Krell KAS amplifier hum


The amps are directly plugged into their own dedicated 20A outlet.  Is there a "conditioner" I can use to minimize/eliminate the mechanical hum I'm hearing through my MBL 111f speakers.  I'm not looking for a multiple outlet item, rather, one which plugs directly into the wall and into the back of each monoblock.  Please advise and thank you in advance.

Best,
Jose
jg2077
Confusing post.

Mechanical hum cannot be coming from the speakers. Not unless they are powered. Are MBL powered? I don't think so. So if the sound of hum is coming from the speakers then its electrical,and probably due to the amps being on a different and unnecessary line.

If the humming is physically coming from the amps themselves, you can hear the transformers making noise, then they are vibrating due to DC offset. The fix is either find the cause and turn it off, or plug the amps into a DC offset eliminator.
And a DC offset eliminator may not completely remove the mechanical hum from transformers. This has been my experience with one particular amp, and after trying everything possible the only culprit remaining was delamination "rattle" from loose windings over time.  The DC offset measures helped but didn't fully eliminate my issue with the tube amp.  Every other amp I've ever plugged into the same circuit, tubed or SS, never hummed a peep using the same speakers/system.  Sometimes it's simply the amp and not DC riding on your line.
MBL 111f speakers are not powered.  The hum i am hearing is coming out of the speakers once i power on the amplifiers.  Therefore, it is the wall outlets or amplifiers that are generating/transferring the hum into the speakers.  Can someone recommend me a product to minimize the hum?  One (product/solution) they have personally used to good effect?  Thank you in advance.  These amps require (necessary) 20A power supply lines.  I had 2 dedicated lines installed, one for each monoblock since my room only had one 15A line.  

@millercarbon - the intent of my query was never meant to be confusing.  Thank you for pointing that out.

Best,
Jose  
@ - three-easy-payments
I believe the hum IS being created by my amps.  I am hoping there is a solution available.  The hum is only audible when the music is not playing, even at low levels.

Thank you,
Jose
@jg2077   This Emotiva CMX-2 at just $129 is a good starting point to see if DC offset can be removed.  It comes with a 30-day return so not much risk.  You will know right away.  It my case it removed about 50% of the transformer hum.  

https://emotiva.com/products/cmx-2
OP,

I assume you are hearing the hum from the left and right channel speakers.

Hum is probably caused by a ground loop.

Are you using balanced ICs or SE ICs?

How are you feeding 120V power for the preamp and the connected front end equipment?
Another dedicated circuit?
Or an existing 15 amp convenience outlet branch circuit?

Update,  I unplugged the 6m long XLR from each amp.  Then, I powered on both amps.  There was no hum at all. 

@jea48 
All instruments are plugged into a Silver Circle audio, "pure power one 5.0, which is plugged into an existing outlet. 

Pre = ARC Ref 3
Phono = DSA
Tuner = McIntosh MR 67
Transport = ML No31
Dac = ML 36s
TT = Final audio VTT-1 
Tonearms = Talea and Kuzma 4pt

Best,
Jose
This means @jea48 is correct - you have a ground loop hum. I still had my hum with only speakers plugged into amp and no other components connected. You need to go through through a process of elimination adding one component at a time to see which is causing the hum.
@ jg2077

There is a good chance the existing 120V 15 amp convenience outlet branch circuit safety equipment ground is causing the ground loop hum. There is a good chance there is a difference of potential, voltage, between the convenience outlet safety equipment ground and the equipment grounds of the two 20 amp dedicated branch circuit outlets, for the power amps.

For a test try this.

Unplug all front end equipment ICs from the input jacks of the preamp. Including any phono ground wire.
All you want is the preamp connected to the two amps. Nothing else.

Check for ground loop hum through speakers. No hum? Post back.


If you still have the hum, for a test, connect a ground cheater between the plug of the preamp power cord and the wall outlet. Check for the hum. (Make sure the cheater plug ground wire, or lug, is isolated, insulated, from contacting the screw that holds on the wall outlet cover plate).
You can also use a 2 wire IEC power cord if you have one instead of using your existing preamp power cord. You won’t need a ground cheater then.


Check for hum.... If the hum is gone post back.



All was unplugged from the back of the pre, I still have "hum".

I ran a ground wire from the pre power cord (plugged into the back of the power distributor) to the ground of wall outlet the power distributor is plugged into; the hum remains...??

