Update March 16, 2020: final version 2.3 shown in first photo uses three NTC’s rather than resistors to provide thermal safety in case SSR fails.

This is a bare PCB for making the SFP soft start circuit. Photos show assembled unit (user needs to provide parts and assembly). Uses ultra low RDson MOSFETs for switch relay with only 22mOhm resistance and capable of switching 27A at 600v. During initial turn on, a bank of three power NTC thermistors (10ohms each for 30ohms in series) limit current flow. After selected ramp up time (0.5sec, 1sec, or 2sec) the SSR bypasses the NTCs for full power. The SSR will provide up to about 0.7 to 1v more headroom from your amp’s PSU as there is no loss through the traditional NTC when hot still have about 0.5ohm resistance (CL-60 etc). You get a little more amp max power. At turn on and turn off, a voltage supervisor IC quickly issues an open collector signal to use for logic controlled SSR for speaker protection (optional use). Circuit is state of the art design using Toshiba opto isolator to drive the special low RDson MOSFET. All easy to assemble thorough hole parts except for simple 4pin optoisolator and 3pin voltage supervisor. Both are exposed pin SMT and can be hand soldered easily. More info here:

https://www.diyaudio.com/forums/group-buys/350441-soft-feather-pillow-sfp-ssr-soft-start-circuit-gb.html

Boards will be 1.6mm thick with 1oz copper and ENIG finish. Sold as bare PCB each.

This is a DIY circuit board that is connected to AC mains. User accepts all responsibility for use and proper precautions and experience are required to complete and connect the circuit to the amplifier and AC mains. Be sure to follow all applicable local electrical codes. If you are not familiar with connection of circuits with AC mains please consult a qualified electrician for installation. The MOSFETs need to be covered with shrink tubing or other insulator to cover the exposed metal tab which will be connected to live mains AC.

Update Mar. 18, 2021 - there is an alternate MOSFET for the SSR here that’s a lot less expensive:

https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/FCP067N65S3?qs=%2Fha2pyFadujXwI1px%252BYuwuaZKDDXAVpY0IGGVx4nLsFFbw2cmIc8hw%3D%3D

Note that selecting “Prepopulated SMT” option means that the two SMT parts will be installed and you still have to provide and install all the Through hole parts. This is meant as a tune saver for those not interested in doing SMT soldering.