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Cartridge dynamic behaviour
Cartridge dynamic behaviour
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Old 16th February 2019, 08:51 PM   #481
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by jackinnj View Post
Gonna try outboarding this for the HP3577 and Bode100
Hi Jack,

Allow me to give you a simple tip.

With 50 Ohm for R19, the equivalent input noise at R19 for a 10Hz filter @3Khz and with the OPA2604 will be 38nV.
For a SNR of 35dB to give an accurate measuring result, the Signal will then have to be 2.1uV at R19. With this 50 Ohm resistor, voltage over the Cart will then be 400uV.

When replacing R19 by a 5K resistor, equivalent noise wil be 46nV with a 10Hz filter. For the same 35dB SNR you will need 2.6uV signal at this spot.
However signal over the cart will now be 5 uV, or 80 times lower as with the 50 Ohm resistor.

Your 3577 will calculate a wrong Inductance for the coil, that only has to be corrrected with a factor 100.


Hans
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Old 17th February 2019, 04:06 AM   #482
davidsrsb is offline davidsrsb  Malaysia
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Originally Posted by luckythedog View Post
Hi Davidsrsb I've been waiting to find time to properly work through your recent stuff, and still not there yet. Every encouragement, the action begins perhaps -30dB below that, if it happens at all, IMO. Appols if already covered.

LD
I have got usable results, showing no impedance or phase changes at all with the source at -50dBU, which means -80dBU at the sense resistor
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Old 17th February 2019, 08:28 AM   #483
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by davidsrsb View Post
I have got usable results, showing no impedance or phase changes at all with the source at -50dBU, which means -80dBU at the sense resistor
Hi David,

Great results so far, already down at ca 60uV on the Cart.
If possible, could you show the same recording as in #470, but now for the -80dBu at the sense resistor recording.
Iím especially interested in how much more noise there is on the curves compared to the earlier recording at -40dBU.

Hans
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Old 17th February 2019, 08:45 AM   #484
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by jackinnj View Post
Gonna try outboarding this for the HP3577 and Bode100
Jack,

Your design probably still needs suitable output caps, because DC going to the 3577 should be avoided.
The 2604ís offset can be as high as 5mV.
Thatís 500mV DC at the output with 40dB gain 🤗
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Old 17th February 2019, 11:07 AM   #485
davidsrsb is offline davidsrsb  Malaysia
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Originally Posted by Hans Polak View Post
Hi David,

Great results so far, already down at ca 60uV on the Cart.
If possible, could you show the same recording as in #470, but now for the -80dBu at the sense resistor recording.
Iím especially interested in how much more noise there is on the curves compared to the earlier recording at -40dBU.

Hans
A -80dBU measurement.
Again impedance is scaled by 1/5, so the maximum value is really 50k
I used 2M samples, 4M gets an out of memory error on a 8GB PC
The TLV2372 is a noisy opamp, the only rail to rail I had to hand.
Also a metal box would be a good idea.
Attached Images
File Type: png G1042_Z_x5_80.png (203.3 KB, 56 views)
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Old 17th February 2019, 05:02 PM   #486
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by davidsrsb View Post
A -80dBU measurement.
Again impedance is scaled by 1/5, so the maximum value is really 50k
I used 2M samples, 4M gets an out of memory error on a 8GB PC
The TLV2372 is a noisy opamp, the only rail to rail I had to hand.
Also a metal box would be a good idea.
Hi David

Impedance is calculated as: Rsense*V(r)/(V(a)-V(r)).
V(a) and V(r) both being contaminated with noise of a certain percentage.
When V(a) and V(r) in the denominator are coming close, the error in the calculation of Z becomes bigger and bigger.

That's what I think that can be seen in your image.
The higher the frequency, the closer V(a) and V(r) become and the higher the noise in the calculation.
Say V(a) is 100 and V(r) is 95, both carrying +/-1% noise.
V(r)/(V(a)-V(r)) may now vary between 94/(101-94) and 96(99-96), or between 658 and 288. The original error of 1% has now become +/-39%.
The only remedy to keep the voltages apart, is to increase Rsense.
This won't increase the noise, because your Cmos amps will stay to be the dominant noise contributors.

So my question is to prove the theory and to get a more accurate measurement, could you repeat the measurement with Rsense 20K or even 50K.


Hans
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Old Yesterday, 12:16 AM   #487
jackinnj is offline jackinnj  United States
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Cartridge dynamic behaviour
Thanks Hans.

Am considering using the ADA4817 which is a single with 4nV/Rt Hz noise, and using the headers to adjust the gain via parallel resistors. ADA4817 is very high bandwidth so needs particular attention to layout.

Yes, the HP3577 is not happy with DC on the input. The Bode 100 is more tolerant.
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Old Today, 08:16 AM   #488
gpapag is online now gpapag  Greece
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Cartridge dynamic behaviour
Quote:
Originally Posted by davidsrsb View Post
A -80dBU measurement.
Thanks David.
As you’ve said, your 0dBu is 0.75Vrms.
Accordingly, –80dBu is 75uV.
For the G1025 (Sensitivity 6.5 mV +/-1.5 dB, 1 kHz @ 5 cm/sec) this corresponds to a signal level close to 40dB below the recording reference level of 5cm/sec.
This is adequately low, no need IMO for going lower.

George
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Old Today, 10:27 AM   #489
Hans Polak is online now Hans Polak  Netherlands
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@ LD,

It was you who came with the idea to investigate whether inductance is dependant on signal level for an MM Cart.
Question is now, has this been answerred to your satisfaction with proper accuracy and low enough level ?

Hans
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