Many thanks for all advice given, 
Jose
@jea48 is the best there is for troubleshooting hum. You need to follow his instructions to the letter. Remove the power conditioner from the system. You should only have the monoblocks and the preamp hooked up and plugged into wall receptacles.

jea48 said:
You can also use a 2 wire IEC power cord if you have one instead of using your existing preamp power cord. You won’t need a ground cheater then.
That won’t work..... The male ground pin on the IEC inlet on preamp would be in the way.


jg2077 OP41 posts

12-11-2019 7:52pm

All was unplugged from the back of the pre, I still have "hum".

I ran a ground wire from the pre power cord (plugged into the back of the power distributor) to the ground of wall outlet the power distributor is plugged into; the hum remains...??

Many thanks for all advice given,

That won’t work.  Adding a parallel ground path will solve nothing.

Use a ground cheater on the ARC preamp power cord plug. (Nothing connected to the inputs of the preamp. Again just the preamp and the 2 amps. Nothing else)
You need to lift, float, the equipment ground from the chassis of the ARC preamp. That will break the ground loop circuit.

By chance do the Krell amps have a ground lift switch on the rear panel? If so the switch will lift the signal ground of the amps from the safety equipment grounded chassis.
Just a note. At this point I am hoping the problem is the equipment ground of the 15 amp convenience outlet circuit. The ground cheater on the ARC preamp power cord will tell us that for sure if it breaks the ground loop and hum.

Take both mono block out of the circuits, connect a headphone amp to the ARC ref3 output and listen if hum still exist?

In some case, hum is caused by upstream equipment.
Is there a "conditioner" I can use to minimize/eliminate the mechanical hum

Beautiful amps BTW.

There is no conditioner I know of that can take idle current plus music playing current into MBLS’s of a pair of KAS Krell monoblocks and not be near it’s limit.

If your hearing hum through the speakers.
KAS Monoblocks can give hum by their very nature of being mono blocks, they should BOTH be plugged into the same heavy duty mains outlet to stop any earth loops through the house wiring.

Also use some "earth cheater plugs" on the preamp and source/s so they get the earth from the Krells through the interconnects, this way you stop any more earth loops through the house wiring.

Cheers George
georgehifi Said:
If your hearing hum through the speakers.
KAS Monoblocks can give hum by their very nature of being mono blocks, they should BOTH be plugged into the same heavy duty mains outlet to stop any earth loops through the house wiring.

jg2077 OP41 posts12-11-2019 5:54pmUpdate, I unplugged the 6m long XLR from each amp. Then, I powered on both amps. There was no hum at all.

@georgehifi ,
Yes I do realize the test above isolated the two amps from one another therein breaking the connection of the two amp’s power supply’s interactions (circuit ground/signal ground) but probably more importantly the two amp’s dedicated safety equipment grounding conductors.
Also worth mentioning is how did the designer of the Krell KAS amplifier connect the circuit ground/signal ground to the chassis of the amp where the AC mains safety equipment ground is bonded, connected to.



georgehifi Said:

Also use some "earth cheater plugs" on the preamp and source/s so they get the earth from the Krells through the interconnects, this way you stop any more earth loops through the house wiring.

Exactly.

Though there may be other factors at play. Process of elimination. The OP has to start somewhere. If we are assuming the hum is caused by a ground loop, imo, we need to isolate the 120V 15 amp wall outlet convenience outlet branch circuit safety equipment grounding conductor from the equation.

Why I picked the 15 amp convenience outlet circuit is because to me it has the highest probability for causing the ground loop hum.

Factors to consider?
Total length of the branch circuit wiring from the electrical panel to the wall outlet.
How many outlet boxes are in the run.
How were the equipment grounding conductors spliced together. (Wiring method used)
What other electrical loads are connected to the branch circuit. Possible small leakage currents from the hot line to equipment grounding conductor that may produced/caused by a connected load/s.


@ jg2077

ground cheater

https://www.homedepot.com/p/Commercial-Electric-15-Amp-Single-Outlet-Grounding-Adapter-Gray-U-09/203...

Put a piece of tape on the bottom side of the ground tab/lug, or over the outlet cover metal support screw, so it can not touch, contact, the equipment grounded receptacle metal screw that supports the outlet cover plate.




georgehifi6,400 posts

12-12-2019 12:32am


Also use some "earth cheater plugs" on the preamp and source/s so they get the earth from the Krells through the interconnects, this way you stop any more earth loops through the house wiring.

