When trying to capture faint acoustic phenomena, are vacuum tube amplification designs superior to ICs? In this type of application sensitivity and low-noise amplification are the desired objectives. Should I be satisified with typical modern low-noise op amps, or if I want the best possible performance do I need to dredge up old books and plan for having a custom vacuum tube constructed?

  • \$\begingroup\$ If an IC LNA would let enough power gain up to 50W it would be awesome but I think they are not capable of it. Thats why amphies are mostly designed of Vacuum tubes. Vacuums let high power outputs and less noise comparing to transistors. \$\endgroup\$ – Alper91 Feb 7 '16 at 18:56
  • 3
    \$\begingroup\$ What sort of microphone? What source impedance is your signal? There is no one-size-fits-all answer. \$\endgroup\$ – user16324 Feb 7 '16 at 18:58
  • \$\begingroup\$ @BrianDrummond So you are saying that for some kinds of microphone vacuum tubes are better, but for others, ICs? That does not make sense. \$\endgroup\$ – Tyler Durden Feb 7 '16 at 18:59
  • 2
    \$\begingroup\$ Show me a design that is equally quiet fed from a <1 ohm ribbon and from 1 Gohm in parallel with 30pf, and I will concede you have a point. \$\endgroup\$ – user16324 Feb 7 '16 at 19:01
  • \$\begingroup\$ The answer can only be no; for recovering extremely quiet signals I would consider a microphone array and digital signal processing for directional separation. \$\endgroup\$ – pjc50 Feb 7 '16 at 19:12

In outline, it depends on the signal source, i.e. the type of microphone.

There are some very low noise vacuum tubes.

There are some low noise IC amplifiers, but not many.

There are also discrete semiconductors, both bi-polar and JFET, and these are often the best choice for an input stage, possibly using an IC for the later gain and output stages.

Among vacuum tubes, the 7586 (Nuvistor) has a good reputation. (Ditto some makes of 6060 triode if I recall correctly, and, I think, the V301 if you have a reliable source of WW2-era components...).

To use a valve as a low noise amplifier, you have to remember that it actually has quite a high noise resistance, so it achieves its best noise figure at a high source impedance. You can after all keep the grid circuit impedances arbitrarily high.

So a valve operated as a cathode follower is a good choice to provide current gain when fed from a capacitor mic capsule (typically 30pf). It is necessary to bias the input with a grid leak resistor in the region of 1 Gohm or higher (consider the RC time constant and you will see this determines the LF performance of the mic). See Neumann U47 etc.

But for a low source impedance such as a ribbon microphone, the only way the valve can achieve low noise performance is by matching that source impedance to the mic's noise impedance with a high ratio step-up transformer.

A good replacement for a vacuum tube in similar applications (high source Z) is a suitable low-noise JFET. Get hold of the NatSemi "Discrete Databook" from 1978 : now a rarity but containing more good info than you can find almost anywhere else - including noise voltages vs frequency and current for a range of devices.

You'll find that some relatively large area FETs (aimed at switching) have relatively low noise voltages. Effectively these are multiple small JFets in parallel. See rms summation of noise sources... Pay special attention to "Process 55" (2N5459) operated at drain currents of 1mA or more.

For low source impedance, as far as I know, you still can't beat bipolar transistors - usually PNP, and usually medium power (again, relatively large area). Even the BC214 isn't bad (see the Nat Semi databook again) but some designers recommend the Hitachi 2SC2547 (and if you need an NPN equivalent, I think the 2SA1075). With these, at currents around 10mA, you can reduce the noise resistance of the transistor to somewhere in the 10-30 ohm range.

I'll leave you to convert that to nV/rtHz and compare with the best opamps you can find, or the AD797...

Between vacuum tubes and discrete semiconductors, each used to its best, I doubt you'll find more than a dB or so difference in noise level.

Ultimately, of course, you are bound by the noise figure of the mic capsule itself, i.e. the Brownian motion of the air molecules hitting it. Again, a large area capsule is quieter (sensing a theme here?) at the expense of inferior HF performance (when its dimensions exceed 1/4 wavelength of the sound signal). There have been twin-diaphragm mics (large LF, small HF) with internal crossovers to overcome this downside.

And as @pjc50 notes, any further improvement beyond this comes from, effectively, multiple microphones in parallel : with DSP to not only overcome the disadvantages of their spatial dispersion but also offer advantages such as synthetic beamforming.

  • \$\begingroup\$ Here is a nice article that expands on some of the ideas you put forward in this answer. One point it makes that seems to be worth repeating here is that one must be careful about the input capacitance of JFETs; generally speaking there is a trade-off between noise density and capacitance for the reasons you outline here. Some more detailed information and microphone preamp designs are in this application note for the Linear Systems LSK489 low-noise JFET. \$\endgroup\$ – Oleksandr R. Feb 8 '16 at 1:32
  • \$\begingroup\$ My first thought on reading this question was that tubes are hot, and so there would be more thermal noise than a semiconductor. I was interested to see that there are circumstances where this could be mitigated. \$\endgroup\$ – user56384 Feb 8 '16 at 2:53
  • 4
    \$\begingroup\$ @nocomprende that is not the right way to think of it. Tubes are hot and have a high impedance, but electrons in them move in vacuum under the influence of a static electric field, without being randomly scattered by thermal fluctuations as in a bulk conductor. Thus tubes do not suffer from Johnson noise except to the extent of their small internal resistance. They are still subject to amplifying thermal noise from the external circuit, of course, and still exhibit shot noise due to the discreteness of electrons. \$\endgroup\$ – Oleksandr R. Feb 8 '16 at 3:34
  • \$\begingroup\$ @OleksandrR. Good article : one detail I'd forgotten was why PNP transistors were preferred when I looked into this : the article points to Rbb (base spreading resistance) as the critical parameter, and it used to be the case that PNP transistors had lower Rbb (which the article, ironically, doesn't mention despite discussing similar devices, e.g. BC549 is a good complement to BC214). Whether that's still true in newer processes I can't say. \$\endgroup\$ – user16324 Feb 8 '16 at 12:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.