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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
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almarg
9,670 posts
 
Today the Halcro designer writes:-

There were 3 faulty parts. Each input has a common mode choke to reduce noise. Both the phono stage chokes were damaged. There was also an electrolytic capacitor which had failed. Some of the other caps were also showing signs of aging (these parts are known to degrade over time). I took the liberty of replacing all of these caps as a preventative maintenance measure.

I have seen the choke fail on an unbalanced input before. This was caused by a fault in the equipment which was connected to it. If there is a large voltage between the ground and signal connection these chokes can fail. Perhaps you could check your turntable? I must say it is hard to imagine how a turn table could cause this problem.
I too can't imagine how the choke failures could have been caused by the turntable. The only semblance of an explanation I can think of is that the abrupt eruption of the very high level oscillation, caused by feedback, may have in turn caused a very large voltage transient to be generated across the chokes as a result of inductive kick (chokes are inductors). And perhaps that high voltage transient somehow coupled into whatever circuit the electrolytic capacitor is located in (perhaps it serves as a filter for some internal DC supply voltage), causing or hastening its demise.

FYI, see page 6 of this document for an explanation of what a common mode choke is.

Best regards,
-- Al
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halcro OP
2,664 posts
 
Hi Al,
I mentioned to 'Mr Halcro' that I had the 'ground' cable of my Victor TT, attached to one of the DM58 monoblock amps and could THAT cause the problem?
It is possible that the grounding on the power amp may cause a voltage difference that could burn out the chokes. I would also be interested to hear if the feedback problem went away as well.
Is his reply.
What do you think?

Regards
Henry
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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
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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.
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