A "Dummy" Cart For Phono Preamp Noise Testing

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mediatechnology
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A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

I just recently bought two of the Dayton belt drive turntables - one as a gift the second to experiment with. They come with an Audio Technica AT-VM95E cartridge and I decided to mount an AT-96 on mine and store the VM95E.

Before I put it away I measured the inductance and resistance and low and behold the VM95E's measured inductance, at 534 mH, was close enough to the "standard" value of 500 mH I decided to put it into service as a dummy cart for noise tests. Most of my previous noise comparisons were made with a Stanton 681 mounted on an arm.

The DC resistance of the VM95E was nowhere near the required 1350Ω, at 517Ω, so I added two 412Ω resistors in series with each terminal. The cart resides temporarily in a ginger tin to provide electrostatic shielding.

To make a long story short one of my experiments was to measure the Flat Moving Magnet Preamp with the test cart and compare it to a shorted input, 2K resistive source, cart with 47KΩ/294pF termination and whatever equivalent resistive source it took to equal the same noise level as the cart.

The gain of the flat preamp is 28X or 28.9 dB. The reason for this particular gain is to produce -23 dBFS (relative to a 2V RMS 0 dBFS) with a 5 mV output cart at a 1 kHz 5 cm/s (lateral) reference. -23 dBFS relative to 2V RMS is about 140 mV RMS. 140 mV/5mV is 28.

A shorted input measurement is a relatively worthless MM phono preamp spec except that it provides a meaningful measurement of input voltage noise.

A 2K termination was chosen since it's used by JRC to characterize the "DD" rank of NJM2068 in an RIAA-equalized preamp. 2K is close to the reference cart's DCR and can be used to estimate current noise but not as well as the dummy cart which, as we shall soon see, presents a much higher source impedance.

Here are the measurements. They are unweighted, flat and can't be directly compared to an RIAA de-emphasized measurement.

Shorted input: Ein -119.8 dBu, 1.02 µV (20 Hz - 20 kHz BW)
2KΩ||47KΩ||200pF: Ein -117.5 dBu
Dummy Cart||47K||294pF: Ein -110.7 dBu (5Hz - 20 kHz BW)(94 pF of cable C)


The resistive input termination that produces -110.7 dBu is 30K||47K|200 pF.

Thus the test cart in parallel with the termination has an Rnv equivalent to an 18.34K resistor (measured) or 16K5 (theoretical.)

Using 16K5 and subtracting the termination in parallel shows the cart to have a source impedance, over a 20 kHZ BW, that can be approximated by a 25KΩ resistor. 18K34 works out to be around 30KΩ.

FFT of the spectrum.

Image
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JR.
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by JR. »

IIRC I used 1.5k resistor for nominal source impedance, of course that won't be accurate at HF....

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AnalogJoe
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by AnalogJoe »

Do you have pics of your cart?
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mediatechnology
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

I'll post pics pf the cart. Most have a small ground strap to an electrostatic shield connected to the right channel ground. On this cart its internal and the plastic body floats from the headshell.

What's interesting is that if I limit the bandwidth to 10 kHz the cart "wins" by about 2.6 dB; if the bandwidth is 10-20 kHz the resistor wins by about 2.6 dB. The noise contribution of the cart over the full bandwidth is equal to the resistor.

We all know that there's an inductive rise in impedance with frequency but I never knew what resistance it took to equal the cart's Rnv.

I'll apply RIAA and measure to see what an equalized Rnv is.
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mediatechnology
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

I just had an "ah ha" "duh" moment with this experiment.

Where the noise curve of the cart crosses the noise curve of the resistor the complex impedance of the cart equals that of the resistor.

In the curve I posted with Rsource at 30K1Ω, the crossover point is about 9 kHz.

I'm using the right channel of the cart which measured 556 mH. The reactance at 9 kHz is 31.45K. The impedance is around 31.48K owing to the 1350Ω DCR.

Given the fact that the preamp and converter gains between the left and right channels may be off by 0.1 dB or more and the noise floors of the two channels different by perhaps a couple of a tenths of a dB it seems to work out pretty closely.

Here's Amazon's pic of an AT-VM95E. It's an entry level cart.

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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

When RIAA is applied it looks like 10KΩ is a good Rnv approximation for the cart.

In the last couple of octaves the curve is approaching the noise floor of the converter. The noise would otherwise follow the RIAA curve.

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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

I decided to zero in a little closer using the PCM4222 A/D. The absolute dBu figures aren't totally accurate due to the very low input impedance of the PCM 4222 EVM board I'm using loading the 100Ω output impedance of the preamp. The relative levels between channels are accurate.

The first FFT compares the left and right measurement channels with 7K5Ω across both inputs:

Image

This FFT compares a 7K5Ω resistor to the cart:

Image

The overall noise levels between the first FFT and the second may be due to the fact that the shielding was better placed over the preamp and RIAA board.

In the second FFT it looks like 7K5Ω is a better overall approximation for a cart Rnv value when RIAA EQ is applied.

The total Rnv, with 47KΩ termination in parallel, is 6K47.
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by AnalogJoe »

Seems like a good cart, the very significant increase in THD is interesting.

Also, the performance of that Scarlett is stellar. Which generation are you using?
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by mediatechnology »

AnalogJoe wrote: Wed Jan 03, 2024 11:21 am Seems like a good cart, the very significant increase in THD is interesting.

Also, the performance of that Scarlett is stellar. Which generation are you using?
Considering that it was bundled with the turntable which sold for $179 it seemed like a bargain.
THD?

The second set of FFTs were done with the PCM4222 EVM.
The Scarlett is a 2i2 Gen 3.
I want a Gen 4 or a E1DA COSMAC.

The FFTs were averaged 35 times.
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AnalogJoe
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Re: A "Dummy" Cart For Phono Preamp Noise Testing

Post by AnalogJoe »

mediatechnology wrote: Wed Jan 03, 2024 11:47 am
AnalogJoe wrote: Wed Jan 03, 2024 11:21 am Seems like a good cart, the very significant increase in THD is interesting.

Also, the performance of that Scarlett is stellar. Which generation are you using?
THD?
I am sorry, I just realized it is 60 Hz hum and its harmonics, I just saw more harmonics and my mind went into THD mode.
The second set of FFTs were done with the PCM4222 EVM.
The Scarlett is a 2i2 Gen 3.
I want a Gen 4 or a E1DA COSMAC.
I have a Gen 3 and I was wondering if I should replace it with a Gen 4. Any good reasons to do so?

Regarding the COSMAC, I didn't know that one, I just made a Google search and it seems it has ultra low THD+N, have you used it before?

The new QA403 line of interfaces also seem quite amazing, they have even lower THD+N than the QA401, which I own. I am really considering buying a QA403, although it is significantly more expensive than the 2i2 Gen 4.
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