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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
almarg's avatar
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almarg
9,670 posts
 
He's obviously far more familiar than I am with the grounding schemes in the preamp and the amps, but FWIW I can't envision how that could have caused the problem. Especially given that the setup has been working well for a long time, with many different MM cartridges. And there have never been any symptoms when the stylus is elevated above the record.

That said, have you tried grounding the TT to the preamp? (I realize it is a little further away). That is of course the more usual arrangement, and the symmetry it would provide between channels (given that the amps are monoblocks) would seem to make it the more logical approach, if in fact it makes any (sonic) difference at all.

Best regards,
-- Al
halcro's avatar
halcro OP
2,664 posts
 
That's what 'Mr Halcro' says also......
I don't think the grounding wire is long enough......but I'll try it without any grounding.
There was none on my TT-81 and certainly none on the Raven.
dgarretson
2,599 posts
 
Hello Halcro, I'm inclined to accord with ct0517 that

"if you want to keep that white shelf, the Victor/armpods in that TT location – you need to add a level on top of that shelf that the Victor and armpods all join to
for isolation. This will then become the common platform aka plinth for that whole turntable system"

The spiked Victor plinth is likely conducting mechanical vibration from earth or some combination of all those vibrating components in a row. The most effective approach will be isolation-based, perhaps similar to the softly sprung platform that I use to elevate all the separate pods of my VPI. In this case the 150 lb. sandbox/plinth assy floats on six soft springs with a resonance of around 2-3 cps. A heavy maple or marble platform could be substituted for the sandbox.

http://cgim.audiogon.com/i/vs/i/f/1284313597.jpg

It might first be helpful to check if the vibration occurs with the Victor elevated on a pillow.
atmasphere's avatar
atmasphere
11,861 posts
 
David is right- you need to do something about mechanical isolation for the 'table.

If the preamp really was damaged by something to do with the ground, then both the preamp and amps need to go back to Halcro to get the problem fixed. There is no way the grounding of the amps should have any ability to do what they did, unless the amps are improperly grounded.

Now I can think of a way that that might happen- if the amps don't have a proper ground, and if the preamp has no ground, then the ground cable for the phono *might* be the only solid ground. If the system was driven to high power, there might have been some significant ground current exercised on account of the power generated by the amp. If the phono preamp was the only path for that (through the ground of the 'table), components in the phono section could have been damaged. Mind you, this is only conjecture as I have no idea how the Halcro stuff is set up; this is simply a possible scenario how such a thing could have happened.
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.
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