2. analog
  3. 1346375193

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
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.
atmasphere's avatar
atmasphere
11,861 posts
 
Just to be clear there is no way any cartridge could make enough voltage on its own to damage a phono section.
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.
halcro's avatar
halcro OP
2,664 posts
 
I received the repaired DM10 back from Adelaide and re-inserted it in my system.
I must admit that even before testing it.....I was 90% sure that the turntable 'ground' to one of the DM58 Monoblocks was indeed the problem?
All the symptoms fitted including the 'coup de grace' when I placed the solid copper 1.8Kg Micro Seiki Cu180 'mat' under the TT-101 in direct contact with the metal base of the cage.
Without understanding electrical theory........I assume the potential voltage difference was increased enormously by this action which explains the fact that when I turned on the preamp........there was an immediate 'explosion' (even though the volume was turned down) which 'blew the chokes and capacitor?
I disconnected the 'ground' wire and placed the LOMC FR-7f cartridge and turned up the volume gradually.
Although there was a low 'hum' as the volume increased.......there was no 'feedback loop'.
I then connected the devilish 420STR, placed it on the vinyl and gradually increased the volume.
No hum and no 'feedback loop' even when the volume was at maximum!?
Problem solved.......and as so often occurs.......I was the problem!!? :-(
More to discover