Forums - hum problem

Alan H Senior Member Posts: 63	Re: hum problem Reply #15 - 11/18/09 at 22:58:52 Must try that. btw SLL, I would re-route the brown wire that is touching the black 1kohm resistor - that baby gets extremely hot in use!
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sll Verified Member Posts: 10	Re: hum problem Reply #16 - 11/19/09 at 18:35:19 Thanks Alan It doesn't touch it
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Alan H

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Posts: 63

Re: hum problem
Reply #17 - 11/19/09 at 21:43:52

OK SLL,

But what is going on here? Looking at the schematic, the two brown wires you have added simply parallel a trace on the board. Also, why use two when they are connected together anyway so surely one would be just as effective.

If this cured your hum, do you think the circuit board was damaged?

Just trying to reason this through. I have used one wire temporarily and the hum was still there. Strangely the sound did appear slightly clearer with this extra point-to-point wiring, but I will have to try it in/out a few times to see if that is just psychology because I seem to be a real sucker for reporting subtle/non-existent changes as improvements.

In case any readers of this thread get the wrong idea, my zkit hum is at a level which is not obtrusive, and I am absolutely loving the sound of this amp!

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« Last Edit: 11/19/09 at 21:48:42 by Alan H »

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dank

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pair of dual 18
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Re: hum problem
Reply #18 - 11/20/09 at 13:37:18

SLL

Glad to hear your hum problem is fixed. Did you ever try solid wire rather than stranded? In my case it worked much better. I would be interested to know if solid worked different from stranded in your case. When I first investigated the problem, I was using light guage clip on jumper wires (that look similar in size to the brown wire you are using) and found that the more jumpers I clipped on - in parallel - the more the hum was reduced. A good size solid wire did the same job as about 6 of my jumper wires, and a single good size stranded wire did no better than a single small gage jumper wire.

DanK

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Alan H

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Posts: 63

Re: hum problem
Reply #19 - 11/20/09 at 14:47:23

Hi Dank,

Did this low-resistance bypass eventually reduce the hum to zero? ie Low enough to be able to run the amp fully open and control the volume with a passive pre-amp?

I'm now wondering about bypassing that trace completely - it runs the full length of the board - and locating the RCA inputs close to the Pot, directly wiring them with very short runs of solid silver.

Alan

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« Last Edit: 11/20/09 at 15:05:02 by Alan H »

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Alan H

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Posts: 63

Re: hum problem
Reply #20 - 11/20/09 at 20:16:41

OK - tried fully bypassing that trace. Mounted the RCAs right beside the pot, and connected them directly. Kept one of the original grounds grounded - as per schematic.

Well it certainly changed things. Now constant hum - the same at all volumes. Loud buzz if input is disconnected. Music is there.

So, plugged in soldering iron, and undid all the changes. Now back to before - hum varying with volume, acceptable level.

Will give this bypass wire thing a go I suppose. Nothing to lose.

(and soldering-iron burn on fingertip - that'll teach me to rush things)

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sll

Verified Member

Posts: 10

Re: hum problem
Reply #21 - 11/21/09 at 03:34:13

Alan, Dank
I used solid wire from the beginning.
Yes brown wires running in parralel with board wires.
I use two wires to separate two channels.
I don't think my board damaged.
I recognized two types of hum . first one changes with volume changel another don't. My brown wires solve first type of hum for me.
Second type been solved by eliminating ground loops.
So my picture has some ground loops and I've removed them later.
So ground from my RCA jacks connected to taper only (no connections to case or board).
So some hum left but it is audible on max volume only. Usually my volume control is around 1PM. No hum at all on this volume

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Alan H Senior Member Posts: 63	Re: hum problem Reply #22 - 11/21/09 at 15:06:54 Cheers Dank. I suspect I could get away with 1pm on the dial if my speakers weren't so flippin' sensitive! It's 11am for me ... OK, got some solid core here - let's try ... Did it. And yes it helps a lot - thanks guys. Still not sure why ...
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Alan H

Senior Member

Posts: 63

Re: hum problem
Reply #23 - 11/23/09 at 23:07:43

I have now resoldered the solid core bypass wires. What a difference!

There is now NO hum which varies with volume control. There is a small amount of background hum which is unchanging at all volume levels. This may simply be the price of tubes, and is only audible from my listening position at night (when it is very quiet), no music playing, and listening intently. So no real deal here.

