Power cords, a trip into the unknown!

[fas42]

I haven't tried running the amp with the lid off. If you think it could matter, I will give it a try.

While you're at it, if the buzz is still there, try applying some light pressure with your hands or something elese (taking appropriate care, of course!) on the cover of the transformer in various directions, increasing and decreasing the pressure applied. This changes, or may change, the mechanical loading on various parts of the transformer and amp, and may give you some clues ...

Frank

 

[fas42]

You can look at the spectrum of your mains easily. Get any power transformer and prepare a probe making a turn or two with insulated wire around the core (easier to do with a toroidal transformer) connect the two ends to the ground and signal terminals of a jack plug. Connect the plug to the line input of a sound card, and using any free spectrometer software you can look at the spectra of your mains in safety. .

I've posted this before, but if you want to get a sense of what your component's power supply has to deal with at the mains input, this gives you an idea: http://www.acoustica.org.uk/other/mains_noise.html

Frank

 

[mep]

If it's a mechanical hum, you might also see if running it upside-down helps. Hanging the transformer sometimes changes the vibration pattern, which can then be damped. Another way I reduce mechanical hum from a transformer is to put one spike directly under the bolt that secures the transformer, and then balance the amp using squash balls. This used to work for me when I had a Classe amp back home in Singapore.

Gary-I love ya brother, but I'm not turning this 140lb amp upside down and running it. The top of this amp is like a chimney with all of the heat coming off of it. I see too many things going wrong with flipping this amp upside down.

 

[Orb]

Gary-I love ya brother, but I'm not turning this 140lb amp upside down and running it. The top of this amp is like a chimney with all of the heat coming off of it. I see too many things going wrong with flipping this amp upside down.

Yeah broken toes or fingers for one

 

[mep]

I just plugged the KBL into the same outlet as the KSA-250 and it made no difference.

 

[fas42]

I just plugged the KBL into the same outlet as the KSA-250 and it made no difference.

If there is definitely hum coming from the speaker drivers, not just the physical case of the amp, then it indicates a ground loop problem. Try unplugging the source components, and shifting those mains plugs across also. Plus, you've got that grounding setup that came from Roger: try taking that entirely out of the picture. Some trial and error experiments could be useful ...

Frank

 

[DaveyF]

Mark, is it possible that the hum you are hearing is inherent in the design of your Krell? I know now that the hum I hear in my Rowland 8 is inherent in its design and required a complete mod of the power supply to fix...( which as I said did away with the dreaded hum, but also made the amp less revealing and ever so slightly congested, IMO).
I do think that one has to question why in this day and age ( or even 10-15 years ago when our amps were new), the reason why a high end amp would have any propensity to hum; particularly since some of the very inexpensive receivers and amps that are commonly available ( then and now) are totally quiet.

 

[Steve Williams]

Mark

If it were me I wouldn't be turning anything upside down. If it occurs at AC outlets on different circuits its an amp issue. It's under warranty. I would send it back to Krell.

 

[DaveyF]

+1

 

[mep]

Before I send this beast back to Krell, I'm going to make every effort to eliminate the hum. Ray said it didn't hum at the Krell factory and he thinks it is set-up related. If the power transformer ends up being *bad*, that will not be covered by any warranty. And remember, we are also talking about $500 or so in order to ship it round trip which also wouldn't be covered. The damn thing sounds great, I just want to be rid of the hum/buzz. It's funny that the more expensive the gear gets, the more problems you tend to have.

As for turning it upside down, I already told Gary that isn't happening. Even if that cured the problem, I would sell it before I would have my amp sitting upside down so it didn't hum.

 

[fas42]

Mark, perhaps you haven't read this: http://www.stereophile.com/solidpoweramps/191krell. Talks about the hum issue, and other people have it as well with the big Krells. Solutions that worked for other people are DC blockers, and re-lacquering of the transformer.

Frank

 

[amirm]

Mark, perhaps you haven't read this: http://www.stereophile.com/solidpoweramps/191krell. Talks about the hum issue, and other people have it as well with the big Krells. Solutions that worked for other people are DC blockers, and re-lacquering of the transformer.

