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Practical limit to tube based phono preamps [message #96236] Mon, 19 December 2022 20:12 Go to next message
gofar99 is currently offline  gofar99
Messages: 1909
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi, (this could go in either this forum or the tube one) In my never ending quest to get rid of hum and noise I seem to have hit an end point in tube based phono preamps that do not use global NFB. There seems to be what is essentially a practical limit to what can be done without going to extremes. The current preamps are in the -85 to -90dbv range for MM/MI use. That is really quiet BTW. Well below the noise level of a really good and clean LP. Depending on what study you check that value is in the -65 to -75dbv range. With a preamp 15 to 25 db quieter than the best you can get off a LP that is really quite sufficient. I however, am a fanatic about hum and noise. If I can hear it or measure it then it is too much. The limits of my test gear is right around -105dbv. I can get solid state phono preamps close to that but they use NFB. So getting to the limits...I removed everything that had AC on it and all power supply filtering from one preamp and made it into a two box design. I figured that it ought to be quieter...nope. When I compared it to one that is a single chassis I use in my main system the differences were really slight. Under a single db for noise and nearly identical for signal related 60HZ hum. Power supply hum and its harmonics were in both cases within a db of each other. Even with getting selected low noise tubes I have apparently reached the limit in this sort of application. (for those who are interested in this the difference between standard JJ tubes and three different selected low noise ones was only about 1.5db) So what I concluded was that the tubes and passive components were the remaining factors and cause for the wide band low level residual hiss. As I indicated in the beginning none of this is audible. My system has a digital (passive) preamp and the usual listening level is at about -40db according to the display. I have to go to -3 or -4 to hear anything right at the speakers. This level of gain would greatly exceed the ability of the amps to deliver. For now I will have to be satisfied and sit back and enjoy the music.

Good Listening
Bruce
Re: Practical limit to tube based phono preamps [message #96237 is a reply to message #96236] Tue, 20 December 2022 13:39 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18695
Registered: January 2001
Illuminati (33rd Degree)

Dude, that's so cool. One of the most useful explorations in tube audio - getting noise down.

Assuming full audio bandwidth as highest priority, reducing noise is only superseded in importance to reducing distortion, and I'm not even sure they're in that order. Once harmonic distortion is under 3% or so - provided its spectral distribution naturally descends in orders - this small amount of distortion isn't really all that unpalatable.

Getting a low noise floor is definitely one of the holy grails in tube audio.

So from what you said, none of the noise - or very little - is related to your amplifier's power supply. To me, that's huge because I find power supply hum to be most annoying. Hiss is less distracting, especially if it is at a very low level. And I'm not sure it can be cured because it's part of the package, being an issue of colliding electrons. It's not like power supply ripple or a ground loop, it's inherent in any circuit with resistance.

At some point, white noise can't be reduced. It's not a qualitative thing - better components won't help here - the only way to reduce it is to reduce bandwidth or reduce circuit impedance or both. And since bandwidth is a sort of a constant (20kHz for audio), we are limited in what can be done at that most fundamental level.
Re: Practical limit to tube based phono preamps [message #96238 is a reply to message #96237] Tue, 20 December 2022 19:50 Go to previous messageGo to next message
gofar99 is currently offline  gofar99
Messages: 1909
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi Wayne, indeed since my very first project I have tried to reduce hum and noise. All my projects now are low distortion typically in the 1% range except for power amps at full output which can get to about 4% just before going tilt. (they flat line because of the CCS) The line stages, phono preamps and headphone amps are in the 1% range. All are wide band as well. The phono preamps are nearly ruler flat on a RIAA curve and extend to about 35K. The line stages can get to 75K with good tubes and capacitors. The power amps are within 0.5db of flat from (depending on the actual size amp) 10-15 HZ to roughly 35K. With the minor bit of NFB off (it is there to prevent resonances above the audio band in the power amps) they typically will reach 65K at only 2-3 db down. BTW I use the NFB in place to insure stability with my electrostatics as they look like huge capacitors and drop to 1 ohm at 20K. Never heard and issue...but just to be safe. This does remind me of my first attempt at a phono preamp in 2009. Fairly similar to the topology of the present ones. It could detect a hum source from 5 feet away. I went through about 5 different variations then and finally got something in the -60 to -65db range. OK, but not good enough. It took about 5 years to get something I liked. Even then it was only about -70. Those extra db in the newest ones took a lot of trial and error. A lot of things matter but grounding and component placement are the top two. I don't recommend folks design their own phono preamps unless they have a lot of experience and patience. I nearly gave up.

Good Listening
Bruce
Re: Practical limit to tube based phono preamps [message #96269 is a reply to message #96238] Wed, 11 January 2023 17:47 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 97
Registered: May 2020
Viscount
A few thoughts for the newbies out there.

1. Tubes produces internal noise from a variety of reasons.

2. The resistors associated with said tube also produce noise.

A. The cathode resistor noise will be amplified by the tube's gain.

B. The grid resistor noise will also be amplified by the tube's gain.
This resistor is generally much larger than the cathode resistor and
will generate more noise.

C. The "stopper" resistor will produce noise that will be amplified by
the tube's gain.

D. The source, itself, will generate noise which is fed into the tube
and will be amplified by the tube's gain.

E. From my understanding, the Vishay naked resistors produce the lowest
noise. After that a good wire wound (non inductive for audio)is
preferred. Both are expensive, but reduce noise.

