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 In Moving Coil Preamp Using the ZTX

Post by mediatechnology » Thu Mar 17, 2016 3:23 pm

Would using a 1246 (+6dB) allow you to drop the resistances even lower...?
Good eye.
I've thought about that and this is one of those cases where it might.
Though it intuitively seems wasteful to throw away gain in a preamp, the NF is about 2 dB better when its running at 56 dB in the INA vs 50 dB in the INA. (Rg is 1R vs 2R2).
So throwing away 6 dB gain in the cross-coupled common mode rejector might measure slightly better.
How linear is hop off resistance of the gain trimpot? Probably OK is you don't measure anything.
It's very poor, about 2R. I'm not entirely keen on having it.
The pot is only useable with high output carts requiring relatively low gain. I put it in to cover the >500 uV ones.
Using the pot below 10-20 Ohms isn't practical.
If it were to cover the full range it would be very twitchy.
That's why I have the 56 dB and 46 dB resistors fixed values and in series.
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Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Fri Mar 18, 2016 3:16 pm

Some noise measurements of the ZTX851-based Balanced Input Moving Coil Preamp.

Image
Balanced Input Flat Moving Coil Preamp Using the ZTX851 and NJM2068DD.
Output noise FFT, Gain 50dB Rsource 3R3.
0 dBFS is +8 dBu. Ein -140 dBu approximately 0.54 nV/√Hz. Note hum contamination.


No super-duper exotic parts here:

Image
Flat Moving Coil Preamp Balanced Input ZTX851 NJM2068DD Protoboard Test Circuit

The trim sets the input stage DC balance.
The ZTX851 are thermally coupled (sort of) using heatshrink.
The two THAT1240 provide common mode rejection and balanced output.
The TL072 servos the THAT1240 outputs to pin them to 0V baseline.
Rgain (2R2) is shown poking up between the transistor emitters.
Rsource (3R3) is out of the frame at the end of a short length of STP cable.

Image
Balanced Input Moving Coil Preamp using the ZTX851
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by mediatechnology » Sun Mar 20, 2016 1:14 pm

I did some additional measurements with 56 dB gain and with the hum field nulled.

Rsource is 3R3
Rgain 1R
2X ZTX851 and NJM2068.
The OPA1612 performs identically to the NJM2068DD.

Ein of -143 dBu.
Voltage noise density 3.67 nV/√Hz
Env = 8.3 Ohms

Image
Flat Balanced Input Moving Coil Preamp 2X ZTX851 NJM2068 Gain 56dB Rsource 3R3. O dBFS is +8 dBu.

The noise power sum of Rs+rb+rb+Rg (values equal to 3R3+2+2+1) is -143.492 dBu. (Assumes an rb of about 2 Ohms per ZTX851.)
The measured noise performance is pretty close to the theoretical being only about 1/2 dB worse.
For a 3R3 moving coil cartridge the noise figure should be about 4.5 dB.

Paralleling two additional ZTX851 - four total - provided about a 0.3 dB improvement.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. » Sun Mar 20, 2016 1:21 pm

nice....

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

Post by mediatechnology » Sun Mar 20, 2016 1:34 pm

Thanks John.

I need to find a simple way to thermally-bond the two transistors.
Right now they are heatshrunk together.
Works OK and the DC drift is manageable but air currents create swings with periods in the tens of seconds.
The output servo deals with that drift but it could be less.

The actual 1/f noise of these devices appears very low...
Most of what we're seeing below 100 Hz is soundcard.

I'm thinking about epoxying the ZTX851 pair in a small metal tube or bonding them to a metal tab to give them some thermal mass and equilibrium.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. » Sun Mar 20, 2016 1:44 pm

If they are heat sunk to a common sink and inside a chassis, the internal air temp should be relatively slow changing (speculation) after everything reaches equilibrium.

If the servo can keep up with it, doesn't seem like a problem. I presume there is a HPF in the stage that will receive this signal so any very LF dithering will be filtered out?

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

Post by Gold » Sun Mar 20, 2016 1:54 pm

mediatechnology wrote:Thanks John.
I need to find a simple way to thermally-bond the two transistors.
I think winding thin wire around them like wire wrap wire would work and be reversible. Gluing them together would make troubleshooting and repair more difficult.

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

Post by mediatechnology » Sun Mar 20, 2016 2:15 pm

I think winding thin wire around them like wire wrap wire would work and be reversible.
Just gave me the idea of stuffing them into the coil of a slightly undersized shorth length of spring.
Like an Accutronics reverb spring.
Was also wondering about using adhesive copper foil or even an aluminum foil strip with heatshrink over it.

John: The servos do HP filter it out and it only becomes obvious if I'm moving around the bench or fanning it.
I don't see much at the output, which is HPF at 1.6 Hz, but I do at the test points.
Inside a box it ought to be fine.
Having them coupled better should help initial DC balance trimming too.
Without the servo I see long term drift referred back to the input of about +/-600 uV with my crude heatshrink-coupled pair.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold » Sun Mar 20, 2016 4:50 pm

mediatechnology wrote:
I think winding thin wire around them like wire wrap wire would work and be reversible.
Was also wondering about using adhesive copper foil or even an aluminum foil strip with heatshrink over it.
I don't know if the adhesive will take the heat. The heat shrink will act as an insulator so you would have to use a bunch of tape. I could send you a length of 1/4" copper tape. I was thinking some sort of spring clip might work if it's not steel.

Two small plates on either side of the transistors with holes for screws at the outer edge. Then tighten the screws and nuts to squeeze the plates together.

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

Post by mediatechnology » Sun Mar 20, 2016 4:58 pm

Two small plates on either side of the transistors with holes for screws at the outer edge. Then tighten the screws and nuts to squeeze the plates together.
That's a good idea.
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