Noise

[Jas0_0]

Hi all,

Could someone clear something up for me?

I want to convert flat non RIAA signal from my turntable to digital using my RME ADI2 Pro FS and apply RIAA digitally. However I have been warned off this on other forums because I’d be amplifying the noise of the preamp and AD converter by up to 20dB at low frequencies.

Is this a genuine concern? Most preamps and the RME have a SNR far higher than the <70dB of a record, so I’d have thought any noise added between cartridge and computer would be lost in the record’s noise.

Or is noise cumulative as you go from unit to unit?
Many thanks in advance,

James

 

[RayDunzl]

Have you tried it?

This is a Focusrite interface, preamp -> ADC idle noise, with the gain knob all the way up:

The noise level goes down as the gain is reduced, -140dBFs at minimum. You'd likely use a position someplace in between.

 

[Jas0_0]

Not yet. I am just looking for advice ahead of potentially buying a flat phono pre (or mic pre + resistor) to understand theoretically whether it’s worth investing.

 

[Frank Dernie]

Hi all,

Could someone clear something up for me?

I want to convert flat non RIAA signal from my turntable to digital using my RME ADI2 Pro FS and apply RIAA digitally. However I have been warned off this on other forums because I’d be amplifying the noise of the preamp and AD converter by up to 20dB at low frequencies.

Is this a genuine concern? Most preamps and the RME have a SNR far higher than the <70dB of a record, so I’d have thought any noise added between cartridge and computer would be lost in the record’s noise.

Or is noise cumulative as you go from unit to unit?
Many thanks in advance,

James

I use a digital RIAA correction. It is way quieter than any analogue RIAA correction (which also boosts any noise in the bass, of course) since most modern ADCs have a noise performance 30 to 40dB better than the LP you are playing anyway.
What you do need though is. an analogue input to the ADC with the correct impedance to match your cartridge.
IME my RIAA stage is better than the high-end analogue one I had before and is built into my amplifier.

 

[sergeauckland]

Matching a low output moving coil cartridge is very easy, as any decent microphone amplifier will have something like a 1.5k input impedance, and is balanced. Moving Magnet cartridges will be more difficult to match as they're usually designed for a 47k input impedance. Even so, that's not impossible to deal with if you have the circuit diagram of the ADC device's microphone input. Ultimately, you could put a MC step-up transformer backwards on the mic input so act as a step-down, which will step up the impedance, but it won't do much for the noise. You could use a resistive attenuator to get the impedance up, but the noise penalty will still be there.

Using a MC cartridge and a decent microphone pre-amp/ADC is by far the easiest approach.

S.

 

[Soniclife]

Moving Magnet cartridges will be more difficult to match as they're usually designed for a 47k input impedance.

MM is also picky about capacitance, where MC isn't.

 

[Soniclife]

Have you tried it?

This is a Focusrite interface, preamp -> ADC idle noise, with the gain knob all the way up:

View attachment 69711

The noise level goes down as the gain is reduced, -140dBFs at minimum. You'd likely use a position someplace in between.

The issue won't be in amplifying the noise in the electronics, but in amplifying the noise from the cart being dragged through a valley of vinyl.
But as Frank points out, something has to amplify that noise, it's the whole point of the correction.

 

[scott wurcer]

Hi all,

Could someone clear something up for me?

I want to convert flat non RIAA signal from my turntable to digital using my RME ADI2 Pro FS and apply RIAA digitally. However I have been warned off this on other forums because I’d be amplifying the noise of the preamp and AD converter by up to 20dB at low frequencies.

Is this a genuine concern?

No, this AES presentation makes the dynamic range issues clear. http://www.channld.com/aes123.pdf

 

[Joachim Herbert]

The adi-2 pro fs lacks a microphone amp, so this probably will not work with a mc cartridge. I bought a babyface pro fs for that reason, but every decent auudio interface with microphone amp will do. I use vinylstudio to do eq, editing and declicking. Also have izotope rx6 for high quality cleanup, but vinylstudio does a nice job. It also comes with tons of eq curves for all kinds of historical records (which can be applied post recording).

