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 In Moving Coil Preamp Using the ZTX

Post by Hans » Fri Jul 15, 2016 5:43 am

JR. wrote:Just to be practical 8x BF862 is still lower Ciss than 1x IF3601.

The 862 appears to be a HF device.

JR
There are two 220pF capacitors at the input of the circuit diagram, these components should be left in case of using the IF3602.
And indeed the BF862 has in this specific application a gain-bandwidth that is ca. 2.5 times higher as the ZTX851, to my opinion nothing to worry about.
Since there is no Spice model for the IF3602, I can't simulate and measure its performance in more detail.
The only real benefit over the BF862 lies in the comfort of not having to match 8 individual Fets out of ???, making the price difference relatively small.
But apart from its higher noise, the BF862 seems to be an excellent Fet.

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans » Sat Jul 16, 2016 6:03 am

Here are the freq. responses with the ZTX851 and the BF862 both with the same gain of 40dB.
By only changing the 330p cap to 110p, responses are identical.

Hans
Freq.jpg
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. » Sat Jul 16, 2016 9:49 am

I find it difficult to worry about a -3dB point of 1Mhz

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans » Sun Jul 17, 2016 1:45 pm

JR. wrote:I find it difficult to worry about a -3dB point of 1Mhz

JR
Neither do I.

What I do worry about is the feedback resistor of 99,5 Ohm, that no OPA can drive.
On the other hand, to achieve 30dB overload op to 20kHz, Gain cannot be more than 100.
To solve this, the 2 Ohm resistor could be increased, or a buffer placed behind the OPA to supply enough current.
None of both is acceptable.

Therefore another solution could be found by making an amp module that can be paralleled.
Module.jpg
Module.jpg (80.69 KiB) Viewed 11443 times
This module only has 2 Fets, so matching is way easier than it is to match 8 Fets.
the original 2 Ohm resistor can now be increased to 8Ohm, and the 99,5 Ohm resistor to 400 Ohm.
Just before clipping, a positive current of 36mA flows into the 400 Ohm resistor, a current that a LM4562 can handle.
Since clipping is not symmetrical around zero volt, a supply voltage of +15V and -19V hast to be used for this head amp, to achieve the 30dB overload margin before clipping.

With 4 modules in parallel, the outputs are to be summed into OutS+ and OutS_.
From hereon, the signals can be handled like before.
The Bipolar version produces 33nV A-weighted noise versus 60nV A-weighted for a version with 4 Fet modules, or only 5 dB more noise.
Bip-Fet.jpg
Bip-Fet.jpg (238.87 KiB) Viewed 11443 times
The beauty of a modular system with piggybacked modules, is that either Bipolar or Fet modules can be inserted, and either one or up to four modules.

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. » Sun Jul 17, 2016 3:03 pm

While I'd be inclined to roll my own discrete buffer between the op amp output and 99 ohm feedback network but for an off the shelf solution consider a LME49600 buffer capable of 250ma (I've never used one but similar parts have been around for a long time).

I don't keep current with the modern uber op amps but they have upped their drive current to facilitate lower Z feedback networks, to realize their incredible input specifications.

JR
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans » Mon Jul 18, 2016 3:25 am

JR. wrote:While I'd be inclined to roll my own discrete buffer between the op amp output and 99 ohm feedback network but for an off the shelf solution consider a LME49600 buffer capable of 250ma (I've never used one but similar parts have been around for a long time).

I don't keep current with the modern uber op amps but they have upped their drive current to facilitate lower Z feedback networks, to realize their incredible input specifications.

JR
A module with just 2 bipolars/fets + a dual opamp 5532/4562 is very affordable and quite a bit more flexible because you can take exactly the amount and type of modules needed for your application.
Bipolars in 4 steps from 0.17 to 0.33 nV/rtHz and Fets from 0.60 to 1.20 nV/rtHz also in 4 steps could be chosen from.
For an MC application even the noisiest of them all, still has a SNR of 76 dB-A ref 0.5mV@1kHz, an overload margin of 30dB up to 20kHz, sub nA input current and linear phase up to 20kHz.


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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by mediatechnology » Wed Jul 20, 2016 4:22 pm

Sorry I'm late to this party.
Sometime back in mid-July I didn't view a post and the board stopped sending me notices.
A lot has happened since.
You would need at least 12x IF 3602 duals to get the same low noise!!
So if you are looking for the lowest possible noise, the ZTX851 can’t be beaten.
Very cool! Thanks for doing the sims.
The ZTX951 PNP might be slightly quieter though the gain resistor starts to dominate.
The ZTX851/951 was certainly Horowitz and Hill's recommendation.

