A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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

Post by terkio » Mon Mar 11, 2019 2:04 pm

mediatechnology wrote:
Mon Mar 11, 2019 10:12 am
I'm thinking about a way for the builder to Vbe match using the preamp as a jig.
Great.
The output of the servo tells how the two transistors match.

Actually, it tells about a mix of Vbe mismatch and Beta mismatch.
In this design with low base resistor values ( 499 Ohm ) the circuit is mostly measuring the Vbe mismastch. ( Beta mismatch is swamped )
As an alternative, using large base resistor values ( >10K ) this circuit would measure, mostly the Beta mismastch.
I am thinking here, about using this circuit for a mic preamp designed with a large input impedance.
There are other sources of unbalance ( Resistor matching, Op amps input offset voltage ); Swapping the two transistors should give a mean to know about those, a mean to calibrate the test jig.

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

Post by mediatechnology » Mon Mar 11, 2019 3:46 pm

I think I'll use a Vos "deltaVbe" method that defeats the servo (to eliminate measurement lag) and read the raw output Vos with gain applied to magnify it. I'll have to try matching with various methods though. Matching them in-circuit seems best.

For the trial matching run I used Ian Fritz' jig. http://dragonflyalley.com/synth/images/ ... 11_144.pdf

At higher currents, about 5.5 mA nominal in this front end, there's a thermal effect that delays final measurement. With Ian's jig, at lower currents, I was able to get a settled measurement pretty quickly. I handled the transistors with tweezers to keep from heating them up and then covered them both with bubble wrap to reduce air currents. I'll have to figure that out.

I'll soon know how the ones I matched with Ian's jig do in the preamp.

I think the overall net servo "gain," as drawn, is actually 0.5.
The cross-coupled THAT1240 output has a differential gain of 2.
The bridge attenuates by 4.
Net servo gain seems to be 0.5.

EDIT: Looked at this again. Deboo has a gain of 2 so the original net servo gain is "1."

I took your suggestion terkio and removed the two fixed lower bridge arms. (20KΩ)
I then used the servo output, with CM DCV pinned at ~0V, to drive the bottom of the bridge with 10KΩ resistors.
The upper arms are 2X 10KΩ to +11V.
The lower arms are 2X 10KΩ to servo + and - output.
That raises the gain by 2 and reduces the output Vos by half, with this particular transistor pair, to about 100-130 uV.

EDIT: Now the servo gain is "2." The Fc with a 0.47 Deboo C is now about 15 Hz.

Note that the OPA2277PA that I'm using has a max Vos of ±20 µV at room temp.
The bias current is ±2.8 nA max at room temp.
With input resistors of 100KΩ the Ib*R error can be as high as ±280 µV - much higher than the Vos term.
So we're in the ballpark of what the OPA2277 is capable of.

Instead of removing the servo IC to defeat it for test, the Deboo C can be shorted.
The servo common mode output voltage will be maintained at 0V but servo correction is defeated.
Doing this magnifies the total error to simplify measurement.
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terkio
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by terkio » Mon Mar 11, 2019 6:34 pm

mediatechnology wrote:
Mon Mar 11, 2019 3:46 pm
At higher currents, about 5.5 mA nominal in this front end, there's a thermal effect that delays final measurement. With Ian's jig, at lower currents, I was able to get a settled measurement pretty quickly. I handled the transistors with tweezers to keep from heating them up and then covered them both with bubble wrap to reduce air currents. I'll have to figure that out.
I forgot the thermal effect, that is not in the Ian's jig thanks to using Rc 100K.
Your preamp as a matching jig could use Rc much higher than its present Rc 1K
Note that the OPA2277PA that I'm using has a max Vos of ±20 µV at room temp.
At the top of the datasheet, it says 10uV, then in the detailed caracteristics, max Vos of ±20 µV. I better use 20uV.
The bias current is ±2.8 nA max at room temp.
With input resistors of 100KΩ the Ib*R error can be as high as ±280 µV - much higher than the Vos term.
So we're in the ballpark of what the OPA2277 is capable of.
I do not get it. Is this the Ib in Ian's jig or the Ib in your preamp ?
Instead of removing the servo IC to defeat it for test, the Deboo C can be shorted.
The servo common mode output voltage will be maintained at 0V but servo correction is defeated.
Doing this magnifies the total error to simplify measurement.
Yes you need to bypass the servo.
EDIT Oups, I misundersood. You short the Cap of the Deboo....Nice trick.
For my project of mic preamps:
For matching transistors I will leave the servo as is. The servo output voltage does give a magnified total error measurement.
The trouble is the 5.5mA Ic that induce thermal effect. The reason is to do the matching in the conditions of the final circuit, including the effect of Beta.
BTW Is the 5.5mA good for a mic preamp, you said you might go as low as 1mA. How is ZTX851 noise going versus Ic ?

