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?
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Your system pictures can be misleading, but it appears you have your turntables in corners of the room with the loudspeakers within inches of them. It could be that the combination is responsible for the feedback. Can the turntables be moved?
Please re-read the symptoms.
1. There is no music playing. The turntable is not even switched on.
2. It doesn't happen with most MM cartridges.
3. It doesn't happen at all with the the Raven turntable.
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.
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.
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.
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
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,
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
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
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?
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
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.
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.
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.
Thanks Chris,
Food for thought.
Funny about the 420STR?
Halcro
Funny about the 420STR?

Henry
The 420str was able to do for me what no other cartridge was able to do - MM or MC so far. IMO - there is no perfect TT due to the medium being flawed. This 420str highlighted the design flaws in two of my turntables. Have you actually tried it on your belt drive Raven yet ?


I had a somewhat similar problem when running a prototype drive for a strain gauge cartridge. I unexpectedly blew out a tube in my linestage. The root of the problem was poor grounding in one of the prototype's RCA connectors that caused hum.

I don't know if my experience helps you, but could it be a purely mechanical problem that is exacerbated by the requirements of a moving coil cartridge that are not common to a moving magnet one?
The Halcro DM10 went to my local technician who....when he opened it up FOUND THIS
The Halcro designer in Adelaide said that the thick paint overlay was done to protect against patent infringement 'copying'....and also to prevent oxidisation?
My local technician said it's probably more likely there to prevent anyone other than Halcro, from working on the circuit boards?
In any case.....without the circuit diagram.....my local technician could not repair the unit so off to Adelaide the preamp went.

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.
Perhaps I need to tell him of the 'feedback problem' :-(
And how do I ensure it doesn't happen again?
Or could perhaps some faulty parts have caused the 'feedback' in the first place?
Wouldn't you be better off to sell it whilst it's still going. What happens if the designer gets run over.
Dear Henry, Everyone knows what 'wishful thinking' means but no one want to give up this dream about the 'Nirvana for cheap'. That is, I think, why so many of us own +/- 20 of those old MM carts. The fact that there are no problems of any kind with your Raven may entail the answer to your question.
Kind regards,
What happens if the designer gets run over?
That's exactly what I've been wondering Dover.........
Ahh.....Nikola,
You think the Germans have better quality control :~}
You may be right?
How about the Americans?
Well, dear Henry, Americans lost the control of their domestic and foreign debts, their jobs, Irak and Avganistan,etc., while we all in the European Union hope that the Germans will liberate us from the Euro crisis. They are really in control and I am not sure who actually lost the II WW. The Britons who hope to earn some money with tourism ( their industrial capability you can see by the Linn LP-12) teach their kids : don't mention the war to any German tourist.

Regards,
It doesn't look that way from where I sit. National debt(s) and lack of jobs are problems everywhere in the so-called industrialized world these days, are they not? In southern Europe the problem is far worse than anywhere in the US.

On topic, I agree with Dover, if Mr. Halcro in Adelaide will not at least give you a schematic, I would consider dumping the DM10 at this juncture. But you also own the amplifiers, as I recall.
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
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
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
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.
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.
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.
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.
Just to be clear there is no way any cartridge could make enough voltage on its own to damage a phono section.
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.
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!!? :-(
Dear Halcro: Good to hear you are happy again!.

regards and enjoy the music,
R.
Aigenga, Yes.

Halcro, good to hear things are up and running! Hard to be without tunes...
Pretty quick turnaround by Halcro. Do you live across the street from the factory?
Thanks guys,
I appreciate all your help and support during this harrowing process.

Yes.....I'm lucky to live in Australia where Halcro was designed and built.
The longest processes were transporting the preamp back and forth to Adelaide (850 miles).....and the cost of that exceeded the repair costs.
Don't know what you would do if you lived in another country though?....

Now to listen with fresh ears to compare the sound to that of the beautiful valves I have become used to over the last 4 weeks?