Oscillation spike

SDA1C

Under what conditions can a turntable, cartridge specifically, cause a high frequency oscillation spike type of reaction by the amplifier. Almost a feedback and self perpetuating squeal while not noticeably, as of yet, damaging the amplifier?

I am obviously happy to clarify but am not sure how else to describe the anomaly.

Thanks, 1c

SDA1C

???
NO Ideas eh?

[Deleted User]

Hello 1c,
From your description I would suspect a connection related problem causing a transient spike. This could be at the headshell's terminals, or if it is a removable headshell where the pins mate with the barrel of the arm. Or, where the tonearm's connecting wires plug into the tonearm and the RCA connections of the tonearm cable into your phono preamp.
Is it coming from one channel or both? What turntable/tonearm are you using?
Regards, Ken

SDA1C

Thanks Ken,

It is a Technics SL-1360 table with a new AT cartridge and stylus (I can get the model if you require). I have replaced the cables with Kapton Kables at a length of around 15 ft to include new ground wire continuous. I have inspected the head shell and tonearm neither of which show any signs on crossed or touching wires. I connected the TT to a separate Pre and speakers having the same result, though controlled and turned off quickly this time, with the squeal beginning at or near 50% volume. I have checked continuity at the RCA's with the following result,

Ground to any other point. 0
Outer to outer on rca 0
inner to inner on rca 0
Outer to inner on RCA .356 for the left and .362 for the right when the head shell is in place. 0 when the shell is removed on all circuits. I have the same readings to the pins on the head shell.

I have slowly, with no active source, turned my TGP3 to wide open volume with amp activated and have absolutely no his or hum. Perfect silence in all inputs with the normal outstanding sound from the Oppo as an analog source. No damage to any other component is noticeable at this point. This occurrence smoked,and I mean smoked!, 6 rd-0194, (4) in front (1c) and (2) in rear (RTA11T) channels in dual stereo mode in a matter of .75 to 1.0 seconds at a very moderate volume (-35 to -40 db) playing Hello again by The Cars on vinyl. The center channel is off in Party Mode and thus survived, ironic eh, lol.

[Deleted User]

Hello,
To damage six tweeters must have been a really nasty sounding spike. I'm not too familiar with Kapton Kables, how are they attached to the Technics? Did you have to solder them to the turntable directly or were female RCA plugs attached to the table's chassis? Fifteen feet seems a bit long for turntable connections, sometimes a high frequency rolloff can happen if there's too much series capacitance. Were there some characteristics of these cables that led you to choose them? Did you have to extend the ground connecting wire from the turntable to the phono preamp? If so, how was that done?
The AT cartridge should give the internal DC resistance in it's specifications, having .356 seems low for the combination of the cable's resistance and the internal resistance of the cartridge.
Regards, Ken

SDA1C

It sounded a lot like NEO entering the Matrix. I Have my table in the corner and didn't give a second thought to the cable length. I used them on suggestion from another Polkie but to his defense I don't think I said they would be 15 ft. I replaced the ground with a one piece of 18 ga. wire. They are soldered to the board inside of the chassis of the TT. I have in the past run the table at much higher volumes with out incident with a temporary spliced piece of ground wire. I didn't think there would have been any issues with a better piece of wire. Guess I was wrong. The resistance in the cartridge free from the tone arm is .010 less giving the entire length of cable .01 ohm or resistance. Does that sound right?

[Deleted User]

Hello,
Something's not quite right, Audio Technica cartridges are moving magnet types and have an output impedance around 1.2k Ohms not a fraction of an Ohm. It might be a good idea to look at where the cables have been soldered under the turntable. Is there a way to take a photo of the connections?
Ken

SDA1C

I found the cartridge seems to dead short on the right side pins. From the + pin to the outer metal chassis there is 0 resistance. From the - pin on the right side there is .356, the left + is infinite and the left - there is .360 I will take a pic of the soldering shortly but I have a feeling it is in the cart. Would you agree? Could I have caused the failed cart?

SDA1C

Well I took out an old but still functional cart and on the right side it measures 0 to the right positive from the outer metal skin so I guess that's not it either. Also, the cables are 10'6" and the ground is 11' 3". It must be the stars alignment as the photos are fuzzy as well. There are no continuity abnormalities, that I have found, in the cabling.

[Deleted User]

Hello,
It is a little difficult to see much detail from the photos, but the wires that go through the tonearm should be, from right to left, red, green, white and blue. This would correspond to right channel positive, right channel negative, left channel positive and left channel negative. The Kapton Kable looks to be a four conductor cable that has a shield wrapped around it, correct? If so, how is this shield terminated?
Regards, Ken

SDA1C

The color coding and corresponding inputs at the tone arm check out OK. There are 2 cables that are 2 wires each. I connected them the same way as the ones I removed that had no shield other than the plastic sheathing. So to that I would say the outer wrap on the cable is not terminated other than wrapped with heat shrink. I have played without incident with the same cables. I am not sure the shield would take a terminal or solder very well, it seems to be coated with some sort of polymer. Should the shield be to ground?

[Deleted User]

Hello,
Thanks for the clarification. One approach is to use a shielded coaxial cable for turntables, but not having any shielding is probably not a problem if there's not a great deal of RF around. Does anybody in your neighborhood happen to have a particularly strong CB radio or ham radio that you know about?
Regards, Ken

SDA1C

There are not any strong radios in the area. A friend of mine with a ham station is the closest around 6 miles away. I did in the past have a base with an unfiltered linear that would wreck a TV nearly a mile away but that was under perfect conditions. I am aware of the effects of RF and can say with a fairly high degree of certainty that nothing like that is in my general area. I have not heard bleed through in the past. I did however recreate the squeal on a different pre amp/carver 900 receiver/ I am about to put the headshell on a different TT and see if that changes anything. I have not tried that yet due to the cables needing to be resoldered to the other table prior to connection to the preamp.

SDA1C

I thought I posted this already...I swapped the cables and the HS. Noise followed. I will try new cables and see if that gets me anywhere.

SDA1C

While poking around under the cover of the first TT (SL-1360) one of the tone arm wires popped off as if it were brittle in the solder joint. I am assuming this is the cause not an effect of the disturbance. I did find with the second TT (SL-23) that I had a larger gain at the pre amp before feedback, it sounds quite a bit clearer and, after a bit of Deoxit, has a very consistent speed.

I do wonder...is there a point in every electronic device that feed back will occur? And if so, Is it possible that I simply broke that threshold making my wallet cry wee wee wee all the way to the bank? lol