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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
timeltel's avatar
timeltel
830 posts
 
Regards, Halcro: Condolences for your loss.

So MMs don't hummm, IMs & MCs do? Leads me to consider EMI & coils---

Wiki, "Electromagnetic radiation":
"According to Maxwell, currents directly produce a magnetic field, but it is of a magnetic dipole type which dies out rapidly with distance from the current.
*In a similar manner, moving charges being separated from each other in a conductor by a changing electical potential (such as in an antenna) produce an electric dipole type electrical field, but this also dies away very quickly with distance.*
Both of these fields make up the near-field near the EMR source. Neither of these behaviors are responsible for EM radiation. Instead, they cause electromagnetic field behavior that only efficiently transfers power to a receiver (cart?) very close to the source, such as the magnetic induction inside an electrical transformer--- or the feedback behavior that happens close to the coil of a metal detector."

Just to throw this thought out, if distance from the motor influences strength of interference, would shielding help?

Peace,
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
halcro's avatar
halcro OP
2,664 posts
 
Thanks all for your concern and contributions.....particularly Al and the Professor (Timeltel).
I was hoping Al would have some theory behind this and the Professor's thoughts are most interesting.
I had my own theory here:-
The problem is structure-borne feedback as is mostly the case with turntables?
Every supporting shelf is under 'stress' to some degree depending on its material, thickness and load, but most importantly.......on the 'span' and 'type' of span.
In my case with the wall-mounted shelf.........the cantilevered nature plus the heavy load and wood core structure sets up a particularly 'nasty' stress pattern.
What I have recently discovered is that 'stress' in any material....... creates a subsonic 'sound-field' within that material which is directly related to the 'level' of stress.
I previously thought.....wrongly I must admit.....that lower frequencies could not 'bridge' the pointy end of a spike support?
It is now known that they can and that spikes in fact 'couple' rather than 'de-couple'.
The 'stress-induced' sound waves in my cantilevered shelf are being transferred to the thin metal surround of the Victor which in turn passes them on the platter surround, motor and spindle support which the stylus then 'reads'?
This can be verified by placing the stylus on a record without turning on the motor......and then turning up the volume of the preamp until one can induce a loud low-frequency feedback loop through the speakers.
It was in the act of trying some forms of 'isolation' that I 'blew' the phonostage.
I am awaiting the arrival of Stillpoints Ultra Minis to really test whether my theory 'holds water'?
My fear was......that it doesn't solve the problem and I am suddenly without any theories?!
Moving the turntable may then be the only option although it is difficult for several reasons?
I was just musing on the input I have received so far......one of my Vandersteen 2Wq subwoofers is directly behind the speaker under the turntable in question and is consequently right in the corner.
Could this be the cause of the low-frequency excitation with the higher gain stage of the LOMCs?
Alas....I must await the return of my preamp to test this theory?
Thanks again gents....and any further thoughts are most welcome?
Henry
halcro's avatar
halcro OP
2,664 posts
 
Another clue........
With the stylus not in contact with the vinyl........there is no feedback, which indicates to me that:-
1. It is not an air-borne problem
2. It is not EMI or RFI
3. It is not motor induced
4. It is not coming through the armpods
5. It is coming through the platter

But when I change the mat from pigskin to rubber to copper.........there is no difference where I would expect some?
The Raven is in exactly the same situation vis-a-vis speakers, walls and subwoofer.......yet is immune due to the Stillpoints/plinth/platter or motor locations?
There is an answer somewhere.........?
I hope?
almarg's avatar
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almarg
9,670 posts
 
I would not reach conclusions 1, 4, and 5. It seems very conceivable to me that low frequency sound from the speaker or the sub, that is conducted through the air, is inducing a very slight vibration in the tonearm. That in turn would induce a slight vibration in the cartridge body, which would cause the cartridge to output a signal mainly, and perhaps only, when the stylus tip is prevented from vibrating in unison, such as when it is resting on a record.

I wouldn't attach any significance to the fact that the other table doesn't exhibit the problem. It is the happenstance of the combination of a great many electrical and mechanical technical parameters and variables, many of which are unpredictable, that results in the oscillation.

Best regards,
-- Al
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