CR-620: Headphones hum/ground loop hunt

thoresui

thoresui

Active Member
Hi,

this is something like part II of my CR-620 restoration thread (https://audiokarma.org/forums/index...e-current-adjustment-without-reaction.926792/).

After re-cap, cleanup, numerous resolderings and double-checking the unit still has 50 Hz hum and its harmonics (mostly 100 Hz) present on headphones output.

The 100 Hz is more prominent without LOW filter and 50 Hz is more prominent with LOW filter turned ON.

It is pretty tricky to describe the hum only from my listening so I finally decided to measure it with REW to have "bigger picture" about what is on there and what are differences between channels.

My current plan is to perform at least some of the following tests/tweakes (will see how much time and energy I will be willing to put into this):

- parallel grounding with wires as seen in https://www.audiokarma.org/forums/i...t-yamaha-cr-620-rebuild.892540/#post-13253680
- additional transformer shielding?
- ac wires twisting?
- complete re-rerouting of sensitive cables?
- isolating the PCBs from chassis and new star grounding with wires

Background measurements (zero volume):

L_with_LOW_zero_volume.jpg L_without_LOW_zero_volume.jpg R_without_LOW_zero_volume.jpg R_with_LOW_zero_volume.jpg
 
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The L channel with louder hum is closer to transformer on pcb. Which could explain why there is more hum.
 
One observation about transformers, there exist at leat two versions. One has end caps/covers and one has "only" belly band (my multivoltage unit has the one without cover):

us_transformer.jpg multi_voltage_transformer.jpg
 
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Is the hum only on the headphone output? I would bypass the jack if possible and compare measurements. It sounds like this was there prior to your recap. I’m not familiar with the receiver, but usually the headphone output is parallel to the main output with power reduction circuit. I would check that circuit, provided the hum is not present on the main output.
 
Removed the bottom chassis "cross beams". Resoldered all joint under the vertical one. Experimented with additional ground wiring as replacement for cross beams path.

No relevant improvement. Cleaned once again the connection points (pcb-crossbeam mount points).

In the service manual was the wire to the tuner pointer depicted as twisted. Experimented with the wire position and twisting. No improvement.
 
Replaced small round bridge rectifier. Wanted to check if the output wave changes when measuring with scope. No relevant improvement. Since the new part had slightly better specs left it there.

The regular noise floor without low filter is "fair enough" considering the construction/conception. The last thing which started it all is the hum with LOW filter ON. Will let it sit for a while.
 
thoresui said:
Replaced small round bridge rectifier. Wanted to check if the output wave changes when measuring with scope. No relevant improvement. Since the new part had slightly better specs left it there.

The regular noise floor without low filter is "fair enough" considering the construction/conception. The last thing which started it all is the hum with LOW filter ON. Will let it sit for a while.
Click to expand...
Assuming you are in EU so the 100Hz does indicate a ground loop issue.
can you try the following:
  • get a wire whip long enough to reach across different points on the amp chassis
  • solder at the input of the headphone amp ground point (or at the input of the actual amp if applicable)
  • use the other end of the wire to test / ground to chassis at different points until you find the sweet spot.

I feel your frustration, ground loop issues are a PITA to resolve.

Just solved for a similar issue on a Yamaha C-1 headphone amp. It was a nightmare...but it is under control now.
Good luck!
 
@ConradH yep, it helped to lower noise floor without the LOW filter. I have even tried to revert the mod to be sure that I did not introduced some new issues with it. I have used insulated wire bended over the pcb edge instead of enameled one routed through hole.

@rottalpha yes 50Hz and 100Hz are the problem in this case (EU). So lets say in my case I would connect one end of the testing wire to the AUX rca ground and will probe various ground points in the unit to see if helps with the hum with LOW filter engaged?
 
thoresui said:
@ConradH yep, it helped to lower noise floor without the LOW filter. I have even tried to revert the mod to be sure that I did not introduced some new issues with it. I have used insulated wire bended over the pcb edge instead of enameled one routed through hole.

@rottalpha yes 50Hz and 100Hz are the problem in this case (EU). So lets say in my case I would connect one end of the testing wire to the AUX rca ground and will probe various ground points in the unit to see if helps with the hum with LOW filter engaged?
Click to expand...
yes, you need to remove the noise without the help of the filter. Turn the filter off.

Pick the ground point at the input of the headphone amp to solder one end of the test wire and test grounding at different points of the chassis with the other end of the wire. If the input for the headphone amp happens to be the same input for the amp, and based on your prior description, it sounds like it is, then use that ground point...or AUX IN, if it is applicable.
Also, if you serviced the amp recently and the hum surfaced after the service, check all your chassis screws to make sure they are tight.