 

[Joachim Herbert]

No, this AES presentation makes the dynamic range issues clear. http://www.channld.com/aes123.pdf

Interesting paper. Also consider that records are limited to a dynamic range of less than 70 dB which would translate to 12 bit resolution.

 

[Jas0_0]

Have you tried it?

This is a Focusrite interface, preamp -> ADC idle noise, with the gain knob all the way up:

View attachment 69711

The noise level goes down as the gain is reduced, -140dBFs at minimum. You'd likely use a position someplace in between.

Sorry my answer above was to "Have you tried it?" as when reading on mobile that was the only thing that showed up for some reason.

Please excuse my ignorance (I'm currently on a steep learning curve!) does the graph suggest that the available dynamic range at full gain on the Focusrite is greater than the dynamic range of a well-pressed record? And that the dynamic range of the Focusrite will only improve as gain is reduced? So I can be safe to assume that a decent preamp (mic or flat phono) plus my RME should be good enough?

 

[Jas0_0]

Matching a low output moving coil cartridge is very easy, as any decent microphone amplifier will have something like a 1.5k input impedance, and is balanced. Moving Magnet cartridges will be more difficult to match as they're usually designed for a 47k input impedance. Even so, that's not impossible to deal with if you have the circuit diagram of the ADC device's microphone input. Ultimately, you could put a MC step-up transformer backwards on the mic input so act as a step-down, which will step up the impedance, but it won't do much for the noise. You could use a resistive attenuator to get the impedance up, but the noise penalty will still be there.

Using a MC cartridge and a decent microphone pre-amp/ADC is by far the easiest approach.

S.

Thanks for this. I've heard that at line level, the input impedance should be at least 10x the output impedance of the source. Is there a similar rule of thumb for matching MC carts and preamps?

 

[Jas0_0]

The adi-2 pro fs lacks a microphone amp, so this probably will not work with a mc cartridge. I bought a babyface pro fs for that reason, but every decent auudio interface with microphone amp will do. I use vinylstudio to do eq, editing and declicking. Also have izotope rx6 for high quality cleanup, but vinylstudio does a nice job. It also comes with tons of eq curves for all kinds of historical records (which can be applied post recording).

Great, thanks. I am hoping to perform digital RIAA on the fly when playing records - does vinylstudio do this or is it designed for playing digital LP recordings?

 

[sergeauckland]

Thanks for this. I've heard that at line level, the input impedance should be at least 10x the output impedance of the source. Is there a similar rule of thumb for matching MC carts and preamps?

Yes, pretty much the same. As long as the input impedance is 10x the cartridge's DC resistance, that's fine. If I remember correctly, Ortofon reckon even 5x is enough. What this means is that loading conditions for low output MC cartridges are pretty non-critical as their internal impedance is so low, and pretty much purely resistive.

MM cartridges have a very reactive impedance due to the coils which have something like 500-1000 ohms of resistance plus a very significant amount of inductance that reacts with the capacitance of the arm cables and phono stage input capacitance.

High Output MCs are that bit more critical of loading as their coils have a lot more turns, but as the coils are small and air-spaced, their inductance is low so again not as critical of capacitance as a typical MM cartridge.

S.

 

[levimax]

For MM I use short leads to an inexpensive DIY balanced "transmitter" https://sound-au.com/project87.htm and changed the input resistor from 10K to 47K. Works like a charm feeding the balanced input of my ADC (Steinberg UR 22 MK2) and I then use convolution for RIAA EQ. The ADC has plenty of gain and is very quiet compared to my phono preamp and the balanced lines eliminate noise issues I was having.

 

[RayDunzl]

Sorry my answer above was to "Have you tried it?" as when reading on mobile that was the only thing that showed up for some reason.

Sometimes I add to a reply in the first couple of minutes. You caught me.

does the graph suggest that the available dynamic range at full gain on the Focusrite is greater than the dynamic range of a well-pressed record?

