Another Finished LM4780 GC

hongrn · 25th October 2005, 03:28 AM

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

hongrn · 25th October 2005, 03:30 AM

Everything is properly grounded.

Evilsizer · 25th October 2005, 03:42 AM

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

hongrn · 25th October 2005, 03:52 AM

Quote:

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 · 25th October 2005, 07:42 AM

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 · 25th October 2005, 09:20 AM

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 · 25th October 2005, 12:28 PM

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

jackinnj · 25th October 2005, 12:43 PM

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

Upupa Epops · 25th October 2005, 01:37 PM

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 · 25th October 2005, 01:45 PM

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.

25th October 2005, 03:28 AM	#1
hongrn diyAudio Member Join Date: Mar 2005 Location: Placentia, CA, USA	Another Finished LM4780 GC 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

25th October 2005, 03:42 AM	#3
Evilsizer diyAudio Member Join Date: Nov 2004 Location: Texas	i take the hum is there with the volume all the way down? how bad is it with your playing music? __________________ "I Love To Camp!" "Comensing Shock Therapy!"

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25th October 2005, 03:30 AM	#2
hongrn diyAudio Member Join Date: Mar 2005 Location: Placentia, CA, USA	Everything is properly grounded.

25th October 2005, 07:42 AM	#5
kittikun diyAudio Member Join Date: Dec 2004 Location: Bangkok	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

25th October 2005, 09:20 AM	#6
youyoung21147 diyAudio Member Join Date: Apr 2005 Location: Belfort	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

25th October 2005, 12:43 PM	#8
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	It would help if the input weren't so close to the transformer. Torroids do radiate.

25th October 2005, 01:37 PM	#9
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	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...

25th October 2005, 01:45 PM	#10
fcel diyAudio Member Join Date: Sep 2001 Location: LA County	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.

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