need simple anti-thump


i'm building a medium-sized power amp & don't need a complex relay-type of anti-thump.

Years ago I saw a simple circuit using a fet on the very input of the power amp; I recall that this device momentarily shorts the onrushing turn-on votage, then from here on it doesn't count.

Anybody seen this circuit?

I think it's also possible to extend this circuit to double as overload protection for the input.

current limiters

Some people use current limiters, sold by GE as CL-## where the number refer to different resistances. See the datasheets on These are thermistors that decrease in resistance as they get hot. The GE originals are black. The oriental manufactured similar devices (found in the inputs of switching power supplies like PC power supplies) are shaped like a disk capacitor and are green. I found a 2.5 ohm resistance device that goes to 1.7 ohm in front of a hair dryer, in a 450 watt PC power supply. I just installed a CL-90 with over 100 ohm cold resistance in series with the power transformer of a mixer preamp, fused for 1 amp but probably using a lot less. It cut the turn on click I was experiencing if I turned the mixer on before the power amp. Some people, including the power supply manufacturers, wrap these devices in plastic to ensure they don't go cold if the music goes soft (on an amp).

The circuit I want comes between the input cap & just before the input darlington pair.

I have just built the classic 1977 NS/Hitachi mosfet amp using a 2sk1058 & 2sj162, to 50 watts. I used the usual permanent ink to hand-draw the PCB artwork & hand-etched the excess copper.

I found that 2n5401's & MJE 340/350's work as well besides the 2sa872 & 2sd756/sb716.

I still can't upload the PCB design so others can try their hands but I'm working on it.


2sa1302? I thought they's been discontinued cuz of fakes from the East!. Years ago I upgraded a friend's power amp by putting a 1302 & it's complement rather than replacing the broken ones which were not as good as these.

I built one anti-thump from Slone's & I might try the one from Self. But this morning I looked at my old book the Electric Guitar Handbook & I found one circuit with the fet anti-thump, by Gibson.
On/off auto mute

The circuit is attached in this post.
Watch to both 1N4007 diodes. They have to be connected BEFORE the bridge, not as is in this drawing.


  • APEX turn on-off mute.jpg
    APEX turn on-off mute.jpg
    172.4 KB · Views: 324
Hello, has anyone got any new ideas on this?

I have a new design/idea. R2 can be a multi-turn trimmer that controls the turn on delay. R3 is the coil of a reed relay (for preamps - audio signal with low voltage/low amps) or relays (for speakers). R8 is to discharge C2 when power is off.

I have thought, and have seen, ideas using transistors instead of relays for protection, but have found that relays are the best solution to really mitigate or eliminate anti thump. My opinion, and I have used this solution and it works fine.

Muting with delay w/ Reed Relay 1 - Multisim Live


  • Muting with delay w_ Reed Relay 1-schematic (1).png
    Muting with delay w_ Reed Relay 1-schematic (1).png
    157 KB · Views: 170
I ended up building the attached for a preamplifier. 15 Vcc power supply. Relay has two NO and two NC contacts. I setup the L and R channels connected to ground when NC. It works fine. This circuit consumes less than 0.5 W. Delay is approx. 3 seconds. C2 is 1uF WIMA film. C2 and R2 control turn on delay.


  • Muting with 3s delay w 960 Relay 12Vcc.jpg
    Muting with 3s delay w 960 Relay 12Vcc.jpg
    132.7 KB · Views: 120
Jhofland designed a very nice anti thump (2 second delay) and speaker DC protection like this. The anti thump was an RC delay (R108/C108) which trips a comparator to activate an optoisolator that drives a pair of low RDSon MOSFETs as solid state relays. The whole thing operates from the +ve rail of the amp through a voltage regulator to provide a fixed 15v. So any voltage 18v up to 100v can work. Circuit draws 20mA in idle mode.


Works very well and simple to make.
Last edited: