Strange sounds when power down gainclone


2007-11-16 3:38 pm
Hi all,

I am getting a very strange sound on one speaker channel of my stereo lm4780 when I power it down. It almost sounds like a ripping sound that is drawn out for about a second. I reconfigured my ground setup to a better star ground setup but am unsure if the strange sound was happening before or after i did that. Anyone know what is causing this and how I can fix it??

Hello synthius, if I understand your description correctly, it sounds kind of like capacitor discharge. I would think you would hear it in both channels. If the amp doesn't have any output muting, then when the amp is powered down, the caps discharge and cause that sort of sound in the speakers. My Adcom GFA-6000 does that when I shut it off. It's not something to worry about unless it gets really powerful. Otherwise it's just a nuisance. Remember, I said if I understand, I'm sure that others with more experience in amp design in general and chip amp in particular will offer other thoughts.




2007-11-16 3:38 pm
Thanks for the response dave. As long as its not something that will damage my speakers I guess i can forget about it. It would be nice to know if there was a resolution as this is my senior project and I don't want the professors to hear that sound when i turn off my amp at the end.:cannotbe:
Hello Synthius, Since this is a senior project, you should take some time to try and diagnose the source of the noise. That will look real good to the professor evaluating you for a grade. A start would be to get an o'scope and hook one channel to the output of your amp, in parallel with your speaker. Hook other channel(s) to the power supply cap(s) and watch both signals when you turn off the amp. This will give some idea if I am correct or talking gibberish. A 4 channel digital storage scope would really be good here. It is somewhat interesting that only one channel in your amp does this. Do you have separate filter caps for each channel?



P.S. The reason I suggest doing actual measurements is that it shows you have good troubleshooting skills and can reason things through. A lot of teachers will give you a better grade for demonstrating that you know how to use what you learned, not just take tests.;)


2007-11-16 3:38 pm
hey guys,

The strange sound is actually coming from both the left and right channel of the lm4780.

I will try putting the resistors from input to ground and get back to you on the results

Also, can someone explain the purpose of RB1 and RB2 on IC1? Are they used to set the input impedance to ~1kohm?

Thanks for the help
the 'typical application' shows only a rudimentary circuit.
Better to use the 'auxiliary amplifier application circuit' for a standalone amplifier.


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One of my LM1875, maybe the 3rd one I made, has a curious noise. Instead of the usual bit of static, this one waits. . . It bides its time. . . and just when I'm sure its not going to do it. . . "Ker Boing!!!"
That scares the daylights out of me. I think its hilarious. That's the one I made with antique parts just for the fun of finding out if there's a difference. ;) Oh yes there is.
In most cases, shutoff noises are grounding or power. In my case, its this: [IMGDEAD][/IMGDEAD]

Greg Erskine

Paid Member
2002-01-05 11:56 pm
Good one daniel. :D

hi synthius,

Usually strange noises after powering off the amp are the PSU filter caps discharging. You can use bleeder resistors across each filter cap to give them an alternative path to discharge.

Also, these chips have a mute feature. So if you wanted to do it properly, you should mute the chip before turning off the power.



2007-11-16 3:38 pm
Hello Synthius, Looking at your schematic, it might help to figure out a way to use the mute function, this should eliminate any noises like you are hearing from getting to the outputs. I still think it would be good to diagnose the source of the noise, college professor like that.

Yes, I will have access to an oscilloscope tomorrow and will see what is exactly going on within the circuit when powered down. I'm hoping for a quick and dirty fix to this as I am running out of room within my enclosure for additions and It would be rather difficult to modify my setup as it is. Everything is screwed down and is not the easiest to remove without removing many other boards.

Maybe a double pole power switch--one side cuts power, while the other side cuts power to the mute circuit (thus engaging the mute switches in the chips)? Edit: This is also possible with a relay.

I'm always curious when I see a fan in exhaust position. I want to know if there is an air intake. Is that located underneath the heatsink?

Hey, anyway, if you want more room, then that heatsink could be located onto the exterior of the enclosure where it wouldn't need a fan.

Althogether, that's quite the impressive project.
synthius said:
just noticed the bleeder resistors are not shown in the schematic but they are in the circuit

Hey, it looks good. I see KBPC bridges. You sure did your homework. ;)

Actually, I'm extremely jealous about homework that requires jamming out at 3 a.m. Wow!!

Anyway, there's an exhaust fan, so are there an air intake holes underneath the heatsink? I ask this because I'm a computer network guy (before I got tired of it and quit) and I've seen so many computers with an exhaust fan, yet no possibility of cool air entering the enclosure. Oops, stuck computer. ;)

By the way, CoolerMaster (availability makes incredibly quiet fans, including the 1800RPM model with blue LEDs for fun. Quiet fans are good for when you don't have temp based control.

Other way, could use a super-fast fan like Zalman 3k RPM, two bimetallic springs and two power resistors, corresponding to off, low, high, based on temperature. Analog power makes buzz-free fan. Inexpensive bimetallic springs with connection terminals can be found in inside attic ventilator fan thermostatic activator boxes, at the hardware store. That's even more fun with LED fans because the light level changes on temperature.

On the speaker output for the subwoofer channel, you might want to hook up both speaker terminals, so to avoid a do-nothing connection on the back panel. Just claim that it can run 1 or 2 subwoofers.

Thinking about that front panel, you could make an addition on front, like a bit of thin Birch panel or veneer, sanded with #0000 steel wool and sprayed with Rustoleum Clear Lacquer (several very, very thin coats) should come out about the same color as the creamy color steel, but much extra "zoot" factor. ;) There's a lot of other similar color materials at the home improvment store or hardware store. Actually, any "warm" color could potentially work in harmony. Just preview before you glue.