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

Post by mediatechnology » Tue Mar 12, 2019 5:27 am

The bias current figure and the 100K refer to the OPA2277 and its input resistors.
Their Ibias*R term is higher than the Vos alone.
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terkio
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by terkio » Tue Mar 12, 2019 6:21 am

I see, good catch. For some odd reason I thougt the OPA2277 was a JFET input, so I did not care about Ibiais.
OPA 2277, Ib spec: 0.5nA typ, 2.8nA max. This widespread between Typ and Max is pretty anoying.

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

Post by mediatechnology » Tue Mar 12, 2019 8:22 am

I went back and looked and I used 10KΩ 1% resistors I ratio-matched to 0.01% for Ian's jig.
After looking at matching in situ at the high Ic currents vs using Ian's jig I've concluded that Ian's jig will be easier.
Devices matched using his jig still match well in the preamp.
I only tested maybe 4-5 pairs and the results were consistent enough I went on to something else.

Here is the latest draft schematic:

Image
Drawing updated March 15, 2019.

A summary of the changes:

I added more jumpers to provide a +62 dB gain option. (+56 dB unbalanced output.)
(Note VR1 is only useful at lower gains when J6 is open due to the hop-off resistance of VR1.)

Added J8 to defeat the servo.

Removed R17/18 so the servo now drives the lower arms of the bias bridge.
This decreases the attenuation of the bias network from "4" to "2."
Overall servo gain now appears to be (and measures) "2."

Added R8 and R9.
Why?
The ZTX851, being a power device, has large geometry.
Cobo is about 45 pF typical. (The PNP ZTX951 is much larger at 74 pF typ.)
In comparison a 2N4401 is <10 pF.

The large Cob requires a large C4 and C5 for stability.
Slower op amps, such as the NJM2068 require an even larger C4 and C5, around 330 pF.

R8 and R9 isolate the transistor's output capacitance from the summing node allowing C4 and C5 to be reduced in size.
Banging the preamp with a square wave shows increased stability that eliminates ringing and overshoot.
Some op amps do not require R8/R9 and large C4/C5 combinations so in many cases R8 and R9 can be 0Ω.

At +62 dB gain I can't measure a difference in noise performance with R8/R9 at 0Ω or 1KΩ.

Using the NJM2068 and a particularly "bad" pair of ZTX851 I was able to get improved waveform fidelity with a 1000Ω/220pF than with 0Ω/330pF.
With a 1KΩ/220pF combination the preamp will run with just about any op amp including an NJM2068 or MC33078.
My first choice is a 5532.

I'll do a table of what combinations work best.
The R8/R9 trick was used in the Amek 9098 mic preamp.
I've also seen it used in a noise test jig that I recall posting earlier here.
That jig used an AD797 and the values were much smaller.
It's better to have spots on the board for them and not use them then need them and not have them.
I'm willing to pay for 4 extra holes.

One other thing I might add is that the bridge Vos adjustment method really only works properly (as shown above) when the bias Q-point voltage is half the collector supply. (5.5V.)
If the values of the lower arms of the bridge are lowered to reduce Ic, the differential servo gain is reduced.
I don't see that as a limitation here, but I need to think through the consequences in a different context.
A servo that introduces a common mode voltage might be better.

One thing that the Deboo offers is that it has a passive input pole and uses a single C.
A Deboo with a Vcm input might be useful later on.
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billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue Mar 12, 2019 9:12 am

Just needs ref and clip LEDs to be perfect. Ref comes on at 0dB off a test disk and clip when the output hits 0dBFS for the ADC.

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

Post by mediatechnology » Tue Mar 12, 2019 9:21 am

Thanks Bill, great idea.
Maybe the TL431 winky for A/D clip?
I think your up-and-coming PCM4222EVM has clip LEDs so you have even more to look forward to.

I think I figured out a simple way around using the bridge at something other than 1/2 the collector supply using a sub-regulator.
That keeps all the bridge arms at identical value and the servo gain predictable.

OTOH:
The ZTX851 has about a 1-10 mA range of optimum Ic.
The ZTX851's Cob also bounds the value of the collector load so the best solution might also be changing Rcollector to change current.
499Ω to 4K99Ω would be the range for about 1 mA to 10 mA Ic.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue Mar 12, 2019 9:32 am

mediatechnology wrote:
Tue Mar 12, 2019 9:21 am
Thanks Bill, great idea.
Maybe the TL431 winky for A/D clip?
I think your up-and-coming PCM4222EVM has clip LEDs so you have even more to look forward to.
So it does. I hadn't spotted that. I need to nag as it's sitting on a bench in a TI lab somewhere. slightly OT question on that. Do you know what the input filter is set to? They don't say which is annoying as there are a number of sample rates supported and the filter will only be right for one of them!

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

Post by mediatechnology » Tue Mar 12, 2019 10:34 am

The high-pass filter to remove DC offsets or the anti-alias filter?
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