Yes. My experience with ADC (limited) is that they add distortion near the top of their range (big signal), so you might trim a few dB off the top.

So I can be safe to assume that a decent preamp (mic or flat phono) plus my RME should be good enough?

Top - left channel cheap phono pre (with RIAA) into Focusrite
Bottom - right channel direct from MM cartridge into Focusrite with a little more on the input volume knob (no RIAA)

An LP playing music

Don't expect the black (current signal) to align.

The level of the graph may look low, but the Peak Sample is not.

---

Same connections as above, idle signal (noise floor) compared to prior peak

Top - cheap RIAA preamp idle
Bottom - MM direct idle

 

[Jas0_0]

For MM I use short leads to an inexpensive DIY balanced "transmitter" https://sound-au.com/project87.htm and changed the input resistor from 10K to 47K. Works like a charm feeding the balanced input of my ADC (Steinberg UR 22 MK2) and I then use convolution for RIAA EQ. The ADC has plenty of gain and is very quiet compared to my phono preamp and the balanced lines eliminate noise issues I was having.

I’m intrigued by this as my current setup is MM (I was thinking of upgrading the cart as part of this) however I am no electrical engineer. Do you or anyone know of somewhere that I could send this schematic and have it made by someone with professional-grade soldering skills?

 

[levimax]

I’m intrigued by this as my current setup is MM (I was thinking of upgrading the cart as part of this) however I am no electrical engineer. Do you or anyone know of somewhere that I could send this schematic and have it made by someone with professional-grade soldering skills?

An option that might work for you are these buffers from Neurochrome. https://neurochrome.com/collections/buffers-preamps/products/universal-buffer. These come assembled and they have 48K ohm input impedance and SOTA performance.... probably overkill for a TT but it won't hurt anything.

 

[Jas0_0]

An option that might work for you are these buffers from Neurochrome. https://neurochrome.com/collections/buffers-preamps/products/universal-buffer. These come assembled and they have 48K ohm input impedance and SOTA performance.... probably overkill for a TT but it won't hurt anything.

Thanks - am I right in thinking that all it needs is a 15V power supply and a suitable case and you’re good to go? Guess it could even be mounted inside my turntable plinth if there was space. Would this take care of the capacitance needs of MM carts?

Sorry to bombard with questions, but just one more thing - I understand phono cartridges are inherently balanced, so could this be wired with balanced ins from the cart to balanced outs as well?

 

[sergeauckland]

An option that might work for you are these buffers from Neurochrome. https://neurochrome.com/collections/buffers-preamps/products/universal-buffer. These come assembled and they have 48K ohm input impedance and SOTA performance.... probably overkill for a TT but it won't hurt anything.

Thanks - am I right in thinking that all it needs is a 15V power supply and a suitable case and you’re good to go? Guess it could even be mounted inside my turntable plinth if there was space. Would this take care of the capacitance needs of MM carts?

Sorry to bombard with questions, but just one more thing - I understand phono cartridges are inherently balanced, so could this be wired with balanced ins from the cart to balanced outs as well?

Looks like a good product, but it's designed for unity gain, and although gain can be adjusted, there's no mention of how much gain it can provide. You will need something like 40dB (100x) for a MM cartridge and 60dB (1000x) for a low output MC cartridge. The manufacturers may be able to advise.

As to loading, you can adjust the capacitance the cartridge sees simply by putting some small value polystyrene caps across the input terminals and either measuring the result if you have suitable equipment, or by ear.

As to balancing, MC cartridges are balanced, MMs seldom are, as the coils are encased in a screening can, typically mumetal, so the capacitance of the windings to earth differs between the inner part of the coils and the outer. Some cartridges like Shure V15s, connect the screening can to the earthy side of one channel, unbalancing the cartridge, although that connection can normally be removed. Others may or may not do this. nevertheless, using the balanced differential input has some merit even if not properly balanced.

S.