Freshly completed build - questions re: hum and blown fuses in power amp

[cacofonix]

Hiya

Just completed my moreplay build and was wonderful to hear the results of my efforts

I've got a couple of things that I could use some help with.

Questions (beginner knowledge level)
1. I've got some speaker hum coming from both channels and I've tried a couple of things to diagnose per the manual

DAC (peachtree) -> Krell 400xi power amp -> speakers == no hum
DAC ->  Moreplay -> power amp -> speakers == hum

So its either the moreplay or a faulty monoprice rca cable causing the possible ground loop?

I've also tried plugging in the moreplay into my main system with McIntosh power amp and its less obvious but still there.

I found 2 spots where I'd forgotten (missing instruction?) to solder the ground lugs near the input jacks - figuring this might have something to do with it - but soldering those did not help alleviate the noise.

Are there any suggestions on how to systematically approach this - can i use my DMM to find the source of the noise?



2. (The more scary question) The fuse in the power socket bit of my krell power amp just blew for the second time. Unfortunately I dont remember the exact steps I took the first time - but this second time. I just powered up my equipment while testing the output RCA jack that usually goes unused
(note: powered up with left output jack being used = no blown fuse
          power up with the right output jack = blown fuse)

Would this be cause by short somewhere in the moreplay?

Now i need to buy more fuses before I can listen to it again and I'm a little worried about plugging the moreplay in.

The second may be the reason for the first question. I'm looking for guidance on how to diagnose both issues.
(See attached pics of my wiring at the i/o section)

 

[Thermioniclife]

Is the 2.2uf coupling cap shorted placing dc on the input of the Krell?

 

[Paul Birkeland]

Can we see some overall build photos of your Moreplay? There should be absolutely no noise at all with the Moreplay turned all the way down. 

Did your Moreplay pass all its voltage checks? 

 

[cacofonix]

Thanks for helping!

Answers to questions
- The noise persists independent of volume level
- It did pass all resistance and voltage tests per the manual at the end. I should probably repeat them post making some fixes.
- @Thermioniclife - I dont know how to answer your question but I'm assuming that was for someone that knows what they're doing.

See attached photos of my completed build with some closeups

Let me know if those help and/or if i should take better pictures in better light

 

[Paul Birkeland]

With the volume all the way down, the output of the preamp is a dead short, so any noise that you're getting at the power amp may be from an external source under those conditions.  Just to double check, with the volume all the way down, what is the DC resistance between ground and the center pin of each output RCA jack?

 

[cacofonix]

The peachtree and the krell amp have been plugged in and working without noise for a while and the only explicit change I just made was plugging in my moreplay.
Removing the moreplay from the chain cause the noise to disappear (or at least get minimized to where I cant hear it?)

So you think that the moreplay may just be highlighting the noise from something else?

I will measure resistances like you asked and get back to you with results

Embarrassing question: When measuring DC resistance like you ask - do I need the unit powered up? My intuition says no - but I want to confirm before sending you erroneous results

 

[Paul Birkeland]

No, you don't need the unit powered up. 

 

[cacofonix]

Resistance measurements with black MM lead attached to ground pin of IEC module

Output jacks closest to IEC module
R5 = 239.8
L5 = 239.8

Further from the IEC module
R4 = 23.1
L4 = 23.1

Tiny bit of ~ 0.1 fluctuation

 

[Paul Birkeland]

Something you could try is temporarily wiring in a jumper wire across the 22R/UF4007s/0.1uF cap to hard ground the audio ground to the chassis.

 

[AB2KH]

The two bare copper ground drain wires that attach to the independent ground lug tab do not look to me to be soldered properly.

Chris/AB2KH

 

[cacofonix]

Thank you guys. Keep the suggestions coming ...

@Paul - trying to make sure I understand this

Something you could try is temporarily wiring in a jumper wire across the 22R/UF4007s/0.1uF cap to hard ground the audio ground to the chassis.