The sims might be adjusted to make the collector loads 1K since the trim, at center, adds 100R to each leg.
But the measured and the sim are very, very close.
The simulation and the real world measurement are thus only 0.6dB apart !!
And it looks like this!

Image
Flat Moving Coil Preamp Balanced Input ZTX851 NJM2068DD Protoboard Test Circuit
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Wed Apr 25, 2018 5:47 pm

I just revisited the ZTX851 MC preamp and decided to test a servo proposed by olafmat.

viewtopic.php?f=6&t=697&p=11280#p11280

This is olafmat's servo applied to a "classic" input stage.

Image

I used a DIfferential Debbo like the one here:

Image

Works like a champ with the ZTX851 front end.

Since it was on my mind I also measured the Ios of three ZTX851 pairs I had assembled.
I measured 0.233 µA, 0.1 µA and 0.436 µA.
Not enough MC cartridge current to worry about IMHO.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Mon Apr 30, 2018 8:35 am

The servo works great.

About the only issue I found was that large offset input pairs (≈1mV) tended to have less power supply rejection.
Originally I thought the collector loads were the source but it turned out it was the reference divider strings.
A simple capacitance multipler solved that problem.

In a typical preamp the reference string is common to both op amps.
Noise there appears in common mode.
With the servo approach I borrowed from Olaf, a bridge is formed which has both a common mode and a differential offset correction component.
When the bridge is pulled harder to one side, common mode rejection of power supply noise apparently decreases from increased common mode to differential conversion.

FWIW the servo injection is high-level and not subject to differential gain which is cool.
With the values I chose the servo attenuation is only "4."
An input pair with 1 mV Vbe mis-match subject to 60 dB gain produces 1V offset at the output.
The differential offset voltage to correct this is only -4V or +/-2V at the servo outputs.
The servo correction voltage at the op amp non-inverting inputs is reflected at the input transistor collectors to equalize the emitter voltages and currents.

Ultimately I decided that I wanted a better-defined voltage for the collector loads and reference dividers than a cap multiplier could provide.
I looked at simple solutions and decided on a TL431 shunt regulator and an 11V sub-rail for the collector loads and reference dividers.
Cohen's circuit had two red LEDs to lower Vcc to about 11.3V with a 15V rail.
The reason he gave - a good one - was to limit the common mode range the following 5532 would see.
Having an 11V sub-rail seemed like a good idea not only from a noise perspective but also mindful of the following op amp's CM range.

The TL431 shunt regulator worked great. Along the way I decided to try an odd-ball idea that came to me and I became distracted: viewtopic.php?f=6&t=960

Now that I have that out of my system I'll do some preamp drawings for us to look at.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Mon Apr 30, 2018 5:14 pm

This is a draft schematic for the latest Flat Balanced Input Moving Coil Phono Preamp using the ZTX851.

Image
Flat Balanced Input Moving Coil Phono Preamp using the ZTX851.

Printable copy: https://www.proaudiodesignforum.com/ima ... ZTX851.png

The circuit above is virtually identical to the original in terms of noise performance.
The servo has been updated to a Differential Deboo and a TL431 shunt regulator added to improve PSRR.
The design is completely DC-coupled.
A high pass filter response is derived by a servo.
C7, a 0.47 uF film capacitor, sets the single-order High pass response 3 dB point at about 10 Hz.

The circuit is based in the "Cohen" philosophy which keeps the impedances low. http://www.proaudiodesignforum.com/foru ... 161&p=1343
R8 and R9 sink emitter current to eliminate additional noise sources.

R13, R14, R15 and R16 form a bridge circuit to set the collector current at a nominal 5.5 mA each.
R17 and R18 along with the Deboo integrator rebalance the bridge to correct offset error by skewing the collector currents.
The servo forces the emitter potentials to be equal reducing the delta-Vbe x gain error to millivolts at the final output.

I have to give credit to Demrow and Cohen for the topology and low-noise philosophy.
Horowitz and Hill, along with Brad Wood (Bcarso RIP) alerted me to the low rbe ZTX851.
"Olafmatt," here on our forum, handed me the servo on a silver platter for which I'm very grateful.
Thanks also to Hans for the sims and JR for the input.
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