Krell Saga

[Steve Williams]

Mark, if the hum is still there with the new PC, I would send it back to Krell.

 

[Phelonious Ponk]

I bit my tongue all during the Krell stories, not because I don't think they make good amps, but because I just didn't think you needed them. Sell them. Embrace the PLs. Be happy.

Tim

 

[MylesBAstor]

Time for an update. As some of you may have seen from some of my other posts, I’m having a hum/buzz issue with the KSA-250 amp. You hear it through the speakers, the level doesn’t change with volume, and it’s always there. It is fairly low in level, but since my room is so quiet, it still annoys me. It has raised the noise floor of my system which is the exact opposite of where I’m trying to take my system.

Ray who is in charge of repairs at Krell says it sounds like the power transformer is starved for current. Ray was supposed to send me a Krell 20A power cord when they returned the amp to me and they forgot. The PC I’m using now is made by PS Audio and I don’t like the way it fits into the Krell IEC connector. It sags. I doubt this is the problem though, but we will see when the new PC arrives from Krell. Meanwhile back at the ranch, Joe Abrams has kindly offered to lend me an MIT Z-Trap which Joe thinks will solve the problem. We shall see.

In the meantime, I’ve decided to remove the KSA-250 from service and revert back to my pair of Phase Linear 400 Series 2 amps. They are stone-cold silent and my noise floor is super low with them in the signal chain. They also have a bit more air on top than the KSA-250 and might be a little quicker on their feet. If you believe in the concept of speed, the PLs are speed monsters. The bottom line is that no matter how much I like the sound of the KSA-250, I’m just not willing to put up with the noise it’s adding to my system. Even though you can’t hear it when music is playing, you know it’s there so that means it’s playing with your mind while it’s playing music.

If I can’t solve the hum/buzz issue, the Krell KSA-250 is getting kicked to the curb. The ironic thing is that I moved to SS in order to free myself up from the issues of dealing with vacuum tubes. None of the tube amps I have owned over the years has ever had any type of hum/buzz. They were all dead quiet.

I know some people shudder when I say I’m listening to a pair of PL 400 Series 2 amps, but if you shudder, I promise you that you have never really heard them. I haven’t owned any “super amps,” but I damn sure have owned some great sounding ones. Some of the amps I have owned are the ARC D-76, ARC D-76A, ARC D-79, ARC D-70 MKII, ARC VT-100 MKII, Quicksilver Audio V4s, Quicksilver MS-190s with triode input boards, and once upon a long time ago, a Dynaco ST-70. I can tell you that the PL amps hold their own and then some. I’m bumming that my KSA-250 makes noise, but I’m not bumming that I had to go back to the PLs.

IIRC, you're using subwoofers with your Def Techs? There's no chance that there's some sort of interaction between the amp and the xover? Years ago, the Krell 300s had a hum problem running Infinity RS1bs and it was an interaction with the active xover. Now I don't know about the earlier amps but it might be something to check out.

 

[mep]

Thanks guys. First of all, I made a liar out of myself last night because I ended up putting the KSA-250 back in the system last night. The small amount of hum/buzz that I have been complaining about is overshadowed by this amp's greatness. I started my listening session last night with the PLs, but I just couldn't live with what I was not hearing (and I don't mean the hum/buzz). Here is the next suggestion(s) that I'm going to try from Galen Carol:

"Mark,

You have a ground loop. Unplug the inputs to amplifier to verify; if it goes away it is a ground loop. The next step is to reconnect the input to the amp and begin unplugging each component connected to the preamp, one by one, to see if the hum is eliminated at some point. If you find that disconnecting a particular component stops the hum, then use a Hum-X on its power cord. If you end up disconnecting every component from the preamp (now only the amp and preamp are connected) and the hum remains, then plug the preamp into a Hum-X."

Another very common reason for a ground loop is a cable TV or satellite connection to the system. If you have one, disconnect it first, as that's likely the culprit. You'll need an isolation transformer on the cable feed to combat the problem.

The small amount of hum or buzz you hear from inside the amplifier comes from the transformer, and (unless you can hear from several feet away) is probably normal.

 

[Steve Williams]

Mark

Does it go away with a cheater plug?

 

[mep]

Steve-No, it doesn't go away with a cheater plug. I tried that first thing. I haven't tried what Galen told me to try yet and that is the next step.

 

[Steve Williams]

I thought that if it is a ground loop hum, using a cheater plug will make it go away

Send it back to Krell and demand that they look at it while it is under warranty

 

[mep]

Steve-I have more tricks to try first! You act like shipping the KSA-250 back to Krell is like lifting a paper weight, boxing it, and dropping it off at the Post Office. Shipping the Krell KSA-250 is an expensive 2-man ordeal that involves lifting, grunting, minor bruises and injuries, and around 8 hours driving back and forth to the freight fowarder on two round-trips.

 

[Steve Williams]

Steve-I have more tricks to try first! You act like shipping the KSA-250 back to Krell is like lifting a paper weight, boxing it, and dropping it off at the Post Office. Shipping the Krell KSA-250 is an expensive 2-man ordeal that involves lifting, grunting, minor bruises and injuries, and around 8 hours driving back and forth to the freight fowarder on two round-trips.

Tell me about it. You're preaching to the choir

I shipped 3 Krell 650's for upgrade to the 750Mcx

It was all worth it

 

[microstrip]

Steve-I have more tricks to try first! You act like shipping the KSA-250 back to Krell is like lifting a paper weight, boxing it, and dropping it off at the Post Office. Shipping the Krell KSA-250 is an expensive 2-man ordeal that involves lifting, grunting, minor bruises and injuries, and around 8 hours driving back and forth to the freight fowarder on two round-trips.