Use of ground cheaters for testing purposes only.

ICs signal grounds/small wires should never be used for safety equipment grounding conductors. In the event of a ground fault event not only could the ICs be damaged but worse yet audio equipment itself. Also worth mentioning in well designed audio equipment the circuit ground/signal ground is not connected directly (hard wired) to the chassis of the equipment where the safety equipment grounding conductor is bonded, connected. In many cases a small wattage low resistance resistor is used in series with the connection to connect the circuit ground/signal ground to the chassis.


Jim



You could try the Ebtech HumX, but you might need two.
Thank you all for taking the time.  After my first failed attempt last night, I decided to plug all "in" and listen to music instead :-)

Tonight, I will try what has been recommended.  Something else I forgot to mention; I have a cable line and box in this room.  The cable line runs parallel to the XLR (they do not touch) and is also plugged into the same 15A outlet the power distributor is plugged into...

with kind regards,
Jose
Post removed 
@ jg2077,

Does the cable box connect to the audio system, in question, in any way by wire type ICs?

Does the cable box 120V power cord have a 3 wire grounding type plug? If yes just for the heck of it unplug it from the wall convenience outlet tonight before you start disconnecting things for testing. Even if there is not any ICs connecting the cable box to the audio system.

A question that should have been asked at the start of this thread. Have you always had the hum in this audio system? If not at what point did you first notice the hum?

.
@jea48  - the cable line plug is a wall-wart and does not have a ground pin.  It is not tied (a part of) the sound system.  I'm not sure when i first heard it...you see, I'm alternating amplifiers (also use ML 33H) and did not pay it much attention.  I will certainly try what you are suggesting and report back.  Thank you for contributing.

Respectfully,
Jose
Update - the pre is plugged into a common 15A outlet; the amps are plugged into a dedicated 20A outlet and ALL else is unplugged.  I still hear the same amount of hum.

I installed a "cheater plug" between the pre and wall outlet,  I hear less hum than before, about 70% less.  I cannot hear hum from my listening position (10ft) away.

The "common" 15A leg has 7 outlets, 3 of which are being used.   1 outlet in particular has my humidor plugged into it. Unplugging everything from this 15A leg yields no change.

Thank you, 
Jose
Some thoughts that have occurred to me after reading through this thread:

1) Since these amps are huge high powered monsters which draw lots of current and require a dedicated 20 amp line for each monoblock, I’m guessing that the electrician probably installed the two lines on opposite AC legs. Jim (Jea48), some years ago you had provided us with a link to a paper by Henry Ott and Bill Whitlock which explained that powering interconnected equipment from opposite legs is conducive to ground loop issues. And of course the grounds of the two amps are interconnected via the preamp.

If the additional experiments the OP will be doing don’t lead to a solution, based on your expertise on such matters do you think it would be reasonable to have the electrician put both amps on the same leg, per the suggestion George made earlier? Or is that likely to unbalance the legs to an undesirable degree?

2) An alternative approach that would certainly resolve a ground loop issue involving the amps would be to insert a single-channel Jensen transformer between the preamp and each of the amps. Based on numerous experiences with Jensen transformers that have been reported here and elsewhere I would expect the sonic effects of doing that to be very minimal, although perhaps not imperceptible. The two transformers would cost something like $250 each.  A very short additional XLR cable would also be required for each channel.

3) I’m also a bit suspicious of the preamp. I infer from what the OP has said that the hum does not occur when the Mark Levinson 33H amps are used with the same preamp. However based on the last paragraph of this page of Stereophile’s review of the 33H it sounds like the input stage of that amp may very well ignore the XLR ground pin (pin 1), while the Krell amps (like many and perhaps most other amps which provide XLR inputs) may not. So the response of the two amps to an issue in the preamp which would inject hum via that ground path could very well be different. Also, the preamp in question apparently uses a 6H30P tube and either a 6L6 or a 6550 (depending on when the preamp was manufactured) in a voltage regulator circuit in its power supply, which supplies B+ to its output stage. And it appears that the filaments of those tubes are powered with AC, which adds to my suspicion. So if at all possible I would suggest that the OP try a different preamp, as an experiment.

4)

I installed a "cheater plug" between the pre and wall outlet, I hear less hum than before, about 70% less. I cannot hear hum from my listening position (10ft) away.