Frank

The man is right:

"I've encountered one problem with the Krell that is not an issue with the Mark Levinson: hum. The No.23 is dead quiet, either with the three-conductor grounded power cord or the two-conductor Distech Power Bridge II cord I use. The KSA-250 always produces some hum (audible only with the ears an inch or two away from the speakers), regardless of three- or two-conductor power cords (floating the ground with two-conductor seems to help). There's no way I'm getting a ground-loop problem from somewhere else in the system, since the hum is present even without any input connection. Dan at Krell has suggested that I'm a victim of very dirty and RF-contaminated AC mains. He's probably right, as I've experienced significant improvement to some front-end components through use of an Adcom ACE-515 AC Enhancer. Krell has suggested I try power stabilizers from Tice and Titan to eliminate the power-amplifier hum problem. If and when this occurs (depending on the status of my bank account, as well as available floor space), I'll be sure to let you know."

I would say this is faulty design if so many people are seeing it. The machine should work with the kind of power people are likely to have at home.

 

[amirm]

This part of the review cracked me up, in light of what Mark said earlier:

"Murphy's Law strikes again
One hundred and forty pounds is a hell of a lot of amplifier to haul around, especially when it arrives at the front door 30 minutes before I have to be at a symphony rehearsal. What makes the situation even more maddening is when the darn thing doesn't work. That's exactly what happened with the first review sample. The first thing I noticed was that both positive five-way binding posts at the left channel output were defective, and would slip upon tightening, therefore preventing a solid connection. Perhaps that's why the sound was so covered, and the amplifier soundstaged so poorly. Even after six hours of warmup time, the situation had not changed, so I called Dan at Krell, and was instructed to immediately return the unit. That evening I was admitted to the hospital, since I could not walk, sit, or stand up. Don't carry this amplifier by yourself.

About four weeks later, the second review sample arrived. This time around, I paid the air freight driver to wreck his back, so that I could listen pain-free. From the first moment after startup, I could tell that this was a beast of another color. It worked."

 

[mep]

Weight-wise, the KSA-250 has to be respected because it will hurt you. That’s why I’m in no hurry to even think about hauling it back out of my listening room again. I had to have a neighbor help me dolly it into my listening room (which is downstairs) when I first bought it. I had to have another neighbor help me get it back out of my listening room and into my Ford Edge so I could drive it to a freight forwarder in Indy so I could ship it back to Krell for repair. I had to have the first neighbor come over and help me get it out of my Edge and dolly it back into my listening room after it came back from Krell. There is just no good way for one man to pick this amp up even if lifting 140lbs is no big deal for you.

I read that review before and I remember Lipnick hurting his back, but somehow I missed the hum portion of the article. Both Steve and Micro have owned the KSA-250 before. Do you guys remember it having hum issues?

 

[MylesBAstor]

This part of the review cracked me up, in light of what Mark said earlier:

"Murphy's Law strikes again
One hundred and forty pounds is a hell of a lot of amplifier to haul around, especially when it arrives at the front door 30 minutes before I have to be at a symphony rehearsal. What makes the situation even more maddening is when the darn thing doesn't work. That's exactly what happened with the first review sample. The first thing I noticed was that both positive five-way binding posts at the left channel output were defective, and would slip upon tightening, therefore preventing a solid connection. Perhaps that's why the sound was so covered, and the amplifier soundstaged so poorly. Even after six hours of warmup time, the situation had not changed, so I called Dan at Krell, and was instructed to immediately return the unit. That evening I was admitted to the hospital, since I could not walk, sit, or stand up. Don't carry this amplifier by yourself.

About four weeks later, the second review sample arrived. This time around, I paid the air freight driver to wreck his back, so that I could listen pain-free. From the first moment after startup, I could tell that this was a beast of another color. It worked."

IIRC the 250 was the first Krell amplifier where Dan employed newly developed wave soldering techniques. All the bugs, unfortunately, weren't worked out

 

[DonH50]

Did you read the article that Frank posted a link to? That diode/cap circuit in the article is straight out of the Bryston designs. Schematics of almost all the Bryston amplifiers are available here.

Doing too many things at once... I skimmed it, then promptly forgot about it. Shoot, hardly an excuse, since now that I see it, I remember using that circuit in some other designs. The diodes mean you can use big LV caps, I had forgotten about that trick. I also now remember a transformer winding going south, taking out the diodes, and subsequently filling the inside of the amp with paper and foil from the caps... The transformer smoked, not sure we ever figured out who done what to whom, but it was a mess.

Thanks - Don (should bag techie stuff until after Christmas, too much rehearsing!)

 

[Phelonious Ponk]

Have you hooked the PLs back up yet?

Tim

 

[mep]

I had the PLs hooked up while the amp was back at Krell being repaired and I hooked them back up once just to confirm that I have no noise in my system when they are in the signal chain.