Krell KAS amplifier hum

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