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Feedback blew my phono stage?

Ok....I don't know what's going on here but here's my set-up
HALCRO
On the right hand side is my 'nude' Victor TT-101 DD turntable supported on spikes surrounded by 3 three tonearms on separate solid bronze armpods.
The 3 arms are connected separately to the Halcro DM10 preamp's phonostage via 2.5 metre long balanced Cardas Golden Ref phono cables.
With MM cartridges (I have around 30).....there are no problems.
With LOMC cartridges however......if I rest the stylus on the record without the motor turned on......I can induce a low frequency feedback through the speakers if I turn up the volume sufficiently?
Nothing I did to support the turntable differently.....could eliminate this problem so I have simply been playing MM cartridges through this turntable.
Recently.....I mounted an Acutex 420STR MM cartridge on the SAEC tonearm and discovered that it produced the same feedback symptoms as the LOMCs?
In experimenting with differing support methods for the Victor.....I forgot to turn the volume down before hitting the 'mute' button to 'OFF'....and an almighty noise knocked out my phono stage and tripped the protection on the left channel Halcro monoblock.
The 'Balanced' XLR inputs for the phonostage now produce no output whilst the RCA inputs produce a feeble output in the right channel only.

Any Gurus here with some ideas for the cause of this problem would be much appreciated?
Incidentally......no such problems with the 3 tonearms on the Raven AC connected to the same preamp inputs....even with LOMCs?
128x128halcro

Showing 3 responses by aigenga

aigenga
131 posts
 
Henry,
I am in agreement with Almarg who said:

"The reason that the problem does not occur with the other turntable is most likely that the characteristics of how it transmits acoustic/mechanical feedback to the cartridge are simply different, as a function of frequency and otherwise."

To this I would add that you need to dampen the JVC as much as you can - especially the base.

I am also in agreement with everyone who said that the corner placement is a problem. But here I would say that if you put sound-absorbing pads on both corner walls behind and to the sides of both turntables you will help your situation considerably. My turntable is surrounded on three sides by folded towels that amount to 1 inch of sound deadening thickness. My turntable is also close to a corner and behind my left speaker - no feedback. See photo:

http://s1106.photobucket.com/albums/h373/Garya1/Nude%20Turntable%20and%20DIY%20Arm%20Pod/?action=view¤t=11.jpg

Finally, as you have discussed (in other threads) your shelf is not good enough for the purpose.

Lots to think about. Lots to do. Lots of luck to you.

Gary
aigenga
131 posts
 
Halo, I assume you are referring to the ground cable attached to the bottom of the turntable rather than the tonearm/cartridge ground. I have tried the TT ground on and off and never found a difference. Mine has been off for the last year without issue.

I don't believe that this is an electrical problem somehow caused by the turntable. Stay focused on structure (the shelf) and air borne vibrations picked up by the turntable and the tonearm and then concentrated on the cartridge. Your shelf is insufficient and the turntable needs extensive damping. I am positive that improvements to your shelf will be audible in your Raven table as well as the JVC.
aigenga
131 posts
 
Atmasphere, is that true even when a feedback loop is in effect that amplifies rapidly? I've never heard this on a turntable but certainly on a microphone and on a guitar pick-up. It gets loud very fast.
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