Ground loop problem

[johnsonad]

Team,

I'm in need of a little assistance.  Normally I can fix ground looks pretty easily but this one is kicking my butt.

My Eros tape with only the output RCA's connected between the Eros and preamp with the power turned off (no connections to the tape deck) induced a significant 60Hz hum when using a three prong power cord.  This hum is there also if it is connected via RCA and grounded to the tape deck.  If I put a cheater socket on that cable Eros power cable, the hum goes away.  I tried a different outlet, same level of hum.  I tried it on an independent power conditioner, same hum.  I tried changing RCA's, same hum. 

In the past, I had an issue with the deck doing this, not the Eros and to get around it I used a cheater plug on the deck's power cord.  After the move, this has all changed.  All sockets are plugged into a PS Audio Premier Power Plant normally. 

Is there something I need to trouble shoot within the Eros?  If so, please point me in the right direction.  Second question, how unsafe is it to run the Eros without a grounded power cord if say connected to the Power Regenerator? 

Thanks for the help.

Aaron

[Doc B.]

Need more info. At what end are the interconnects' shields connected to the plug shell? And have you tried grounding the ground post of the Eros to the preamp chassis?

[Paul Joppa]

Also, what is the power amp? Is it grounded?

[johnsonad]

Dan and PJ, thank you for the responses.  I'll give you details of the system, answer your questions and describe the trouble shooting.

Presently the tape path is, Otari>Eros>Pioneer C-Z1 preamp (extra gain stage for the Eros and my current phono stage)>PB custom preamp (6J5 shunt regulated, cap coupled to B7 output transformers)>Rythmik subwoofer (single with 80Hz HPF)>Paramounts (grounded and not used for the test, all hum was coming from the sub).  My system has very low gain and I need to run a second gain stage for the Eros otherwise I'm nearly at max volume on the PB preamp.  This wouldn't be a problem if there wasn't the hum.  This hum is present but predominately volume dependent.  In describing this problem, the PB preamp is at full volume.  I compare two different channels on the PB preamp to establish the baseline of hum in my description.  Also, the C-Z1 preamp, which I use as the phono stage and as a secondary gain stage for the Eros has independent volume and it's at a level where the phono and Eros have gain equal to ear of what the digital channel has on the PB preamp.  That volume is at roughly the 1 o'clock position.  I use the Aux input for the Eros and the phono for obviously phono on the C-Z1, there is also a tuner input of which nothing is connected to. 

The C-Z1 is a Japanese preamp, two pronged power cable.  It has a grounding post and this is where the phono is grounded and where I ground the Eros in the trouble shooting below.  I'm using a step down transformer which is three pronged and is plugged into the PPP. 

Dan's question: 1.  The interconnects from the Eros to the C-Z1 are the Bottlehead interconnects and I believe they are grounded on both sides.  I built them to the manual.  I do have a pair with a floating ground on one side and earlier I tried these in both directions but this before I grounded the Eros to the C-Z1. 2. I did after your recommendation and I'll get to that in the trouble shooting below.

PJ's questions: The Rythmik subwoofer is grounded and I'm running both channels through the sub to use the HPF before the Paramounts to the Altec 755A's.  As stated above, the Paramounts are switched off and all hum levels are evaluated through the sub woofer.  Earlier I did try plugging the sub into a different outlet and tried the cheater plug on it with zero change.  It is dead quiet with the input disconnected and at a very low level when using a different channel of the PB preamp at normal volume levels. 


Trouble shooting.  With the Eros independent of the tape deck, connected to the C-Z1 (on Aux channel) with the BH interconnects and using a three pronged power cord and the PB preamp at max volume this has the highest level of hum.  If I change channels on the C-Z1 preamp to phono, the hum decreases but not to the level if I change channels on the PB preamp.  If I leave the Eros connected via interconnects to the C-Z1 and remove the power cord, the hum level decreases to baseline and is similar to changing the channel on the PB preamp. 

I made a grounding wire and grounded the Eros to the C-Z1 grounding post.  This significantly lowered the hum in the Aux channel input were the Eros is connected.  Of note, it raised the hum level when I change the C-Z1 input to phono or tuner which is a channel with no current input.  This was tested by connected and lifting the ground on the Eros while switching the between the channels on the C-Z1. 

I then connected the Tape deck via RCA's and grounded the tape deck to the Eros with a grounding wire from the Eros to the C-Z1.  The hum level stayed the same (lower level).  Of note,  If I lifted both grounds off of the Eros and touched them together leaving the Eros out of the ground loop, it reduced the hum to the same level as if the C-Z1 was grounded to the Eros. The tape deck did not increase the level of hum.

I hope this helps some and thank you for your help thus far!

