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A trip to the dark side?

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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 Mar 2023 at 2:55pm
The 9TA stereo cartridge outputs 70mV approx. so you'd think it would just need to be padded down, and a 1 Meg resistor in series would take its output down to about 3mV.

Unfortunately that won't do the job.

A ceramic is constant amplitude, and a "modern" phono stage is for constant velocity pick-ups.

So, here goes me trying to explain how they work in an easily understood manner...

The ceramic shares one thing with an electronic cig lighter. Apply pressure to a crystal and it outputs a voltage.

The crystal used in a ceramic is lab grown, a piezoelectric material, called lead-zirconium titanate (see: http://www.roger-russell.com/sonopg/sonopc.htm - warning: not secure)

Being "rock" it doesn't conduct but behaves like a capacitor, and in this case is about 600 pico-farads, +/-10%.

So, the bass response is decided by the input impedance of the amplifier it's connected to.

If 1 Meg like the grid resistor of an old valve input, the bass frequency response is

1/2pi * 0.0006 * 1 = 265Hz (-3dB), which is sh*t!

What is needed for the correct reproduction of an RIAA record - remember, it has bass boost turning over at 50Hz - is 

1/2pi * 0.0006 * 50 = 5.3 Meg!

For most domestic situations where small loudspeakers are used, especially bookshelf's, then  a 4.7 Meg input impedance is OK.

But, how do you get that? Perhaps a valve input could be run with a 4.7 Meg grid resistor? I don't know because I'm not red-hot at valves.

What I did was to make a preamp of small gain, using a JFET input stage op-amp. A TL071 would do (TL072 is the dual version).

So, if you all want me to scribble a circuit diagram I'll be happy to oblige.

Now, the point about constant amplitude is it's already flat. So here the mid frequency shelf of the record comes in. When looked at as constant amplitude that shelf represents a 12dB cut from 500Hz to 2122Hz.

So, without the preamp compensating for that, it's like having a dull top-end.

Therefore, you'll need some NFB EQ to give some boost from 500Hz and then tail it off at 2122Hz.

Around 10kHz, the cartridge loses it. It's crosstalk is about the worst it gets, and so it cannot paint as precise a stereo image as good as a magnetic, but it isn't that far off.

But the cartridge itself isn't affected by mains frequencies because it isn't inductive, so it might well sound cleaner in the bass region.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2023 at 8:01am
Whenever we get on to subjects that could have happened, but were blocked every which way possible, it makes me think of Black Sabbath's "Lord of This World."

PZT (lead-zirconium titanate) would make for an excellent phono cartridge, and make life easier for the vinyl listener, but is lead, and lead is banned (except for rainwater ducts on roof's) because, as waste in landfill, it reacts with rainwater and pollutes the ground (so why is it used for rainwater ducts?)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote CageyH Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2023 at 10:15am
Zinc has mostly replaced lead for the job here in France.
Kevin
European loan coordinator, based near Toulouse, France.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2023 at 5:27pm
Let me explain something about microphones you may not know about.

In any venue where there's an induction hearing loop, a dynamic microphone can make that hearing loop have howl-round (feedback).

There is no acoustic coupling, so how's that happen?

A dynamic mic is a magnetic mic, and the induction loop is --- inductive!

So the signal is induced into the dynamic mic, and it howls round by mutual inductance.

And that's why venues with a hearing loop - an induction loop - don't use dynamic mics. They use condenser mics.

So you'd think that progress would have seen the magnetic cartridge bite the dust, because it so easily picks up hum, and we'd have ceramic cartridges by today.

However, as you know, CD supplanted vinyl in such a big way that there was no money left in developing things for vinyl further - just penny millionaires making magnetic cartridges and boasting how good they and their antiquated products are.

And I didn't mind too much, because I could make phono stages for magnetic cartridges, and that kept me gainfully employed. Wink


Edited by Graham Slee - 25 Mar 2023 at 5:28pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2023 at 6:15pm
This is an answer to Mike (Mikeh), but it clarifies what I've been saying so I thought it was worth including it for all here:

Sonotone came up with a passive matching circuit to match up the ceramic cartridge with a magnetic cartridge preamp.

Does it work?

Ceramic bass depends on the CR filter made between the cartridge's source capacitance and the input impedance of the amplifying stage.

