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Lowest noise amps

Discussion in 'Electronic Design' started by [email protected], Aug 18, 2005.

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  1. Guest

    An old friend swears that vacuum tubes for audio have less noise than
    the best op-amps. Is this true? This sounds completely untrue to me.
    I've seen op-amps around 1nV/sqrt(Hz) in the audio range.

    What type of voltage amplifier has lowest noise for audio? As far as I
    know, it's an op-amps or a FET, right? OK, some op-amps have FETs, but
    I've seen some 1/4nV/Sqrt(Hz) FETs.

    Paul
     
  2. Pooh Bear

    Pooh Bear Guest

    No - but good toobs are actually surprisingly quiet.
    Discretes will outperform even the best op-amps. Best noise I've seen is
    0.3nV / rt Hz for a discrete fet.

    Graham
     
  3. Jim Thompson

    Jim Thompson Guest

    It's all about power. Goose up some monolithic designs and you can do
    just as well noise-wise. But I've done as good as 1nV/rt-Hz in
    squeezed current situations.

    ...Jim Thompson
     
  4. Guest

    Check this out. I found 0.1pV/rt-hz gain of 500 transformer
    preamplifier. The datasheet shows 0.1pV/rt-hz for both transformer and
    buffer, it says you can bypass the buffer for a gain of only 100. I
    wonder what the input noise is without the buffer! You can probably
    get even better cores than that. I've heard that cobalt core materials
    have nearly zero Barkhausen noise.
    http://www.thinksrs.com/downloads/PDFs/Manuals/SR554m.pdf

    Paul
     
  5. Guest

    Even better yet, supposedly this guy built a 65pV/rt-hz 5Hz-100kHz
    amplifier:

    J. Lepaisant, M. Lam Chok Sing, D. Bloyet
    Low-noise preamplifier with input and feedback transformers for low
    source
    resistance sensors
    Rev. Sci Instrum. 63(3), March 1992, p2089

    Is that just a low noise transformer?

    Paul
     
  6. Dave

    Dave Guest

    I've only just skimmed it, but it looks like a typical non-inverting
    buffer, where some of the output is coupled back to the input in
    antiphase, to give negative feedback.

    Instead of setting the gain with resistors like in a normal op-amp
    would, with gain.

    G = 1+R1/R2

    it is done with transformers. So the gain must be set by the turns ratio
    of the transformers.

    The actual circuit has FETs in front of the op-amp.

    The Stanford data sheet someone pointed to earlier shows the noise
    figure to be 1dB at optimal frequencies. As an RF engineer, I think in
    noise figures and temperatures.

    A noise figure of 1dB is equivalent to an internal noise generated by
    the preamp equal to a resistor at 75 Kelvin.

    http://www.satsig.net/noise.htm

    A decent UHF (500MHz) pre-amp can achieve noise figures of around 0.3dB,
    or 21K, so produce between a third and quarter of the noise of that
    Stanford device.

    Those UHF premaps I talk of usually have less noise at lower
    frequencies, but they might be next to impossible to keep stable at
    audio frequencies. With gains to well over 30GHz, they tend to oscillate
    somewhere I suspect.
     
  7. Clive Tobin

    Clive Tobin Guest

    This may be close to being true in high-Z circuits as long as there is
    no high sound pressure level, shock or vibration.

    But remember that tubes are microphonic and any physically induced
    noise level in the audio range can be way higher than the thermal
    noise, even leading to oscillation if a preamp tube is close to a
    speaker.
     
  8. The noise figure is 1 dB, which is not all that good.
    It is only great at 10 Hz to 10 kHz
    It also distorts and causes harmonics at low level signals, because of
    non-linear magnetic, and they do not list a spec for it either.
     
  9. Pooh Bear

    Pooh Bear Guest

    Input impedance of 0.5 ohms won't be any use for audio ! Rubbish frequency
    response. This is designed for entirely different applications.