EDIT forgot a very important callout:

before you do any of the above, short the input of the amp at the wire to board connection. Short both input channels to ground, then listen to the headphones. If the noise disappears completely, then comes back after you remove the "short" then you have a ground loop issue. If you still have noise with the input shorted at the amp board, then you have PSU noise coming in and that is a different problem to solve.
 
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Yes, CR-620 does not have separate headphones amplifier. The headphones is just attenuated regular output.

Do not understand the "without the filter" recommendation, noise floor without it is acceptable. Engaging the LOW filter creates some loop and the 50Hz hum goes up (see pics in the beginning of the topic). My first guess would be that I should experiment with the test wire with the LOW filter in ON state.

The unit had some other problems before service and I really do not remember if the problem with LOW filter was present before. The screws are checked and tight. But I can double check of course.
 
I've seen various receivers with more hum in the headphone output than is desirable. It's sometimes caused if the phone jack is mounted to the chassis, as that location isn't usually the best ground point. If the jack is isolated, and most aren't, then you can experiment with wiring. There's also the ground point used by the attenuator, which is often sub-optimal. The signal to hum ratio at the headphone jack should be about the same as the main output, but they often aren't. I never spent a lot of time with the headphone output on the 620, so can't offer more than general comments. Less sensitive headphones will help! BTW, if you do some measurements with your headphones, you're likely to find the level of hum you can hear is remarkably small in terms of mV or uV of signal, which makes you appreciate how perfect the "simple" attenuator has to be.
 
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thoresui said:
Yes, CR-620 does not have separate headphones amplifier. The headphones is just attenuated regular output.

Do not understand the "without the filter" recommendation, noise floor without it is acceptable. Engaging the LOW filter creates some loop and the 50Hz hum goes up (see pics in the beginning of the topic). My first guess would be that I should experiment with the test wire with the LOW filter in ON state.

The unit had some other problems before service and I really do not remember if the problem with LOW filter was present before. The screws are checked and tight. But I can double check of course.
Click to expand...
I meant, filter ON or OFF, you should have no noise.
 
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thoresui said:
The unit had some other problems before service and I really do not remember if the problem with LOW filter was present before. The screws are checked and tight. But I can double check of course.
Click to expand...
I would not be surprised if the unit had this issue all along. To @ConradH point, the more sensitive headphones would reveal the background noise at full gain, more than others. This was my realization with the mighty C-1 after I tested 4 individual C-1 units.
C-1 design btw, has a fully insulated headphone jack from the chassis and an independent headphone amp and independent supply rail for the headphone amp...still, some noise at full gain is present and when using sensitive headphones. At the same time, nothing at the speakers since that is a different amp with an apparently more effective ground loop. As a matter of fact, the line stage of the C-1 is dead quiet, which made the noise at the headphone amp stage stand out even more....what a PITA.
I hope you solve the CR issue and document the fix.
 
Performed three rounds of tinkering with test wire. Connected ground from the input side with various other ground points on PCB and on chassis.
Without any significant results.

Only improvement happens when I ground the chassis via the grounding screw for turntable directly to mains ground. And the improvement is only for LOW filter OFF state.

There is definitely still some loop with LOW filter ON, but on need to focus to hear it when the unit is properly covered and wired in (with connected source device, grounded via some other device or directly with separate wire to mains ground).

Would be nice to know if this is a design flaw (if other CR-620 have the same problem) or there is something special in my unit.

Other projects/units are piling up and I need to declare this CR-620 as "good enough" (AKA non-improvable with my current skill set and knowledge :)).
 
rottalpha said:
EDIT forgot a very important callout:

before you do any of the above, short the input of the amp at the wire to board connection. Short both input channels to ground, then listen to the headphones. If the noise disappears completely, then comes back after you remove the "short" then you have a ground loop issue. If you still have noise with the input shorted at the amp board, then you have PSU noise coming in and that is a different problem to solve.
Click to expand...

quoting myself....
did you have a chance to short the input of the amp (both channels) to ground at the amp board input? doing it at the PCB wire to board connection and not at the other end of the wire, is important.
  • If you did perform the above, did you still hear the noise on the headphones (agnostic of filter ON of OFF)??
  • If you did not hear any noise after you shorted the input to ground (if the noise went away) then you can continue to search for the ground loop issues.
  • If you can still hear the hum with the input shorted at the amp board, then you need to look at your PSU and stop chasing the ground loop. In the case of the PSU hum, I would start looking at rectifier and size of the smoothing caps.
 
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