Krell KAS amplifier hum

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

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

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

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