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Comment by fallingfrog | Hacker News Reader

I usually put a single 470 ohm resistor in line with the gate of a discrete jfet in common collector mode as the first gain stage in my projects. Once you boost up the signal voltage it’s way easier to maintain a good signal to noise ratio.

The resistor is there to prevent the jfet from being burned out by over voltage on the gate, which is very sensitive to static electricity. But, I can easily hear the difference if I put a 10k resistor there instead. It’s really important to get that first gain stage really, really quiet, a discrete jfet has a better noise floor than an op amp or a regular transistor.

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> ...a discrete jfet has a better noise floor than an op amp or a regular transistor.

This used to be true, but you can get really good low-noise op amps these days.

What is your favorite?

I don't really have a favorite op amp. There are so many good ones. Some audiophiles like to swap out op amps and talk about how good different ones sound, but I think that's a bunch of hogwash. If you believe that choosing the right op amp for an audio project has an impact on the sound, I'm the wrong person to ask about it.

There are some exceptions, like if you need a microphone preamp with 60 or 80 dB gain, but I have no experience in that area.

For audio, it's hard to go wrong with the OPA1654 for DIY projects that are not cost sensitive. It draws a lot of current. For battery power I'm intrigued by the TLC2264 which, while having somewhat more noise, has very low operating current and rail-to-rail performance.

Uh, for a looong time already. The LT1028 was available in the eighties and afaik, still unsurpassed (in terms of voltage noise, it's unfortunately a sucker in terms of input current and current noise, so it's for low impedance applications only and it's fairly expensive). The cheaper OP-27 is also old and still available. The challenge is to find a low-noise OpAmp with high input impedance where earlier hybrids with discrete JFets fronting a low noise OpAmp were often used. These days its rather a challenge to find low-noise discrete JFet pairs and one has to use an integrated OpAmp instead (the causal chain might be reversed there).

Yes, I built a guitar pedal that was designed around JFET gain stages and the JFETs had to be sourced from a specialist. I wonder if they're still being made. Most of the discrete JFETs I see available are for switching applications.

I bought 4000 J201’s when I saw they were becoming obsolete, I still have most of them. But, I think there are better choices out there anyhow, that’s just what I’m used to using. They’re really easy to bias.

Low noise JFETS are hard to find today. Devices like the Toshiba 2SK369, 2SK117, 2SK170 etc. went out of production years ago for being mostly THT and possibly non ROHS devices, while purely functionally speaking they would still be great parts. They can be seen online on Ebay, Aliexpress, etc. but those are almost always relabeled fakes; real ones are hard to find and not cheap. Before learning about counterfeit components the hard way, I bought two bags of 2SK117 from two vendors on Ebay: they were identical, but when fit in a circuit, I had to recalculate the drain and source resistor with all the parts from one vendor because it clearly had different specs than a real 2SK117. Ugh! ...lesson learned. Somebody suggested the use of the BF256 JFET in audio circuits, even in RIAA phono cartridge preamps. It is a RF part (the natural successor and direct equivalent of the venerable BF245) but it seems it is also a low noise part capable of working down to audio with no issues. It went out of production more recently than other models and some vendors still have stocks available. Probably not as quiet as the others though.

Linear Systems still makes JFETs and all sorts of other discrete parts that everyone else stopped making.

Indeed the LT1028 datasheet shows a BF862 JFET at the front end of a photodiode preamp using the LT1018.

The reason why we're at a plateau of op amp noise performance is that the current crop of chips are operating very close to the theoretical limit for both bipolar and JFET input devices.

Ironically I use a LT1028 in a homemade circuit for measuring the noise of things that are more noisy.

Also, a lot of the newer low noise op amps want much better frequencies characteristics and more GBW as they're primary application is sensors. Take for example the ADA4817. It has very comparable noise characteristics but is quite a lot faster.

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