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PostPosted: Wed Jan 11, 2017 9:52 am 
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Hi all, I've also posted this over on diyaudio, but thought I'd take a shot here too. I have a DCB1 preamp build which has been working great but has suffered from a very slight buzz. I've tidied up the wiring and did some deduction and have come to the conclusion that I have a ground loop issue somewhere. Here's the rough signal flow:

- 2 sets of inputs go to a source selector switch, which then feeds a volume pot, which finally feeds the input of the DCB1
- 2 sets of outputs. One is fed by a passive split off of the source selector switch to run my headphone amp (thus monitoring what I've selected with the switch), and the other is fed by the DCB1 output.

So, I have signal grounds on both sets of inputs and both sets of outputs (RCA jacks). I have 2 signal ground lugs on the pot (Noble). And I have a signal ground on the input of the DCB1 board as well as a signal ground on the output of the DCB1 board.

Does anyone have any thoughts as to the best way of connecting all the grounds?

Cheers!


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PostPosted: Wed Jan 11, 2017 10:23 am 
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Since it's a DIY, you could probably do a star ground. There are several sites on-line describing the procedure.

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PostPosted: Wed Jan 11, 2017 11:14 am 
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Are the RCA jacks and headphone jack isolated from the chassis
and what is the chassis ground scheme


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PostPosted: Wed Jan 11, 2017 11:42 am 
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The signal ground and chassis ground are separated in the DCB1, yes. However, I checked all the other gear too and, as it turns out, in every other piece of equipment connected to the DCB1 the chassis and signal grounds are not separated (continuity between AC input ground and RCA sleeve). So by the time everything is hooked up, no, the signal and chassis grounds are no longer separated in the DCB1.

I thought this could be the source of the loop so I tried tying the signal ground to chassis ground in the DCB1 but it made no difference.

Chassis ground is very simple in the DCB1: ground lug on AC input is bolted to the chassis.

Also, in all this swapping around and cycling the power amp (McCormack DNA1) repeatedly it seems that the quality of the "ground loop" has changed (i.e., a bit quieter now) so I'm no longer 100% sure that's what I'm dealing with. With shorting plugs on the power amp's input I get no buzz at all. Lifting the ground of the power amp at the IEC cable makes no difference. Unplugging all other equipment makes no difference. Removing the DCB1's IEC cable makes no difference: if the DCB1 is connected to the power amp, and whether or not it is actually connected to AC, I get the buzz. Moving the DCB1 physically makes no difference. I tried a different cable between the DCB1 and power amp, and no difference.

Puzzled!


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PostPosted: Wed Jan 11, 2017 12:05 pm 
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So actually I just tried another thing that I haven't tried thus far which is to plug in one of my sources (phono pre) directly to the power amp to present it with a load, but to take the DCB1 out of the equation altogether. Unfortunately the buzz persists. I guess that means the power amp is the issue here and not my DCB1 build. Bummed out...


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PostPosted: Wed Jan 11, 2017 12:33 pm 
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Location: St.Catharines, ON, CA
tiller wrote:
Hi all, I've also posted this over on diyaudio, but thought I'd take a shot here too. I have a DCB1 preamp build which has been working great but has suffered from a very slight buzz. I've tidied up the wiring and did some deduction and have come to the conclusion that I have a ground loop issue somewhere. Here's the rough signal flow:

- 2 sets of inputs go to a source selector switch, which then feeds a volume pot, which finally feeds the input of the DCB1
- 2 sets of outputs. One is fed by a passive split off of the source selector switch to run my headphone amp (thus monitoring what I've selected with the switch), and the other is fed by the DCB1 output.

So, I have signal grounds on both sets of inputs and both sets of outputs (RCA jacks). I have 2 signal ground lugs on the pot (Noble). And I have a signal ground on the input of the DCB1 board as well as a signal ground on the output of the DCB1 board.

Does anyone have any thoughts as to the best way of connecting all the grounds?

Cheers!

Follow the signal.ONE single ground (0V ref) wire from both rca input lugs to selector switch to volume control lug then to input ground on circuit board.Does this make sense?
You shouldn't have individual ground wire's from each lug of the rca's,selector switch ,volume control going to the input ground on the circuit board.
I assume no provisions marked on the circuit board for selector switch and volume pot.

