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 » Fri Oct 25, 2019 3:42 pm

Hi, I put it in the assembly pdf but forgot to put it in the original post.
Thanks for bringing that to my attention.

https://www.mouser.com/ProjectManager/P ... 657a690b73

Note that the BOM only lists 4X ZTX851. In order to Vbe match two pairs you'll probably want to order at least 10-12 devices.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Sun Nov 17, 2019 4:10 pm

I just finished building a ZTX851 moving coil preamp board and thought I would post the noise spectra.

Image

I haven't measured the actual gain - it usually runs just under 62 dB so we'll call it 61.5 dB.
The PCM4222EVM A/D loads the 100Ω output by about 0.7 dB.
Factoring those things in we have an Ein of about -141 dBu.
It's probably just a tad better.
This gives us a 1.65 dB NF for a 10Ω Rsource.
485 pV√Hz.

The total Rnv is 14.5Ω.
Rsource + Rgain = 11Ω.
rbb estimated is 3.5Ω/2= 1.75Ω.
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Radardoug
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by Radardoug » Wed Nov 20, 2019 12:32 am

Any chance of curves with your other termination resistors? Those are certainly very good results for ten ohms.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Wed Nov 20, 2019 5:15 am

There are some 30Ω curves in the original thread.

This is what it looks like with a 30Ω termination of a real DL-103||100Ω.
mediatechnology wrote:
Tue Jul 02, 2019 4:49 pm
I thought I would grab some quick shots of the low-level hum performance of the preamp and the "normal," differential mode, hum pickup of the DL-103.

The preamp wiring is STP/balanced/floating to the head shell.

The first image is an FFT with the DL-103/head shell unplugged from the arm to get a background noise level of the preamp with all tone arm wiring connected. The noise level in the first frame is determined by the internal 100Ω termination.

Image
ZTX851 Moving Coil Preamp Hum Signature Headshell Removed, Flat, Gain=58dB.

The next image is with the DL-103 and headshell plugged into the arm with the arm on the rest.

The hum shown here is the 1-2 mG ambient field inducing differential mode hum into the DL-103's coils. The noise floor lowers about 4 dB because the 40Ω cart is now in parallel with the 100Ω termination. (Note that common mode rejection cannot reduce differential mode hum in the source.)

Image
ZTX851 Moving Coil Preamp Hum Signature DL-103 on Armrest, Flat, Gain=58dB.

As the arm is moved onto the platter the DL-103's orientation to the field can be seen to change:

Image
ZTX851 Moving Coil Preamp Hum Signature DL-103 Over Platter, Flat, Gain=58dB.

The DL-103's hum is probably about normal and is inaudible under surface noise.
In an area with a lower ambient hum field it might be lower.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Wed Nov 20, 2019 5:24 am

Found these plots that I did before it was a PC board:
mediatechnology wrote:
Thu May 10, 2018 6:55 am
Bill - I'm hoping to keep it the same board size as the MM preamp which is 3x4".

Here are some noise measurements with the Protoboard moved off the workbench about a foot and the hum component from a nearby 100W linear supply's hum "vortex" nulled. I can't turn that supply off - it powers the PCM4222EVM used to make the measurement. So for the time being I null it by rotating the Protoboard. The hum signature looks similar when the MC preamp is powered from a pair of 12V batteries.

The PCM4222EVM used for measurement loads the MC preamp's 49.9Ω X2 build-out resistors by about 0.65 dB so add that to the measured results for the actual output noise. Though the displayed measurement bandwidth is 5 Hz -22 kHz the measurement bandwidth is 20-20 kHz. The FFTs were 65 kpt, 20 sample average.

Image
ZTX851 Moving Coil Preamp Noise Floor Balanced 30Ω Source Impedance 54 dB Gain. Ein approximately -136.46 dBu.

Image
ZTX851 Moving Coil Preamp Noise Floor Balanced 10Ω Source Impedance 54 dB Gain. Ein approx. -138.66 dBu.

Image
ZTX851 Moving Coil Preamp Noise Floor Balanced 1Ω Source Impedance 54 dB Gain. Ein approx -140.46 dBu. Approx. 0.5nv√Hz.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by Gold » Wed Nov 20, 2019 11:25 am

The quietest setup I got with the MM pre was the DL-103 with a step up transformer. It was being compared to an AT-150 and a V-15MKV. There was a few dB less noise at equal output level. It was mostly RF/EMI stuff. This was with only about a foot of shielded twisted pair between tonearm and preamp. I chalked it up to theof the CMRR transformer.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Wed Nov 20, 2019 4:35 pm

What I've noticed comparing the Flat Moving Magnet Preamp/Stanton 681 combination versus the Flat Moving Coil/DL-103 is that the EMI of the MM tends to be centered in the 6-10 kHz region at the resonant frequency where the impedance is highest. The balanced input really lowers the hash in that region some of which appears to be line-related high-order harmonics. (Dimmed LED lighting for example.)

The DL-103 on the other hand has its hum at lower mains frequencies and low-order harmonics. It doesn't take much loop area to see hum that is not common mode. (-160 dBu at the input, 7.75 nV, isn't a lot of signal.) As shown in the FFTs the DL-103 itself generates a fair amount in my 1- 2 mG ambient hum field. I don't suppose I should complain however since its so far below surface noise as not to matter.

There's not much we can do in the preamp for differential mode hum in the generator so having the cart itself in a low hum field is a big benefit. That holds true for both types.

The MM preamp benefits from just about any kind of metal box since electrostatic fields are the prime source of hum.

On the other hand metal doesn't seem to help shield the MC preamp from hum at all. It needs double box mu-metal to shield magnetic fields.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by billshurv » Wed Nov 20, 2019 6:10 pm

There has been a discussion on this recently. Hans it seems lives not only in a low EMI area, but has his lab setup away from big transformers and gets a lack of 50Hz pic up that has people raising eyebrows. Makes not a blind bit of difference to listening to vinyl, but nice from a low noise measurement perspective. He also tried waving an electric drill near a balanced phono board to confirm that the CMRR did help and measured a worthwhile reduction in pickup.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by Gold » Wed Nov 20, 2019 7:08 pm

That makes sense Wayne. I still don't have an FFT setup so I wouldn't have seen that. I find RF/EMI much more objectionable than hum. Hum at least is a constant level and frequency while RF/EMI changes over time. I can detect RF/EMI at very low levels under music as harshness. Kind of like truncating PCM without dither. I don't have the same reaction to low level hum.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Wed Nov 20, 2019 7:09 pm

Thanks Paul and Bill.
This is the FFT I was looking for.

The red plot is the Stanton 681 with the MM preamp balanced, the blue plot is unbalanced.
RIAA EQ is applied.
Looks like the range is more 2-10 kHz with 3 kHz looking to be the impedance peak.
The audible difference in the noise floor is significant.

Image
Comparison of the Flat Phono Preamp with a Stanton 681. The red trace is with common mode rejection realized using the balanced input.
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