A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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

Post by Hans » Wed Sep 19, 2018 3:57 am

Also what does the simulator tell us for 3.3Ω?
Self gets a 6.4 dB NF at 3.3Ω with his 3X 2N4401 MC preamp.
A 3.3 Ohm Resistor produces 33.1 nV from 20 Hz to 20 Khz

With R collector 1K, noise is:
Rs 3.3 / Rg 3.3 : 63.1 nV NF=5.6 dB
Rs 3.3 / Rg 2 : 59.4 nV NF=5.1 dB
Rs 3.3 / Rg 1 : 52.6 nV NF=4.0 dB

With R collector 500 Ohm, noise is:
Rs 3.3 / Rg 3.3: 56.8 nV NF=4.7 dB
Rs 3.3 / Rg 2 : 52.6 nV NF=4.0 dB
Rs 3.3 / Rg 1 : 49.1 nV NF=3.4 dB

So in all cases this circuit beats the Self design.

Hans
Last edited by Hans on Wed Sep 19, 2018 7:10 am, edited 1 time in total.

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

Post by mediatechnology » Wed Sep 19, 2018 6:26 am

Thanks for doing the new sims Hans.

How did rowing go?
Hans wrote:
Wed Sep 19, 2018 3:21 am
Here is the answer to your first question concerning the ZTX851, noise specified from 20Hz to 20 Khz:

Rsource 1 Ohm/Rg 1 Ohm:
Noise Free Transistor: 48.9 nV
2 ZTX851 : 49.1 nV
1 ZTX851 : 49.3 nV

Rsource 1 Ohm/Rg 2 Ohm:
Noise Free Transistor: 52.3 nV
2 ZTX851 : 52.5 nV
1 ZTX851 : 52.6 nV

Hans
OK, so it looks like we're at rock-bottom near its best noise floor and are seeing other noise sources dominate.
A 3.3 Ohm Resistor produces 33.1 nV from 20 Hz to 20 Khz

With R collector 1K, noise is:
Rs 3.3 / Rg 2 : 59.4 nV NF=5.1 dB
Rs 3.3 / Rg 1 : 52.6 nV NF=4.0 dB

With R collector 500 Ohm, noise is:
Rs 3.3 / Rg 2 : 52.6 nV NF=4.0 dB
Rs 3.3 / Rg 1 : 49.1 nV NF=3.4 dB

So in all cases this circuit beats the Self design.
I forgot to include Self's 3.3Ω Rg which would make this one a little noisier.

Looks like I need to build a tidier prototype to see what one measures under better conditions.

My instincts tell me Self's circuit could be built with a single ZTX951 (or an 851 with bias reversed) and perform just as well as 3X 2N4401. Doug didn't know about the ZTX851/951 when he wrote Small Signal Design. None of us did. Thanks to Horowitz and Hill and the late Bcarso (aka Brad Wood) I learned about them.

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

Post by Hans » Wed Sep 19, 2018 7:14 am

I added in the previous posting als the outcome for both Rs and Rg 3.3 Ohm to keeps things together.
And you are still below Doug's 6.4 dB.

Hans

P.S. Rowing was great with 4-6 Bft.

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

Post by Hans » Wed Sep 19, 2018 7:35 am

Just out of curiosity, I changed the ZTX851 for the ZTX951, and turned 11V into -11V.
The result was in all situations, including the quietest version with 500 / 1 / 1 Ohm, less than a percent or > 0.01dB

Hans


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

Post by Hans » Wed Sep 19, 2018 10:12 am

What op amp is in the first "socket?"
When comparing, only change one component at the time, so for both ZTX851 and ZTX951 an Opa1642 was used.
Somewhere I have the idea that you don't like this amp, am I correct ?

However, after having corrected the Spice related bug for the Opa1612, I have made the overview below with this very amp.