So, they reckoned by loading the cartridge more heavily, the bass would be cut at the same rate as a magnetic phono stage provides bass boost. Thereby equalising the cartridge using a lower impedance load.

Unfortunately the 9TA capacitance isn't very well channel matched.

Add to that the tarnish that builds up the rear plug and cartridge socket contacts, and the capacitance can be as low as 450pF on one channel and 650pf on the other (measured).

Therefore, the bass cut off varies considerably, and the stereo image is very imprecise.

The only way I managed to get good performance was to do a purpose made preamp for it, as implied in my earlier post.

The output of the preamp was line level, so then simply went to a line input on the amp or preamp.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lucabeer Quote  Post ReplyReply Direct Link To This Post Posted: 26 Mar 2023 at 9:51am
I get the point about amplification of low voltages. In my work (automotive engineer), it can be a nightmare to process signals by suppressing noise while retaining accuracy. Even signals in the 2 V range!

So yes, that's always been one of my HUGE doubts about MC: how is it possible to process a 0.2 mV signal, to amplify it x1000 and still get something not only "usable", but which is supposed to sound better than the signal from a MM cartridge which only needs x100 (which in itself is already quite a feat)? All things equal, I would always choose a device that starts with a tenfold advantage in signal strength at the source, rather than the weaker one!

It baffles me: amplification is NOT free. If you amplify 1000x, you get ten times the noise and crap that you get by amplifying 100x, and that is even with the most perfect theoretical amp you could design on paper. With a "real" one, it would actually even be worse (bandwidth, phase margins, etc).

So, again, there's something fishy in the "MC is always better" line of thought that I too often read on some forums. If only for that extra 10x amp factor, there's something that doesn't convince me.



Edited by Lucabeer - 26 Mar 2023 at 10:31am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 26 Mar 2023 at 11:39am
Originally posted by Lucabeer Lucabeer wrote:

I get the point about amplification of low voltages. In my work (automotive engineer), it can be a nightmare to process signals by suppressing noise while retaining accuracy. Even signals in the 2 V range!

So yes, that's always been one of my HUGE doubts about MC: how is it possible to process a 0.2 mV signal, to amplify it x1000 and still get something not only "usable", but which is supposed to sound better than the signal from a MM cartridge which only needs x100 (which in itself is already quite a feat)? All things equal, I would always choose a device that starts with a tenfold advantage in signal strength at the source, rather than the weaker one!

It baffles me: amplification is NOT free. If you amplify 1000x, you get ten times the noise and crap that you get by amplifying 100x, and that is even with the most perfect theoretical amp you could design on paper. With a "real" one, it would actually even be worse (bandwidth, phase margins, etc).

So, again, there's something fishy in the "MC is always better" line of thought that I too often read on some forums. If only for that extra 10x amp factor, there's something that doesn't convince me.


It's good to have your control engineering knowledge on here, and for you to share analogies.

In an (ICE) car you have a small nominal 12VDC. Maximum power is reached when the load equals the battery internal resistance (possibly at cranking), so the voltage there is 6VDC. But then there is the charging current plus the regulation required for the F-V converter known as the alternator. Therefore your sensors and comparators have a hard time of it, but they expect you to do fantastic things.

I learned the hard way through making plug in temperature controls for plating lines, and the demands for process chemicals were often +/- 1.5C. If the installation electrician did not comprehend cable power losses, I'd end up the other end of the country trying to combat relay chatter and subsequent burn-out.

But now we find ourselves in a land of make believe where a smaller signal is supposedly better than a larger signal.

As an engineer, what do you see? (your mind's eye already sees it)

But the customer has been promised nirvana if he (she etc) will just believe that worse is better. Then lots of money can change hands.

The problem I faced was in having to go along with the "belief" (lies) by making phono stages capable of resolving the demanded musicality while fighting the known sources of noise.

There were often unknown sources of noise outside my control - similar to the problem where "the installation electrician did not comprehend cable power losses". You have the customer living in this modern age surrounded by interference generators, but the customer doesn't see them because he doesn't have the experienced mind's eye, and neither should he have.

I do not know what kind of mind those who promote moving coil have, but I do know that those minds are bereft of the comprehension of signals and noise. They may be excellent mechanical engineers with great understanding of mechanical and gravitational forces, but that isn't enough.

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