| 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
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| hongrn |
Everything is properly grounded.
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| Evilsizer |
| i take the hum is there with the volume all the way down? how bad is it with your playing music? |
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| hongrn |
| 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... |
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| 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 |
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| 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 :) |
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| fcel |
| I have seen the attached GC grounding diagram. May be this will help? |
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| jackinnj |
| It would help if the input weren't so close to the transformer. Torroids do radiate. |
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| 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... |
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| 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. |
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| 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. |
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| 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 ? :xeye: |
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| Peter Daniel |
| I don't know why you asking me, those are not my PCBs. |
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| moamps |
| quote: | Originally posted by fcel
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. |
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| Upupa Epops |
| I see two ground loops in each channel... |
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| moamps |
| quote: | Originally posted by Upupa Epops
[B]....This wiring diagram is wrong...I see there several ground loops... |
Use bigger glasses. |
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| youyoung21147 |
Yep, IMHO it isn't the best thing you can find. It is not exactly what I call "short path" :D !
And furthermore, the small signal ground doesn't look like being separated from the PSU ground, and then connected to it through one wire. |
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| Upupa Epops |
| OK, some people can be blind... ;) |
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| hongrn |
| quote: | | Good looking amp ! Is it bridged or paralleled ? |
It's bridged. Ca fait bon de parler avec un Francais. Merci.
Hong |
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| 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 :D
Did you use the National schematic ?
Did you match manually some resistors to match the gain ?
Good luck with your grounding issues :smash: |
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| hongrn |
I used a kit from hifidiy.net.
I also got help from Aaron in the PE forum to rewire the amp and get rid of the hum. Left to resolve is a slight buzz when a source is connected to the RCA inputs. The buzz is absent with the Alps pot turned all the way down, but clearly audible when volume is up (without music) and between quiet passages. I know my input cables are long because of the position of the pot (on the front panel), so that may be an issue. I'll try to shorten them and see what happens.
I think I got a problem with one of the channels, in my case, the right one. It would play fine for about 30 minutes, then buzz loudly and send my right woofer into a scary outward excursion. I have to shut off the amp and let it cool off before it will play again. I have switched speakers to test the amp, and it is a problem with the right channel. Which component is defective? Do you think the LM4780 chip is giving me problems?
Thank you.
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| wintermute |
Hi Hong,
the fact that it is heat related probably indicates a bad joint. It also sounds like it is resulting in DC on the output! Another (VERY REMOTE) possibility is that you have too much solder on a couple of joints which aren't quite touching, but when it heats up the solder is expanding and causing a short. I'd say take the board out and examine all solder joints with a 10X magnifying glass, alternatively if you have a digital camera with good macro (and can get good lighting without too much reflection) take a closeup and look at it on screen :) A high res macro can show you all sorts of horrors that to the naked eye seem fine ;)
here is a good guide on soldering ---> http://www.epemag.wimborne.co.uk/solderfaq.htm I found a better one once before which had closeups of bad and good joints but I can't find it now :( probably have a bookmark on my pc at home though.
Tony. |
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| hongrn |
| Thanks for your help Tony. I'll double-check all my soldering joints again. |
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| Upupa Epops |
| Make quite different internal instalation : give transformer beside front panel and both modules shift to the back side. At this case you get much more shorter signal and speaker wires ;) and you will be satisfied... ;) |
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| javven |
Better than a digicam for PCB joints is a good scanner. A bit of good celophane and you won't scratch the glass. Put your PCBs solder-side down on the scanner with the lid up. Then preview and scan normally. 1200 or more DPI (don't exceed your scanner's hardware resolution) shows up most things nicely. I know coin collectors who do this.
Is that much heatsink even required? I've seen much less. Nothing wrong with erring on the side of caution, to be sure.
