Flat Balanced Input Moving Coil Phono Preamp Construction Information

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
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mediatechnology
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Sat Dec 07, 2019 12:57 pm

JP. wrote:
Sat Dec 07, 2019 11:50 am
JP. wrote:
Fri Dec 06, 2019 6:01 pm
In practice the MM setup has more noise. Not in practicality, but at full volume 60Hz and harmonics dominate from the cartridge. Not so with the MC setup.
This is solved. The headshell on the EPA-501H is composite, and the AT-150MLX lower body is not electrically connected to the upper. I used the the original ground jumper and a piece of 32 magnet wire to ground the lower body to the armwand.
Good find!
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Sat Dec 07, 2019 1:07 pm

I did some gain calculations that I will need to confirm by measurement.
Assumes -60 dB input pad and medium impedance load at the output. (49.9Ω X2 build-out.)

Jumper settings

J4, J5, J6, J7 linked = +62 dB gain. Output = +2 dBu

J4, J5, J6 linked. (J7 open) = +56 dB gain. Output = -4 dBu

J4, J5, J7 Linked. (J6 open) = +52 dB gain. Output = -8 dBu

J4, J5 linked. (J6 and J7 open) = +49.6 gain. Output = -10 dBu

I've never found one to produce exactly 62 dB gain due to the very low 1Ω Rg and stray resistances. Usually it's about 0.4 dB or so shy.

The gain equation for the front end is 1+[(316Ω*2)/Rg].
The differential output, if used balanced, adds 6 dB gain to the front-end gain.
Make certain that if an unbalanced load is connected that the unused output floats.
Otherwise there is a 49.9Ω short to ground.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by billshurv » Sun Dec 08, 2019 5:53 pm

That's interesting as I had wondered what would happen if the MM case shield were left floating. Now we know. I have two arms with no way of grounding the cartridge body without running an extra wire along the tube (or attacking with a file in one case). Handy to know it does actually work without even if not the lowest possible noise.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by JP. » Sun Dec 08, 2019 9:43 pm

Maybe, maybe not. After grounding the case I removed the tab to take a ‘before’ measurement. The interference wasn’t there anymore. I’ll need to experiment more with it later.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by grufti » Sun Dec 08, 2019 11:03 pm

Stunningly good, thank you Wayne. I just played music through my MC board for the first time ... and it can only get better. I had the your MC board sitting on the bench with a Mean Well supply about a foot away. My 5R cartridge was loaded with 100R and the tonearm cable surgery had uncovered a botched repair by a previous owner which I sort of managed to patch for the moment. You know what: the connection from the turntable was still extremely quiet and the music was captivating.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Mon Dec 09, 2019 7:49 am

Thank you!

If I may ask "what is the output level of the cart and how much gain did you need?"
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by grufti » Mon Dec 09, 2019 11:59 am

0.34mV @ 5cm/sec. I used the +52dB gain setting yesterday and applied more gain after software RIAA. It turns out that I could have easily used all +62dB gain for the record that I tested with.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Mon Dec 09, 2019 12:27 pm

Thanks for the info.

0.34 mV and +52 dB gain would look to provide an output of -15 dBu at 1 kHz 5cm/s so with a hypothetical converter overloading at +9 dBu that would be -24 dBFS. With a +15 dBu OL point (4V RMS) it would be -30 dBFS.

With a 4V RMS OL at the A/D it looks like you could use +56 or +62 dB gain depending on how bright the recording is.

With a 5Ω cart you might get an even better NF with +56 or +62 gain since that puts Rg << Rsource.

I'm using +56 dB with a DL-103.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by grufti » Mon Dec 09, 2019 2:10 pm

This is a cartridge that I thought had one dead channel after another MC phono amp experiment. I installed it for the test figuring that it's broken already so who cares if something else goes wrong. It's actually in great shape. So now I have a 20th century Ortofon Jubilee that's still in great shape. I kept it all this time because Ortofon offers a repair service where they install a new motor. It's just a bit pricey. I was extremely happy with that outcome.

My A/D OL is at 4V RMS so +62 might well be my best option. I'll experiment.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Mon Dec 09, 2019 3:10 pm

Let's see what kind of NF we can get with the Jubilee's at 5Ω, 0.34 mV (1kHz 5cm/s) and 62 dB gain.

For 62 dB gain Rg=1Ω
5Ω Rsource noise is -145.7 dBu (20kHz BW)

The preamp Rnv is 3.5Ω (2X ZTX851) + 1Ω (Rg) = 4.5Ω

In the test here I figured the Preamp's Rnv was 5Ω:
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 0.367 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.
So let's use 5Ω for the cart and an Rnv of the preamp of 5Ω for a total Rnv of 10Ω.

Rnv 5Ω is -145.7 dBu
Rnv 10Ω is -142.7 dBu
Which gives us a NF of about 3 dB.

I've got some 1Ω Rsource measurements somewhere that might provide a more accurate Rnv of the preamp.
https://ka-electronics.com

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