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PostPosted: 26 Sep 2017, 08:31 
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Joined: 27 Dec 2014, 14:15
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Hello there,

As the title says, I've got a wee hum problem with an ACF-2 tube buffer I've just built, using 6SN7 tubes. Bloody things sounds really nice but the hum is killing me (what has been heard cannot be unheard).

First, I thought this was a ground loop (or an earthing issue). However, I looked at it from all angles and I don't see any loops. The chassis is tied to mains earth right at the input. The board is connected to the chassis from the house ground pad, through a little board I made myself, that has a power 10-ohm resistor and a small cap in parallel, together with 4 diodes in parallel as well (pairs of two). I've tried disconnecting the board from the chassis (leaving the chassis tied to earth), without any success.

The input/output jacks are isolated from the chassis (verified this with my multimeter) and signal grounds are paralleled all the way to the board. I have shielded part of the signal wires, but the shield is connected to signal ground only at one end.

From this, I started to suspect this is heater hum. I've measured it to be "exactly" 50Hz and I also noticed the hum goes away almost instantly after I cut power to the unit (however, some 30s after that, the power amps start to hum terribly loud - not particularly a problem as I never use them like that).

My mains transformer is a Hammond 369AX, which has a 6.3VAC secondary. It's currently wired in the Full-Wave Bridge configuration and I'm thinking the regulator doesn't have enough input voltage to output properly filtered DC, hence the noise. I am considering reworking this into the Voltage Doubler config, while keeping the reg output at 6.3V. It should dissipate a lot more voltage, but I would assume the DC should but much cleaner.

In any case, I would appreciate your input and opinions. Also, if required, I can provide some pics tomorrow


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PostPosted: 27 Sep 2017, 16:37 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Can you post a photo of the insides of the unit? It might assist in finding the problem.

Good listening
Bruce

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PostPosted: 28 Sep 2017, 01:03 
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Joined: 27 Dec 2014, 14:15
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Thanks, Bruce!

Here you go. This is a slightly older picture where the mains earth is disconnected from the case, but its connected now.

I have a strong feeling you're familiar with the schematic, but if needed, I can supply some pictures of that as well.

Image


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PostPosted: 28 Sep 2017, 01:22 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Hi,

Have you tried another pre with the amp? Possible this thing doesn't play nice with your amp.

Could be a ground loop in the interconnects, too. Some preamps don't play nice with our Rogue Audio amps unless you break the ground on one of the interconnects! :blush:

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 28 Sep 2017, 01:35 
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I’ve connected the DAC directly to the power amps (AES SixPac) using the same interconnects and it was almost silent (nothing audible unless you out your ear to the speaker). However, I have no idea if the Chord Crimsons I’m using have the ground conected at both ends (I would assume not, but...). I have my own interconnects which have the ground (not signal return) connected at one end only. I can try them as well just to be sure.


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PostPosted: 28 Sep 2017, 08:38 
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Hi, I don't see a photo. The site has some restrictions on size and it may not have come through. All I see is the word "image" but no link.

Good listening
Bruce

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PostPosted: 28 Sep 2017, 09:54 
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Joined: 27 Dec 2014, 14:15
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Ah, blimey. Looked just fine on my side.

Anyway, here’s a link to the album where I’ve got the photos:
https://photos.app.goo.gl/R7opqWOFxMHGjboL2


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PostPosted: 28 Sep 2017, 10:26 
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The pic was up here last night for me as well, but not now :confused:

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 28 Sep 2017, 10:52 
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Hi, The link works. My first thought is that you have a lot of fairly lengthy signal carrying wires that are not shielded. Twisiting some can work, but they need to be tightly twisted. The pairs of red and black signal wires might be ok for low impedance circuits but I have found them to be unsuitable for nearly all tube circuits. Also verify that the Alps pot is grounded. I have found in some circuits it can be to the chassis, and in others it must be isolated from it and have a separate ground to a good signal ground. I believe you said you used a 10 ohm resistor, small capacitor and 4 diodes to isolate the chassis. I would change that to 120-150 ohms and a 0.1uf (0.22 is OK) type X2 capacitor and delete the diodes. Somethins they help and sometimes not. Easy changes and may solve the problem. Where do the power supply ground and heater circuit connect to ground. Can you post a copy of the schematic?

Good listening
Bruce

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PostPosted: 28 Sep 2017, 11:30 
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I can tell you that the Alps pot is not grounded. The signal ground goes directly to the PCB and the pots case is isolated from the chassis. I have noticed there’s some HF noise when the pot is not at min or max (noise is highest mid way).

I’ll post schematics tomorrow, as I’m out of town today.

Otherwise, I’ll try to twist tighter and shield them, but I need to change the jacks. The ones I have have the signal return on a washer inside, which makes them impossible to remove once the wires are twisted and shielded.

Thanks a lot for you help guys. I hope one or more of your advices fix my issue.


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