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Can someone recommend a good op amp that can run on a potentially noisy 6.3V tube amp heater supply?

So desired / required characteristics are:

  • Single supply < 6.3V (although I think the 6.3V is RMS so the real voltage might be more like 8.9V)
  • Very good PSRR
  • Low noise (so preferrably bipolar and not something like "LinCMOS")
  • Unity gain stable
  • Decent power necessary to drive possibly long cable

I want to create a rack mount unit with a bunch of tube preamps and I need to buffer the output using probably a simple inverting op amp stage with a very large Ri / Rf ratio (like 1M / 10k) to step down the ~230Vpp of the vacuum tube anode to ~2.3Vpp on the op amp output.

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  • \$\begingroup\$ If you're going to make a rack-mount unit, you really don't have many excuses for not giving yourself a dual-voltage supply. You can find space to fit multiple transformers into the unit, relieving the need for a single complicated custom transformer with multiple windings. \$\endgroup\$ – Kaz Jun 12 '13 at 18:11
  • \$\begingroup\$ The ADA MP-1 (classic guitar pre-amp design from circa 1987) does it all with one transformer with multiple secondary windings: 190V supply for tubes, +/-15V regulated dual rails for audio, and an 8-9V unregulated line which is used for tube heating, and which is also regulated down to 5V for digital. \$\endgroup\$ – Kaz Jun 12 '13 at 18:15
  • \$\begingroup\$ Also: do the tubes really need 6V for their heaters, or is that because you're using a center tap to run filaments in parallel on 6V rather than in series on 12V? \$\endgroup\$ – Kaz Jun 12 '13 at 18:20
  • \$\begingroup\$ Even though the fillament supply may be noisy, that doesn't mean this noise has to make it all the way to the opamp supply. Rectifying and filtering the fillament supply is really not hard. I'd be more worried about what its ground reference is. If the tubes have heaters completely separate from the cathodes, then it could be floating or referenced to something inconvenient. \$\endgroup\$ – Olin Lathrop Jun 12 '13 at 19:12
  • \$\begingroup\$ @Kaz It seems to me the only thing that +-15V gives you is 15dB of headroom. And if the tube amp is for guitar it's probably operation near or in overdrive so you don't really need that much dynamic range. If the op amp has a PSRR of 130dB or so, I can't think of an excuse for not using the heater supply. The dual supply would simply be redundant. There must be a good 5V single supply op amp out there that would serve perfectly as a line out in absence of a power amp stage. \$\endgroup\$ – squarewav Jun 12 '13 at 19:58
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Three comments:

1) You'll want to regulate the voltage down to a stable voltage, say 5V, using a high-line-rejection regulator. A simple linear regulator with a couple of capacitors can provide > 80 dB rejection here.

2) You're presumably using a tube pre-amp to get a sound that has significant tube (saturation?) distortion instead of possible transistor (zero-crossing?) distortion. Why would you then introduce transistor distortion from an opamp on the output signal? Isn't that kind of counter-productive?

3) All tube characteristics (linear and non-linear) can be modeled efficiently in a low-cost DSP, to within limits of measurability (which are more sensitive than human ears.) So there's probably a much cheaper and easier way to get the sound you want, with less energy waste and smaller space.

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  • \$\begingroup\$ 1) Using an op amp with a good PSRR (power supply rejection ratio) is effectively the same as using a regulator. 2) Op amps do not introduce any noticeable distortion. If the signal is a guitar preamp, I seriously doubt anyone could actually measure any effect of the op amp. 3) Tube non-linearity is not trivial and not easy or cheap to model in DSP. I really don't think DSP is anywhere near as good. I have had Boss GT boards and a PodHD. They're no where near as good as the real thing. \$\endgroup\$ – squarewav Jun 12 '13 at 20:14
  • \$\begingroup\$ Correct. Just because you have some "good" nonlinearities from tubes in the signal chain doesn't mean that it's acceptable to have "bad" nonlinearities, such as output stage crossover distortion from an op-amp. Luckily, there are op-amps that have vanishingly small distortion, so they will transparently capture whatever the tubes put out. The situation might be, say, 2% THD from the tubes, versus 0.002% from the subsequent op-amp. The tube is, effectively, used as a signal processing effect, whereas the op-amp is part of the machinery to realize the effect (and not itself an effect). \$\endgroup\$ – Kaz Jun 12 '13 at 22:20
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    \$\begingroup\$ @ioplex Regarding "Using an op amp with a good PSRR is effectively the same as using a regulator": Not really. Even the best PSRR rating you can get from an op amp is not comparable to the performance of a decent linear regulator. Further, PSRR of the op-amp is one thing, injected supply noise into the rest of the op-amp support circuitry is another factor, and one you are apparently ignoring. \$\endgroup\$ – Anindo Ghosh Jun 13 '13 at 6:29
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I've used AD8606s in pretty noisy applications fed with tenuous supplies but the only disadvantage is that they are 6Vdc max voltage rated so a dropper resistor and zener maybe be a good idea to stop them over-stressing. They have 8nV per root hz Vn which is probably good enough, 10MHz GBP, 80mA drive capability and are RRIO. PSRR is 90dB up to 1kHz. Driving cable should not be a problem but a small output resistor would prevent excess ringing on transients.

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protected by clabacchio Jun 13 '13 at 6:00

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