Gary


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PostPosted: Wed Jan 11, 2017 12:41 pm 
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Thanks Gary, that is pretty much what I have going in that case, except the grounds bypass the switch since there's no ground lug on it. RCA ins are bussed and go to the vol pot which goes to the input signal ground on the board, RCA outs are bussed and connect to the output signal ground on the board.

As per my last post I think this is wild goose chase anyway. It's looking more like the issue is with the McCormack.


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PostPosted: Wed Jan 11, 2017 3:02 pm 
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tiller wrote:
Does anyone have any thoughts as to the best way of connecting all the grounds? Cheers!

To answer this question - the audio ground should use a star grounding technique.

The audio ground from the input RCA's - output RCA's - power supply and amp
should all focus on 1 node - typically near the input RCA's.
As such, all the RCA's, power supply and amp are all referenced the same "O Vdc "

The ground from the power supply should NOT go directly to the amp and then the RCA's.


To answer the question about the buzz when your McCormick power amp is used

if your McCormick amp has its ground "lifted" then that is not the issue.

But a loop may also be formed by the preamp ==> R interconnect shield ==> power amp ==> L interconnect shield


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PostPosted: Wed Jan 11, 2017 3:06 pm 
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Thanks for the information. What do you mean by "lifted"?


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PostPosted: Wed Jan 11, 2017 5:07 pm 
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tiller wrote:
Thanks for the information. What do you mean by "lifted"?


"Lifting" means isolating the hydro earth from the audio ground.
Typically this is done with a pair of back to back power diodes, a resistor and a cap.
I don't recall if it needs to be a an AC rated cap.
There are other methods of doing this.

This article explains loop hum issues and "Lifting"
http://sound.whsites.net/earthing.htm

Also, this photo helps show the physical layout of star grounding.
Note that the power supply ground should be connected to the audio star ground node.


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PostPosted: Thu Jan 12, 2017 7:40 am 
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Thanks for the great info Underhill.

I checked all my gear when I was trying to diagnose this and all of my equipment, with the exception of the DCB1, shows continuity between chassis ground and signal ground. I added a reference in the DCB1 between signal ground and chassis ground (as suggested in your star grounding diagram) but it made no difference to the noise I am experiencing. So, no, the power amp is not "lifted".

As I said in my previous post, I get the buzz whenever there is a load present at the input of the amp. I have tried a couple difference pieces of equipment now, connected to AC and not connected to AC, and whenever the McCormack sees a load I experience the buzz.

My next step (as suggested by someone on diyAudio) is to temporarily increase the output impedance of the DCB1 to see if the nature of the buzz changes. I'm also thinking about reaching out to McCormack, but I'm not hopeful that will result in much.

Your input is appreciated!


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PostPosted: Thu Jan 12, 2017 8:16 am 
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Tiller,

Is the Mccormack a DC amp?No input coupling cap.

Gary


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PostPosted: Thu Jan 12, 2017 9:25 am 
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Good question! I don't know for certain, but a quick google leads me to Stereophile's original review that says:

Quote:
The circuit is a direct-coupled design employing a DC servo. JFETs are used in the input stage, with a bipolar cascode stage and MOSFET drivers.


So, I guess, yes, it is a direct-coupled amp. Why do you ask?

Cheers


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PostPosted: Thu Jan 12, 2017 9:56 am 
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Your DCB1 kit seems to be DC coupled also.Thought maybe some DC junk might be getting in that way.

Also if pushing/pulling alot on the amp's rca inputs might have jarred something.
Input circuit board looks to be in close proximity.
Maybe get in with a chop stick and flex that board,the servo chip etc...with amp running.Snug up bolts and screws and reseat anything with a connector (amp off )while your in there.

Just some thoughts.

Gary


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PostPosted: Thu Jan 12, 2017 10:01 am 
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Not a bad idea. On the other hand though, I have tried connecting other sources directly to the McCormack -- not direct coupled sources I might add -- and I get the same thing. It's also quite a high-frequency buzz.

Quite puzzled on this one :/


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