- The 22R/UF4007s/0.1uF bundle goes from 23L to 32L per instructions
- So you're saying to temporarily just attach some jumper wire on either side of that bundle?
- This would cause a hard ground of the audio ground?

Once I do this - what should I do to test? Check voltages? or plug into amp and cross fingers that it doesnt blow another fuse?


@Chris
Pretty sure I've soldered them but I'm plenty capable of shoddy work. So I will reflow / resolder those two to make sure

 

[Paul Birkeland]

I would try it with the Peachtree first to see if the noise you're observing has gone away.

 

[cacofonix]

ok - i followed your suggestions

Changes I made

1. Re-soldered 2 copper wires to ground lug per Chris

2. Temporarily connected bare copper wire across  22R/UF4007s/0.1uF bundle

3. Tried with just laptop and peachtree in chain
Macbook source (using headphone to RCA) --> ( R1/L1) Moreplay (both outputs) --> Peachtree aux in --> Speakers

No noise, no blown fuses, no fireworks

What do you suggest next? Try the original chain with the temp wire in place? (I bought 2 new fuses so I have 2 attempts )

Macbook source (using usb) --> Peachtree DAC (use Pre out) --> (R1 / L1) Moreplay  --> Krell amp --> Speakers

 

[cacofonix]

next test continuing from previous attempt

Changes

1. Original / desired chain
Macbook source (using usb) --> Peachtree DAC (use Pre out) --> (R1 / L1) Moreplay  --> Krell amp --> Speakers

Keeping gain low I powered up and didn't get any blown fuses

However, the speaker hum / noise returned

Music played and volume and balance all seemed to work

So, it seems

Peachtree + Krell = no noise
Moreplay + Peachtree = no noise
Peachtree + Moreplay + krell = noise (however, no blown fuses this time)

Thats an improvement right?!

So - what next - what do I do with that temporary wire bypassing the 22R/UF4007s/0.1uF bundle?

 

[Paul Birkeland]

I have found that for some components with two wire power cords, having a hard connection between audio ground and earth ground can be helpful (which is what that bypass wire is doing).

With the Moreplay driving the Krell, is there noise now with the volume pot turned all the way down?

 

[pboser]

Can you try another interconnect?  I've had some of those Monoprice interconnects with RCA plugs that are so tight that it seems like a choice between a good connection and breaking something.  It's always a good idea to rule out the simple things in these frustrating troubleshooting situations.
Best of luck with it!

 

[cacofonix]

@Paul
- Yes, there is still noise with Moreplay driving the Krell amp
- I find it odd that the noise persists with volume all the way down and even with Moreplay powered off!

i.e.
- If I connect the Krell amp to the peachtree = no noise
- If I connect the Krell amp to nothing = no noise

If I connect the Krell amp to the Moreplay = I get noise

Note: I also get this to reproduce with my main systems which is using an MC462 Power amp - but im not using that to test regularly.

Thankfully - no fuses have blown now - I am being a lot more careful about powering things down and starting up with volume pots all the way down. Any guesses for what was causing that short?

Any thoughts on what to do or try next?

@pboser - I just bought new interconnects from WBC to try that theory as well but the noise persists across interconnects. You are right about the monoprice ones being rather tight

 

[Paul Birkeland]

- I find it odd that the noise persists with volume all the way down and even with Moreplay powered off!

Try this again, then slip the power cord out of the Moreplay while it's off.  I would bet the noise goes away.

 

[cacofonix]

You are correct / win the bet @Paul - your drink of choice on me

With the moreplay powered down, but connected to the Krell, I still got hum

Then when i slip the power cord out of the Moreplay - the hum is gone

note: both devices are using 3 prong plugs plugged into adjacent plugs on a power strip I've been using here for a bit

So, what that tell you? and/or what can I try to do next?

 

[Paul Birkeland]

The Moreplay will probably be perfectly quiet on a cheater plug.  We don't typically recommend/like doing this, but it may be a good option in your case.