Universal Amplifier Soft-Start Power Supply Board


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There are lots of soft-start power supply boards out there. Most of them are kind of cheap and crappy, so I developed this versatile soft-start board for my own amplifier projects. It will work equally well for non-audio projects with large transformers.

Uncommonly, it provides transformer primary AC connection terminals right on the board. Most soft-start boards only switch the incoming mains AC, and from there you must break out to a terminal block for the primary connections. Conveniently, this board provides terminal connections for up to four 120V primary coils. (Up to four transformers, or two dual-primary transformers. Other configurations are easily hacked.)

Soft-start action is provided by a NTC thermistor in series with the mains AC, that is shorted out after a delay of 1.1 seconds.

Hand-assembled and tested, by me, in Madison, WI. All parts are chosen for maximum reliability from premium brands like Panasonic, TE, WAGO, TRIAD, TI, Dale, Ametherm, etc.

  • Up to 2000VA worth of transformers
  • Connect up to four 120VAC primary coils. Jumpers set 120V/240V operation. Since most transformers have two 120V primary coils, this typically means the board can handle up to two power transformers, which is perfect for dual-mono amplifiers.
  • Thermistor is a huge, high-quality Ametherm MegaSurge 32, for maximum over-kill. The default 10-ohm value may not be optimal for your application, so you should test the charging time-constant of your particular transformer and capacitor combination, and install a different value thermistor if needed, or adjust the time-constant of the 555. The relay should click in after the capacitors are charged to at least 2/3 full operating voltage. I can supply a 10-ohm or 5-ohm thermistor, please specify when ordering. Other ohmic values on request.
  • Soft-start delay circuit is based on the good-old NE555 timer. Delay time is easily customized by changing either R_TC or C_TC. Let me know if you want something other than the default 1.1 seconds.
  • For long service life, the power supply is a traditional transformer, bridge and filter cap followed by a linear regulator. (I’m aiming for 30 years lifetime or more.)
  • WAGO Cage-Clamp terminal blocks provide a firm, gas-tight connection that will never come loose. (Accommodates up to 12ga solid or stranded wire)
  • 30A bypass relay made by TE Connectivity
  • Surge protection MOV
  • 0.1uF X2 Line-snubbing capacitor across mains input
  • 10nF spark-arrestor capacitor across switch terminals.
  • Includes (4) 12mm metal stand-offs, and (8) M3 screws.
  • Board uses double-thick 2oz copper. All AC connections use super wide traces for minimum resistance and heating, sometimes employing both layers of the board.
  • For safety, a minimum of 2mm clearance is maintained between all live AC traces and other conductors. AC and DC sides of the circuit are isolated by milled slots in the PCB.
  • Power LED terminal block provided. Lights only when relay is engaged.
  • Requires a high-current switch to be installed in amplifier front panel. I recommend paralleling both poles of a 16A DPST toggle or rocker switch.

NOTE: You must provide your own AC mains fusing ahead of this unit. This fuse should be as close to the power inlet as possible, preferably integrated into the power inlet itself. For safety, I emphatically recommend the use of an earth-grounded chassis. Rod Elliot has an excellent article on the subject, and also describes methods to avoid the noise commonly associated with 3-wire components.

Maximum Transformer Size – ~2000VA

This is a conservative rating based on tests of several combinations of power transformers and capacitor banks. The largest configuration I tested was 2500VA charging 120,000uF. (Two 1250VA transformers from Adcom GFA-565’s charging four 35,000uF capacitors to 85VDC.) During testing of this configuration, the thermistor withstood 50 cycles of charge/discharge without damage or overheating. I also simulated some worst-case scenarios, such as the power cycling on and off, while the thermistor was still hot from the previous power-up.

Fault-Tolerance scenarios:

The soft-start circuit is designed to cope with fault conditions such as a brown-out. It will operate correctly down to 70VAC without relay chattering. Relay cycling is limited to 1.1Hz, the time-constant of the 555. If power is interrupted for more than about 100ms, the relay will click out, and the soft-start will engage again after 1.1s. If the relay should fail, then the thermistor will simply heat up and become nearly zero ohms, and the amp will continue to operate, though with an undesirable, variable, small resistance in series with the mains. The operator will be alerted by the absence of relay clicking. If the thermistor should fail open-circuit, the soft-start circuit will not operate, and the relay will not attempt to close the connection. If the thermistor should fail short-circuit, then the power switch will take the brunt of the switching, and will arc and probably burn out after some number of cycles. But it should be obvious that the soft-start is not working, as your lights will be dimming much more than when the soft-start was operating. And if by some weird chance the thermistor were to fail half-way and become high-resistance but still conductive, then the soft-start circuit might still receive enough current to operate the relay, which would then be asked to take the full brunt of the in-rush current. It’s a 30A relay, so it should be able to tolerate that for some time.

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