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 1 response by johnsonwu

johnsonwu
469 posts
 
Short the input RCA jacks and see if the hum is there.
If so its the preamp, if not its your CD player or cables.

Not your power amp.

Yes some tubes are microphonic, some tubes hum.
Carefully swap the L and R channel tubes and see if it moves with the tubes. If not, then send the amp back to Audio research and have them check signal grounding.

I find Marakanetz's post about Audio Research recommending NFB resistor change to be shocking. Changing NFB to tune overall gain can shift the oscillation pole frequency and can cause stability issues.
More to discover