A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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mediatechnology
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Thu Sep 20, 2018 1:52 pm

For the DL103 with an output of 0,3mv@5mV/sec, S/N including the Cart becomes an excellent 75.0 dB.
Good to know!

I would just make Rg 20Ω for +36 dB as you did.

You don't ever want to add external resistors to the line receiver inputs if you expect to maintain CMRR.
The absolute values of the internal resistors also vary by ±20%.

You can use THAT1246 and lose 6 dB.
On the MM preamp I tried that and it doesn't really improve noise.

Hans
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by Hans » Thu Sep 20, 2018 3:26 pm

I would strongly advise against making Rg 20 Ohm. It will partly ruin your careful design.

With Rg 20 Ohm, Flat noise will be 173.3 nV and after Riaa+A-weight 69.9nV with Rs 40 Ohm and Rload 100 Ohm.
Circuit with the DL103 will than have 72.6 dB S/N ref 0.3mV@5mV/sec

Keeping the original Rg 2 Ohm, but replacing each That1240 by an Opa1612 with two 10K and two 1K resistors of 0,01% as a 20 dB attenuator
Flatnoise will be 111.2 nV and after Riaa+A-Weight 44.8 nV (again with Rs 40 Ohm and R load 100 Ohm).
With the DL103 you will now have 76.5 dB S/N ref 0.3mV@5mV/sec, an improvement of almost 4 dB and still with an excellent CMRR.
It would be a shame to simply throw 4dB overboard after having squeezed every potential noise contributor to its minimum.

Hans

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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Fri Sep 21, 2018 7:52 am

Been awhile since I looked at MC gain for flat recording and 36 dB isn't enough to be useful anyway so I'm not sure I proposed a valid case.

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