OPENAMP1 - MM phono preamp open project

For those who might be interested, I have prepared an open project of the phono preamplifier for MM cartridge. It has very low noise and very low distortion. Gain is 40dB/1kHz. Output is capable to drive 50 ohm load, so one may directly connect headphones via 100 - 220 ohm pot. Please feel free to download documenation from

Preamplifier for MM cartridge
 
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Mooly, thanks for the kind words.

MLStrand56, not yet as a kit. But PCBs will be available by the end of January 2013. I have been very busy this year with industrial automation projects, namely plasma spraying, and have had almost no time for audio. Now in December I finally got some time for audio projects. Let's see, at the moment I cannot promise the kits.
 
The buffers seem an unusual addition (in a good way) and should make for total compatability.

The buffers make 2 or 3 improvements:

1) feedback network may use lower impedance resistors (100 ohm between -IN and GND) to get lower noise. This results in higher capacitances, that are easily driven with buffer.

2) buffered output is much less sensitive to output cable type and length.

3) output is able to drive headphones.

To demonstrate buffer capabilities, please see attached distortion measurement of this MM preamplifier into 50 ohm load.
 

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  • openamp1_loop_50_ohm_load.pdf
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I hope some of you might appreciate the measurement I am posting now, as measurements like this are almost never shown, for understandable reasons. It is the noise of the preamp with real cartridge at the input. The cartridge is Shure M35X, turntable output cable connected to the input of the phono preamp. The tonearm is above the vinyl record. 0dBFS = 2Vrms.
 

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  • noise_phono627+Shure.pdf
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You are right Pavel, the noise of this structure is not the lowest.
Around 1.4nV/qHz. In praxis the noise is of a very benign character.
The cartridge has to have an output of better then 0.5mV though.
The noise can be reduced by 6dB when we parallel 4 LMEs for a total of 8 per channel.
The circuit has other advantages. It has no common mode distortion and it is totally hum free because it is balanced. I also like that the feedback of this semi discrete INA is not connected to ground.
 
Most good MC cartridges have a " floating" construction. Benz, Lyra, Ortofon, to name a view.
Some tonearms make a connection of one of the pins of the cartridge to the tonearm wand though, like Rega. I think that is false but it can be rewired.
Optimum is when the cartridge pins are brought out plus any " metal" part that the arm maker wants to be grounded. My turntable has a different " philosophy" altogether.
I have a Spiral Grove SG1 with Centroid arm. Here the turntable is made from metal ( aluminum and steel ) and the tonearm is made from carbon fiber. The metal parts of the turntable are connected to main earth. Anyway, even when you connect an unbalanced ( say not floating ) arangement to my MC head design the chance is high that hum will be suppressed.
 
I hope some of you might appreciate the measurement I am posting now, as measurements like this are almost never shown, for understandable reasons. It is the noise of the preamp with real cartridge at the input. The cartridge is Shure M35X, turntable output cable connected to the input of the phono preamp. The tonearm is above the vinyl record. 0dBFS = 2Vrms.

It might be nice to post spectra with the cartridge compared to shorted input- that way, you demonstrate that the preamp is not the limitation (assuming that's not the case!). What's the reference level for the noise graph?

Nice demo of how good an IC preamp can be. :up: