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

Post by mediatechnology » Tue Jan 08, 2019 3:10 pm

This discovery was a complete accident...

I had been measuring some glass 3AG 1/4A fuse's DC resistance. (About 2.8Ω for fast-blow.)
One of them happened to come apart after unclipping it from the 4 wire meter.
I thought the glass tube was cute enough to save and set it aside on the workebench.

As I was picking up the end caps to throw in the trash I looked at them and thought "Would two ZTX851 transistors fit in here?"
Yes they do!

Back-to-back the E-line ZTX 851's fit neatly in the end cap.
This may be the slickest and most easy-to-find way to thermally couple the devices yet.

It actually looks like a real "3AG 1/4A" dual transistor.
Much more real than the "nut-case" version.
viewtopic.php?f=6&t=783&start=50#p9394

Image
Two ZTX851 thermally-coupled by a 3AG fuse end cap.
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Gold
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by Gold » Wed Jan 09, 2019 1:11 pm

Cool! McMaster-Carr sells a few different potting compounds is small quantities. It’s under Adhesives for Electronics.

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

Post by billshurv » Sun Jan 13, 2019 4:55 pm

Whilst the nut is a bit more steampunk that is a very neat solution1

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