3. Concerning 120hz AC hum, I prefer at least 4 stages of filtering, 5 is even
better. My preamplifier and phono stage have 6 stages. I never use an
inductor anywhere as it causes non-linearities due to
2 factors. (If I did, it would be only in the early stages of filtering.)

A. One is inductance which is frequency sensitive. It would appear as
a straight diagonal line in the circuit below.

B. The other factor is dc resistance, which is not frequency sensitive.
Attached is a simple circuit. The choke/inductor has 200 ohms winding
dc resistance. Notice the curve where dc resistance becomes dominant.

C. Me and another music lover have experimented between choke/inductor and
resistor and the sonic change is quite substantial.

Distortion being defined as the altering of sound in any way.

With speaker crossovers, dc resistance tends to be minimized.

cheers

pos

/forum/index.php?t=getfile&id=3293&private=0





Re: Practical limit to tube based phono preamps [message #96270 is a reply to message #96269] Wed, 11 January 2023 18:02 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18695
Registered: January 2001
Illuminati (33rd Degree)

Interesting stuff. To me, that's the "holy grail" of tube amps. Rare indeed is the tube amp that is dead black quiet between musical passages.
Re: Practical limit to tube based phono preamps [message #96271 is a reply to message #96270] Thu, 12 January 2023 21:45 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 97
Registered: May 2020
Viscount
Wayne Parham wrote on Wed, 11 January 2023 18:02

Interesting stuff. To me, that's the "holy grail" of tube amps.
Rare indeed is the tube amp that is dead black quiet between musical passages.
I thought I would do a little more research that might help..

The Vishay Z, S102, tx2352 resistors have approximately
10 nano-volts/volt across the resistor, current noise.
Another figure given is -40db. Expensive.

Mills non-inductive wire wounds equal or pretty close. Expensive

Metal films seem to Max out at 200 nanovolts/volt (0,2 microvolts/volt)
I do not know the averge for all metal films. Based on 0,2, that would
be approximately -14db.
Inexpensive.

Metal oxide inexpensive and
finally carbon types inexpensive. I saw figures of 10 microvolts/volt
for carbons.

cheers

pos
Re: Practical limit to tube based phono preamps [message #96272 is a reply to message #96271] Thu, 12 January 2023 22:11 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18695
Registered: January 2001
Illuminati (33rd Degree)

Amazing how much noisier the metal film resistors are.

I wonder how much difference one might see in various tubes. I haven't noticed much difference in the amps I run, until the tubes get close to end-of-life. And then the noise they begin to make isn't white noise, but rather intermittent whistles and pops. Some start drawing more current too. So but I'm not considering what the tube does as its sunsetting. I'm more interested in what it is doing when it is working properly.
Re: Practical limit to tube based phono preamps [message #96274 is a reply to message #96272] Fri, 13 January 2023 16:37 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 97
Registered: May 2020
Viscount
From my understanding, ragged edges, rough wire surface, impurities in
the material/copper cause eddy currents and is one source of noise. I can't
remember which resistor company discussed this aspect though.

A couple of decades ago, the finite number of NOS vacuum tubes and prices
worried me. So I searched, tested and found the small signal JJs not only
had the lowest harmonic distortion I had ever measured, -79db below the
fundamental, but the sound quality was the most accurate/natural.
I have gobs of Bugleboys,PQs, and others, but the JJs won out with visitors
as well.

(I am not affiliated with JJ in any way, shape, or form. I am also retired
for some 10 years from designing, but still tinker.)

I am not saying harmonic distortion is the only criteria, but it sure
helps. Sovteks, EH and others are ok, but they are not NOS and not JJs.

Turned out the harmonic distortion of the JJ E88cc (prob others as well)
was some 1/10th (-19db) that of any other tube I have ever tested.
The materials used are also excellent.

Still, dealing with such small signals as the OP, one has to deal with
the noise and hum. For me, it is by far the noise over hum.

Tubes with high Rp and RL (plate resistances and plate resistors) tend
to have the highest noise levels as well.

I know, kind of a diatribe. Thoughts just run together sometimes.

pos




Re: Practical limit to tube based phono preamps [message #96275 is a reply to message #96274] Fri, 13 January 2023 17:18 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18695
Registered: January 2001
Illuminati (33rd Degree)

I'm with you there. The most annoying noise to me - by far - is power supply hum. If a tube amp keeps that down below audibility, it's a big deal to me. That's so rare. This is especially true when you stack all tube phono stage, preamp and power amps, and drive high-efficiency speakers. That combination is tough to get dead-black quiet, but it's so important.
Re: Practical limit to tube based phono preamps [message #96277 is a reply to message #96275] Sun, 15 January 2023 22:13 Go to previous messageGo to previous message
positron is currently offline  positron
Messages: 97
Registered: May 2020
Viscount
Wayne Parham wrote on Fri, 13 January 2023 17:18

I'm with you there. The most annoying noise to me - by far - is power supply hum. If a tube amp keeps that down below audibility, it's a big deal to me. That's so rare. This is especially true when you stack all tube phono stage, preamp and power amps, and drive high-efficiency speakers. That combination is tough to get dead-black quiet, but it's so important.
So true. I personally designed at least 4 stages of filtering in my amps pre output stage, and I went from 5 to 6 stages of power supply filtering for my preamplifier. I use 2 chassis design, and 6 stages of filtering in my phono stage as well.

Cheers

pos
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