A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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KMN
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Re: Out-of-band and Flux Loop Tests

Post by KMN » Thu Apr 25, 2019 10:27 am

mediatechnology wrote:
Mon Apr 01, 2019 9:41 am
I'll leave a 5 mm space on the PCB for a 1 nF film C for problematic RFI situations but its not required for stability.
This post is a bit late to the party.

Did you ever try with moving magnet inductances? Maybe some of your customers, couldn't imagine who they might be, would want to try running one that way sometimes.

I was wondering if an RC snubber would offer more flexibility for killing oscillations for more extreme circumstances.

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mediatechnology
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Re: Out-of-band and Flux Loop Tests

Post by mediatechnology » Thu Apr 25, 2019 12:22 pm

KMN wrote:
Thu Apr 25, 2019 10:27 am
Did you ever try with moving magnet inductances? Maybe some of your customers, couldn't imagine who they might be, would want to try running one that way sometimes.
I never even considered it since the noise current would likely be too high and performance would be poor.

The high output 1-2 mV output moving coil carts might be a better test.
I've setup 1-2 mV low output MM carts with the MM preamp (using an NJM2068) and they've been pretty quiet.
For high output MC carts the MM preamp may also be a better choice.
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mediatechnology
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Thu Apr 25, 2019 3:32 pm

Image

Wow, just wow.
I was going to make some minor mechanical adjustments and level measurements but just had to let the entire second side of "Crime of the Century" play out.
Holy sh*t!

No turning back.
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billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Thu Apr 25, 2019 5:10 pm

Sounds like it works :)

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mediatechnology
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Noise Measurements with a Denon DL-103

Post by mediatechnology » Thu Apr 25, 2019 6:09 pm

RIAA Equalized noise measurements.

1 kHz gain = 68 dB.
DL-103 Rterm is 100Ω.
"5 cm/sec lateral" reference level (actually 3.54 cm/s RMS left or right-only) is +2.0 dBu.
(Checked with an STR-100; DL-103 channel balance was about 0.5 dB.)
There is a 0.7 dB approx insertion loss for the FFT A/D.
The induced magnetic hum field can be physically nulled on both channels to better than -70 dBu.

S/N ratio relative to 5 cm/s is approximately 72 dB unweighted and 78 dB A-weighted.

As can be seen most of the remaining noise is hum contamination which could be improved by a better enclosure.
Some may also be normal mode differential hum from the cart itself.
Relative to disc surface noise its incredibly quiet.

Image
ZTX851 Moving Coil Balanced Phono Preamp Noise Measurements With Denon DL-103 On Armrest.

Image
ZTX851 Moving Coil Balanced Phono Preamp Noise Measurements With Denon DL-103 On Armrest, A-Weighted.
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billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Thu Apr 25, 2019 6:52 pm

Damn mains hum gets everywhere! Time to rewire your your turntable with a balanced cable run?

But looking very good so far!

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

Post by mediatechnology » Thu Apr 25, 2019 7:21 pm

It is a balanced cart run but the source of the field are 4 big-ass APC and Belkin UPS's.

Its a big field too: The null point is the same angle on the bench or near the turntable which is another 4-5 feet away.

I just played some hammered dulcimer music and the transient response of the DL-103 is incredible.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Thu Apr 25, 2019 7:30 pm

Ah assumed that phono to 1/4" adaptor board in your picture was part of the TT to preamp wiring, which looks like std low screen coverage coax. I had noticed the UPS in the background. If that is on then things are looking very good as well below vinyl noise and a non issue unless you are one of those odd types who likes to turn the volume up then put their ears up to the speakers.

Glad you like the 103. I've never had the pleasure as sold one broken one I did get before getting a change to retip it, but always viewed it as the flathead v8 of the cartridge world. Fun out the box and lots of mods you can do to make it better.

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

Post by mediatechnology » Thu Apr 25, 2019 8:38 pm

Those RCA leads are the output connection to the monitor amp.

I need to line the Hammond pot metal enclosure with some mu-metal sheet.
I get that low-level hum with the cart unplugged and it internally terminated in 10Ω and can null it by moving the enclosure.

Would like to remind everybody that the 1 kHz gain is 68 dB but the 60 Hz gain is 84 dB owing to RIAA EQ.
So a 60 Hz hum spur at -70 dBu in the output is actually -154 dBu (50 ppm or 16 nV) at the input.

I also remembered that there are two pieces of cast iron vent pipe in the wall the preamp backs up to.

Between the Flat Preamp and EQ board are 50 feet of balanced interconnect and DC power.
Same identical setup as the MM rig.
The noise performance however, whether its on the bench or at the end of those long 50' leads, is the same.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Thu Apr 25, 2019 9:09 pm

I just ran a quick FFT checking the hum floor as I move the arm at various positions over the platter.

I can change it by several dB depending on where it is on the disc so some of this looks to be differential "normal mode" hum picked up by the DL-103 itself. No amount of common mode rejection will eliminate that.

At the armrest there are large differences between channels.
Over the platter they're fairly equal.
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