A Flat Moving Coil Preamp Using Paralleled Amplifiers

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JR.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by JR. »

mediatechnology wrote:Richard published his design in 1981: viewtopic.php?f=6&t=749&start=14

Leach came 18 years later.
I recall seeing the Leach design published back in the late '70s or early '80s, not much later than the 1981 reference.

I played with it at the time and didn't pursue, while it was very interesting. IIRC I tried to discuss the topology with Fitchen (co-author of Low Noise design text) and he had no useful observations about it. He was a professor at a nearby college at the time.

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ricardo
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

mediatechnology wrote:Richard published his design in 1981: viewtopic.php?f=6&t=749&start=14

Leach came 18 years later.
The original Leach article was in Audio, 1979 or 1980. A friend of mine built it and was disappointed so I said I would do him a better one.

All I did was to throw some bits out & change some values :mrgreen:
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mediatechnology
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Flat Moving Coil Balanced Input Preamp Using the OPA1612

Post by mediatechnology »

I had Brown Dog/Cimarron make me some OPA1612 to DIP adapters to try in the Flat Moving Magnet Preamp board.

I wanted to see how the OPA1612 worked both as a moving magnet preamp as well as a moving coil application in the balanced instrumentation amp configuration here:

Image
A Flat Balanced Input/Balanced Output Moving Magnet Phono Preamp

The moving magnet performance was not optimal due to the higher current noise of the OPA1612 compared to the NJM2068, NJM2114, NJM5532 and OPA2134.
The results were not a big surprise.

As a moving coil preamp the OPA1612 turns out to be pretty decent.

I modified the resistor values to lower noise making Rg 5 Ohms (total) and Rfb 499R X2.
R4 and R5 were made 0R; R2 and R3 were 499R.
The single-ended output gain is 46 dB.
Rsource was 10R. Cterm 0.
The measurement bandwidth was 20 kHz.

Image
OPA1612 Moving Coil Preamp 46dB Gain 10R Source

The blue trace is the output of the preamp after the THAT1240 common mode rejection stage.
The red trace is the converter residual noise.

The ein looks to be about -131 dBu, 218 nV.
The noise density is about 1.5 nV/sqrtHz.

Though not sub-nV pretty decent for an un-assisted op amp.
The OPA1612 has respectable performance in this circuit as a moving coil preamp.
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