Zen amp turn-on thump

get a 10 second time delay relay rated at 10 amps or more and a 50 watt 5 ohm resistor. wire the resistor in series with the power transformer primary and use the relay contacts to short out the resistor after the 10 seconds are up. This method was used back in the 50's to reduce the effect you are having. If the thump is still to much try using a higher value resistor like 15 ohms.
I'm building this amp now.

I'm not really worried about the turn on thump damaging the circuit, just my speaker.

It looks like you could short the output of the amp for 10 seconds. Assuming the circuit doesn't care that you're shorting the output.
Then you don't have that ugly relay circuit in the path after it opens.

What do you guys think?

The time delay relay circuit will both protect your speaker and amp. If you want to use a thermister instead of the resistor go ahead. The relay contacts that short out the thermister/resistor keep the line voltage loss to zero. The theremister will keep droping the line voltage like a low value resistor which it of course it is. Thermisters that will run hot all the time like in a Aleph will fail in a few years because their on resistance will gradually get higher as they break down from each heating cycle.


Paid Member
2000-10-10 7:27 pm
That's right. There would be loss. I guess that's why they (thermistors) are in parallel? To reduce the resistance?

Is there really ZERO loss when using a relay?

What kind of relay? DPDT, SPST? Can you recommend a brand or place to look for one?

I'd like to try both methods.
Use a amperite # 115NO0B relay available from Newark Electronics as cat # 46F7518. The coil is connected accross the switchred 110vac and the contacts connect across the resistor/thermistor. The relay contacts would have a resistace of less than .1 ohm vs the thermister having several ohms when at opperating temp. Also see my thread conserning lightning protection.