Did you try disconnecting the preamplifier and shorting all inputs with a shorted XLR plug, first directly connected to the mains and after with a cheater plug?

 

[garylkoh]

IIRC, you're using subwoofers with your Def Techs? There's no chance that there's some sort of interaction between the amp and the xover? Years ago, the Krell 300s had a hum problem running Infinity RS1bs and it was an interaction with the active xover. Now I don't know about the earlier amps but it might be something to check out.

That is definitely a possibility. The Def Techs have a built-in subwoofer. Some power amps don't like the extremely high impedance at the low frequencies (almost an open circuit to them) of semi-active speakers. If it still buzzes with no inputs connected, and the power cords removed from the speakers, then you're out of luck - the Krell won't work with the Def Techs - or probably any loudspeaker with a built-in subwoofer.

 

[fas42]

That is definitely a possibility. The Def Techs have a built-in subwoofer. Some power amps don't like the extremely high impedance at the low frequencies (almost an open circuit to them) of semi-active speakers. If it still buzzes with no inputs connected, and the power cords removed from the speakers, then you're out of luck - the Krell won't work with the Def Techs - or probably any loudspeaker with a built-in subwoofer.

I don't follow that, Gary. If the amp doesn't like a high impedance presented by the sub-woofer at the low frequencies, then I don't see why a simple solution wouldn't be to place a resistor of an inbetween value across the appropriate point on the speaker. Something like a 200, or 1k, should do the trick, I would have thought ...

Frank

 

[mep]

micro-Do you mean shorting out both the RCA and XLR inputs of the amp?

 

[mep]

That is definitely a possibility. The Def Techs have a built-in subwoofer. Some power amps don't like the extremely high impedance at the low frequencies (almost an open circuit to them) of semi-active speakers. If it still buzzes with no inputs connected, and the power cords removed from the speakers, then you're out of luck - the Krell won't work with the Def Techs - or probably any loudspeaker with a built-in subwoofer.

Gary-I'm not understanding your post. I would think that if I disconnected the power cord for the amp built into my speakers and the buzz went away it would tell me the Krell KSA-250 was incompatible with the speakers.

 

[garylkoh]

I don't follow that, Gary. If the amp doesn't like a high impedance presented by the sub-woofer at the low frequencies, then I don't see why a simple solution wouldn't be to place a resistor of an inbetween value across the appropriate point on the speaker. Something like a 200, or 1k, should do the trick, I would have thought ...

Frank

Because then it would short out his built-in subwoofer.

 

[microstrip]

micro-Do you mean shorting out both the RCA and XLR inputs of the amp?

The RCA is in parallel with the positive phase of the XLR - just short the XLR with a XLR plug with the 1 2 and 3 shorted.

 

[garylkoh]

Gary-I'm not understanding your post. I would think that if I disconnected the power cord for the amp built into my speakers and the buzz went away it would tell me the Krell KSA-250 was incompatible with the speakers.

Mark, some amplifiers are unstable on open circuit (which is why most manufacturers recommend connecting up the loudspeakers before turning the amplifier on). Your loudspeakers, because it has a built-in subwoofer amplifier has a high-pass passive crossover to protect the midrange drivers from low-frequency energy. This combination results in a very high impedance (it can be up to 100k ohms) in the low frequencies (where the subwoofer is working).

Removing the power cord for the amp built into your speakers removes the possibility of some ground loop/interaction between the Krell and the speakers. There could still be some ground loop between the Krell and your source components, which is why microstrip suggested removing the inputs and shorting them. I don't think that shorting the inputs is necessary with the Krell which should have capacitor coupled inputs.

 

[fas42]

Because then it would short out his built-in subwoofer.

What??! What about the Krell output stage hooked up to this same point, which probably has an impedance of milliohms, from the point of view of the subwoofer: that's not going to cause a problem to the subwoofer, but the 200 ohm resistor will?

Frank

 

[mep]

Mark, some amplifiers are unstable on open circuit (which is why most manufacturers recommend connecting up the loudspeakers before turning the amplifier on). Your loudspeakers, because it has a built-in subwoofer amplifier has a high-pass passive crossover to protect the midrange drivers from low-frequency energy. This combination results in a very high impedance (it can be up to 100k ohms) in the low frequencies (where the subwoofer is working).

Removing the power cord for the amp built into your speakers removes the possibility of some ground loop/interaction between the Krell and the speakers. There could still be some ground loop between the Krell and your source components, which is why microstrip suggested removing the inputs and shorting them. I don't think that shorting the inputs is necessary with the Krell which should have capacitor coupled inputs.

You know it’s funny that we are now bringing the built-in powered subs into the equation. My ARC VT-100 MKII had an issue where it sounded like it was oscillating with my speakers at very low frequencies and I wondered then if the subs were causing that problem. The damn thing is that my old PL 400 Series 2 amps which only have skinny two prong power cords are quiet as a church mouse with my speakers.

 

[garylkoh]

What??! What about the Krell output stage hooked up to this same point, which probably has an impedance of milliohms, from the point of view of the subwoofer: that's not going to cause a problem to the subwoofer, but the 200 ohm resistor will?

Frank

If you put a 200 ohm resistor across the input of the subwoofer, you would short it out and the subs won't work. The output impedance of the amplifier has nothing to do with it.