J112 j-fet n-channel alternative in SMD package for muting audio?

[Nrkb]

i'm working on this muting circuit shown at #2, works well with j112 in thru hole package but i need to make this design in smd layout. I found mmbfj112 which is smd but it seems not really available easily. What j-fet in smd package would be easy to find for the next few years for this muting circuit ? I don't really find one that has the same specs, i could modifiy some parts of the muting circuit to accomodate though if you have a better idea. Thanks! https://sound-au.com/articles/muting.html

[Terry Bites]

There is an sm version, MMBFJ112.
The MMBFJ108, MMBFJ111, MMBFJ112 and MMBFJ113 are all very similar.
Many jfets will work in this circuit as long as their Rdson is low enough compared with R1 and R2.

[Kleinstein]

The MMBF4392 is rather similar to the J112 with mainly minimal slower switching, but nothering to worry with audio. There are also BSR57 with essentially the same chip inside. The difference between J111 , J112 and J113  or the corresponding similars from the other series is in the threshold and this can make a difference. There is quites some overlapp and scattering. So some of the J111 and J113 would actually also meet the J112 specs, but not all.

[RoGeorge]

i could modifiy some parts of the muting circuit to accomodate though if you have a better idea. Thanks! https://sound-au.com/articles/muting.html

Have you tried using reversed BJT instead of JFET?

Now here's a very interesting set of curves from way way back!! This is a 2N3904 operating upside down or reverse, with the emitter and collector exchanged. So Vce is now from the emitter which is positive to the collector, and Ic now flows into the emitter and out the collector toward ground thru the current sense resistor which is 100 ohms.

Back in the very old days (60s) we utilized certain transistors upside down or reverse as analog switches because they had such a low saturation voltage when operated reverse.

https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4421512/#msg4421512

Learned about this only couple of days ago, should work but didn't tested myself for audio.
Note the very low Vce saturation voltage for the BJT (red trace is the Vsat of reversed Q2):

[dietert1]

Behringer had this in some of their audio devices. Disadvantage is DC coupling into audio path, i mean something that can destroy a speaker while turning on or off. For a redesign one can use an IC with multiple mosfet switches to save space.

Regards, Dieter

[Benta]

Using a JFET for audio muting is a very 80s concept.
Today, an analog switch is the way to do it.

[boB]

Using a JFET for audio muting is a very 80s concept.
Today, an analog switch is the way to do it.

Absolutely !  Muting via analog switch is great if it works.

The applications where I used to use a JFET for output muting was for reducing the "thump"(transient) from the turn-on of the circuits when the amplifier is already powered up to amplify that thump.  The JFET also does not need to have a split rail supply to handle the output swing if that might be necessary.  An output capacitor may also be needed for anti-thump apps.

JFETs are normally ON so they may fit this application better being used as a simple shunt switch.

An analog switch could be used as well either in a shunt configuration if it is normally ON or a series configuration if it doesn't have a power-up transient while the supply is rising.

I don't remember how well simple analog switches work for this application ?  Been a long time.

boB

[Nrkb]

Yes said that about the mmbfj112 in my post but it looks like it's harder to find in stock

[Nrkb]

ok thanks i'll check theses 2

[Nrkb]

I need it to be dc coupled actualy.
One could add some cap to block dc if needed isnt it?

[Nrkb]

You're probably right but the pcb is already very tight on space, i can't add an IC that needs other stuff around it to work.
Do you have a schematic idea you can link? Maybe i can consider it for future designs

[Nrkb]

yeah that's the point, i need to slow down the muting to avoid pops, i'm sure it can be done with analogue switch ic's but probably twice the component count and space used on a board

[Nrkb]

BSR57 seems out of stock everywhere probably not in stock anytime soon.
I'll try with MMBF4391LT1G

[3roomlab]

PMOS simulation back to back for sanity check
the NMOS works too (+ gate)
however MOS have Crss capacitance (larger than JFET), some AC signal may get thru
but then again, the JFET leak looks huge

[Kleinstein]

With the currently difficult stock situation a suitable way around is looking which JFETs are available in stock and than check, which of them are fitting. There are application notes about the JFET families (e.g. Fairchild AN6609) that list similar types. The J112 is process 51, which is a very common one. As a switch chances are the treshold is not that critical. The MMBF4391 still a relative high voltage (e.g. -10 V relative to the signal)  to be off for sure. So this can be a limiting factor. The lower threshold ones would have less trouble turning off, but slightly high R_on, which may be acceptable.

[SiliconWizard]

yeah that's the point, i need to slow down the muting to avoid pops, i'm sure it can be done with analogue switch ic's but probably twice the component count and space used on a board

It wouldn't need more components. Apart from a small bypass cap maybe. And you can find pretty small packages that wouldn't be larger than a JFET in SOT-23. Oh and the JFET would require a negative voltage to control it, could be annoying too. So uh, yeah. These days the real criterion if you're in a hurry is availability.

This is available in large quantities, in TSOP-5 package:
https://www.mouser.com/ProductDetail/onsemi/M74VHC1GT66DTT1G?qs=3JMERSakebp33l%252BR2qIiZQ%3D%3D

Just a thought.

[Nrkb]

That switching from this IC can be slowed down to minimise pops and clicks occuring when it switches? It's pretty audible when switching low frequencies sinewaves on and off

[Terry Bites]

I love the 80's

[edavid]

BSR57 seems out of stock everywhere probably not in stock anytime soon.

It's actually very well stocked for a JFET - it actually has 2 good sources - why don't you buy it from Arrow?

https://www.findchips.com/search/bsr57