% being in the
audiophool class. I don't recall any recent AES treatment of the
subject.

My own view is that since transistors have pentode-like output
characteristics, there is little point in comparing them with triodes.
And if you compared them with pentodes, VERY carefully adjusting the
measurable parameters to be equal, there wouldn't be an appreciable
difference in the subjective effects of overload.

Urban myth...About triggering an old type photgraphic flash bulb from a
mic in a bass dr

No problem at all with a carbon microphone. You don't even need the bass
drum - a loud shout would do it.

 

[Jaye Gallagher]

Have a look at Rod Elliott's Webpage. I've build his Low Balanced
Microphone Preamp (http://sound.westhost.com/project66.htm) myself.
I'm running it from 2 9V Blocks (one for +9V and one for -9V) and it
works just perfect. Added two 2k resistors from +9V to balanced inputs
followed by two capacitors as phantom power supply.

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.

 

[Allan Herriman]

I always associated the "excess" noise at unity gain with the highish
5K feedback resistor causing the noise. Change this to zero ohms, as
in a voltage follower, and the noise would (might?) decrease
significantly ,(I havent done the sums,)

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

 

[Allan Herriman]

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?

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

 

[Jaye Gallagher]

Mmm, this does look pretty much perfect for my purposes (Though the

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.

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.

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.

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 = 333mA

2300mAh / 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.

 

[Allan Herriman]

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.

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