2. amps-preamps
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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
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.
marakanetz's avatar
marakanetz
5,473 posts
 
Rrog,
I believe that preamp should be minimalistic and perform as close to passive as possible. LS15 is quite opposite to any amplifiers it drives.
And yes I've upgraded to Mytek 192 DSDDAC/preamp/headphone + Musical Fidelity XPS for analogye mainly for excellent dynamic capabilities and bunch of digital inputs. If I'd use ARC on my rig, I wouldn't be able to turn volume past 8:00. Can you imagine how dynamics would suffer? Nah I'd like to have linear performance on nearly all volume levels as I have right now. Donno about youall.
All components have to be driven with comfort -- forget the luxury because price and model has nothing to do with system synergy.
Have you ever seen BMW 5 series with plush seats and manual sunroof/side windows? I guess I don't mind driving it... You?
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.
marakanetz's avatar
marakanetz
5,473 posts
 
That single resistor is a feedback resistor. Altering this resistor from default setting to some magnitude may bring the amp onto oscillation or out of stability and another level of microphony(good point to check btw). The best what can happen is sacrificed dynamic range and transparency. I believe and know that tubes are not microphonic and amps/preamps are. There are plenty of good designed tube preamps with no microphony and my eyes and hands had seen them. Did anyone have an experience with AES by Cary little tube preamp?
As to BMW 5 or 3 series, in other than US countries people are still paying top dollar for these automobilles, but have still options not to have all extra items such as auto-trans, rear climate, auto-seat, power windows etc...
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.
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