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    How much additional noise is normal when adding a phono stage to a preamp?

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    Post by rjpjnk on Sat May 11, 2019 10:11 pm

    My SP13 and ST70 is very quiet with my 99dB Klipsch Heresy IIIs. Of course I can hear some noise, but it is minimal and I really have to be close to the speakers to hear it. I am very happy with the noise floor.

    When I engage the PH12 phono board, however, there is a considerable increase in noise. I have always thought this was normal since every phono amp I've ever heard had this same sort of rushing noise, but I thought it would make for interesting discussion.

    I figure the best way to discuss noise would be to show some measurements rather than try to describe it.

    Below are two frequency spectrum measurements captured with REW software and a calibrated mic. I think this set was captured with the mic close to the woofer of one speaker as I was focusing on hum at the time. The input signal is full spectrum pink noise. Don't let the big numbers scare you, these are actually very quiet. As I mentioned, the line stage alone is almost dead silent to the ear. But these plots show the spectral content of the little bit of noise that does exist.

    I don't have an absolute reference level to give meaning to the dB scale shown, so they are really only useful in a relative sense.

    The first shows the SP13 line stage alone with my VTA ST70. In this configuration the aux input is selected and the volume is set to 0dB. The 0dB position was determined beforehand as the volume knob position at which the bench measured output signal is exactly the same voltage as the input signal. Physically, this happens to be very close to the 12 O'clock position of the volume control in my unit, which is a nice design (Thanks Roy!)

    As can be seen, the noise is dominated by 60Hz and all its harmonics, i.e.: 120, 180, 240, 300, (360 missing), 420... These are all clearly evident. But as I mentioned, you really can't hear it much.

    The second plot slows what happens when the PH12 phono board is engaged. The volume control is still at the 0dB point. The noise is now significantly louder in the speakers.

    The third plot shows an overlay of the first two plots for comparison. You can see that with the PH12 added the noise floor has increased roughly 7-10dB in the frequency range from 70Hz to 500Hz. I am just wondering if this is normal. I'm guessing this is actually pretty good performance considering we just added 40dB more gain to the mix. Still, 10x more noise definitely moves it into the realm of easily audible. Is this just a fact of life for all phono stages?

    Clearly the spikes are due to 60HZ power line noise leaking into the circuit, and that could possibly be improved but I have not had any success so far. (Any suggestions welcome!) But the residual noise below this (thermal?) is also increasing by the same amount in this frequency band. I believe this is the "rush" component.

    In both cases the inputs (Aux and Phono) were grounded at the RCA jacks on the back of the preamp during testing.

    How much additional noise is normal when adding a phono stage to a preamp? 33951682188_c816f02d13_z

    How much additional noise is normal when adding a phono stage to a preamp? 47039437194_2b0308a972_z

    Overlay of both SP12 alone and SP12+PH12
    How much additional noise is normal when adding a phono stage to a preamp? 32885292557_81ff17abd6_z
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    Post by Guest on Sun May 12, 2019 10:14 am

    spectrum analyzer aside, as you mentioned, a tube phono stage is always somewhat 'noisier' than a tube line stage.
    However, this noise should not be audible under normal listening conditions. If the phono inputs are shorted to ground at the RCA sockets, the only time a phono stage noise should be audible is with the volume cranked right up.
    In your build, what wiring is used or the phono input. I prefer to use Mogami style shielded audio cable for the phono input and not a twisted pair which is more than adequate for the other inputs.
    If memory serves me right, the PH12 does not have its on 'on board' B+ supply?
    If that is the case, you may want to look at adding a simple RC filter stage for the PH12 B+. Nothing fancy, I would try a 100R 1W resistor and a 100uF 350V electro cap, see if that makes things a but more quiet.
    Adding 0.47uF 50V film caps on each + & - filament leg right at the tube socket pins down to ground also goes a long way to make things quieter and even adding a 0.1uF 50V film across the +/- filament supply can help.
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    Post by PeterCapo on Sun May 12, 2019 11:37 am

    Some tubes are noisier than others. You might consider contacting McShane Design about a quieter set of tubes for your phono section.
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    Post by rjpjnk on Sun May 12, 2019 11:57 am

    Erhard, The PH12 I have is the most recent version and does have its own regulated supply onboard.

    As for the internal input wiring, it consists of 3-4 inches of coaxial cable from the RCA jacks directly to the PH12. For the testing the inputs were shorted at the RCA jacks.

    And yes, I only really notice the phono noise when the volume is very high and there is no input. For this test the volume was near the halfway point (exactly 0dB gain for the SP13). To my ears the phono noise is not intrusive at this setting, but of course the mic can easily detect it.