    Remember you asked about *voltage* amplifiers. These usually have high input
    Z.

    Graham
     
  10. Pooh Bear

    Pooh Bear Guest

    And RF practice is also entirely different to audio.

    Graham
     
  11. Ken Smith

    Ken Smith Guest

    The Interfet will do about 0.25 if you run selected ones at about 10mA.

    I've often wanted to see what the noise of a great big power MOSFET would
    be when it is running at a modestly high current.
     
  12. Pooh Bear

    Pooh Bear Guest

    Funnily enough, the device I had in mind was from Interfet. Someone here
    posted some info about them so I went and got the online data. Looks very
    interesting.

    As you say, you need to use some quite serious current to get those low noise
    figures. Typical audio mic preamps using discretes run the input devices at
    around 2-3 mA each to get better than 1dB noise figure re: 200 ohm source (
    typical microphone impedance ).

    Graham
     
  13. And the 1/f Noise of those semiconductors goes skyrocking at "near DC"
    (In means of Audio Frequencies)

    Jorgen
     
  14. Pooh Bear

    Pooh Bear Guest

    Depends on the device a bit. Some are better.

    I've never known the parts that make it into pro-audio to be troublesome in
    practice.

    Graham
     
  15. Ken Smith

    Ken Smith Guest

    Watch out in the capacitive micro-pre-amp. The gate capacitance of the
    very low noise JFETs is enough to effect the linearity.

    At 2mA, I think I'd look at the LSK170 for my low noise FET.
     
  16. Ken Smith wrote...
    I just purchased 100 Toshiba 2sk170, and measured samples at 1 and
    3mA: under 1nV. I like that their input capacitance is lower than
    many others with similar or lower e_n. I purchased mine from MCM.

    It's easy to devise circuits that eliminate any linearity effects
    from the JFET's gate capacitance changing with drain voltage.
     
  17. Ken Smith

    Ken Smith Guest

    This is not the distortion I was refering to. In a capacitive microphone,
    the sound moves one plate of a capacitor by the amount I'll call X. For
    small spacings between plates, the capacitance of the capacitor varies as
    1/X. If the charge on the capacitor is fixed, the voltage varies as 1/C.

    You end up with 1/(1/X) or simply a voltage that varies with X.

    If the gate capacitance is high, you end up with 1/(Cgs + 1/X) and hence
    distortion.
     
  18. Although in a 'follower' circuit, the effect of the gate-to-source
    capacitance is considerably reduced. (Gate-to-ground capacitance is not
    affected unless you bootstrap the surrounding metalwork)
     
  19. Adrian Tuddenham wrote...
    Using a high-gain source-feedback circuit effectively bootstraps
    the JFET's source, eliminating most of its high Ciss gate-source
    capacitance, with an effectiveness approaching a follower circuit
    if enough loop gain is present. A cascode configuration reduces
    the Crss gate-drain capacitance, and bootstrapping the cascode off
    the source drives that already small capacitance toward zero. All
    that's left from the JFET's capacitance is high-frequency e_n-Cin
    noise, which is not canceled out by configuration or the feedback
    circuits (this means one musn't get carried away in choosing too
    large a JFET, just because it has a nice low voltage-noise rating).

    The 2sk170 presents a nice compromise for many applications. For
    example, I'm using them in a sensitive capacitance position gauge.

    The 2sk170 with its under-1nV spot voltage-noise level isn't best
    employed in a follower circuit, because this low noise level would
    force one to follow the JFET follower with an unusually-low-noise
    BJT amplifier, such as one using Rohm's 0.55nV 2sd786 transistors.
    That's not appealing, compared to a properly-designed common-source
    JFET amplifier that provides all the advantages without the pain.
     
  20. Ken Smith

    Ken Smith Guest

    Yes, an amplifier where the feedback to the source is taken from much
    later in the circuit is a better way to go. You usually want to follow
    the cascoded pair of JFETs with a low noise PNP stage.
     
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