Microphone preamp instead of phono preamp

[markstein]

Hi all,

I was inspired in another thread to compare performance of my RME UFX II mic input vs. my ProJect DS3 B phono preamp.

Setup is as follows:
- Technics SL1210GR with AT 33 PTG II MC cartridge
- Balanced wiring between MC cartridge and preamp
- RME ADI 2 Pro is used as control unit and volume controller
- Test record: "Vinyl: Check - New signals and recordings to test and adjust vinyl equipment" (Tacet)

- Scenario 1) Technics SL -> ProJect DS3 B (50dB gain, 1kOhm impedance) -> RME ADI 2 Pro analog in (+4dBu) -> RME UFX II AES in -> 4x analog out to 2.2 speaker system

- Scenario 2) Technics SL -> RME UFX II mic input (65dB gain, 3.4kOhm impedance) -> PC with Acourate Convolver (RIAA convolution, 6dB gain@1kHz) -> RME ADI2 Pro USB in -> RME UFX II AES in -> 4x analog out to 2.2 speaker system

Measurements of AES signals (both scenarios level matched by 0.2dB) are attached.

Conclusion:
- Scenario 2 has approx. 8-12dB better noise performance than scenario 1. Even at max. volume I can her almost no low frequency noise with my ear near the loudspeaker which would be clearly noticeable with the ProJect.
- When playing a 1kHz test tone, this advantage disappears in the vinyl noise. Measurements look pretty much identical.
- k3 and k4 seem to be a touch lower with the ProJect. Higher k seem lower with the RME.

I was not able to measure headroom with my setup. Subjective impression is that the ProJect emphasizes pops more than the RME setup. The RME setup subjectively is much more quit and fluid with a "blacker" background than the ProJect setup.

Although I did not want to be dependent on a PC during playback and therefore build my rather complex and expensive system with focus on standalone usage, I am seriously thinking to sell my ProJect preamp, which btw. is a really good device and great value.

Best regards, Mark

 

[somebodyelse]

https://linearaudio.net/record-replay-riaa-correction-digital-domain is mostly about the DSP side, but includes circuit designs to improve matching of cartridges to mic inputs. It looks like it's no longer available separately now - you need to buy the whole volume in print, or all volumes on USB.

 

[Pluto]

The core requirements for a MC gramophone pickup amplifier and those for a microphone are broadly similar. Input Z of a few hundred Ω or less, typical gain of, say, 50-60dB, and the lowest possible noise figure. In both cases a balanced input topology (in conjunction with the appropriate wiring) is helpful in order to minimize the amount of electro-magnetic garbage that can be easily picked up at levels of -50dBV or less.

In this kind of scenario it is important that the front end exhibits the lowest possible noise and that this ultra low noise front end gets you up to a level that can be expressed in terms of volts rather than milli (or, heaven forbid, micro) volts. The overall level of noise is largely determined by the front end because any noise introduced by that front end is itself amplified by however much gain exists later in the chain.

This all suggests the likelihood that the noise performance of the RME mic. amp. is somewhat better than that of the other device.

 

[markstein]

https://linearaudio.net/record-replay-riaa-correction-digital-domain is mostly about the DSP side, but includes circuit designs to improve matching of cartridges to mic inputs. It looks like it's no longer available separately now - you need to buy the whole volume in print, or all volumes on USB.

Question is whether MC cartridge loading down to, lets say, 100-150 Ohm, is required at all.

See the following links for background info:

UFX II mic input for MC phono - input impedance (Page 1) — FireWire & USB series — RME User Forum

MC loading. Myth?

Technically not a myth. :biglaugh:OK, you added that after my post above.... I was going to say "EDIT: But I'm still keeping an open mind here...."...

forums.stevehoffman.tv

--> Ralph Karsten

 

[DVDdoug]

NOTE - For anyone considering a mic preamp, it ONLY works properly with moving coil pickups. Mic inputs are low impedance (typically 1K) and moving magnet pickups expect a specific 47K Ohm load with some capacitance. There is an RME interface with 47K input so it can be used with a regular phono pickup and I believe it has digital RIAA equalization built-in. I assume it could be used as a preamp without a computer attached.

