2. amps-preamps
  3. 1369076228

Tube pre picking up feedback/vibration?

Recently got a pair of Magnepan 3.7's and a Sanders Magtech.
Have an old ARC LS15, newly retuned.
Have noticed a muddiness in the bass, kind of a one tone bass at times. When I tap on the preamp cabinet, get a distinct sound transmitting to the speakers, so
I gather I'm picking up some feedback. The pre is rather cole to the speakers, and it doesn't have the isolation of more current models.
Can you guys help with suggestions to stop this?
I'm considering a new preamp, but never noticed this with my previous speakers.
Are there isolation devices to deal with this, tub dampers, etc???
Changes the wires from the Pre to the amp, Putting in my Verbatim wires in place of the Nordost, and that helped some. but want to remove this source of pollution from the system, obviously.
Thanks
Carl
brrgrr

Showing 6 responses by rrog

rrog
1,958 posts
 
Marakanetz, I find your BMW 5 series analogy especially useful since that has been my car of choice for more than 10 years. Come to think of it I have never seen a 3 series with the lack of options you mentioned either.

BTW Audio Research volume pots can be easily adjusted by simply changing a single resistor.
rrog
1,958 posts
 
The resistor change is Audio Research's recommended fix for adjusting volume control setting. Ideally the volume control should be in the 11-1 o'clock position at your desired listening level.
rrog
1,958 posts
 
Remove the top of your preamp and with the system on tap lightly on the tubes. The microphonic tube will be located by using this method.
rrog
1,958 posts
 
ARC checks their tubes, but tubes can change. I am sure they will gladly replace the defective tube.

"LS15 has poor design because their tubes are overbiased for the highest unnecessary voltage gain."

Whose opinion is this? Audio Research designs their preamps for performance. Isn't that what you want?
rrog
1,958 posts
 
"LS15 has poor design because their tubes are overbiased for the highest unnecessary voltage gain."

This is from a guy who uses a Classe 30 and a Bryston 11B. Two solid state preamps that are inferior to the ARC LS15. Maybe you should consider a new preamp.
rrog
1,958 posts
 
Newbee, I used EH tubes supplied by Audio Research in an SP-8 MKII with completely different results. That preamp with EH tubes became as neutral as any preamp I have heard. The OP may have a microphonic tube or the preamp is not setting on a solid surface. Audio Research is making an effort to minimize the effects of the type of surface their equipment is setting on with built in isolation. I think this is a smart move on their part.
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