Alternatives to J201 and MPF102?

[Kirfew]

I'm building a pedal that needs a J201 or MPF102 in a buffer, but my local electronics store doesn't carry either of those JFETs, but they do have 2N3819's and BF245A's. Would these work, or are there others that would be okay?

[ppaappoo]

If are for a buffer you can use any fat with the same pin out o 180° like BF245

[Toney]

2SK30

[edvard]

I'm building a pedal that needs a J201 or MPF102 in a buffer, but my local electronics store doesn't carry either of those JFETs, but they do have 2N3819's and BF245A's. Would these work, or are there others that would be okay?

Those will work just fine for source-follower buffers.
Mind the pinouts, though.
2N3819 has the gate on the center pin, MPF102 and J201 have the gate on pin 3.
BF245A has gate on pin 1.

[meffcio]

2SK30

+1, it sounds really good to me.

[JDoyle]

At 9V and with a 10k source resistor, almost any JFET will work in a buffer/follower circuit. With higher voltages and/or lower source resistances one has to ensure that Idss*Rsource is greater than V+ or the circuit will prematurely clip when the JFET can't turn on any harder to continue following the signal on its input.

Regards,

Jay Doyle

[Cliff Schecht]

I've been meaning to try the U310 in some of my stompboxes. It's really an RF JFET (although so is the 2N5457) but IIRC the noise figure is something like 1.2 dB at the RF level and stays low throughout the audio band.

[zambo]

if you are near a radioshack they carry both mpf1o2 and j201 strangley enough. I know i was shocked to find them..

[caspercody]

I have used the BF245's in place of these Jfets. Just check the pin layouts.

[PRR]

> I've been meaning to try the U310 in some of my stompboxes. It's really an RF JFET ... IIRC the noise figure is something like 1.2 dB at the RF level and stays low throughout the audio band.

Try it; you have the insight to get it going.

> It's really an RF JFET

That just means there is a smaller device which is cheaper and good-enough for audio.

An RF device has a small RC product, various capacitances against channel resistance. In most audio, JFET capacitances are moot because swamped by wiring capacitances; we often ADD capacitors to limit bandwidth. Likewise in audio we only need small currents (and thus high resistances) or BIG current (to drive low resistances at high power, higher than jellybean sizes will carry).

So you pay too much. A penny. Maybe a dime. Possibly a dollar.

The penny or dime difference may only matter for thousands or millions; not in DIY. If you need less than a crate, someone's gonna charge you 2 bits for picking onesies/twosies from a box, and 2 bits more for the half-box that never gets sold.

U310 is 2 to 6 volts and 24 to 60mA. We often run audio FETs at less than 1mA. If a zero-bias U310 flows 24-60mA, we may have to smack it with nearly -6V to choke it down to 1mA. And we need nearly another 6V source-drain to pinch the channel, and some more voltage for load.... we're far above 12V needed which is awkward in 9V pedals.

> noise figure is something like 1.2 dB at the RF level

At what source resistance?? RF designers have the luxury of transforming to any impedance. At 105MHz, the ~~10pFd of input and Miller capacitance is near 170 ohms. While much of that can be tuned away, the useful source resistance can't be over 1K or so.

Guitar tends to be 5K in bass and ~~~100K up high. Volume and tone networks tend to 10K-200K. That's a different world from 1K and lower.

A specific DE-merit (if the specs are true) is the 15pA gate leakage current. That seems high for an audio FET. Some of that leakage appears as shot-hiss in the source impedance. At 1K, about zero. At 100K, maybe enough to hear. And maybe the 15pA is a CYA worst-worst-case reject spec.

For low audio hiss you want 1/Gm to be considerably lower than source impedance, and input current to be far-far smaller than signal current. Little JFETs do well with guitar sources. The U310 may do better for sources below a few K.

Switching? We rarely care about a few hundred ohms, so 35 ohms suggests a big die and higher cost.

Phase-shifter? Now the low ohms is counter-productive; we must use bigger caps. And in 9V work, weed-out the high Vgs(off) devices.

The min 24mA Idss suggests we could drive 600 ohm loads to over 10V peak (with 24V supply). There's other ways to skin that cat, but this can work.

[PRR]

A snippet from the Vishay U310 datasheet suggests why we may not want a high-current FET in most audio.



http://www.datasheetcatalog.org/datasheet/vishay/70237.pdf

U310 input voltage noise is near 4V/rtHz at 1KHz, around 550uV over 20KHz. Guitar self-hiss is over 1uV, generally closer to 2uV. This fat FET has no significant hiss advantage over a smaller FET (which may be easier to bias).

[goulashnakov]

if you are near a radioshack they carry both mpf1o2 and j201 strangley enough. I know i was shocked to find them..

Most Rattyshacks I've visited have MPF102's, but I've never seen one who'd carry J201's!  Indeed that IS a shock!  Must've been a decision by locale store management.  Good call on his/her part.

Also worth noting, an electronix store near my school had mostly NTE products, including a few of their own work-alikes for FET's.  NTE457 drops in for 2N5457.  NTE458 is (allegedly equivalent to) J201.  So, yeah, if you happen on a place selling NTE's, there ya go.