Scenario 2 has approx. 8-12dB better noise performance than scenario 1. Even at max. volume I can her almost no low frequency noise with my ear near the loudspeaker which would be clearly noticeable with the ProJect.

Noise would be my main concern but don't just crank them up to the maximum because the setups will have different maximum gains. It's the signal-to-noise ratio that's important so they should be both be adjusted to the same signal volume (which it looks like you did).

 

[markstein]

Hi DVDdoug,

When using a MM cartridge, the RME ADI 2/4 will do the job standalone. A nice device.

And yes, I adjusted both scenarios to the same signal level, and the S/N ratio with the RME is clearly better.

 

[Bernardo]

Hi DVDdoug,

When using a MM cartridge, the RME ADI 2/4 will do the job standalone. A nice device.

And yes, I adjusted both scenarios to the same signal level, and the S/N ratio with the RME is clearly better.

I have just bought the RME ADI 2/4 to act in part as a phono preamp, and can confirm it works wonderfully well for that, despite RME not marketing the device for this function.

 

[markstein]

Brief update:
You can further dramatically enhance the low frequency groove noise by mono summing the low frequencies below 100-200 Hz during RIAA equalization (see red curve in measurement attached vs green curve with full stereo signal). This should be no issue for stereo imaging as low frequencies are cut in mono on vinyl anyway.

This feature is also called LF X-feed.

I have created a linear phase LR 4th order filter @150Hz and am blending the mono sum below 150Hz with the stereo signal above 150Hz in Acourate Convolver.

(Notice: The 0.5dB volume difference in my configuration is to compensate the channel imbalance of my MC cartridge)

 

[Balle Clorin]

The input capacitance of my Focusrite Solo interphase is much higher than my RIAA and it makes it useless for Vinyl and accurate Vinyl measurements. Can be used as a source but not suitable as an input device

 

[markstein]

I guess you are referring to MM cartridges, not MC where loading capacitance should be almost irrelevant?

 

[/dev/null]

This is something I am looking into too. Since doing flat transfers and doing all the equalisation in the digital domain sounds like it could be a good way to go.
Also using RME stuff for preamping is appealing as I can trust their specs and quality, as opposed to 'audiophile' phono-preamps.

But I cannot really get my head around the input impedance stuff and if it really matters?

I have a MC cartridge too, with these impedance specs:

Internal impedance, DC resistance: 6Ω
Recommended load impedance: > 10 Ohm

The load impedance spec seem to suggest it does not matter, as long as it is over 10 Ohm?

 

[cgallery]

Am I correct in assuming flat amplification is a bit of a challenge because amplifying the already boosted high frequency content requires lots of headroom? Or can a single op amp build with RIAA in the feedback be modified to provide a decent result?

 

[Balle Clorin]

Get a Parks Audio Puffin with Toslink out or Waxwing , and you can do anything. I use a HifiMe Toslink to USB converter from Puffin to record vinyl to PC at 96k/24bits,
Yes the Parks Audio can work as a Preamp and ADC to PC too.

 

[cgallery]

I was just sort of curious about any challenges, I'm satisfied with my phono stage (which feeds into MiniDSP, so I have lots of flexibility for cartridges that are peaky), and I don't really record much.

 

[markstein]

See my post above with a link to "MC loading myth?"

You should ask yourself if
- MC cartridge loading affects the performance of the cartridge itself?
- MC cartridge loading affects the phono preamp primarily by damping HF resonance and reducing distortion?

If you connect your MC cartridge to the RME mic preamp with a balanced wiring (I would recommend doing so), you can load the cartridge by adding a resistor between mic amp XLR connector pins 2 and 3 as confirmed by RME (link to the RME forum is provided above as well).

 

[DVDdoug]

Am I correct in assuming flat amplification is a bit of a challenge because amplifying the already boosted high frequency content requires lots of headroom?

Maybe... But with normal audio high frequencies are much weaker than lower frequencies. That's part of the reason, if not THE reason, for RIAA EQ and NAB tape EQ. And generally headroom isn't much of a problem with analog circuits (unless you hit the limits of a power amplifier).

