Short of sand

Firstly, my apologies. It’s been long overdue to get back on this blog. I receive several emails from people asking me to share more. I hope I will, when I get the time and energy to get back on hi-fi audio projects. I still work a lot on synthesisers which keeps me very busy when I catch a glimpse of free time from my hectic day life.

We’ve all been hit by the short of semiconductors. Yet, the HV precious parts are in shortage as well. Luckily some of the key parts like LDN150 and IXTP08N100D2 are back in stock, not sure for how long. These are key parts for building any CCS or hybrid mu-followers for audio circuits.

I received an email yesterday asking whether we could use the 2SK2394D as a replacement of the belated BF862 for low current anode loads. On this email, Pawel shared also a noise measurement made by a DIY Audio member (Sampler) who have done a fantastic comparison shown below:

© by Sampler – extracted from DIY Audio

Nice to see the performance of this jFET overall. Its specifications are similar to the 2SK3557 which I suggested as replacement to the BF862 years ago. The 2SK3557 seems to have slightly higher gm, and noise figure is similar to the 2SK2394D.

The CPH3910 has double NF, which can be seen on the measurement as well. Yet, is available and a good option for a preamp stage. Obviously, for a RIAA stage, I’d use either the 2SK3557 or the 2SK2394-D.

Both devices are drop in replacements for the CSS or gyrator PCBs. You will always have to add the protection zener on-board between drain and source to ensure the devices operates within the maximum VDS voltage levels.

For those who have missed my ETF lecture from 2018, here it is an extract on this topic which is very relevant:

Over the last years, I experimented extensively with different FETs in the hybrid-mu follower circuit. As there are very few through-hole (TO-92) jFETs still available, I also explored other options including newer enhancement MOSFETs with high GFS and low CRSS which proved to work really well as lower devices of the Cascode pair.

I’ve used extensively the BF862 for preamps for currents under 20mA due to the IDSS limitation. The BF862 is now EOL, so there are other similar options like the 2SK3557 and CPH3910 which work and sound very well.

For currents above 20mA (e.g. 4P1L) I use the BSH111BK or BSN20BK which will have at least 200-250mS of transconductance above 20mA.

For the top device, I found that the ubiquitous DN2540 isn’t the best choice for this position. There are other depletion MOSFETs in the IXYS IXTP family that have a higher VGS (off) level which is normally above -2V. On the right curve tracing example you can see the difference between these MOSFETs. Either way, we need to be careful here as MOSFET parameter variance is significant – VGS(off) being one of those parameters. For example the DN2540 datasheet quotes a 2V range between – 1.5 to 3V for VGS(off). On the other hand, the IXTP3N100D2 has the same 2V variance but from -2.5V to -4.5V!.

Ideally, you would like to manually select them by testing their VGS(off) level.

From “DHT Preamplifiers”, ETF 2018 lecture by Ale Moglia.

Luckily we have some sand for a bit more time 🙂

Enjoy