Phono stage hiss - Measurements

[Confused]

The issue of phono stage hiss has been widely reported on this forum.  One problem is that the subjective reports seem to be very inconsistent, one persons "I don't notice it when listening" is another persons "this is unacceptable in a product like this and is ruining my enjoyment of vinyl".

At the recent Devialet event at Oxford Audio, Devialet's Mathieu Pernot made the suggestion that anyone who is suffering from the issue could take a digital recording of the phono stage and send it to him for analysis .  A "needle drop rip" without the dropping the needle bit, if you like.

I had some spare time this morning, so I thought I would try this.  One problem I have is that I do not have access to a Laptop PC or similar at the moment, or at least not one that I can load Audacity onto for USB digital captures.  I do have a PC in the same room as the hifi, but this is a long way away from the hifi, so I ended up using a 10m USB extension cable.  Not ideal, but it did work.  I mention this because it is quite well known that USB cables of this length are not really suitable for audio, so this might have some bearing on my results.

As a first test, I digitally recorded one track from an LP.  Playing this back, it sounded fine, so at least the overlong USB cable works, even if it is not ideal.

I then tried a 20 second recording of just the phono stage. The phono stage settings were RIAA 1976, 0.39mV, 30 ohms.   Playing this back at high volume, yes, it sounds like the hiss you get from a Devialet phono stage.  So far so good.  

I then tried doing a "plot spectrum" frequency analysis in Audacity.  The results were not quite what I was expecting:

   

It looks like the "hiss" content is concentrated around 30Hz.  The problem is that it sounds nearer to white noise than anything at 30Hz, so this is very puzzling.  Concerned that maybe the 10m USB cable was picking up something odd, I tried a capture with the USB cable disconnected from the Devialet, and got absolutely nothing, which I guess is what you would expect in the digital domain.  I then remembered Mathieu's advice at Oxford, disconnect the everything from the phono stage RCA inputs.  The above capture was taken with my TT still connected.  I disconnected the cable from the TT completely, so absolutely nothing connected to the Devialet's phono input, and tried again, the result is below:

   

This is actually quite an interesting result, because it does seem to show some extra grunge that the TT is picking up being removed from the second capture.  However, the plot is still concentrated around the 30Hz area, so I remain a little baffled.  I suspect that I am doing something wrong in Audacity, maybe the Audacity experts out there can suggest something?  I am completely new to Audacity so might well be making some newbie errors.  One thing I can say is that the digital capture subjectively sounds exactly like the phono stage hiss, so this much is fine and so I can't be doing too much wrong.  

Unless someone can point out the potential idiotic error I have made, I plan to send the capture to Mathieu to see what he can make of it.  Meanwhile, if any other forum members could capture the phono stage hiss, it would be fascinating to compare results.

EDIT: One last thought, if anyone wants a copy of my "hiss" WAV file, I am happy to share, just send me a PM.

[Confused]

I sent a copy of my "hiss" wav file to the ever helpful @thumb5, who has performed an Analysis using Oceanaudio.  I have to say that I much prefer the Oceanaudio analysis, this is showing content up to about 5KHz, whereas the Audacity plot shows nothing above about 150Hz, or at least it appears to show nothing, which is highly misleading.  The Oceanaudio analysis looks far more like what I hear, if that makes sense.

As follows:

   

   

Many thanks to Ian for his help with the above.

I am hoping that one or two forum members can perform a capture from their own phono stage, it would be fascinating to compare results.

Now I am happy that there is not something idiotically wrong with my capture, shall send a copy of my wav file to Mathieu Pernot.

[thumb5]

Here are my results - this is for a D1000 Pro/CI with open phono inputs.

The phono stage settings are MC with max level 1.6 mV and high resistive loading, RIAA 1976.

(sorry for the over-sized images)

   

   

Apart from seeing some spikes at 50 Hz, 100 Hz and 200 Hz, presumably picking up some 50 Hz mains hum and harmonics thereof, the general shape of this looks similar to what @Confused posted.  The levels are higher though: I guess this is largely because I have high resistive loading.  The point being that with only 47 k-ohm across the input there is much more scope for noise pick-up than with the 30-ohm loading @Confused uses, which nearly short-circuits the input.