Regards,

Aaron

[johnsonad]

Thoughts? 😊

[Paul Joppa]

This chain of six (!) devices is way too complex to give a short answer. We must simplify first.

How about a shorting plug at the Eros input, then Eros to the sub with nothing else connected. If it still hums then we can go into more depth with these elements. If the hum goes away, then you can add one item at a time until it reappears. Both should be plugged into the same power circuit.

Presumably the sub has a level control; be sure to check if it has any effect.

[johnsonad]

Will do tomorrow and reply. Thanks Paul!

[howardnair]

Aaron use the cheater plug your neutral and grounds are tied together at the panel. If you were running a power saw in a puddle you would want to be concerned  also sometimes subs which have a switching power supply can cause issues interconnects with a floating ground may help mine are all floating grounds. And am I correct in understanding that you are running a preamp into a preamp if so that will almost always cause a problem

[johnsonad]

All right, here is my mornings test results…  First abbreviations:
Z1= C-Z1 preamp
EG= Eros with three pronged grounded power cord
EF= Eros with cheater plug on the power cord
DG= Otari with three pronged grounded power cord
DF= Otari with cheater plug on the power cord
GW= ground wire

Overview.  The Subwoofer is grounded.  All power cords are connected to the PS Audio Premier Power Plant including the sub. I used a Radioshack sound level meter, C weighted at 12” from the subwoofer for all results listed.  The PB preamp is used for all test results listed and it is grounded via the power cord.  All levels listed are with max volume on the preamp. 

Background hum measured in CH1 with the Mac Mini attached bia Ifi Nano DAC 63
Background hum measured in CH2 from the C-Z1 with phono selected 66

Series one, Eros connected to the preamp via BH RCA’s
CH3, EF, no connections to the deck (RCA or ground wire) 95
CH3, EG, no connections to the deck 97
CH3, EG, GW only to the deck 88
CH3, EG, DG and plugged in, no connections between deck or Eros 91 (this one was interesting!)
CH3, EG, DG and plugged in, GW between deck and Eros, no other connections 91
CH3, EG, DG, RCA’s connected between deck and Eros, NO GW 102
CH3, EG, DG, RCA’s and GW between deck and Eros 97
CH3, EG, DF, RCA’s and GW 79
CH3, EF, DF, RCA and GW 76

Series two, Eros connected to the C-Z1 preamp via BH RCA’s and the C-Z1 connected to the PB preamp via RCA’s.
CH2, Z1, EF, no connections to the deck 93
CH2, Z1, EF, GW from Eros to Z1 92
CH2, Z1, EG, no connections to the deck 94
CH2, Z1, EG, GW from Eros to Z1 89
CH2, Z1, EF, DG no connections between deck and Eros 91
CH2, Z1, EF, DG, RCA’s between Eros and deck 113
CH2, Z1, EF, DG, RCA’s and GW between deck and Eros 107
CH2, Z1, EF, DF, RCA’s and GW between deck and Eros 74
CH2, Z1, EF, DF, RCA’s and GW between deck and Eros, GW from Eros to Z1 72

CH2, Z1, EF, DF, RCA’s and GW between deck and Eros, GW from Eros to Z1, volume on PB preamp at normal listening level 56
CH2, Z1 phono selected at the same volume as tested for the Eros for normal listening level 53

My primary question is, is there hum in the Eros that I can address?  It would be nice to be able to ground the Eros and the deck but until then, I’m going to float both of them as this gives me the lowest hum level.

[johnsonad]

Thoughts?

Also, I cleaned the grounds on the RCA's and polished the tube pins of the Eros, no improvement though they were getting dirty.

Thanks!

[Doc B.]

My thought from the start is that it would be a lot easier, at least for me, to sort this out by standing in front of the system and trying stuff. There is so much equipment involved that it is pretty difficult to come up with all the possible combinations of grounding connections - though that two prong preamp seems to be the red herring. And hearing the changes in the character of the hum as different setups are tried can also help to determine the cause - buzzy hum is created by grounding issues, soft hum by power supply or electromagnetic interference.

As for hum in the Eros, there is of course some small amount of hum in everything. If you only hear hum and no tube rush it may be a little higher than normal. If the hum is as low as the tube rush and a soft sound you are pretty much at the noise floor limit for the tube. If it is buzzy you still have a grounding problem. If it is not buzzy, typically the repro head and cabling picking up hum is the culprit if you can hear more than a minimal amount at high volume settings. But the EF86s can also pick up hum from other power transformers.

[johnsonad]

Thanks Dan.  Given my tiny listening room, I'm unable to move the deck and Eros away from other transformers at the moment and from what you wrote, that sounds like a likely culprit.  I've got one channel with just a little tube rush and the hum is soft, not buzzy.  It sounds great and that's all that really matters.  Having time off work is causing me to chase things I wouldn't normally have the time for!