Perhaps multiple issues are present, namely some combination of a ground loop involving the preamp (when the cheater plug is not used); the use of opposite AC legs to power the amps; and possibly also a problem in the preamp, perhaps involving excessive filament-to-cathode leakage in one of the tubes in the voltage regulator circuit I referred to.

Regards,
-- Al

Al, thank you for such a lengthy explanation. 

I plugged EVERYTHING as before, with one exception,  there is a "ground cheater " between the pre and power distributor. The hum is 50% less than first posted.

I tried both amps on the same outlet - no change.
The pre uses a 6L6 tube.
Maybe, I will try some of the grounding devices suggested.  Funny thing is,  I've been wanting to upgrade my pre...😀

Respectfully, 
Jose
Al (almarg),

I was hoping you would chime in at some point.

Them Krell amps are pigs and are power hungry when pushed hard.

I looked over what you posted and will respond tomorrow.

I noticed the OP posted after you and he tried some additional tests. One, he plugged both amps into one duplex 20 amp dedicated circuit and it didn’t make any difference. Maybe both dedicated circuits are fed from the same Line, leg.

Did you look at that Silver Circle audio, "pure power one 5.0 power conditioner?
https://positive-feedback.com/Issue33/silvercircle.htm

Jim

Rule No1: Only put these sort of amps direct into the wall.

Those Krells of yours have an incredibly "stiff" (low esr, low impedance) power supply and great output current delivery that’s why they can drive anything even the worst speaker load yet still get great controlled bass, this is only got by plugging direct into the mains.
To put any power conditioner between the Krells and the wall outlet and you introduce series resistance which lower the power supplies stiffness and that bass and ability to deliver huge current is diminished.

Cheers George
@georgehifi

The Krell amps do plug directly into the wall outlets. Each amp has its own dedicated 120V 20 amp branch circuit.

The " Silver Circle audio, "pure power one 5.0 power conditioner" feeds the ARC preamp and the front end equipment.
All instruments are plugged into a Silver Circle audio, "pure power one 5.0, which is plugged into an existing outlet.

Pre = ARC Ref 3
Phono = DSA
Tuner = McIntosh MR 67
Transport = ML No31
Dac = ML 36s
TT = Final audio VTT-1
Tonearms = Talea and Kuzma 4pt
Existing outlet is a 120V 15 amp convenience outlet branch circuit. (More than likely 14ga wire)
The "common" 15A leg has 7 outlets, 3 of which are being used. 1 outlet in particular has my humidor plugged into it. Unplugging everything from this 15A leg yields no change.


The " pure power one 5.0" has a 5KVA isolation transformer making the unit a separately derived power system. Though the secondary 120V output is isolated from the input mains power the mains wall outlet’s safety equipment ground basically passes through the " pure power one 5.0" and connects to the equipment ground contacts of the units output duplex receptacle outlets. (The designer/manufacturer, just a guess, added filtering or what ever to/for to clean up noise that may be on the equipment grounding conductor entering from the mains wall outlet but there is nothing the " pure power one 5.0" can do about resistivity of the equipment grounding conductor from the mains electrical panel to the wall outlet.

The same mains wall outlet safety equipment grounding conductor is used for bonding,connecting, the grounded neutral conductor of the separately derived power system to the earthed grounding system of the main electrical service. (Technically because of the KVA size of the unit using the branch circuit safety equipment grounding conductor for the earth grounding (Bonding Jumper) does not meet NEC code.)

I wouldn’t be surprised if there isn’t a difference of potential, voltage, between the "pure power one 5.0" output neutral and the neutral at the 120V 20 amp dedicated branch circuits outlets for the two Krell amps.

jg2077 12-12-2019 8:59 pm EST:
... the pre is plugged into a common 15A outlet; the amps are plugged into a dedicated 20A outlet and ALL else is unplugged. I still hear the same amount of hum.

I installed a "cheater plug" between the pre and wall outlet, I hear less hum than before, about 70% less. I cannot hear hum from my listening position (10ft) away....


jg2077 12-12-2019 10:14 pm EST:
I plugged EVERYTHING as before, with one exception, there is a "ground cheater " between the pre and power distributor. The hum is 50% less than first posted.

I tried both amps on the same outlet - no change.

Am I correct in understanding that in the first of these two experiments the preamp was plugged directly into the wall outlet, without the power conditioner? If not that should certainly be tried, as Lowrider had suggested earlier.