[thumb5]

[I wrote a long, rambling post and somehow managed to delete it!  Argh, will try to put it together again.]

Musing on the shape of the noise spectra both @Confused and I posted, it reminded me of the RIAA playback equalisation curve:



That being the case, I wonder if what we're hearing is just white noise that's been RIAA equalised?

Doing a bit of Googling, I found an interesting article that talks about theoretical noise limits for phono stages: https://www.renardson-audio.com/phono-1.html.  This quotes an earlier article published in Wireless World (1972) from which:

...ignoring amplifier noise the signal to noise ratio for a 50Hz to 20kHz bandwidth relative to 2mV at 1kHz was 58.5dB for circuit A and 72dB for circuit B.

The results are for a phono stage with a 47 kΩ resistive loading, and take into account the RIAA equalisation.

The "circuit B" mentioned has the loading connected across the input terminals, which is said to be the most usual design.  Based on something Mathieu Pernot mentioned recently at OAC, namely that the input loading can be easily measured with a digital multi-meter, I guess the Expert phono stage has this configuration.

Those theoretical limits are largely due to thermal (Johnson-Nyquist) noise from the cartridge itself and the loading resistor.  This thermal noise has a spectrum that's similar to white noise.

Working out how much noise a resistor generates just by sitting there at room temperature is quite surprising.  For example, the 47 kΩ loading in my phono stage generates about 4 µV RMS across the audio bandwidth.  In the context of a 1600 µV input level that's quite significant!  In the measurements @Confused and I posted earlier, where the cartridge is disconnected, this thermal noise is going to be present at the input to the phono stage and will be amplified by whatever gain it's configured to use (and then RIAA equalised).

For lower loading resistors, the RMS noise will be reduced (proportional to the square root of the resistance).  So for example @Confused's 30 Ω loading resistor would produce about 32 dB less noise than my 47 kΩ.  On the other hand, his phono stage is set up to be about 4 times more sensitive, which is about 12 dB, so if we look just at thermal noise from the loading resistor, without the cartridge present, @Confused should have about 18 dB better signal-to-noise ratio.

With the cartridge in place the effect is likely to be less marked, but it seems that using a high load resistance could be a contributing factor to phono stage noise.  Next time I get a chance I'll try dropping it down to one of the lower values and measuring again.

[Confused]

A quick follow up on this thread. I received an email from Mathieu Pernot today. Apparently he has analysed the recording from my phono stage, but he is, to quote; "still waiting for similar recordings from the various gentlemen suffering from this issue."

To further quote; "I'd like to compile results across as many users as possible."

I know that @thumb5 (Ian) has sent in his recording. Is anyone else able to do this? It would be great if we could get some more examples sent off to Mathieu.

[Greg]

A quick follow up on this thread.  I received an email from Mathieu Pernot today.  Apparently he has analysed the recording from my phono stage, but he is, to quote; "still waiting for similar recordings from the various gentlemen suffering from this issue." 

To further quote; "I'd like to compile results across as many users as possible."

I know that @thumb5 (Ian) has sent in his recording.  Is anyone else able to do this?  It would be great if we could get some more examples sent of to Mathieu.

I must confess, I have not yet sent a hiss recording to Mathieu and really should get it done, particularly as I was the very outspoken guest at the Oxford Audio Consultants day with Mathieu, and I was the one who raised the issue with him. Unfortunately, I don’t know how to make this hiss recording other than it requires the use of the USB socket as an output and a laptop. I don’t own a laptop. I have a friend (he has no time for instruction of others) who I can confirm will record it when he’s next around. Of course, if anyone in the meantime can give me some instruction on how to do it (wife has a laptop I might be able to borrow and I have a Dropbox account) I’ll certainly get on with it. Any instructions will need to facilitate my digital Luddite ignorance, but, if laid out simply, I am able to follow and action on.