Now I have the Decware permanently at full volume, and control the volume levels with my TVC. Sound is without doubt slightly more focussed this way.

I can't see why this tweak shouldn't be mentioned in the build instructions as a possible solution to try if there is hum. It's cheap, and easily reversed if it doesn't help.

Big thanks to SLL and Dank for finding and reporting this!

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« Last Edit: 11/23/09 at 23:12:36 by Alan H »

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Dean

Verified Member

Posts: 4

Re: hum problem
Reply #24 - 11/25/09 at 23:42:25

Hello Alan,

I am trying to figure out where you used the solid wire and it eliminated the minor hum when mvoing the volume above 11 o'clock? I have the same problem.

I have to say that I really love the sound I am getting... wow.... I have recently added a ZenHead as the preamp and now I can get the volume I want .... Oh my... sooooo nice...

Thanks,
Dean

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« Last Edit: 11/25/09 at 23:43:53 by Dean »

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Alan H

Senior Member

Posts: 63

Re: hum problem
Reply #25 - 11/26/09 at 00:40:52

Hi Dean

I used wire from some solid core mains cable - fairly large gauge, and stiff copper.

Exactly where you connect these depends on how you have built your kit1. I did not use the stock RCA connectors, but put RCA connectors on the case, and wires from them to the circuit board board. So one end of each solid-core wire gets soldered to the ground of each RCA input socket.

I also used a different volume pot to stock, with wires runing to the 6 solder pads on the board. The other end of each solid-core wire goes to the two pot solder points that are at the word "taper" in the audio taper label. I actually attached them directly to the pot.

Pretty much as Dank's second photo earlier in this thread, except he used the stock pot, and attached the wires to the solder pads.

And I agree - the amp sounds fantastic!

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« Last Edit: 11/26/09 at 00:42:36 by Alan H »

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dank

Seasoned Member

pair of dual 18
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Posts: 255

Re: hum problem
Reply #26 - 11/26/09 at 13:57:50

Continued work on what's left of my hum problem reveled that the hum caused by the amp when it is in the system was indeed a "ground loop". I was able to measure 6 mv difference between the chassis of my PC (source) and the shield of the input RCA connectors at the Zkit1. The new 3rd solid wire in the picture reduced this to 0 mv, the lowest volt scale on my meter. While still not totally silent, you now have to strain to hear hum from the listening chair in between songs, and this is with the Zkit1 volume set at max. Most of the hum now is actually coming from the subwoofer amp.

The new 3rd solid wire ties the power transformer neutrals (red/yel and grn/yel) to the two other solid wires previously installed. It did not seem to make a difference to either move the earth ground wire to the new wire (eliminating an inch or two of trace) or to jump straight to the earth connection on the AC receptacle with solid wire, so I left the earth connection unchanged.

Happy Thanksgiving

DanK

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Alan H Senior Member Posts: 63	Re: hum problem Reply #27 - 11/27/09 at 00:34:22 Tried it here, Dank. However this one made the hum very slightly louder in my amp. So I've taken it out.
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HT-EXT Seasoned Member NYUK NYUK NYUK Posts: 272	Re: hum problem Reply #28 - 12/09/09 at 20:13:30 I have just order a couple of these. http://www.dcispec.com/index.html I hope this sovles my plate amp hum but we will haft to wait and see. HT-EXT
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Evo

Senior Member

Posts: 53

Re: hum problem
Reply #29 - 01/10/10 at 11:56:24

I have just ordered this kit and wanted to check a few things before I start about the casing and how to properly ground it.

These are probably really obvious questions, but I figure it is better to ask than to go ahead and do it wrong.

1) Firstly I wanted to mount the PCB on wood. Is there any reason why I can't mount it inside a wooden chassis with the tubes poking through the wood (just like it is mounted in a metal chassis), instead of mounted on top of the wood?

2) I am planning on having the trimpot, RCA and speaker connectors connected directly to the PCB. I am thinking that the only grounding I need is i) connecting the 'GND' and the 'SHIELD' points on the circuit with a jumper, and ii) connecting the 'SHIELD' to the earth from the mains supply connection. Is this right?

3) Anything else I need to be careful about?

Cheers

Matt

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