Strange question: if it's a hobby circuit, it is up to you to decide whether you find it acceptable, and if it is something you made for work, it's up to the marketing department. In either case my opinion and rayma's opinion are irrelevant.
It is possible to do better than 72 dB(A), also with valves, but record surface noise on a clean record is typically around -60 dB RIAA- and A-weighted with respect to 5 cm/s, 1 kHz according to xx3stksm's measurements. Therefore, assuming a moving-magnet cartridge with a sensitivity of the order of 5 mV at 5 cm/s, any further improvement will only be audible during the silence in between records. With a record playing, the record's noise will limit any improvement to less than 0.27 dB.
It is possible to do better than 72 dB(A), also with valves, but record surface noise on a clean record is typically around -60 dB RIAA- and A-weighted with respect to 5 cm/s, 1 kHz according to xx3stksm's measurements. Therefore, assuming a moving-magnet cartridge with a sensitivity of the order of 5 mV at 5 cm/s, any further improvement will only be audible during the silence in between records. With a record playing, the record's noise will limit any improvement to less than 0.27 dB.
Here is a link to a page with a spread sheet where you can calculate the system noise for a RIAA EQ amp.
The biggest noise contributor (outside record surface noise) in any decent system will come from the cartridge
So Just How Quiet is Your Phono Stage?
If your phono stage is doing better than -72 or 73 dB ref 5 mV with a real cartridge source you are doing well. After that, it takes a lot of effort for tiny improvements. And all is lost as soon as the stylus hits the vinyl!
The biggest noise contributor (outside record surface noise) in any decent system will come from the cartridge
So Just How Quiet is Your Phono Stage?
If your phono stage is doing better than -72 or 73 dB ref 5 mV with a real cartridge source you are doing well. After that, it takes a lot of effort for tiny improvements. And all is lost as soon as the stylus hits the vinyl!
Hi, i took a brief look at the spreadsheet and wondered how much low output mc cartridges contribute in noise compared to mm. Since mc preamp are noisier by themselves, their electrical properties differ so much that it may mitigate or worsen noise in comparison to mm. Still there's surface noise off couse but the more advanced stylus shapes do have less surface noise too which may be the most important factor in background noise anyway.
Best wishes, Vincent
Best wishes, Vincent
Thanks for all replies. This is a DIY MM phono amplifier, measured standalone. I use a SUT but it is not involved in my measurements. But compared to a better reel-to-reel tape machine that has around -56dB S/N (Sony TC-645), I think can live with it.
It looks like on the pictures:
Preamplifier - Google Photos
It looks like on the pictures:
Preamplifier - Google Photos
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MC transducers are by and large quieter because the coil resistance is usually much lower but because the output signal from an MC is 20 to 30 dB lower than an MM cart you need a very quiet preamp to get the benefits.
A typical MM cart is between 500 and 1350 ohms and L of 200 to 800 mH (make and model dependent).
MC are 5-40 Ohms (most around 8-20 Ohms) with very low L that doesn’t really play a part in noise over the audio BW.
The design for noise challenges for the two cart types are different. In an MM it’s about reducing noise current primarily and with MC the real focus is on getting the noise voltage as low as possible.
A typical MM cart is between 500 and 1350 ohms and L of 200 to 800 mH (make and model dependent).
MC are 5-40 Ohms (most around 8-20 Ohms) with very low L that doesn’t really play a part in noise over the audio BW.
The design for noise challenges for the two cart types are different. In an MM it’s about reducing noise current primarily and with MC the real focus is on getting the noise voltage as low as possible.
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Actually, for moving magnet, the thermal noise of the 47 kohm termination resistor is usually of the same order as the thermal noise of the cartridge. I know there are people who count the termination resistor noise as cartridge noise, but that is creative accounting, as the termination resistor is part of the amplifier rather than the cartridge (and there are ways to reduce termination resistor noise if you really want to).
Thanks to Bonsai and to the excellent Hifisonix site about ground loops, I could reduce hum to a bare minimum. The following tricks have been applied:
- Input ground loop breaker resistor in the ground path of the phono stages
- Ground loop breaker diode bridge between signal ground and chassis ground
- Negative of DC heater connected to signal ground
- Good quality teflon insulated coaxial cable between the stages
- Separate coaxial cables carrying the signal and the GND to the input stage, the shield of these cables is connected to the chassis.
- Case of the coupling capacitors (Russian PIO) connected to GND
- Aluminium tube shields are completely useless
- 0.3 mm steel plate intended for magnetic shield between mains transformer and the tubes is completely useless (I will try some permalloy sheet)
- Input ground loop breaker resistor in the ground path of the phono stages
- Ground loop breaker diode bridge between signal ground and chassis ground
- Negative of DC heater connected to signal ground
- Good quality teflon insulated coaxial cable between the stages
- Separate coaxial cables carrying the signal and the GND to the input stage, the shield of these cables is connected to the chassis.
- Case of the coupling capacitors (Russian PIO) connected to GND
- Aluminium tube shields are completely useless
- 0.3 mm steel plate intended for magnetic shield between mains transformer and the tubes is completely useless (I will try some permalloy sheet)
Good that you have made progress Laszlo!
For the transformer hum, you would need the screen to enclose the transformer so that it forms a loop. This way the magnetic flux is ‘trapped’ in the loop and does not radiate past the screen as much.
The diagram below shows how the screen works. This happens because the mag field flows in the path of least reluctance.
(For this reason, you must have one and only one connection to the metal chassis. If you have more than 1 you get a current flow between the two connections and that can inject a noise voltage in your circuit).
For the transformer hum, you would need the screen to enclose the transformer so that it forms a loop. This way the magnetic flux is ‘trapped’ in the loop and does not radiate past the screen as much.
The diagram below shows how the screen works. This happens because the mag field flows in the path of least reluctance.
(For this reason, you must have one and only one connection to the metal chassis. If you have more than 1 you get a current flow between the two connections and that can inject a noise voltage in your circuit).
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