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
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
Ag insider logo xs@2x
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
Ag insider logo xs@2x
rauliruegas
13,101 posts
 
Dear Halcro: Oscilations on SS electronics almost always occur and can damage the unit. Things are that several designs were not full tested to be aware on " problems " through the designed circuits.
As Almarg posted " something " exited that kind of circuit oscilations and then the damge in the unit.
Now, a mute function is to mute the output level it does not matters at which position/level the volume is. I can't be sure but maybe the " culprit " is that your unit design has no protection about or was designed not full tested for this kind of problems. An SS design is a lot lot more complex than a tube one and the SS designs are a lot "weak " than the tube ones.

In the other side: do you already tested to put the stylus in your Victor when the Victor is not connected to the wall electric source?, try it and share what happened.

Regards and enjoy the music,
R.
dover
2,250 posts
 
Henry,
What does the low frequency feedback sound like - is it a quick or slow pulsing sound or a constant sound. What sort of frequency would you estimate.
ct0517's avatar
ct0517
2,041 posts
 
Henry,
Very sorry to hear about your preamp. This is further to my
email to you.

IMO - All turntables are a system that needs to work in
unison – Bearing, motor, platter, spindle,
tonearm/cartridge. How all these parts are joined together
differs greatly. They all however at some point need to come
to a common ground/platform/plinth. Your white shelf right
now represents this common ground, and you have indicated
that it is sub-par already. The Raven already has this
common ground – its own plinth.

Again IMO - 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 420str is a beacon for all good and bad and a great test
of a system I found for wires/grounding, motors. If you put
it on a pole in your backyard it will bring in HD channels
for you.

Keep phono runs as short as possible. As I suggested earlier
place your preamp next to the victor and try it that way
when you are up and running.

Good luck.
More to discover