Hear a thump when turn amplifier off!

[lanchile07]

Hello! I just build my second GC LM3875.I used the best parts as possible.the result was a death quiet amplifier,excellent sound.I am very happy with it but. I was worried about a thump in my speakers when I turn unit off (only when I turn it off).and I started looking for an answer.and I found it in the "Adcom" site.
this is what they say about their amplifiers:

There's a "thump" when I turn off my system.
There are usually two reasons for this: The amp has a large power supply and it doesn't use output relays. Let's look a bit more closely at both.

First, a power supply's job is to filter and store energy until the amplifier circuitry needs it. Adcom amps have HUGE power supplies. That's part of the reason they sound as good as they do. But when you turn the system off, that energy has to go somewhere. The "thump" you occasionally hear is the power supply capacitors dissipating the energy. It's not dangerous at all. In fact, it's just another sign that the amp won't run out of steam when you really want serious sounds.

Yes, we could have used output relays that would disconnect the speakers as soon as the amp turned off. That way, you would never hear the thump. But relays, as we've already seen, aren't great for real high fidelity sound quality. If we used them, you would never hear how good our circuitry is either. Is that a sensible tradeoff? We sure don't think so.

Look at it this way: The "thump" is the same thing as a high performance automobile's rumbling exhaust. The sound alone tells you there's something serious under the hood, right

well,I do not Know if this is normal with these GC LM3875.
any comments guys?.

[Dan2]

sounds to me like you have answered your own question.

But what if (hypothetically) u put relays between the power supply caps and the amp?? You could put resistors to ground so the caps discharge through the resistors instead of through the amp..

BTW this is just a question, i have never done that myself

[AndrewT]

Hi,
you could turn the mute pin off instead of disconnecting the speaker. Does the 3875 have a mute pin?
Alternatively, a fast discharge of the PSU caps using resistors can reduce the turn off thump.

[Nuuk]

Does the 3875 have a mute pin?

No.

I am currently using a four channel GC with dual mono PSU, ie one PSU for each pair of channels. One 'side' is fairly quiet when I turn on or turn off power, the other isn't. The only difference between them is the power switches - single pole on one side, double pole on the other.

[Original Burnedfingers]

Yes, we could have used output relays that would disconnect the speakers as soon as the amp turned off. That way, you would never hear the thump. But relays, as we've already seen, aren't great for real high fidelity sound quality.

You will have to prove to me that relays have an effect on the sound. Some of the best amplifiers made use relays. To be perfectly honest here....the chip amplifier while sounding ok isn't the greatest toy in the box. Its quick to make for most people and tends to give the owner that feeling of accomplishing something. I made one just to see what all the excitement was about and my personal opinion is that its about as good as any used $200 amplifier off ebay.

[d3imlay]

What are the power supply rail voltages? There isn't a relay around for less than $100 that can reliably break more than 50VDC under any kind of load.

[AndrewT]

Originally posted by d3imlay
What are the power supply rail voltages? There isn't a relay around for less than $100 that can reliably break more than 50VDC under any kind of load.

that isn't the problem this time.
At switch off the input signal can be muted, without any effect on the sound quality during normal operation.
The signal that needs to be broken at the output should just be output offset + noise + hum and this should be less than 100mVpk for all worst case conditions.
If the chipamp went into a failure mode that made a direct short from supply rail to output then the supply rail fuses should rupture.

[lanchile07]

"Yes, we could have used output relays that would disconnect the speakers as soon as the amp turned off. That way, you would never hear the thump. But relays, as we've already seen, aren't great for real high fidelity sound quality".

You will have to prove to me that relays have an effect on the sound. Some of the best amplifiers made use relays. To be perfectly honest here....the chip amplifier while sounding ok isn't the greatest toy in the box. Its quick to make for most people and tends to give the owner that feeling of accomplishing something. I made one just to see what all the excitement was about and my personal opinion is that its about as good as any used $200 amplifier off ebay.

Well,if you read on my post well .this post is from "ADCOM" not from me.and the thump is not so loud.

[megajocke]

Is it a snap the moment you turn off the switch or is it a thump some seconds later? If it goes snap when you turn off the switch an X-rated capacitor and/or varistor across the transformer primary might help.

[lanchile07]

Well,the thump is four seconds after I turn it off.but it is in only one channel. I checked all the connections and they are all right.there is no hum or any kind of noise at all.the amplifier is clean all the way.but the thump is what I am worry about and again it is in one channel only.I change the speakers but same thing.can it be the chip?.