I've been told not to put my transformer anyplace near input - level connections if possible. My BoZ / Zen integrated setup layout thusfar has both the BOZ and Zen xfmrs on one side of the chassis with all input - level stuff completely on the other side. Is this preferably or just paranoia on my part? Too, what can I use to shield my amp from the transformers? Lead sheet? Copper sheet? Just plain aluminum? |
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| carlosfm |
| quote: | Originally posted by Upupa Epops
Make quite different internal instalation : give transformer beside front panel and both modules shift to the back side. At this case you get much more shorter signal and speaker wires ;) and you will be satisfied... ;) |
Yes, much better.
Also, it could be arranged so that the heatsinks separate the trafo from the amps, so they could be installed across the amp, side to side.
The rectifier boards can also be behind the heatsinks, near the trafo. |
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| wintermute |
| quote: | Originally posted by javven
Better than a digicam for PCB joints is a good scanner. A bit of good celophane and you won't scratch the glass. Put your PCBs solder-side down on the scanner with the lid up. Then preview and scan normally. 1200 or more DPI (don't exceed your scanner's hardware resolution) shows up most things nicely. I know coin collectors who do this.
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Great idea javven :) I used to use my flatbed scanner to "take pictures" of small objects before I had a digicam, but I hadn't thought of that use! much better resolution than my 3 megapixel camera!!
Tony. |
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| jackinnj |
placement issues -- move the transformer to the center of the chasis. mount the rectifier bridge to the rear, on the right side (facing away from you). the I^2R losses for a few cm of power run are next to nothing.
mount the input jacks to the left side of the chasis above the speaker terminals. route the input cabling along the floor of the chasis, to the front and then to the input terminals on the PCB. you can use adhesive backed cable mounting restraints for this purpose.
I assume that the heatsinks are in a "temporary" position -- they should form the right and left hand walls of the cabinet -- you might want to mount them on spacers and use a nibbling tool to make a cutout for the amp pcb's. If you are running the LM4780's full tilt the heat sink size appears correct. the LM4780 really cooks when run for 120W. |
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| carlosfm |
That chassis is tall, it seems that the toroid could be mounted on the front, in a vertical position.
Lots of space, then. :cool: |
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| hongrn |
Thanks everyone. This forum is awsome. Looks like I got work to do.
Best Regards,
Hong |
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| wintermute |
Hi Hong,
I think upapa may have missed that you have a volume control. I agree with his suggestion, but in order to do it properly you will need to mount the pot internaly at the back near the amp boards, and use an extention shaft to drive it. If that is too much hassle then Jacks suggestion I think is the next best solution.
One other question, does the source you are connecting have a three pin plug, or only two?? if three you may have an earth loop.... see here for more info ---> http://sound.westhost.com/earthing.htm Note that IMO if you can get away without implementing the fix Rod suggests that is the best solution, only implement it if absolutely necessary :) when new to electronics it is very tempting to try everything you see, thinking it will be a magic cure..... actually not just when you are new ;)
Of course once you track down the problem with one channel going awol after 30 minutes, you might get rid of the problem completely without additional layout changes necessary (might pay to go over the left channel board as well).
Tony. |
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| javven |
I read something about problems with having inputs and outputs on the same planar mounting surface. Capicitance issues, maybe? Possibly it was just one person's design or perhaps it was merely marketing on some brand's ad I saw...
My plan was to run AC power up one side to the transformer, which will be in front on one side. I haven't seen anywhere to get extensions and the like for mounting a vol. pot in back, so I was going to run my signal - level input up the opposite side of the chassis using twisted shielded pair wire, grounding the shield on one side of the chassis only (probably the back). You could use CMNR to help you with noise rejection but it'd take more work and I don't have the expertise to tell you if it'd help or not.
I like the symmetry of what you have pictured.
However, looking around I see the -vast- majority of amps mounting transformer on one side, input on the other. My guess is there's a reason for this beyond following convention. I've seen a lot of high end equipment with asthetics as a real focus, however and perhaps they've really mastered the art. I don't have the expertise to tell you for sure. |
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