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Turntables and Hum

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Pushpaw View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pushpaw Quote  Post ReplyReply Direct Link To This Post Posted: 24 Feb 2021 at 1:48pm
Maybe I’m looked at things all wrong. If I just took the approach of opening the doors to my perception through chemical alteration, I could listen to any old system and be blown away by it. Maybe the ground hum itself would become like a symphony... they say good audio starts with a good source: in the end, we are the source of the sounds we hear, aren’t we? So we just need to upgrade ourselves. Ha!
P******t Debut Carbon, Goldring 1042, GS Accession MM w/Enigma, GS Solo ULDE w/PSU1, Sennheiser HD6XX, Technics SU-800 IA, Castle Knight 1, PS Audio Ultimate Outlet, Lautus & CuSat50
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Feb 2021 at 3:12pm
Originally posted by Pushpaw Pushpaw wrote:

Maybe the ground hum itself would become like a symphony...


You never tried that? LOL

They say that chewed-up cassette tapes sounded out of this world! LOL
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Feb 2021 at 3:27pm
I often wonder why this stuff never caught on?



It could be that it doesn't attract the right kudos of radio waves.

Maybe it would help if you use this https://www.wires.co.uk/acatalog/ag9999_tape.html

Sleeve it individually, then place it in an overall sleeve. Add phono plugs, and voila!

"A great interconnect to replace arm cables."

"One core is rotated 90 degrees to the other core for effective shielding from interference."

And you think I'm taking the piss?

(perhaps we ought to have a confessions corner?)



Edited by Graham Slee - 24 Feb 2021 at 3:28pm
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pushpaw Quote  Post ReplyReply Direct Link To This Post Posted: 02 Apr 2021 at 3:15am
Of interest for anyone else with a P******t Debut Carbon TT experiencing hum when the stylus/cart is over the record and/or is lowered toward the record (a common issue on this TT I believe) - I upgraded from the metal platter to the acrylic and the hum is completely gone. I still get RF interference from dimmer switches nearby but those are more of a buzz than a hum. The hum is gone. So I can only conclude it had something to do with the metal platter and stylus/cart interacting?

And there’s no longer any static buildup on the TT either which is great. 


Edited by Pushpaw - 02 Apr 2021 at 3:22am
P******t Debut Carbon, Goldring 1042, GS Accession MM w/Enigma, GS Solo ULDE w/PSU1, Sennheiser HD6XX, Technics SU-800 IA, Castle Knight 1, PS Audio Ultimate Outlet, Lautus & CuSat50
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 02 Apr 2021 at 8:19am
I recall reading some audio history where a strongly magnetic high output moving coil cartridge was used on a turntable with a steel platter.

When the cartridge was lowered onto the record, the magnetic attraction was so strong that the cantilever was squashed into the cartridge body. The cartridge remained in intimate contact with the record.

It was a long time ago - the only type of magnetic cartridge in town was moving coil - which might have been around the time of Blumlein (1930s). Those were nothing like the MC's of today, by the way.

The magnets in a MM cartridge are tiny. The stylus causes them to move (some call it "vibrate") in proximity with two coils (two for stereo) and create a minuscule electrical signal. The magnetic attraction to a steel platter might be so weak that it doesn't add much to the playing weight.

Even though the signal is minuscule, it takes hundreds of turns of wire for each coil to produce it. It doesn't take much of an imagination to picture how fragile the wire is. Each coil is an inductor, and the varying distance due to stylus movement causes the variable magnetic field to be induced in the inductors, turning it into electricity.

In the same way as a dynamo gives out an increasing voltage, the faster the bicycle wheel turns, the cartridge output increases with frequency. The 1042's 6.5mV output at 1kHz becomes 1.47mV at 100Hz and 1mV at 50Hz. The phono stage applies more gain at those frequencies as part of its equalisation to bring the output at all frequencies, level.

In the upper mid and treble range, the output increases, and at 10kHz, the output is 31.5mV. A little above 10kHz, the arm cable capacitance and the phono stage input capacitance acts with the cartridge inductance and produces a peak (in the presence of a signal of such frequency). The 47k load impedance damps the peak within about +1dB of what it should be.

Beyond the peak, the signal starts to decay in the opposite direction and at the same rate as it rose. The stylus cantilever has some tuning-fork resonance which "carries" the signal upwards the partial octave required to reach 20kHz.

The cartridge body offers little shielding to the inductive coils against other magnetic fields because any metal shield will influence the velocity of the magnets. Steel has a strong attraction to magnetism, so it would be no use. Aluminium is effective as a shield but has some magnetic attraction (read up about mechanical speedometers).

Although the coil outputs fall after the >10kHz peak, their output reduces and is still slightly viable into the low radio broadcast spectrum (long wave). The cartridge wires, if left unshielded, have a length somewhat longer than the arm, so given an earth plane might readily accept radio wavelengths 1 metre and above.

Radio frequencies having wavelengths between 1 metre and 1000 metres might have a lesser effect, but that depends on their signal strength. For example, Mr H******n, a hi-fi reviewer, lives within a mile of London's Crystal Palace transmitter, but it seems he will not venture into discussing any of the above.

A plastic arm has no shielding, so the cartridge wires are exposed to UHF transmission frequencies, which just about covers every frequency above 200MHz.

The cartridge has no shielding, so might admit long-wave to mediumwave AM transmissions. The input capacitance of a phono stage offers some resistance but is not a short circuit to RF.

The phono stage designer has to make the music listenable, which requires it to perform up to the low MHz as any real amplifier designer knows! At the same time, it has to mitigate radio frequency interference. There are phono stages that might be "slugged down" significantly to block stronger RF, but as a result, slew so slowly as to be fatiguing to listen with.

A ferrous platter is challenging to ground to such a low impedance that the amplification ground "moves with it," not just because of the lubrication but the obvious resistance between the metals; otherwise, they'd not move!

Mutual inductance will ensure a ferrous platter vibrates by the magnetic fields all around it and next to it (e.g. motor).

Dimmers are rectifiers (SCR's - silicon control rectifiers), and all rectifiers fire-off wide-spectrum radio frequency interference due to their abrupt switch off characteristics. The strength of the interference is mitigated in some but cannot be entirely eradicated. They might be made compatible within acceptable limits, but record players are omitted (considered a relic of the past by legislators).

Turntable manufacturers are under no obligation to comply with EMC legislation as there is none. The only legislation is about electrical safety and energy efficiency (EuP legislation).

Whether preamp or power amp, every solid-state amplifier has some crystalline junction because that's how transistors and FETs work. They are the equivalent of a crystal radio with mitigation. There is a limit to as far as this mitigation can go, but turntable/arm manufacturers are not listening.

The earth is a ball spinning in space. There is no such thing as flat earth. I have given this written lecture in several forms over many years. Perhaps one day, somebody will understand it.

Not simple enough for Google-Bot to understand...
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