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.
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-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.
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.
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 ...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.
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
micro-Do you mean shorting out both the RCA and XLR inputs of the amp?
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.
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?Because then it would short out his built-in subwoofer.
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.
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