Jim (Jea48), thanks for the link and your comments about the power conditioner. I also looked at the writeups at the Silver Circle website about the 5.0 conditioner and also at the writeup about the 5.0se. Statements in the writeup about the 5.0se confirm that the 5.0 incorporates filtering of some sort, as you speculated, with inductors being specifically referred to (that are presumably in series with some combination of the hot conductor, neutral conductor, and perhaps even the safety ground conductor, on whichever side of the transformer). Without detailed information about the design I think that all we can say is that the conditioner **might** be contributing to a ground loop issue in this application. And as was suggested the hum should be assessed with the preamp plugged directly into the wall, with its inputs disconnected, if that hasn’t already been tried.

One other thing that seems worthy of note: The OP’s speakers have very low sensitivity (apparently 83 db, although it isn’t clear if that is on the basis of 1 watt or 2.83 volts; if the latter, given its 4 ohm nominal impedance the speaker is only 80 db/1 watt). So with speakers of average or higher sensitivity the hum would be much louder than what has been described. (In saying this I’m assuming that the amount of hum doesn’t vary as a function of the volume control setting).

Best regards,
-- Al

almarg9,232 posts

12-13-2019
9:34am


jg2077 12-12-2019 8:59 pm EST:
... the pre is plugged into a common 15A outlet; the amps are plugged into a dedicated 20A outlet and ALL else is unplugged. I still hear the same amount of hum.

I installed a "cheater plug" between the pre and wall outlet, I hear less hum than before, about 70% less. I cannot hear hum from my listening position (10ft) away....


jg2077 12-12-2019 10:14 pm EST:
I plugged EVERYTHING as before, with one exception, there is a "ground cheater " between the pre and power distributor. The hum is 50% less than first posted.

I tried both amps on the same outlet - no change.

Am I correct in understanding that in the first of these two experiments the preamp was plugged directly into the wall outlet, without the power conditioner? If not that should certainly be tried, as Lowrider had suggested earlier.

Al (almarg),
Yes you are correct the OP plugged the preamp directly into the wall 15 amp convenience outlet and he did not hear any difference in hum.

He then tried the ground cheater between the wall outlet and the preamp. He lowered the hum by 70%. That tells me indeed he has a ground loop that is caused by a difference of potential between the equipment ground at the convenience wall outlet and the Krell amps 20 amp dedicated branch circuit outlets equipment ground.

Almarg said:
Without detailed information about the design I think that all we can say is that the conditioner **might** be contributing to a ground loop issue in this application. And as was suggested the hum should be assessed with the preamp plugged directly into the wall, with its inputs disconnected, if that hasn’t already been tried.

Al, that’s what I thought when I first read this statement of the OP’s.
jg2077 12-12-2019 10:14 pm EST:
I plugged EVERYTHING as before, with one exception, there is a "ground cheater " between the pre and power distributor. The hum is 50% less than first posted.
70% less hum to 50% less using the power conditioner? That don’t make any sense. Then I went back and reread what the OP said.

I plugged EVERYTHING as before, with one exception, there is a "ground cheater " between the pre and power distributor. "

" I plugged EVERYTHING as before," ........ As in all the other equipment he had plugged into power conditioner originally before he started troubleshooting for his hum problem.
Well, that’s a horse a different color.

One of these is causing the additional ground loop hum. One of them, or more, has a 3 wire cord and plug. Also I would guess the way the circuit ground/signal ground is connected to the equipment grounded chassis is definitely different than the way the ARC preamp is done. Your thoughts.......

Phono = DSA
Tuner = McIntosh MR 67
Transport = ML No31
Dac = ML 36s


Last night when the OP was trying different things and when he plugged both Krell amps into one duplex outlet dedicated 20 amp branch circuit, I wonder if he could have been able to plug the "pure power one 5.0" power conditioner into the other unused dedicated branch circuit 20 amp duplex outlet. Then plug the preamp into the pure power one 5.0. Listen for hum, then plug in the front end equipment and check for hum. That would speak volumes.......

Al, I liked your idea about using the Jensen transformers on the balanced ICs between the preamp and krell amps. Not only does it isolate the ARC preamp from the two amps but wouldn’t they isolate the two amps from one another because of the common circuit ground/signal ground connection of the ARC preamp. Your thoughts....
Jim



almarg9,232 posts   

12-12-2019  
 8:26pm  

Some thoughts that have occurred to me after reading through this thread:

1) Since these amps are huge high powered monsters which draw lots of current and require a dedicated 20 amp line for each monoblock, I’m guessing that the electrician probably installed the two lines on opposite AC legs. Jim (Jea48), some years ago you had provided us with a link to a paper by Henry Ott and Bill Whitlock which explained that powering interconnected equipment from opposite legs is conducive to ground loop issues. And of course the grounds of the two amps are interconnected via the preamp.


