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Old 6th January 2011, 06:14 PM   #1
losacco is offline losacco  United States
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Default Phono pre amp question.

Can someone please explain the difference between a phono pre amp and other pre amps?

Thanks
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Old 6th January 2011, 06:17 PM   #2
sreten is offline sreten  United Kingdom
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Hi, a phono pre-amp incorporates the MM RIAA frequency response curves, rgds, sreten.
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Old 6th January 2011, 06:21 PM   #3
Mooly is offline Mooly  United Kingdom
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Phono preamps work with a very low signal from a cartridge and have a non linear response that follows a pre determined "curve". The output level from a cartridge can be in the 100's of microvolts region to a few millivolts depending on type.

RIAA equalization - Wikipedia, the free encyclopedia

Line level sources usually do not need amplifying, just control of volume so that gives rise to the "passive" preamp which is little more than a selector box for different inputs and a volume control/

Old "Din" specced equipment has a low output level of a few hundred millivolts and usually would need modest gain to fully drive a power amp. The response of all line level amps is linear with regard to frequency.
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Old 6th January 2011, 06:36 PM   #4
LAJ is offline LAJ  United States
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The bass is really at a much lower level on the disc because it would take up too much space on the disc if it were recorded flat. So the RIAA curve compensates for this.
Also some preamps have a tape position (NAB) which is for a tape head.
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Old 6th January 2011, 09:02 PM   #5
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Here is schematic of phono corrector for MM head that I like. MM means Moving Magnet, it has higher output level and higher inductance than MC head (Moving Coil). Usually they are specified to be loaded on 47K resistance, but in reality sometimes is better to load them on lower resistor in order to equalize highs using inductance of the head.

This corrector had been designed by Russian designer Alexander Bokarev. It is simple, but very good sounding.

Click the image to open in full size.

Professional turntables usually use MC heads: they have lighter moving structure, but have lower output level so need more amplification factor, so noises and microphonics of input tube play higher role.
I myself currently work on MC corrector. It has input and output transformers. Input tube is very unusual for audio: it is planar UHF triode 6С17К. The tube had been designed and was manufactured for radars in rockets, so it has almost zero microphonics. Noises as well are very low. It has tiny planar grid made of 8 micron thick wires, with distances between wires 18 microns!
The tube is made of Titanium and special ceramic with the same temperature coefficient.
The complexity of usage of the tube is caused by very close distance between the grid and the cathode. It results in input current that is much higher than ordinary tubes have.
Also, one side of filament is connected to cathode so it has to be grounded.
But it is not a problem at all!
No resistors in cathode is good! Bias can be made by adjusting of nominal of grid leak resistor!

Here is the picture of the tube: it is tiny and weights 5 gram only!

Click the image to open in full size.
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Old 6th January 2011, 09:16 PM   #6
DF96 is offline DF96  England
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Actually the bass is at a slightly higher level on the disc (due to the mid-band shelf), but a magnetic cartridge is a velocity sensor so it gives higher output at treble. RIAA corrects for both effects. Not non-linear, but non-flat.
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Old 6th January 2011, 11:56 PM   #7
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Here is typical RIAA curve drawn by Audacity software:

Click the image to open in full size.
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Old 7th January 2011, 12:27 AM   #8
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Now that is a cool tube! How does the size compare to a nuvistor? low microphonics huh? Looks like I need to do some research on that one. Thanks for the tip.
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Old 7th January 2011, 02:13 AM   #9
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Quote:
Originally Posted by mashaffer View Post
Now that is a cool tube! How does the size compare to a nuvistor? low microphonics huh? Looks like I need to do some research on that one. Thanks for the tip.
Actually, nuvistors ring like bells, while this one is grave quiet! It is specified for 500g acceleration (when the rocket starts, I suppose). For vibrations 5-200 Hz it is specified up to 10g: I suppose radar in the rocked have to continue working when it flies, huh?

It was assembled in vacuum, like nuvistors, but I suppose equipment was needed more precise. And it should cost if start manufacturing now much more than vintage Telefunken 12AX7 costs. But they are dirt cheap on ePay currently, I suppose because audiophiles did not discover them yet...


Here is the datasheet:

Click the image to open in full size.

Click the image to open in full size.
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Old 7th January 2011, 02:19 AM   #10
Bigun is offline Bigun  Canada
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Those curves don't look very impressive to me, should be good for adding 'colour' to your sound.

I've seen better for Nuvistors and as far as I know, not all of them ring like bells.
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Last edited by Bigun; 7th January 2011 at 02:21 AM.
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