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Old 10th January 2005, 09:12 PM   #1
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Default what is wrong with my gainclone?

Well after I ruined the lm3875s on the last boards I built a new set of boards, with Brian's suggestions and half of an old box of his. I shorted the last pot on accident (I'm waiting for the group buy) some my rca placement is not final, and neither is the back pannel (I cut a piece of alumium for the back, but have to cut holds for the AC and everything). But the wires are so much shorter and is well shielded, but I still get a loud hum. In this picture you can see that this afternoon after school I added some aluminium foil to a few wires to see if it would cut down on the hum, not at all. I've moved lots of wires all around the box but it doesn't seem to help what can be the problem. I also have checked with the aluminium top on the amp, but it doesn't help either. Here is a picture (cant get it working right now)

Thanks,

Josh
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Old 11th January 2005, 01:27 AM   #2
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I got the picture working.

I've tried moving everywire, but I just can't seem to make it any better.

Click the image to open in full size.

Thanks,

Josh
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Old 11th January 2005, 01:57 AM   #3
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where the hum come from? the transformer or from the speaker?

I do not see the pot on the amp... do you use the amp as "power amp" only?

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David
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Old 11th January 2005, 02:08 AM   #4
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Maybe you have a cold solder joint on one of the boards somewhere? If you aren't an experienced solderer, have a look at one of the guides on the net. (Easily found with google.)

Is the circuit grounded to the chassis at more than one point? Like at the input jacks or speaker terminals?

Loud hum is usually from a poor ground or a ground loop.
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Old 11th January 2005, 02:42 AM   #5
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Default xformer

Did you know that the metal dealie on top of your transformer is upside down, causing your bolt to stick too far up? If your transformer bolt is hitting the cover of your case this will cause a loud hum. Its called shorted turn phenomina. Do a search here for that. Happened to me too. Couldn't figure out which side was up if you know what I mean. Anyway, should not happen with cover off, so maybe that's not it.

I know these problems are very aggravating, you check and recheck and nothing seems wrong. Keep at it!

I would check all component values, use a DMM or check the labels.
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Old 11th January 2005, 03:26 AM   #6
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thanks for the quick responces.

Since I shorted the pot, I'm not using my computer as a source and the volume control on the computer works really well, while the output on my home theater amplifier is either loud, or louder. The hum is coming through the speakers, but its definately the hum of the transformer. I can put my ear up to the torroid and hear nothing except when it is first turned on.

The bolt on the transformer is actaully quite short, only about 4 threads are poking out after the nut, about 1/2" from where the top of the amp sits.

The box is BrianGT's and he helped me with grounding, and disconnecting it does nothing, only make it worse actually.

The boards themselves, all the joints are well soldered (I was never tought to solder and my first boards had too little solder) Brian showed me a few tricks and the amp boards are well soldered, but the rectifier is a different story. I never added solder to those joints, can this cause the hum?

Because of this hum, I cannot properly compare my Denon to the Gainclone, as of now, the Denon sounds a whole lot better, even after about 15 hours of hot burn in which I burned 2 holes in my capet with the dummy loads getting too hot. Since I dont have a pot and I dont like playing music on my computer I tend not to use the amp I built very often.

Before entering the project I did a lot of research, but after I finished my first amp, and then ruined a cap and later had my LM3875s smoke and ruin, I started from scratch and this amp is much better constructed and my knowledge has trippled on the topic, I've read about the phenomina with transformers, but I hadden't heard anything bad about undersoldering.

After the rest of my homework, I'm going to add solder and hopefull come back with good news.

Right now I feel like I'm listening to KLH amplifier, or something cheap. Obviously the amp shouldn't sound like this, what can I add or change?

Also OT, but what about a tube rectifier for Gainclones? Would it work well? Would it give a "tubier" sound to the amp?

Thanks,

Josh
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Old 11th January 2005, 03:47 AM   #7
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well I decided, that the homework could wait a couple minutes, I tried adding solder and the hum is still loud and clear.

Any other suggestions of what it could be?

What is DMM?

Thanks,

Josh
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Old 11th January 2005, 03:49 AM   #8
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so you don't use pot at all?

do you have other source other than your computer? what happened if your computer as a source connected to the Denon?

I wonder whether your source output is might be too high....
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Old 11th January 2005, 03:54 AM   #9
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Quote:
Originally posted by edjosh23
Any other suggestions of what it could be?

What is DMM?
I'm still thinking it's a ground issue.

The amount of solder isn't directly the issue. Sure, you can have too little solder, but usually novices err on the side of too much. In the end though it's the electrical quality of the joint that counts. Have you had a look at soldering guides on the net? There are some that show you what good and bad joints look like.

Guessing isn't much good.

A DMM is a digital multi-meter.
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Old 11th January 2005, 04:46 AM   #10
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I just looked at a few soldering guides the ones that came up on google. I use flux, along with a wet sponge, and all my joints look like mini volcanoes. I used a 45w iron and lead free silver solder. I feel like nothing was done wrong, and everything was up to Brian's standards.

One thing that I think could be a potential problem is AC. The AC live goes across the entire enclosure to the on off switch and over all the DC wires but the cross perpendicular and the wires are about 1" above the DC wires. When I cut the final back plate The AC outlet will be higher in the enclosure, and more towards the center, along with the fuse. All sources that I have are my DVD player, my dads DVD player, his tape player, and his reciever.

I have sound blaster extigy and I'm using it to run the amp. I can use front speaker out or headphone. Both have the same ammount of hum.

The computer works fine though the Denon.

I can also use a discman, in which hum is maybe a little bit less loud, but I also cannot completely turn down the music, with the ear up close you can still hear the music faintly. But the hum is there none the less.

I just don't think its a grounding problem, because everyone says that the way Brian recomends on his website works with his PCB, and this is the way that he did for me when I was with him. This is also a copy of the box used in the user guide for his PCB.

All voltages have been checked with my multimeter.

I thought I should mention, with no input there is still a loud hum.

Thanks,

Josh
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