Choose JFET for Audio Amp Design: J112 vs J113

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When designing audio amplifiers, the choice between J112 and J113 JFETs depends on specific circuit requirements, particularly current handling and gate voltage sensitivity. The J111 is noted for having a higher Idss, which may be advantageous if higher current switching is needed. Adjustments to biasing and compensation are essential when substituting JFETs in existing designs, especially when increasing the current to around 2mA. Some users have reported better sound quality with JFETs compared to BJTs in audio applications, although JFETs are becoming less common in modern designs. Experimenting with different JFET models, including dual matched pairs like the LSK489, can yield beneficial results in audio amplifier performance.
yungman
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I am designing audio amplifiers, I am not familiar with JFETs, how do I choose between J112 and J113

https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-jfets/288?k=jfet&k=&pkeyword=jfet&sv=1&pv1183=u35V&sf=0&FV=1140050%2Cffe00120%2C9780001&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

My original design using BJT is running 1.2mA collector current, which one will fit better?

Also if you get the datasheet of J111, from Fig.5 in the data sheet on input capacitance is lower the more negative the Vgs, meaning choosing J111 is better than J112 as Idss is higher with J111. Is that true?

Thanks
 
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yungman said:
J111 is better than J112 as Idss is higher with J111. Is that true?

It is capable of switching a higher current. It is only "better" if you need that parameter to be larger. If you need it to switch with a lower gate voltage than it is "worse" than the others.

yungman said:
which one will fit better?

Can you share a schematic?

BoB
 
rbelli1 said:
It is capable of switching a higher current. It is only "better" if you need that parameter to be larger. If you need it to switch with a lower gate voltage than it is "worse" than the others.
Can you share a schematic?

BoB
This is the typical "Blameless" front end, I am just going to substitute Q48 and Q49 with jFET to see what is the difference in the sound.
Blameless JFET.JPG


I just change R90 and R91 to very low value like 5.1ohm to get the gain back. Q48 and Q49 are matched and wrapped together by copper tape to track temperature to reduce offset drift.
 
Last edited:
Your biasing will need to be adjusted.

@berkeman can you help here?

Thanks

BoB
 
rbelli1 said:
Your biasing will need to be adjusted.

@berkeman can you help here?

Thanks

BoB
Yes, bias, compensation all needed to be adjusted. At this point, I just want to pick the best fit. Most likely I will increase the current to about 2mA through each of the JFET.
 
rbelli1 said:
Your biasing will need to be adjusted.

@berkeman can you help here?
Sorry, I've only used JFETs for one product design, and had so-so luck with them. I haven't used them since.
 
berkeman said:
Sorry, I've only used JFETs for one product design, and had so-so luck with them. I haven't used them since.
I know, JFET is dying...if not dead already. Only place I've seen using is the first stage of audio amplifiers. I tested opamps with JFET input differential pair vs BJT, it seems to sound a little livelier. I've seen some high end audio amps using JFET as front end also, I just want to try it as I already design the pcb, it's easy to just replace two transistors and listen to it.

Anyway, I ordered all J111, 112 and 113. I just going to try it.
 
Did you ever get it working?

BoB
 
You might want to consider dual matched, very low noise pairs like the LSK489 series.
 
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