Another Finished LM4780 GC

[hongrn]

Hello Guys,

I just finished putting the GC together. Sounds great, lots of power (about 100 watts per channel). However, it's humming (in more ways than one). It's dead quiet when turned on, with the speakers connected. When I connect a source to the RCA jacks, such as a CD player, I get a low hum through both speakers. Disconnect one channel (either left or right) from the RCA jacks, and the hum stops. It only hums when both channels are connected. Any idea why? Thanks.

Hong

An externally hosted image should be here but it was not working when we last tested it.

 

[hongrn]

Everything is properly grounded.

An externally hosted image should be here but it was not working when we last tested it.

 

[Evilsizer]

i take the hum is there with the volume all the way down? how bad is it with your playing music?

 

[hongrn]

i take the hum is there with the volume all the way down? how bad is it with your playing music?

I don't have a pot connected to the amp yet. The hum is there as soon as I connect the CD player. May be I should wait for the Alps pot to arrive...

 

[kittikun]

Hi Hongrn,

Do you connect the groud of two supplies together and also groud at both RCA? They have to be connected, otherwise you will get the hum. Try and let see. Hope this help.

Kittikun

 

[youyoung21147]

Good looking amp ! Is it bridged or paralleled ?

Maybe the grounding should be done this way : put the RCA ground on the chip's ground pin, ground everything that is related to small signals (feedback, input resistor...) on this pin, then link this pin to the PSU ground. I may also suggest using a 3ohms resistor to connect the case to the ground, instead of directly.

Hope it helps

 

[fcel]

I have seen the attached GC grounding diagram. May be this will help?

 

[jackinnj]

It would help if the input weren't so close to the transformer. Torroids do radiate.

 

[Upupa Epops]

To fcel : This wiring diagram is wrong...I see there several ground loops...I'm sorry...
To hong : Input wires you must twisted together, mainly if you lead them along the trafo...
In sumary : When I see this multipcb koncept, I can cry... I'm sorry authors...

 

[fcel]

Upupa Epops,
I personally have not built a GC ever. The wiring diagram is wrong, you said. Then I better delete that diagram from my hard drive.

 

[Peter Daniel]

Forget your star ground.

Connect the grounds of both amp PCB's with heavy gauge wire. Run grounds from rectifiers boards directly to the point central on that wire.

Run signal grounds directly to the boards. I assume each rectifier board serves one rail.

This should fix your ground problem.

 

[Upupa Epops]

Peter, isn't better to make PCB " all in one " ? Or do you mean, that it is not possible ? Do you mean, that all are happy by buiding of this wire nest ?

 

[Peter Daniel]

I don't know why you asking me, those are not my PCBs.

 

[moamps]

The wiring diagram is wrong, you said. Then I better delete that diagram from my hard drive....

The diagram is OK. I'm the author.

 

[Upupa Epops]

I see two ground loops in each channel...

 

[moamps]

....This wiring diagram is wrong...I see there several ground loops...

Use bigger glasses.

 

[youyoung21147]

Yep, IMHO it isn't the best thing you can find. It is not exactly what I call "short path" !

And furthermore, the small signal ground doesn't look like being separated from the PSU ground, and then connected to it through one wire.

 

[Upupa Epops]

OK, some people can be blind...

 

[hongrn]

Good looking amp ! Is it bridged or paralleled ?

It's bridged. Ca fait bon de parler avec un Francais. Merci.

Hong

 

[youyoung21147]

You're welcome

I'm also planning to build a bridged or paralleled LM4780, so I were curious about how yours works, hum noise apart

Did you use the National schematic ?

Did you match manually some resistors to match the gain ?

Good luck with your grounding issues