Al, I think this is the quote you are referring to. For what ever reason the web Link no longer works

"Less than 300 microamps of ground loop current can cause hum as it flows in an unbalanced audio interconnect cable. However, harmonics of 60Hz that are generated from lighting dimmers or switch-mode power supplies sound like Buzzz mixed with a bit of Hummm and are more easily coupled by even smaller currents. Harmonics can add together when equipment is powered from different phases, so clearly there is an advantage to specifying same-phase electrical service to power the electronics systems in most cases....

Any leakage currents on the safety ground wires of split single phase load circuits fed by different phase legs will add together due to the 240V potential difference....

Power conditioners do not solve any of these common problems: Cross phase coupling (doubles hums & buzzes) .... What actually does solve them: Same phase power.

.

Split Single Phase electrical service is most commonly found in residences and smaller commercial buildings, and is commonly used to feed AV equipment. One key advantage that single phase has over three phase is that while harmonic currents are still present, it is not possible for the �triplen� components to add in the neutral. In addition, use of split single phase can result in at least a 6dB reduction in noise floor as compared to three phase if the capacitances of the connected equipment are relatively well balanced. However, any leakage currents on the safety ground wires of split single phase load circuits fed by different phase legs will add together due to the 240V potential difference."

http://www.exactpower.com/elite/assets/pdfs/theTRUTH.pdf


Jim


Gentlemen, please allow me to be COMPLETELY off-topic.
I am humbled by all the thought processes and the suggestions you have provided.  Audiogon is filled with back and forth banter, so much so, that there are many recent posts about it...However, you prove that this is a site where help and assistance is given/provided, Thank you.

@jea48 - Yes, everything was was reinstalled as before.  The one exception was the ground plug installed between the pre and power distributor (hum was reduced to 50% less of when i first posted).  It would be easy to plug the 5.0 into one of the unused 20A outlet, as would be to move the Pre.  Having to Move everything would require MUCH work.  Could I use a long heavy-duty power cord plugged into one of the 20A outlet to power the 5.0 distributor where it sits without moving it?

I have a Coda 02b preamp, I will try it tonight and check for hum.

Respectfully,
Jose
Jose, thanks for the nice words about the nature of this thread. Yes, unfortunately all too many threads seem to devolve into pointless, unpleasant, and unconstructive bickering.

Jim, yes, that is the statement I was referring to in the paper by Henry Ott and Bill Whitlock. Both of them being very distinguished experts, as you of course know.

One of these is causing the additional ground loop hum. One of them, or more, has a 3 wire cord and plug. Also I would guess the way the circuit ground/signal ground is connected to the equipment grounded chassis is definitely different than the way the ARC preamp is done. Your thoughts.......

Phono = DSA
Tuner = McIntosh MR 67
Transport = ML No31
Dac = ML 36s

Even though the MR67 has a two-wire power cord and plug it wouldn’t surprise me if it is contributing to the problem, given its age. Like many other tuners and other components of its era (ca. 1963) it has a small (0.01 uf) capacitor connected between the hot conductor and its circuit ground, and another such capacitor connected between the neutral conductor and its circuit ground. And given its 50+ year age it wouldn’t surprise me if there were some leakage through the cap that is between the hot conductor and circuit ground, which in turn is connected directly to the shell of its RCA output connectors and thence to the circuit ground of the preamp.

Al, I liked your idea about using the Jensen transformers on the balanced ICs between the preamp and krell amps. Not only does it isolate the ARC preamp from the two amps but wouldn’t they isolate the two amps from one another because of the common circuit ground/signal ground connection of the ARC preamp. Your thoughts....

Yes, absolutely. The Jensen PI-XX would probably be a good choice, used in conjunction with a very short low capacitance XLR cable (100 pf max capacitance for the total length of the cable) connected between its output and the amp input. Specs are identical to those shown in this datasheet for the two-channel version. That choice should be confirmed with Jensen, though, and when speaking with them they would probably want to know what the input impedance of the amp is (which I haven’t been able to determine). The input impedance should be at least 10K to be suitable for use with this transformer. The PI-XX is available here for only $150 each.

