A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

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

Post by billshurv » Tue May 15, 2018 8:28 am

It's certainly something to stock up on just in case for personal use. Luckily for my use a lifetime buy is a dozen. Oh and I realised the PNP is actually 14p more than the NPN. Horror.

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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by JR. » Tue May 15, 2018 9:16 am

I apparently missed this the first time around.

One question.... at max gain, how much is the current density of the two input devices shifted apart? This might affect HF CMRR but this is picking a very tiny nit.

Nice circuit.

JR

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

Post by mediatechnology » Tue May 15, 2018 9:27 am

I re-read Horowitz and Hill 3rd Ed. and had forgotten or overlooked a histogram they did showing delta-Vbe for 100 each of the ZTX851 and ZTX951.

The PNP ZTX951 had much tighter Vbe though both were quite good.
The PNP had a flatter beta vs Ic curve but the NPN, at the operating currents we're using (~5.5 mA), had higher beta.
I had forgotten the exact numbers: H&H put the rbb' of the NPN ZTX851 at 1.5Ω and the PNP ZTX951 at 1.2Ω.

With the contact resistance of the Protoboard I may not be able to see the 0.6Ω difference.
For testing I may solder Rgain to the emitters.
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billshurv
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue May 15, 2018 10:21 am

You do have to admire their dedication to the cause to measure that many!

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

Post by mediatechnology » Tue May 15, 2018 10:54 am

They write about testing some 300 N-FETs being "painstakingly done" with the added footnote 55 on page 508.
55 Literally! One's fingertips were rendered quite tender, after lead-forming and inserting three hundred transistors.
After a search I discovered some people using helical compression springs to keep discrete transistors compressed together. A short length of some of the Accutronics reverb spring like the one used here might be perfect. It doesn't have the mass of a nut but its also not permanent.

Before I change anything to PNP I think I want to change the input bias resistors to 1K5Ω each and try it with 150Ω and 200Ω source impedances as a mic preamp.
"We have grown to love our captors and demonize those that try to lead us to freedom."

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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by JR. » Tue May 15, 2018 11:30 am

For one of my kit phono preamps using JFET inputs (P-10), I measured hundreds of JFETs in a fixture for Vgs with a nominal source resistor termination, then paired them up for closest Vgs match.

They actually delivered lower THD in circuit when matched. Unexpected since the topology pretty much holds them inside a closed NF loop, but apparently enough changing conditions to measure a difference on the test bench. Probably not very audible with typical mV level input signals, but better is always better.

FWIW the same topology using bipolar input devices for MC showed no such sensitivity or need for Vbe matching (but maybe I didn't look close enough :lol: ).

JR

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

Post by mediatechnology » Tue May 15, 2018 11:43 am

The P10 was AC-coupled (with the Cc inside the loop) so the Vbe-mismatch didn't matter.

This one is DC-coupled.
I haven't selected for Vbe but high offset parts do require the servo to pull harder.
The difference in collector potential after servo correction is equal to the Vbe error times the DC gain.
A 1 mV Vbe mismatch with 60 dB gain pulls the collectors 1V apart.

1 mV error in the ZTX851 is about the worst I've found though.
I haven't worn my finger tips like H&H did selecting.
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mediatechnology
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Wed May 16, 2018 7:05 am

JR. wrote:
Tue May 15, 2018 9:16 am
I apparently missed this the first time around.

One question.... at max gain, how much is the current density of the two input devices shifted apart? This might affect HF CMRR but this is picking a very tiny nit.

Nice circuit.

JR
Thanks JR. I missed your post or would have responded sooner.

The worst pair I have, at full gain, produces about 500 µA delta Ic (Or ±250 µA per device from a nominal 5.5mA.)

PSRR vs. Servo

Curiously what I found was that this pair, when pulled into correction, had reduced PSSR.
Running the preamp on batteries verified that it was PSRR, not CMRR.

The first place to look is obviously the collector loads so I built a cap multiplier for them.
That did not solve it: The ingress was actually into the reference voltage for the bias/servo bridge. (R13-R18.)
As the servo corrects the bridge gets pulled.

At this point I introduced the TL431 shunt regulator for a low noise +11V reference.
I chose the '431 over a cap multiplier because I wanted both input PSSR and low output impedance.
The '431 solved the PSSR vs. servo problem and gives me an accurate +11V reference with a low dynamic impedance.
The 11V supply also eliminates the LED string used by Cohen et al to limit the common mode range seen by the 5532.