Hans

Noise2.jpg

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

Post by mediatechnology » Wed Sep 19, 2018 7:09 pm

Thanks for the table Hans.
Very informative and ties the data together.

I like the OPA1642 they are a nice FET input similar to the OPA1612 otherwise.
The 5.1 nV√Hz noise voltage seems high for that location though its respectable for a FET op amp.

In MM preamps the OPA1612 is significantly noisier than most due to its noise current.

I don't see a significant noise penalty in the MC preamp using the OPA1612 though.
It may sim higher and may actually be slightly higher but I don't measure it higher if I remember correctly.

The NJM2068 isn't characterized in the usual way though there is a chart JRC publish showing it in the 3 nV√Hz range. They don't show current noise.
I think its current noise is pretty low however which is why it usually measures quieter than a 5532, 2114.

The 2068 and OPA1612 are pretty close in this application so where one has a higher voltage noise and lower current noise the other is the opposite.
In the socket they measure similarly.

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

Post by Hans » Thu Sep 20, 2018 6:06 am

Wayne,

The amp's noise in the first socket plays hardly a role because of the large feedback.
The Opa1612 is giving some 0.1dB less noise contribution than the Opa1642.
Replacing the Opa1612 by a completely noiseless amp shows even less than 0.05dB improvement.
And with also less than 0.05dB profit, there is hardly any benefit in placing ZTX851's in parallel
Some 1 dB can be gained when changing Rcollector from 1K to 500 Ohm as is shown before, but only with an MC Cart with an Rs close to 1 Ohm.
On the other hand, the servo amp better be a low noise Fet version like the Opa1642 as long as large feedback resistors > 10K are being used.
So to conclude, to my opinion what you have is top of the bill in terms of very low noise contribution (the same as a 7.5 Ohm Resistor), low complexity, low current consumption and very low distortion.

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 » Thu Sep 20, 2018 6:37 am

Thank you Hans. I appreciate the compliment.

Its just pure luck for all of us that the ZTX851/951 can be used "off-label" as low-noise transistors.
If I can just get the radiated/induced hum out by building a proper prototype...

Also remembered one of my motivations in lowering collector current was to also lower Ibias.
Ios is pretty low.

I wonder if a lower voltage noise op amp in socket 1 provides benefits at lower gains.
As you know the preamp is flat for RAW recording or playback with RIAA EQ applied in DSP and the typical gain needed is about 20 dB less.

The OPA1612/1642 may test equally at high gain and very low source resistance but what about at gains of 36 dB with a DL-103?
To be honest mine would likely be built with an NJM2068.

BTW the servo noise isn't amplified by the gain and in fact its attenuated by 12 dB.
If the dominate pole were moved to the output the noise contribution of the servo would be even less.

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

Post by Hans » Thu Sep 20, 2018 9:23 am

Wayne,

How do you want to achieve a lower gain from 56 dB to 36 dB, by increasing Rg or by decreasing the 316 Ohm resistors ?
When increasing Rg to 20.8 Ohm, there is no difference in noise RTI between a Opa1612 and a noiseless amp.
Going to 30.5 Ohm for the current 316 Ohm, gives the same absolutely no difference.
But no OpAmp is able to drive a 30.5 Ohm feedback resistor, so this option seems a no no.
Much better would be to place four 81K resistors before the That1240, to let this stage attenuate with 20dB and keep the input stage intact.
In that case, with Rs=40 Ohm and Rload=100 Ohm, noise flat becomes 131.3nV and after Riaa + A-weight 53,9nV RTI .
For the DL103 with an output of 0,3mv@5mV/sec, S/N including the Cart becomes an excellent 75.0 dB.

Concerning the Servo, Spice shows different results in noise performance depending on the chosen amp.
In the first socket the Opa1612 is the winner by a small margin, but for the Servo the Opa1642 is the winner by a larger margin.

The beauty with OpAmps however is that you can swop them as much as you want to find what suits you best.

Hans

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