speaker thump /amp-on question


Better still. Leave the power amp on all the time. Then you will have not more thumps.

As long as the amp has no valves and it's not Class-A, its consumption will be very small.

Als many people report improvements in audio equipment after being on for a long time, so you might have another benefit.

My power amp has on for the last 15 years or so, only being off when the light company cut off the energy for any reason.

There are 2 causes for this loud thump.

1) Sparking caused by main power switch of power amplifier.

Every time the amplifier is switch on or off, a spark jumps across the gap. It can get pretty loud, sometimes loud enough to make you jump out of your chair if you are not expecting it.

2) DC offset at the output of your power amplifier.

This is a much softer thump.

Let us deal with no (2) first: DC Offset.
Disconnect all speakers from amp.
Place probes across amp + and - outputs.
"On" power amp and note reading of voltage. Check for other channel as well. "Off" power amp immediately.

Caution: make sure your red probe is not touching the -ve terminal or the chassis. If this happens, there is a very high likihood that your amp will be destroyed.

Your readings should be below 100mV. Preferably below 10 mV.
If above 1V, your amp is faulty.

As Saakra mentioned earlier, an output relay with a delay will eliminate this sound on power up. I would like to add that this does not solve the problem of DC Offset at the output. It only removes this annoying sound.

We now proceed to cause no(1), which is a very loud thump caused by sparking on the amp power switch.

What I am about to suggest is going to:
1)void your warranty, but more importantly,
2)expose you to the dangers of live 240Vac. If you are not well versed in mains electricity, do not attempt this. Electricity can kill. Its called "electrocution". Get a friend who is qualified to do this if you are not.

Step 1)
Unplug the mains power cable of your amp. Do not just switch off the wall supply. Pull the plug out of the socket.

Step 2)
Open the chasis cover of your amp. At the back of the front panel you will see the power switch. Hopefully, there are only 2 cables soldered to it. This is the mains "LIVE" wire. Check whether there is something connected across these 2 cables. I am refering to the point where the cables are soldered to the terminals at the back of the switch.

If there is nothing there, you have identified the probable cause.

Step 3)
Get hold of one of each of the following "X2 Mains Suppression Capacitor": 472 - 240VAC
103 - 240VAC
104 - 240VAC
Note: Make sure these are "X2" caps. Only "X2" caps are approved for use across mains.

Step 4)
Solder 472 across the 2 cables at the back of the amplifier power switch. Double check that 472 is not shorting to chassis or any other things.

Step 5)
Close cover of amplifier.

Step 6)
Connect one speaker to one channel of your amp.

Step 7)
Make sure your amplifier power switch is in the "OFF" position.

Step 8)
Plug the mains power cord to the wall supply and switch on wall mains.

Step 9)
Switch on power amp with switch at front panel. There sould be very little sound on switched "ON" and "OFF".

If you wish, you can try 103 and 104 to see which value gives you the softest sound on power "ON".

Hi fcel

You can find the part in mouser.


Mouser part no for
472 = 75-F17723472000
103 = 75-F17723102030
104 = 75-F17724102000
They are Vishay "X2" Caps rated at 275V, tolerance 10%.
Will definately work for 120V.

Yes, you can use for pre-amp or any other appliance, as these are mains suppression caps often used to suppress sparking at power switches.


about leaving the amplifier on all the time!

15 years x 24 hours x 365 days=131400 hours

tipical lifetime of a computer grade electrolytic (in Components and Materials Handbook of phillips) is +- 10.000 hours...

food for thougt!!!


Jorge Santos
Component lifetime is a estimate taking into account turning off and on , but the life time can be far greater when left in use. A good example of this is a light bulb that has been on for over 100 years ..
Thats good too, but its dangerous that because your house can be set on fire because of malfunction in amp..
Seriously doubt it. That is why it has an AC fuse , rail fuses, and usually a speaker fuse. It is not like cheap Li-Ion battery chargers which do have a bad reputation for fires. Walk around your house and count how many things are always on. TV, microwave, furnace. Everything with a remote control. How many wall worts? Better yet. when you think everything is off, go look at your electric meter.

Some amps do not have an output relay because slick-page reviewers claim they are audible. Others because they are cheap.
You can get output relay module, some with DC detection, off e-bay. A mains suppression cap is always a good idea. I use old 600V .01 ceramic disks. They predate the X2 rating. Most amps will still thump and it is not due to a small DC on the output, but how everything gets to operating parameters as active devices all have thresholds, both sides of the amp don't come up at the same time.

Rocket, that bulb in a fire house is a bad example. It's "on" is a dim yellow and in bulbs at low temps, they don't outgas much. So lack of thermal shock is a key reason it is still on. Not a good comparison to an amp. An amp, being on and warm 7 x 24 may dry out the big electrolytics faster so a shorter life span. But thermal shock is not good for transistors, so you really can't guess.


diyAudio Moderator Emeritus
2002-09-20 7:20 am
Simple case of omitted standard speaker relay. Many if not most amplifiers have non ideal start up and shutdown behaviour that results in a short but annoying DC pulse on the speakers. Good designers prevent the power on/off plops with a relay in the speaker outputs that switches delayed at power on and switches off very fast at power off. This was eighties design standard in 95% of Japanese amplifiers that were dead silent at power on/off AND did not blow up speakers when they died as the relay circuit was often done with an IC (the µPC1237) that also measured DC at the outputs. So, as a user, you never had your expensive speakers going kaputt because of either power on/off plops or the amplifiers having a very bad day. This was all standard and working OK more than 40 years ago. This seems to be forgotten as many 2022 amplifiers "fart" which apparently is appreciated, have severe power on/off plops and when the famous designers baby has a very bad day it will burn your loudspeakers as well. You can't stop progress :)

So it makes some sense to add the omitted speaker relay. If you also want DC protection: the good old µP1237 IC's are still produced. Most simple way to have it all: silence and safety. Make sure to choose a relay with silver or silver containing bifurcated contacts and not the often made mistake to choose gold/gold plated contacts. There are many of such designs available as kits or ready made PCBs so the challenge is to find a quality one that fits physically in the amplifier. In the case of buying cheap Ebay/chinese stuff then at least make sure to buy a branded relay for the Chang Hui relay and use quality electrolytic caps. Some of the cheap devices use 2 relays so each channel has 2 x bifurcated contacts in parallel mitigating any (exaggerated) negative side effect.


BTW it is OK to use the correct name/type of the device in question. You seem to have a Roksan Caspian but which one? Here a picture of a Roksan Caspian M2 that ... has the relays. Schrack relays even, one of the better brands.

Question: what do you think will happen when that amplifier is fitted with a speaker relay and you switch it on before the preamplifier is switched on?


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