Krell KAS amplifier hum

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