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
sarcher30
596 posts
 
I agree with Onhwy61. TT's in corners is a no-no. In that room I would buy an isolation rack or two and locate them between the speakers. In the same plane as the speakers. Then put both TT's and your front end gear on them. If you get them short enough they won't block the TV. If you had room I would locate the whole front end of the system to the side wall but it does not look practical in your case.

Sorry to hear about you preamp. Halcro is no longer in business that I am aware of. Hopefully you can find someone that can repair it.
sarcher30
596 posts
 
There is no music playing and you get feedback? Do you live next to a busy street or some other source of vibration? You could just move the TT temporarily to a different location and see if the problem goes away.

Is it low frequency feedback? You could also try a rumble filter.
sarcher30
596 posts
 
One possible explanation for why it is occurring mostly with MC's and not MM's is most MM's are high compliance and most MC's are low compliance.
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
Sorry to hear about the damage to the preamp, Halcro.

Concerning the feedback question, a high gain feedback loop that is unstable at some frequency or frequencies does not require much if any input stimulus for oscillations to occur. The reason that the problem occurred mainly with MC cartridges is most likely that you were using a considerably higher amount of phono stage gain with them, and perhaps also differences in frequency or phase response in the phono stage between when the circuitry that provides added gain for LOMC's is switched in and switched out. 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.

So what is probably happening is that some extremely tiny (and inevitable) low frequency vibration causes the cartridge to output a tiny signal, which is amplified by a high gain factor, resulting in an output from the nearby speaker that causes further vibration to be mechanically transmitted to the cartridge, which is further amplified by that high gain factor, etc., etc. The root cause of the problem is that the overall loop is unstable at some low frequency or frequencies, when the overall gain through the loop is above a certain amount.

I therefore agree with the others that the turntable needs to be re-located.

Best regards,
-- Al
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,
More to discover