CMRR

I've looked at CM rejection and it seems pretty good.
It's difficult to take any measurement on a Protoboard too seriously.
In checking CMRR on a Protoboard you normally have two things working against you: Capacitance and sub-Ohm contact resistance.
Its difficult enough testing a modestly-high impedance line receiver until its on a PC board. (18KΩ for a THAT124X.)

The MC preamp has a common mode input impedance of 250Ω.
A tenth of an Ohm makes a huge difference so I'll need boards to make any meaningful CMRR measurements.

ZTX851 as a Mic Preamp

I made some quick noise measurements at 100 and 200Ω source impedances with 1K5Ω bias resistors and gains of about 20 and 56 dB.
The high gain NF were excellent.
At low gain, 20 dB, there is 6 dB gain in the cross-coupled output with high-ish value feedback resistors.
The actual preamp gain is 14 dB.
The output stage noise dominates as expected.

Using the basic circuit as a mic preamp I would consider lowering Ic to maybe 1 mA.
The object is to reduce base current noise for the higher source impedance where it matters more than for an MC cart.
There may not be much improvement but its worth trying.

The other thing I would do for a mic preamp using this front-end would be to make the common mode rejection stages use 2K resistors to lower low gain noise.
For a MC preamp enough front-end gain is required that the output stage noise doesn't dominate and the precision of line receiver common mode rejection stages are preferred.

I did have a look at the ZTX951 datasheet and the Cob is almost double the ZTX851 at 74 pF.
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mediatechnology
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Thu May 17, 2018 2:11 pm

I just did a quick measurement and calculation of the noise current by measuring the output noise floor with the input shorted versus the input open.
An open input has a 998Ω differential termination.
As expected current noise dominates and for all practical purposes the voltage noise can be ignored.
A shorted input is about 20 dB quieter.

The voltage noise (shorted input) at 56 dB gain works out to be 0.517nV√Hz.
The current noise (998Ω termination) at 56 dB gain is about 4.9pA√Hz.

The Ic is 5.5 mA nominal.
I'm pretty happy with the results for a MC preamp at this current.

For a mic preamp lowering Ic to hit a 2 pA target seems do-able.
I may want to use this circuit in the input-capacitorless preamp.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Fri May 18, 2018 12:55 pm

I decided to run some stability tests with the preamp input terminated with "pure" inductance.

In a mic preamplifier using this same topology there are usually current-limiting series resistors to protect from phantom power faults.
Though they do add to the source resistance and noise, their value is usually much smaller than the actual source resistance.
With a 200Ω source impedance the added 10Ω per leg isn't that much of a penalty for the benefit of phantom power protection.

The protection resistors also provide a benefit which isn't talked about much and that's stability.
The resistors serve as "base stoppers" to isolate the input transistors from inductance across the input.
Without low-value series resistors many mic preamps will oscillate particularly if one input leg is grounded.

A moving coil preamp does not provide the luxury of any added series resistance.
Stability with inductance across the input matters.

What is the inductance of a typical moving coil cart?
Relative to a Moving Magnet cart it is low. But what does that mean?
I pulled 10 µH out of thin air as an estimate.

About the only manufacturer that publishes this is AT.
The values range for the ART7 at 8µH to 25µH for the ART9, OC9 and F7.
So my 10 µH guess was in the right order-of-magnitude and at the low end of the AT range.

I have some JW Miller 10 µH and 33 µH molded RF chokes. The DCR is about 0.5Ω and 0.9Ω.

I removed all termination capacitance and differential termination.
Due to 499Ω bias resistors the input differential impedance is 998Ω and the common mode impedance is 249.5Ω.

Does it oscillate with 10 µH across the inputs?
Yes. Bigly.

I increased the input inductance to 33 µH X3 to make it even bigger.
With the preamp broadcasting around 330 kHz I decided to see what would stop it by putting back in what I had taken out.

Here's what works.

1) I added some DCR to simulate the actual cart's DCR. Somewhere between 3.3 and 10Ω it stops.
2) A differential resistive termination somewhere between 1K and 300Ω makes it stop.
3) A differential capacitive termination of 4.7 nF makes it stop.

Either 1,2 or 3 provides stability for both balanced and single-ended input connections. (One input grounded.)
With 2 and 3 installed but the source impedance 2.7Ω (no added cart DCR) it was still stable.
If none of the remedies above are in-circuit the preamp, as expected, makes a nice Colpits oscillator.

Since a cart is always going to have some non-inductive DCR and it will be terminated at least with a resistor, if not an R and a C I think stability will be OK.

The next step is to invert the polarities and try it with the ZTX951.
That will permit direct comparison to the 2SB737 of which I have a few.
"We have grown to love our captors and demonize those that try to lead us to freedom."

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