Of course, my other reason for being slow here is my own solution of using a Longdog Audio MCJ3 Hybrid Valve Phonostage into my D250 at line level which produces for me the very best sound presentation from vinyl. Unfortunately, Matieu’s view on this is what I have done is a ‘ridiculous’ solution. I don’t doubt his opinion, so probably all the more reason for me to get that recording sent off

[Dev_Steve]

Hi Everyone,

I'd love to help out but I'm not super savvy when it come to creating these types of files. I've posted a couple times on this issue and I have always suspected I'm hearing quite a bit of noise because I have an ultralow output cartridge (Ortophon Windfeld Ti). I ultimately had to purchase a SUT to correct the noise issue and even at high volumes, it still exists.

If someone could provide me with step by step instructions (including where to send the file), I'd be happy to assist. Note I would be taking the sample through a Macbook Air. Sorry I'm not more knowledgable on how to do this, but I'm excited to learn!

Thanks all!

[Out of Gum]

Same here. I got a Ortofon Quintet Bronze MC which hisses considerably although i don’t know if it affects the sound when playing. I have a Macbook which i could use to record. I don’t have any specific recording software; can i use quicktime for this? Do i understand correctly that the turntable should be disconnected from the devialet when making the recording?

[Confused]

I am a long way from being an expert in making a recordings from the Devialet's USB, but I have successfully made a recording and sent it to Mathieu Pernot, so in response to the posts above I will try to do an easy "step by step" guide.  I am sure that there are others on this forum that know far more about using Audacity than I do, so this is a kind of idiots guide, written by an idiot.  However, it worked for me, and it should help other beginners to at least get going.  If there are more competent users of Audacity than myself out there, please feel free to chime in with tips and advice.  I can only cover the very basic stuff with my level of knowledge.

I used Audacity software to make the recording, this is widely recognised as being one of the best "free" software packages.  So the first step would be to load Audacity onto your PC or laptop, it is available in Mac or Windows versions, so this should suit most people.

https://www.audacityteam.org/download/

As the idea of this test is to make a recording of the noise from your Devialet's phono stage only, not the noise that your very sensitive turntable, tonearm and cabling might be picking up, you should temporarily remove the turntable's RCA connectors from the phono input of the Devialet.

You then need to connect you Laptop to the USB connection of the Devialet, and select the phono input on the Devialet.

Assuming you are using a Windows Laptop of PC, you need to check that the Devialet "Expert Pro USB" is shown as the input in "Sound Settings", see below:

   

If you then open Audacity, in the pull down menu on the top tool bar you need to set the "microphone" input to "Expert Pro USB Audio", see below.  (or Expert USB Audio if you don't have a Pro)  

   

In the bottom left hand corner of Audacity there is a pull down menu that lets you select sample rate, I used 96000Hz.

Now you are ready to make the recording.  Simply hit the red circular record button on the top toolbar and you will see the recording commence, let it run for maybe 15 or 20 seconds, that is all you need.

Then select "File", "Export Audio" and you can save the recording as a .WAV file.  That's it, you have made a recording of whatever hiss or noise your phono stage may have.

You can play the recording back in Audacity, you may need to turn the volume up a bit, but if you do you should hear a lovely recording of phono stage hiss.

You can analyse the recording yourself in Audacity, or maybe cheat like I did and email your recording to someone who can use Oceanaudio, see the various posts at the start of this thread.

The recordings can be sent to Mathieu Pernot at Devialet:

mp@devialet.com

Note that if you have just made a 15 or 20 second recording, you should be able to send the recording file as a simple attachment.  The file size will be relatively small, so no need to use DropBox or similar.

When you sent your recordings to Mathieu, make sure you mention your phono stage settings used when making the recording, for example RIAA 1976, 0.39mV, 30 ohms, or whatever loading and gain you might be using.

So that is my idiots guide written by an idiot.  Unfortunately this only covers use of Audacity on a Windows PC.  I am sure that the procedure will be similar on a Mac, but I do not have a Mac available to try this at the moment.  Maybe Mac users could advise any subtle differences to the procedure.

I hope this helps, and good luck if you decide to give this a try. 

[Out of Gum]

Thanks Confused! I will have a try.