...I opened random music file in Audacity and then I made a crude reverse RIAA curve with the Graphic EQ (no change at 1KHz). The peaks DID increase by about 10dB, but (surprisingly?) not at the highest frequencies. The spectrum was "more flat" than the original with a peak around 2kHz. (The original peaked below 100Hz.) As long as the gain is adjusted/corrected I wouldn't expect headroom to be a problem. And microphone preamps always have gain control.

Or can a single op amp build with RIAA in the feedback be modified to provide a decent result?

I assume most phono preamps are built that way. I build one with a "special" op-amp (LM382) back in the analog days. There are some very good (low noise) op-amps and it's easy to get 10 volts out of one. A (cheap) preamp that I bought had discrete transistors but that was in the 1970s when op-amps were kind-of "new". Of course, "audiophiles" usually don't like the easy-economical-practical way... They often like discrete circuitry... Even better if it's hand-wired vacuum tubes!

I assume the amplifiers/drivers for viny cutting lathes still use analog equalization.

 

[/dev/null]

Am I correct in assuming flat amplification is a bit of a challenge because amplifying the already boosted high frequency content requires lots of headroom? Or can a single op amp build with RIAA in the feedback be modified to provide a decent result?

Not that I know, but when looking at the RIAA curve it looks like it boosts lows as much as highs are attenuated (20dB). So it might not make a difference if one or the other is boosted/attenuated, as long as it is 20dB for both.

Get a Parks Audio Puffin with Toslink out or Waxwing , and you can do anything. I use a HifiMe Toslink to USB converter from Puffin to record vinyl to PC at 96k/24bits,
Yes the Parks Audio can work as a Preamp and ADC to PC too.

I have looked at those, but the Puffin only got a middling review on ASR. The Waxwing requires an app to operate which is functionally planned obsolescence.

See my post above with a link to "MC loading myth?"

You should ask yourself if
- MC cartridge loading affects the performance of the cartridge itself?
- MC cartridge loading affects the phono preamp primarily by damping HF resonance and reducing distortion?

If you connect your MC cartridge to the RME mic preamp with a balanced wiring (I would recommend doing so), you can load the cartridge by adding a resistor between mic amp XLR connector pins 2 and 3 as confirmed by RME (link to the RME forum is provided above as well).

I am afraid I am not sure if I understand the findings on the "MC loading myth" thread. Loading does not affect the frequency response, but it affects the attenuation? (This by looking at one of the graphs there).

I did read the thread on RME. But I am not sure how I would add a resistor. Could i "jam" one inside the connector on the cable? I do have balanced cables, 5-pin DIN to dual XLR.

 

[markstein]

I did read the thread on RME. But I am not sure how I would add a resistor. Could i "jam" one inside the connector on the cable? I do have balanced cables, 5-pin DIN to dual XLR.

You could add a 1/4W 1% metal film resistor inside the XLR connector to the mic preamp, between signal + and signal - (pins 2 and 3). The resistor value is the cartridge loading impedance.

 

[somebodyelse]

The Waxwing requires an app to operate which is functionally planned obsolescence.

That's not the claimed intention - from https://www.parksaudiollc.com/news.html:

"Will this be another IoT device that stops working one day when Parks Audio goes out of business or decides to stop support?"

• The Waxwing Bluetooth API will be documented and shared with the community.
• Customers can use third-party Bluetooth apps with macros or write their own custom software.
• Third parties may use the Bluetooth API to write, distribute, and even sell their own apps (respectful of the Waxwing trademark).
• The Waxwing File Settings API will also be documented and shared.
• This API openness ensures a long lifespan for the Waxwing, making it not dependent on Parks Audio.
• While Waxwing settings must be changed via a Bluetooth connection, it is still a standalone audio device. So an app isn't required for operation and no annoying tethering is required.(e.g. like with a Bluetooth speaker or "radio")
• No internet connection is needed (other than to initially download an app), and there's no need for subscriptions to services or upgrades.

Having said that, I have yet to find that documentation, and for me that's a showstopper.

 

[undeakyetonde]

Having said that, I have yet to find that documentation, and for me that's a showstopper.

Most likely it hasn't been a priority. Send an email to Shannon about it, just to signal there is interest. Might speed things up.