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Understanding Record Repro

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Topic: Understanding Record Repro
    Posted: 12 Sep 2017 at 6:43pm
Download a copy of this and save it on your computer before it disappears...

http://www.novotone.be/_site/projets/Projet06/Doc01.pdf

It will explain all about the rising output of magnetic cartridges (what I've been trying to hammer home forever). You will then be able to understand the workings of the Accession phono preamp (I hope the patent examiners will also understand... one day... maybe not!).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 2017 at 7:11pm
That's a tough read! Saved for when i'm feeling intelligent . . . Ermm
Bob.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 2017 at 10:55pm
Downloaded and skimmed earlier. A good read (!) that clarifies a lot of the principles taht have been confused elsewhere! It also made me think that records have done well to last so long and still sound so good on a wide variety of musical styles.

Jon

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2017 at 7:42am
This is the image which proves my point. In the text (read PDF) Galo acknowledges there are no graphs showing the constant amplitude (or true) output of a record, only that 'seen' by the magnetic cartridge, but if you draw a line linking the tops of the sine wave signal you will see it.



It may have little use today, but it would have helped me as a young designer working on disco equipment in the 70s! Life is too short, and to have this knowledge hidden away is shameful. There is a prescribed way to do everything in the hi-fi industry, which they decide, and so any other way is 'wrong', and they indoctrinate the customer to only see it their way, and that's that. You are taught to resist any other point of view, and their cohorts do everything in their power to discredit anybody who challenges the industries comfort zone. Obviously it's done to protect the money!

Because of that nobody can see the wood for the trees and when somebody comes along with an invention which equalises the cartridge and record in a 'non-prescribed' way, it cannot be understood - not even by 'qualified' patent examiners! As I said, life's too short.

So, by exchanging what's known to engineers as poles and zeros, will EQ done differently sound different? I am not saying better, but if it is better, it could have been done a long time ago, and maybe it could have improved everyone's vinyl listening, and kept the vinyl industry as it was, because it wasn't broken, and so didn't need fixing (IMO).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2017 at 11:02pm
The Connected World and its effect on the cartridge signal

Here are three images taken with the arm cable plugged into a digital storage oscillscope. Turntable off. Arm in rest. Technics SL1200 MkII.

1. Moving coil cartridge (Hana EL)



The Hana EL outputs 0.5mV but here we can see interference at nearly 14mV (when converted to an rms figure).


2. Moving magnet cartridge (Ortofon 2M Black)



The Ortofon 2M Black outputs 5mV, and here we see interference at nearly 14mV (rms equiv.) again, but it doesn't dwarf the signal as much as with the MC cartridge which is ten times worse.

3. No cartridge - open ended arm cable



And here we see the arm with infinite driving impedance, or open circuit. Approx 10mV in rms terms. It shows that the arm isn't very effectively shielded, and calculations show a 9 inch arm is only effective up to 30MHz. But it is not the sole culprit as with cartridge connected (as shown above) the inteference is worse.

The worst offending frequencies are from around 100MHz and above. Each horizontal division represents 10MHz, and the 'spikes' are about 1/10th of a square or narrower, and the scope can only do 60MHz per division and it being a digital display, it cannot render higher frequencies than around 100MHz all that well, so some of it could be up to 1,000MHz.

Therefore the interference ranges from FM radio transmissions to Bluetooth and WiFi as well as Ethernet over mains radiating from mains wiring.

The industry designs MC stages to handle incoming signals based on what the cartridge output is. Mostly these designs hail from the pre-connected world. An example being a bipolar transistor input stage (discrete or op-amp) which for low noise is not emitter degenerated, and so its input is limited to 60mV P-P, which is 20mV rms.

The screen shots utilised the stop function of the oscillscope, but it was not fast enough to capture the peaks seen in real time. These were considerably higher than each image shows. It is therefore not too difficult to imagine the interference exceeding 20mV in rms terms. When this happens the input clips.

Also, the input slew rate of a conventional bipolar design is not fast enough for such high frequencies. If we take 100MHz and a voltage of 30mV peak (around 20mV rms) we find we need nearly 20V/uS. A low noise bipolar input stage will be just a fraction of this, and it will also clip.

Clipping is really bad distortion. 100%. Frequencies mix and produce new frequencies. That's how AM radio demodulates the audio. When this distortion meets harmonic distortion (remembering many hi-fi stages are designed to produce extra even harmonics believing it sounds 'better') it produces new noises and some reside in the audible spectrum.

These have a tendency to make moving coil reproduction bright, edgy or etched, and even screechy on loud notes.

The stock answer seems to be the use of a relatively large film capacitor across the input (usually 10nF or thereabouts) but capacitors have series inductance which still allows the high frequencies of "the connected world" access.

The connected world is causing problems for low output moving coil users whether they realise it or not. There are plenty of complaints on the web which refer to bright sounding moving coils, but moving coils are not inherently bright!

The problem is how they are interfaced.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 2017 at 2:16am
In The Beginning...

My earliest foray into record EQ was mid-70s trying my best to obtain a smooth sound from a 'noise generator', which is what discotheques were perceived to be by the older generation. The idea the breweries and their inn-keepers had was to equip concert rooms with something that made loud noises, and it would somehow attract young people. It didn't!

The equipment was a complete mismatch. Ceramic (capacitor) cartridges loaded wrongly into microphone mixers whose output was presented to a mono public announcement amplifier, and that would drive a humongous bass horn loudspeaker cabinet. So much for the 'older generation'!

Replacing the mic mixer, amp and speaker helped. But not much sense was forthcoming from the cartridge manufacturers and hobby electronics magazines and hobby components suppliers didn't have a clue either. It all had to be done by process of elimination. The end result was much better sounding but only with bass boost applied.

The circuits gleaned by those supposedly in the know gave a bass roll-off around 200Hz, hence the requirement for bass boost. I eventually understood that the cartridges had a source capacitance of circa 600pF and from the well-worn formula F = 2piCR, a 1Meg load gave 265Hz. But add the cable capacitance of the twin screened cable and it was near enough 200Hz.

Although with a bass boost filter (the one from the PW Easy Build Disco, 1976) it sounded as sweet as a nut, it didn't without, and that is what set me off on my investigations, several years before Mr Galo's article (mentioned first post above).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 2017 at 7:38am
I didn't read the article because it's beyond me but your posts above are very helpful. Probably a naive question but would a metallic (copper or even lead) turntable cover & lined plinth have a chance to screen out HF interference which is doing so much damage via the arm and cartridge?
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