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Moving-Magnet Head Amp - without RIAA
Moving-Magnet Head Amp - without RIAA
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Old 11th February 2019, 01:04 PM   #101
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by Hans Polak View Post
The 1.58Meg resistor was wrong anyhow.
Gain in your case is (1+3.09/0.1) = 31.9. This figure times 47K gives 1.5Meg.


Hans
Sorry, it is 1.5Meg plus 47K.

When you use these two resistors, you can easily try what difference it makes by connecting the low side of the 47K to Gnd.


Hans
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Old 11th February 2019, 01:28 PM   #102
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by rsdio View Post
First of all, the synthetic 47 kΩ impedance is definitely supposed to reduce the Johnson noise, compared to a literal 47 kΩ resistor. I haven't done the research or the math to confirm that, but the texts and articles seem to agree.
With the circuit you have in mind, you can expect an improvement in SNR of between 1dBA and 3dBA, depending on the cart you will be using.
When you tell the exact Cart figures in mH and Ohm I can give you a more precise answer.


Hans
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Old 23rd February 2019, 03:46 AM   #103
rsdio is offline rsdio  United States
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Originally Posted by Hans Polak View Post
With the circuit you have in mind, you can expect an improvement in SNR of between 1dBA and 3dBA, depending on the cart you will be using.
When you tell the exact Cart figures in mH and Ohm I can give you a more precise answer.

Hans
Ortofon Nightclub Mkii : 580 mH, DC 1 kΩ, 8 mV 1 kHz, 5 cm/sec

Thanks!
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Old 23rd February 2019, 10:54 AM   #104
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by rsdio View Post
Ortofon Nightclub Mkii : 580 mH, DC 1 kΩ, 8 mV 1 kHz, 5 cm/sec

Thanks!
Noise in the figures below is RTI, after Riaa and after A-weighting.
Gain from "cooling" in less noise production is 3,2dB.
Without, SNR ref 8mV is 20*log(8mV/671nV)=81.5 dBA and with it becomes 84.7 dBA.
As you can see in the noise spectra, up to 1kHz both are equal, but beyond the cooled version produces less noise.
I do not understand the 330pF cap at the input, much too high for the cabling if this is what you want to visualise.
Try to keep total capacitance under 200pF to start with, before you add anything.


Hans
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Old 24th February 2019, 09:32 AM   #105
rsdio is offline rsdio  United States
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Quote:
Originally Posted by Hans Polak View Post
Noise in the figures below is RTI, after Riaa and after A-weighting.
Gain from "cooling" in less noise production is 3,2dB.
Without, SNR ref 8mV is 20*log(8mV/671nV)=81.5 dBA and with it becomes 84.7 dBA.
As you can see in the noise spectra, up to 1kHz both are equal, but beyond the cooled version produces less noise.
Nice. Thanks.

What if RIAA is done in digital domain and not analog? Does that affect the noise figures at all? (forgive me, but I don't have my thinking cap on at the moment)
Quote:
I do not understand the 330pF cap at the input, much too high for the cabling if this is what you want to visualise.
Try to keep total capacitance under 200pF to start with, before you add anything.

Hans
I'm going purely by Ortofon's recommended 200 pF to 600 pF load, but I'll certainly start at 200 pF. I think that 330 pF was the closest cap I could find at first, but I'm sure that I can locate a 200 pF.
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Old 24th February 2019, 12:37 PM   #106
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by rsdio View Post
Nice. Thanks.

What if RIAA is done in digital domain and not analog? Does that affect the noise figures at all? (forgive me, but I don't have my thinking cap on at the moment)

I'm going purely by Ortofon's recommended 200 pF to 600 pF load, but I'll certainly start at 200 pF. I think that 330 pF was the closest cap I could find at first, but I'm sure that I can locate a 200 pF.
It doesnít matter noisewise whether Riaa is applied anologue or digitally.
Cabling is at least 150pF, so to follow Ortofonís instruction, start with a 47pF cap.
You can always increase this value. Just listen to what difference it makes, that will give you the answer.

Hans
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Old 26th February 2019, 08:14 AM   #107
rsdio is offline rsdio  United States
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It works!

I built the most recent schematic shown, along with a +/-17.5V supply, powered it up and am enjoying great sound!

I don't own the kind of test equipment that would be needed to confirm the S/N ratios, but I might try to do some testing.

If there's interest, I can try to take some photos and post them here.

p.s. I used a 220 pF cap, but have ordered assorted other values from 47 pF to just under 220 pF that will arrive soon.

Last edited by rsdio; 26th February 2019 at 08:31 AM.
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Old 28th February 2019, 04:03 AM   #108
rsdio is offline rsdio  United States
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Here is the power supply. It takes in 12 V AC (I may switch to 9 V AC), from an AC wall wart with a standard barrel connector, and produces +/-17.5 V DC.

I used an AC wall wart rather than a DC wall wart so that I wouldn't have to create a virtual ground. It turned out that the circuit suffers from frequency response issues (in simulation) with a passive split rail / virtual ground setup, although that's mostly because I didn't have room in the tiny case to add an op-amp to buffer the voltage.
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Old 28th February 2019, 02:35 PM   #109
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by rsdio View Post
It works!

I built the most recent schematic shown, along with a +/-17.5V supply, powered it up and am enjoying great sound!

I don't own the kind of test equipment that would be needed to confirm the S/N ratios, but I might try to do some testing.

If there's interest, I can try to take some photos and post them here.

p.s. I used a 220 pF cap, but have ordered assorted other values from 47 pF to just under 220 pF that will arrive soon.
Congrats for a DIY job well done.
You should hear no noise at normal listening levels with the PU arm in the air. So why bother about SNR figures.
Noise from the LP is quite a bit above the noise level of your preamp.
What would be interesting to know, have you cosen for the split resistor solution with 47K+1M5.
In that case it would be nice to know if you notice any difference when grounding the 47K, thereby switching the "cooling" off.


Hans
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Old 2nd March 2019, 10:26 PM   #110
rsdio is offline rsdio  United States
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Originally Posted by Hans Polak View Post
What would be interesting to know, have you cosen for the split resistor solution with 47K+1M5.
In that case it would be nice to know if you notice any difference when grounding the 47K, thereby switching the "cooling" off.
I have a 1 MΩ resistor now, and have reduced the gain of the first stage to 20.1x (+26 dB) to create a virtual 47.4 kΩ load.

I didn't understand what you were suggesting the first time, but I could disconnect the 1 MΩ and solder a 47 kΩ with 1 lead on the PCB and the other lead grounded. I might try that to see if there is any difference, but I doubt I could measure it.
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