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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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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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.

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

Post by terkio » Thu Feb 07, 2019 5:44 am

I am very interested in this thread. I built a mic preamplifier using 4 modules based on the Grahem Cohen double balanced design. I am so found of this design, I wish to build more, but the major trouble is the availability of low noise matched bjts.
I found this: AS194 AS394 Matched NPN transistor pair http://www.alfarzpp.lv/eng/sc/AS394CH.php
Caracteristics look the same as LM194 LM394, but the AS194 AS 394 seem available at reasonable prices..... This looks too good to be true.
What do you think: Is this real or clever fakery ?

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

Post by mediatechnology » Thu Feb 07, 2019 7:34 am

Thanks for joining us here terkio!
I'm glad they started producing the LM194/394.

The AS394 should be suitable for Cohen mic preamp designs but the 1.8 nV√Hz noise voltage alone is too high for a moving coil preamp.
The LM394 has certainly been used in MC preamps but the ZTX851 will outperform it.

The ZTX851/951 have much lower base resistance.
The ZTX851, being a large-geometry part, also has a high output capacitance.
On the other hand the AS394 has better Vbe matching and higher current gain.

I went through a limited number of ZTX851 to Vbe match them and didn't have much trouble matching pairs within 100µV and they're cheap enough you could buy 100 of them to have your pick of the liter.
Once I got a half-dozen pairs I went back and measured the Hfe which ran between 140-200.

I really would like to advance this project.
I've been out of stock on the flat moving magnet preamp for awhile and just got some back in.
With those moving again it makes sense to finish the MC version.

The Protoboard for it, along with a few others, is sitting on my bench.
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terkio
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by terkio » Thu Feb 07, 2019 8:44 am

Thanks for your welcome and fast answer. I forgot to mention the preamps I worked on use two LM394 cross parallelled.
I am impressed by the ZTX851 and the most excellent work you made from it. The noise figure is great. I see matching can be better than the 500uV I figured from reading earlier posts. So 100uV can be expected, this is great, compared to the 50uV typicall for the LM194
Thermal bounding of 2 ZTX851 asks for a copper enclosure fat enough for thermal inertia, a copper tube closed on top, that extends a bit around the leads. The remaining temperature mismatch will creep in from the leads and PCB tracks.
The servo is great to take care of temperature drifts, but: Using a servo, what about artifacts when changing the gain ?
The Rgain of my preamps is made of a rotary switch 24 position commuting on a chain of resistors. Thanks to 'make before break' it switches gains nicely. I am afraid the servo cannot be good at all gain settings and will induce crakling noise when changing the gain and take ages to settle at low gain. Furthermore, I have 33uF caps in front for the 48V phantom power.

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

Post by mediatechnology » Thu Feb 07, 2019 9:04 am

If you have enough ZTX851 to chose from you can get some very close.
The jig I used matched them simultaneously using a differential connection which helps.

The M7 nut method of thermal-coupling does give it a lot of mass.

In this particular design I didn't concentrate on servo settling since the gain is not switched.

I am familiar with servo settling time in a mic preamp.
If you haven't already visited this thread its a rabbit-hole you might want to go down into: https://proaudiodesignforum.com/forum/p ... ?f=6&t=598

At some time in the future I want to graft the ZTX851 front-end and the different method of servo injection used here into the input-capacitorless mic preamp.
Being able to make the servo non-resonant single-pole by eliminating the input coupling caps and isolation of the mic's internal caps (if any) using a downstream servo injection point is a big bonus.
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