    If I get inspired by ideas in this thread I may decide to take another stab at reducing the already very low noise shown. In that case I would start by removing these short coaxial cables completely and directly shorting the PH12 inputs on the board itself to eliminate any possibility of stray pickup. (I did that during initial bench testing but not while connected to my system)

    I think it is interesting that the noise spikes in the SP13 spectrum are all multiples of 60Hz, which leads me to believe that the fundamental was indeed 60Hz line noise and not 120Hz rectifier noise.

    However, when the PH12 is added, the spikes at 120 and its harmonics jump up more than the others indicating the PH12 is introducing a noise with primary at 120Hz *not* 60. For instance, look at 120, 240, and 360. All of these increased by more than the others (say 60, 120, 300 etc that are multiples of 60 but not 120).

    I wonder why 120Hz is getting into the PH12 but not the SP13 as much?

    Interesting suggestion about adding capacitors on the filament legs right at the tube sockets. The filament supply is also regulated DC in this system. In fact there are two independent DC filament supplies, one for R and one for L channel. Would this still benefit from caps at the tube sockets?

    To be clear, I am perfectly happy with the level of noise in my SP13/PH12, but if it were possible to reduce the power line harmonics of course that would be even better. So in summary it seems there are three ways this sort of noise can get into the circuit: B+ supply, Filament supply, and induction into signal wires. Any others?



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    Post by rjpjnk on Sun May 12, 2019 12:01 pm

    PeterCapo wrote:Some tubes are noisier than others.  You might consider contacting McShane Design about a quieter set of tubes for your phono section.

    Peter, can the tubes introduce the power line harmonic spikes shown in the graphs or would they just effect the residual noise floor?

    I wonder if maybe tubes themselves can pickup the EMF in the box and inject it into the circuit? Interesting...


    Last edited by rjpjnk on Sun May 12, 2019 12:04 pm; edited 1 time in total
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    Post by Guest on Sun May 12, 2019 12:03 pm

    rjpjnk wrote:Erhard, The PH12 I have is the most recent version and does have its own regulated supply onboard.

    As for the internal input wiring, it consists of 3-4 inches of coaxial cable from the RCA jacks directly to the PH12. For the testing the inputs were shorted at the RCA jacks.

    And yes, I only really notice the phono noise when the volume is very high and there is no input. For this test the volume was near the halfway point (exactly 0dB gain for the SP13). To my ears the phono noise is not intrusive at this setting, but of course the mic can easily detect it.

    If I get inspired by ideas in this thread I may decide to take another stab at reducing the already very low noise shown. In that case I would start by removing these short coaxial cables completely and directly shorting the PH12 inputs on the board itself to eliminate any possibility of stray pickup. (I did that during initial bench testing but not while connected to my system)

    I think it is interesting that the noise spikes in the SP13 spectrum are all multiples of 60Hz, which leads me to believe that the fundamental was indeed 60Hz line noise and not 120Hz rectifier noise.

    However, when the PH12 is added, the spikes at 120 and its harmonics jump up more than the others indicating the PH12 is introducing a noise with primary at 120Hz *not* 60. For instance, look at 120, 240, and 360. All of these increased by more than the others (say 60, 120, 300 etc that are multiples of 60 but not 120).

    I wonder why 120Hz is getting into the PH12 but not the SP13 as much?

    Interesting suggestion about adding capacitors on the filament legs right at the tube sockets. The filament supply is also regulated DC in this system. In fact there are two independent DC filament supplies, one for R and one for L channel. Would this still benefit from caps at the tube sockets?

    To be clear, I am perfectly happy with the level of noise in my SP13/PH12, but if it were possible to reduce the power line harmonics of course that would be even better. So in summary it seems there are three ways this sort of noise can get into the circuit: B+ supply, Filament supply, and induction into signal wires. Any others?




    OK, no probs. Yes not sure why you're getting those spikes with the PH12 engaged....interesting.
    Yes, in fact my builds also use regulated DC filament and I have 0.47uF 50V film cap on EACH tubes' filament leg AND a 0.1uF 50V across each +/- filament leg. It may be overkill, but this is what helped me eliminate some nose issues a few years back now.
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    Post by PeterCapo on Sun May 12, 2019 12:09 pm

    rjpjnk wrote:
    PeterCapo wrote:Some tubes are noisier than others.  You might consider contacting McShane Design about a quieter set of tubes for your phono section.

    Peter, can the tubes introduce the power line harmonic spikes shown in the graphs or would they just effect the residual noise floor?

    I am not the best person to answer this, but IIRC, some tubes have been said to be more immune to picking up hum as a result of their particular construction. Jim McShane screens his tubes and mentions that some are better for phono stages. If you drop him a line with your observations, I am sure he'd get back to you jimmcshane@mcshanedesign.net
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    Post by rjpjnk on Sun May 12, 2019 12:11 pm

    erhard-audio wrote:

    OK, no probs. Yes not sure why you're getting those spikes with the PH12 engaged....interesting.
    Yes, in fact my builds also use regulated DC filament and I have 0.47uF 50V film cap on EACH tubes' filament leg AND a 0.1uF 50V across each +/- filament leg. It may be overkill, but this is what helped me eliminate some nose issues a few years back now.