Best regards,
-- Al

almarg9,233 posts

12-13-2019
12:48pm


One of these is causing the additional ground loop hum. One of them, or more, has a 3 wire cord and plug. Also I would guess the way the circuit ground/signal ground is connected to the equipment grounded chassis is definitely different than the way the ARC preamp is done. Your thoughts.......

Phono = DSA
Tuner = McIntosh MR 67
Transport = ML No31
Dac = ML 36s

Even though the MR67 has a two-wire power cord and plug it wouldn’t surprise me if it is contributing to the problem, given its age. Like many other tuners and other components of its era (ca. 1963) it has a small (0.01 uf) capacitor connected between the hot conductor and its circuit ground, and another such capacitor connected between the neutral conductor and its circuit ground. And given its 50+ year age it wouldn’t surprise me if there were some leakage through the cap that is between the hot conductor and circuit ground, which in turn is connected directly to the shell of its RCA output connectors and thence to the circuit ground of the preamp.
Al, great point!

Even if the power switch is off there is still one of the 0.01 uf capacitors between one AC mains Line to chassis/circuit ground. Depending on which way the non polarized plug is plugged in to the outlet there is a 50 50 chance the hot mains conductor is on the non switched side.

https://www.manualslib.com/manual/999194/Mcintosh-Mr-67.html?page=14#manual

The OP may want to unplug the receiver and have new capacitors installed. Just a guess he probably wants to keep it original.


Al, as for the Jensen transformers you mentioned the XRL cables the OP is using are 6m long.

Any RF (buzzing) issues using a 6m XRL cable on the OP’s Krell amps?

Jim.

When I get home, I will look for the Krell manual and see if it states the input impedance.  Additionally, I will unplug the tuner.  

Come to think of it, prior to experiencing the hum issue, I was using my Sansui TUX-1 tuner, so it may be the introduction of the McIntosh, (I'm always alternating gear)...

Those Jensen look to be the ticket.

with best regards,
Jose
Al, as for the Jensen transformers you mentioned the XRL cables the OP is using are 6m long.

Any RF (buzzing) issues using a 6m XRL cable on the OP’s Krell amps?

Jim, as you may have seen the length of the cable on the input side of the transformer is not critical, from the transformer’s perspective. What matters from its perspective is just the capacitance it sees on its output side, which for a given cable type is of course proportional to length.

The constraint on the length of the 6 meter cable is that its capacitance should support ARC’s usual recommendation for most of their preamps and line stages of load capacitance of 2000 pf max. For a 6 meter cable that corresponds to around 100 pf/foot. Any decent cable should meet that requirement with a good deal of room to spare, which in turn would almost certainly allow more than enough margin for the unknown and most likely unspecified input capacitance of the amp, that would be "reflected" through the transformer and that is probably small compared to the capacitance of the 6 meter cable.

As far as RF which may be picked up by that cable is concerned, the transformer’s 50 kHz 3 db bandwidth limitation can only help.

Best regards,
-- Al


jg2077 OP47 posts

12-13-2019
11:43am


It would be easy to plug the 5.0 into one of the unused 20A outlet, as would be to move the Pre. Having to Move everything would require MUCH work. Could I use a long heavy-duty power cord plugged into one of the 20A outlet to power the 5.0 distributor where it sits without moving it?

How long is the heavy duty power cord?
25ft or less? If yes that would be fine. Just don’t coil up any excess cord.



Update:

1.  All was left as initially, I only powered up the amp; very low hum from the speakers (ear against the speaker).  Could not be heard from 1 ft. away. 

2.  Unplugged pre from back of distributor and turned ON the distributor.  No change from above. 

3.  Plugged in Pre, the hum increased significantly. 

4.  Unplugged distributor from common 15A outlet.   Ran a 10ft cord from unused 20A dedicated outlet to the distributor  and powered on.  Same outcome as noted above; maybe...slightly less hum.

I will try another Pre and report back.   I have to take my son to pitching lessons. 😀

Regards, 
Jose 
Update:

I replaced the 6L6 tube and left all else the same; the hum can only be heard from 18inches away, without any music. Also, the hum is of a lower frequency.   I can tolerate this for the time being. 

I am going to replace the ARC Ref3.  I haven't made up my mind as to my next preamp choice. 

Thank you all for helping. 

Happy Holidays, 
Jose