Phono stage design considerations part 1: choosing 1st stage tube

[Joshua_G]

So the CCS is ruled out because it lowers distortion?

Please read again what I quoted from Morgan Jones.

And the cathode follower is ruled out just because?

100% negative feedback, when negative feedback suppresses lower harmonics, while it increases higher harmonics.

I think you'd do best to rule out voltage regulators, too, cause they work really well.

Good thinking.

 

[Joshua_G]

It looks to me that possibly some people look only at the THD figure, not minding the spread, or proportion, of the various harmonics.

Possibly, such view may overlook something which has considerable effect on sound quality.

Measurements don't lie, only, at times, it may be useful to consider what to measure and how to look at the measurements results.

 

[Joshua_G]

Back to my initial enquiry in this thread.
Are there any contenders to the 6GK5 for the 1^st stage in a phono preamp?

 

[Joshua_G]

BTW,
The weather here goes bananas. On December 1, it was 28 degrees centigrade, on December 2 it's going to be 30 degrees, with the driest winter in 70 years.

 

[abraxalito]

Possibly the second harmonics masks, at least to some degree, the higher harmonics. I don’t care much why and how. All I care about is the sound quality.

If as you say you care only about sound quality - i.e. listening to music - then it makes little sense to base engineering decisions on what happens with single tones. Only 'audiophile' music (say girl-with-a-guitar) has a sparse enough spectrum for individual harmonics to be considered. Real music fed through electronics non-linearities gives rise to an error spectrum which is noise-like - J_J being my authority on this - so IMD is the primary consideration, not THD on single sines. Higher order harmonics on sine waves may well be important as markers for inferior IMD performance but don't mistake the map for the territory.

P.S. weather here also unusually warm and sunny for December - 15oC.

 

[Joshua_G]

If as you say you care only about sound quality - i.e. listening to music - then it makes little sense to base engineering decisions on what happens with single tones. Only 'audiophile' music (say girl-with-a-guitar) has a sparse enough spectrum for individual harmonics to be considered. Real music fed through electronics non-linearities gives rise to an error spectrum which is noise-like - J_J being my authority on this - so IMD is the primary consideration, not THD on single sines. Higher order harmonics on sine waves may well be important as markers for inferior IMD performance but don't mistake the map for the territory.

Agreed.

I don't dare to ask here people if they measured multiple-tones IMD on particular tubes and/or particular architectures, or topologies. I'm about to be crucified as it is, without asking it.

BTW, who is J.J.?

 

[abraxalito]

James D. (jj) Johnston Home Page

 

[Joshua_G]

Thank you.

 

[Hearinspace]

After looking at your posts I thought you might get something out of these pages. First is from the Sound Practices reading club list and includes the article on the Siren Song, an old design but one that others have used as a starting place for one of their own designs, and the Second is J.C.'s Blog page on a circuit I thought might interest you.

 

[Vinylsavor]

Hi!

It takes much more to achieve good sound than just selecting the right tube.
Since you already ruled out so many suggestions, maybe you should give some more specifics like range of current, rp and mu you are looking for.

I would have suggested the 6SF5 for a restively loaded first stage. I have been using it in my lower cost phono preamps and it works beautifully despite it's low plate current in the region of 1mA. It is available cheaply too:

VinylSavor: Tube of the Month : The 6SF5

I also used the D3a successfully but only in transformer coupled and in LCR EQed phono stages.

Another cheap tube is the 6AM4:

VinylSavor: Tube of the Month : The 6AM4

If you want best sound you should also think about circuit and EQ alternatives as they have a huge impact. In my experience 600 Ohm LCR EQ is giving the best sound.

Best regards

Thomas

 

[TonyTecson]

thomas, have you looked at the ef91/6AM6?

 

[Vinylsavor]

thomas, have you looked at the ef91/6AM6?

No... So many tubes, so little time

 

[esltransformer]

You can start with a ec8010 as Morgan Jones dos.

What transformer you have?

1st stage tube considerations

The phono stage is intended for both MM and MC, probably with SUT (step-up transformer).

It looks to me that the most important characteristic for that tube should be very low noise and microphonic.

On top of that, reasonably low miller capacitance is necessary (for MM cartridges).

Tough signal levels at this stage are low, good linearity is preferred, especially low 3rd and higher odd harmonics distortion levels.

Long life, reasonably low plate resistance, availability and reasonable cost are preferred.

I hope I didn't miss anything here.

All the above relate to anode resistor loading, capacitor coupling to the next stage (including the RIAA equalization network).

What are your recommendations?

Replies based on experience, especially listening evaluations between different tubes, will be appreciated.

 

[euro21]

One of my previous phono stages was based on 12AX7 tubes. It sounded enjoyable, however I want do something much better.

Try decent (I use Sylvania 3 mica) 5751 instead of 12AX7. Slightly less gain, but much better sound.

My phono (used every day): 5751 (green LED bias, 1mA), passive RIAA, 5751 (green LED bias, 1mA), ECC82 (3.5mA) cathode follower.

CCS:
As you can see, this is CCS loaded phono (all tubes!), and almost invisible harmonics.

http://www.diyaudio.com/forums/tubes-valves/246392-c3g-triode-strapped-linearity-question-9.html #85

 

[SY]

Please read again what I quoted from Morgan Jones.

100% negative feedback, when negative feedback suppresses lower harmonics, while it increases higher harmonics.

Again, you're letting your preconceptions prevent you from arriving at good solutions.

Perhaps you could take a look at the distortion spectra of my preamp, where everything is loaded by CCS and point out the 3rd, 5th, or higher harmonics. And there's dreaded cathode followers.

 

[PMA]

Perhaps you could take a look at the distortion spectra of my preamp,

I would also like to provide you with one. And my suggestion would be - get away from tubes as far as possible

 

[SY]

Pavel, -100dB third will drive any right-thinking person out of the room with blood running from his ear sockets.

 

[Merlinb]

If the rp of the tube is low, the i_n of the CCS is not a significant contributor, as long as it's reasonable low.

What matters are the noise currents generated by the tube, and by the CCS. Both of these noise current flow in r_p, so it drops out of the equation (r_p is irrelevant). CCS noise current is sure to be greater than that of a low-noise tube, and even with a non-specialist tube, CCS noise is likely to be at least equal to the tube noise.

Case in point: my MC preamp, which uses CCS loading and is ultra-quiet for a tube design.

And it would be quieter still with passive loading.

 

[SY]

And it would be quieter still with passive loading.

Tried that. Nope. As it sits, it has a 1dB noise figure referenced to the cartridge's Johnson noise. Not to mention the vastly better PS noise rejection.

If we do the sums, a cascoded DN2540 CCS has a current noise of about 14pA/rt Hz. A D3a at 20mA is likely to have an rp of roughly 2k. Mu is 75. So this translates to an input referred voltage noise of 0.4nV/rt Hz, which is completely swamped by the tube's voltage noise since it's uncorrelated.

 

[Salas]

I would also like to provide you with one. And my suggestion would be - get away from tubes as far as possible

What is the full scale zero in dBV? Looks like a logarithmically averaged chart, correct?