    It would be a fairly simple matter for me to add those caps. In the interest of science I may add them to just one channel and then compare the effects.

    This would not be possible on the PH12 because the same tubes are used for both channels I believe. But I could definitely do it on the SP13. I will put it on my list of possibilities for future tinkering sessions. Thanks.
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    Post by rjpjnk on Sun May 12, 2019 12:14 pm

    PeterCapo wrote:
    rjpjnk wrote:
    PeterCapo wrote:Some tubes are noisier than others.  You might consider contacting McShane Design about a quieter set of tubes for your phono section.

    Peter, can the tubes introduce the power line harmonic spikes shown in the graphs or would they just effect the residual noise floor?

    I am not the best person to answer this, but IIRC, some tubes have been said to be more immune to picking up hum as a result of their particular construction.  Jim McShane screens his tubes and mentions that some are better for phono stages.  If you drop him a line with your observations, I am sure he'd get back to you jimmcshane@mcshanedesign.net

    Thanks. This reminded me that often times high gain tubes are shielded for this reason, right? I wonder if tube shields would be of benefit here?
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    Post by Guest on Sun May 12, 2019 12:18 pm

    rjpjnk wrote:
    PeterCapo wrote:
    rjpjnk wrote:
    PeterCapo wrote:Some tubes are noisier than others.  You might consider contacting McShane Design about a quieter set of tubes for your phono section.

    Peter, can the tubes introduce the power line harmonic spikes shown in the graphs or would they just effect the residual noise floor?

    I am not the best person to answer this, but IIRC, some tubes have been said to be more immune to picking up hum as a result of their particular construction.  Jim McShane screens his tubes and mentions that some are better for phono stages.  If you drop him a line with your observations, I am sure he'd get back to you jimmcshane@mcshanedesign.net

    Thanks. This reminded me that often times high gain tubes are shielded for this reason, right? I wonder if tube shields would be of benefit here?

    that is one of the main reasons I only use the 6922 family of tubes for phono preamps, as they have that internal shield, pin 9, which is connected to ground. The other main difference is that the 6922 is 6.3V filament only, so this needs to be kept in mind.
    But if all else fails, a tube shield may help.
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    Post by rjpjnk on Sun May 12, 2019 1:16 pm

    I found another couple graphs that may be of interest. In this figure I compared the PAS 3x with the PH12/SP13. The input here was a 1KHz test tone played from a Radio Shack test LP. (Yeah, they really used to sell those). To setup this test I used a sound pressure meter to set the volumes of the PAS and the SP13 so that I measured the same SPL from each when the tone was playing.

    You can see that the PH12/SP13 has lower noise at both 60Hz and 120Hz compared to the PAS 3X.

    The overall THD for each was about the same but it is interesting that the PH12 has primarily even order harmonics whereas the PAS has a significant 3rd order term at 3KHz. Even orders are generally considered better because they are natural octaves so they sound nicer to us.

    I think this graph show some of the ways the VTA product measures better than the PAS.

    I also have a complete set of THD and Intermodulation distortion measurements for another day... Personally, I am a big believer that all that really matters is how it sounds, but it is still nice to see supporting data that shows *why* it sounds better Smile

    How much additional noise is normal when adding a phono stage to a preamp? 46917661995_7ab6f54be4_c





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    Post by rjpjnk on Sun May 12, 2019 1:27 pm

    erhard-audio wrote:
    that is one of the main reasons I only use the 6922 family of tubes for phono preamps, as they have that internal shield, pin 9, which is connected to ground. The other main difference is that the 6922 is 6.3V filament only, so this needs to be kept in mind.
    But if all else fails, a tube shield may help.

    The VTA PH12 uses the 6922 as well.

    I have some sort of Russian variant of it. I forget the number. Let me go take a look.
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    Post by rjpjnk on Sun May 12, 2019 2:38 pm

    Here are the tubes I am currently running in my PH12.
    6H23n. I ordered them directly from Ukraine.
    Are these any good?

    How much additional noise is normal when adding a phono stage to a preamp? 46918481955_5ba3fddf45_z
    How much additional noise is normal when adding a phono stage to a preamp? 47834797231_498996e7b3_z
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    Post by Guest on Sun May 12, 2019 2:42 pm

    rjpjnk wrote:Here are the tubes I am currently running in my PH12.
    6H23n. I ordered them directly from Ukraine.
    Are these any good?

    How much additional noise is normal when adding a phono stage to a preamp? 46918481955_5ba3fddf45_z
    How much additional noise is normal when adding a phono stage to a preamp? 47834797231_498996e7b3_z

    I think those are 6N23P's which are very good, also 6DJ8's are excellent alternatives to the 6922
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    Post by Peter W. on Mon May 13, 2019 2:34 pm

    Coming late into this as we have been traveling.

    A phono head-amp takes millivolts and makes them into volts, applying far more amplification than even an active line-level pre-amp. Extraneous noise from an un-shorted input jack will get amplified. Shorting the input jack should result in greatly reduced, but not zero, noise.

    Shielded wiring from the input jack to the phono board is helpful. Not always necessary, but helpful. It would always be my choice if given.

    As to the response curves between the PAS and SP - yes, the SP displays slightly better results. But, note even the most extreme spike shows maybe a 3 dB difference. Comparing a 20th century design against a 21st century design, the old 'un is not too shabby.
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    Post by rjpjnk on Mon May 13, 2019 2:47 pm

    Thanks for joining Peter. All of the measurements are indeed with the inputs shorted as described.
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    Post by rjpjnk on Mon May 13, 2019 3:07 pm

    So in general does the 7-10 dB increase in noise floor when the phono section is inserted seem about normal?

    I am referring to the overlay plot in Post 1 (repeated here). Remember this was at the woofer.

    All inputs are grounded.

    [img]How much additional noise is normal when adding a phono stage to a preamp? 32885292557_81ff17abd6_z[/img]

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    Post by Peter W. on Mon May 13, 2019 5:54 pm

    Not only normal, but to be expected as the nature of the beast.
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    Post by tubes4hifi on Tue May 14, 2019 3:30 pm

    I've sent a detailed email to rjpjnk concerning this.
    The PH12 isn't a normal addition to an SP13 preamp, and I suspect that he needs to add a separate filament supply.
    Other than that, the additional noise from the PH12 can be consider normal, and as he says, can only be heard at very high volume settings with no music playing.
    Those 6H23 tubes are the same as EH6922 but not likely been screened by the seller as "low noise" spec'ed.
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    Post by rjpjnk on Tue May 14, 2019 5:43 pm

    tubes4hifi wrote:I've sent a detailed email to rjpjnk concerning this.
    The PH12 isn't a normal addition to an SP13 preamp, and I suspect that he needs to add a separate filament supply.
    Other than that, the additional noise from the PH12 can be consider normal, and as he says, can only be heard at very high volume settings with no music playing.
    Those 6H23 tubes are the same as EH6922 but not likely been screened by the seller as "low noise" spec'ed.

    Nice! Looks like there may be a possibility of reducing some of this power line noise even further. I'm up for trying.

    I ordered an SP12 + PH12, but my SP came with dual B+ and filament supplies, so I thought maybe I got a free upgrade to SP13. But I could be mistaken. Maybe just the power supplies were upgraded? I will post a picture for proper identification. My apologies if I called it by the wrong model number.

    I suppose I could just power the PH12 filaments with a 6V battery or regulated bench supply to see if the noise spikes decrease.

    If they do I will definitely build a separate filament supply.

    Maybe I'll get a chance to try it this weekend.
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    Post by Peter W. on Wed May 15, 2019 11:41 am

    Just a side note in this conversation, given that Phono head-amps are unusual beasts these days. And as always, we are starting with the phrase "Back in the day"...

    Back in the day, a line-level pre-amp S/N would be somewhere at -75 (+/-) dB
    Back in the day, a MM head-amp to pre would be something between -55 & -65 dB.
    MC head-amps would be yet worse.

    Nature of the beast.
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    Post by rjpjnk on Sat May 18, 2019 1:11 pm

    Just a minor update. I didn't have time this weekend to try running the PH12 with an auxiliary power supply, but I did have a chance to make a few quick measurements of the SP12 with and without the PH12 tubes inserted.

    (Side note. Roy confirmed that what I have is an SP12 built on an SP13 board. The difference being that it has SP12 tubes, 12AU7)

    So to recap, the issue we are exploring is the possibility that excessive current draw from the SP12 power supply due to the filaments of the PH12 is causing the hum shown in the measurements above.

    Ultimately, I want to try running the PH12 on a separate supply, but that will have to wait for later.

    For today's test, I decided to measure the noise of the SP12 AUX channel with its input grounded, with and without the PH12 tubes inserted. The goal was to see if the additional current draw of the PH12 was contributing to the hum in the SP12 output.

    The figure below shows the results. The SP12 spectrum with the PH12 tubes inserted (Red) contains a little more noise than without the PH12 tubes (Green). The largest difference (3dB) is seen at 120Hz.

    So what I make of this is that it does appear that the additional current craw draw of the PH12 is causing the SP12 hum to increase by about 3dB. This is a relatively small component of the total 120Hz content, but it is a step in the right direction. More later...

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