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Old 7th July 2012, 11:37 AM   #1
ja2dhc is offline ja2dhc  Japan
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Default Pre-amp for Tube Sound

Hello from Japan !

Based on my experiences, one of the reasons of tube sounds comes
from 2nd order distortion as is far different from solid state amps.

In the sense, I did a circuit simulation of low supply voltage
pre-amp as follows:

Supply Plate Voltage = 34V
(consists of 19V + 15V by Laptop PC Power Supplies)
Used tube = 6N6P, Russian Low rp tube to work under low voltage
Voltage Gain of Pre-amp = 2.6 (approx.)

In order to consist power amp, I added a power booster for
simulation purpose whose spec is as follows:

*Simulator = TINA-TI version 9
*power amp = Ideal OP amp
*GV = 6.2
* Simulation circuit schematic and result are shown in the attached photos.

As the simulation result, THD= 0.4% @1W and 1.3% @10W which are
near same as 211 SET.

For the actual amp, I am going to use Class-D amp as the power
booster.
Attached Images
File Type: gif 6N6-HEADAMP.gif (16.0 KB, 716 views)
File Type: gif pre-distortion-graph.gif (20.8 KB, 699 views)
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Old 10th July 2012, 02:52 PM   #2
disco is offline disco  Netherlands
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my 2C: if you wanted to make a frequency doubler effect, wouldn't it be better to have a constant relation to input voltage?
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Old 10th July 2012, 04:45 PM   #3
kevinkr is offline kevinkr  United States
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Many of us here design low level tube electronics that is extremely linear and has similarly good noise and distortion performance to well designed solid state gear. I'm not particularly interested in any circuit for use in hi-fi that is deliberately intended to color the sound by adding second harmonic as a "sweetener" i.e. a sound effects generator - I suspect many here would agree.

Many tube types are inherently more linear than their semiconductor counterparts in zero and low feedback configurations and that is why I use them.
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Old 10th July 2012, 04:47 PM   #4
Ipanema is offline Ipanema  Malaysia
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How do you draw the load line on datasheet IV curve with such a low B+? Do you generate your own low voltage IV curve?

http://www.lampizator.eu/UPGRADE/RU-6N6P.pdf
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Old 11th July 2012, 04:49 AM   #5
ja2dhc is offline ja2dhc  Japan
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Quote:
Originally Posted by Ipanema View Post
How do you draw the load line on datasheet IV curve with such a low B+? Do you generate your own low voltage IV curve?

http://www.lampizator.eu/UPGRADE/RU-6N6P.pdf
I found an actual measured IV values at a Russian Web site (I forgot the URL).
Those measued values are shown in the attached photo.

I generated a SPICE model for TINA-TI based on the measured values
as follows.

* Generic triode model: 6N6
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Sun Nov 22 23:27:49 2009
* Plate
* | Grid
* | | Cathode
* | | |
.SUBCKT 6N6 A G K
.PARAM X1=0.28728514 X2=0.023818988 X3=-0.69616265
.PARAM X4=0.68300952 X5=13.30831 X6=2.1961627
.PARAM X7=0.0027779074 X8=19.484809 X9=0.0016279111
.PARAM Y1=0.0013889537 Y2=0.0016055263
BK IK 0 V=U(V(G,K)+X1)*X7*URAMP(V(G,K)+X1+URAMP(V(A,K))/X8)^1.5+(1-U(V(G,K)+X1))*X9*(X2*URAMP(V(A,K)))^X3*(X4*URAMP(V (G,K)+X1+URAMP(V(A,K))/X5))^X6
BA A K I=URAMP((Y2*URAMP(V(A,K))^1.5)-URAMP((Y2*URAMP(V(A,K))^1.5)-V(IK)+Y1*URAMP(V(G,K))^1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+.4)))+1E-10*V(A,K)
BG G K I=Y1*URAMP(V(G,K))^1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+.4)
* CAPS
CGA G A 3.5p
CGK G K 4.4p
CAK A K 1.7p
.ENDS
*
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File Type: gif 6n6pmu10.gif (6.0 KB, 555 views)
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Old 11th July 2012, 05:04 AM   #6
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Did you try JFETs? You can get better 2'nd order harmonics than from tubes.
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Old 11th July 2012, 05:04 AM   #7
ja2dhc is offline ja2dhc  Japan
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Quote:
Originally Posted by kevinkr View Post
Many tube types are inherently more linear than their semiconductor counterparts in zero and low feedback configurations and that is why I use them.
I like the sonic of my 211SET whose distortion charachteristics (NFB=3dB)
are shown in the attched photo. THD = 1% at 10W, 2 % at 20W.

Those THD's are alomost identical to 6N6 pre-amp+Booster (my original post).
My 211SET uses 1050V plate supply voltage but 6N6 pre-amp uses only 34V.
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File Type: gif 211THD.gif (17.3 KB, 546 views)
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Old 11th July 2012, 07:24 AM   #8
disco is offline disco  Netherlands
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THD does not make the characteristic of a tube amplifier. When looking at distortion only, it's important to maintain the relation of higher uneven order harmonics with lower even harmonics.
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Old 11th July 2012, 11:23 PM   #9
Loren42 is offline Loren42  United States
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Hello, Japan!

Just an observation, but shouldn't the whole point of a high quality audio preamp be extremely low distortion?

Given you can't hear distortion below about .5% (at least at frequencies above 8 kHz) and any decent tube preamp will likely be an order of magnitude lower in distortion than that, how could you hear it?

That seems to blow the harmonic distortion theory.

However, I have another theory and that is related to a tube power amplifier and the impedance of the speaker it is driving rather than a preamp.

Tube power amps tend to produce more power with a higher impedance load, while transistor amps are the reverse. Since the typical speaker impedance is high at its lower resonance peak, then drops to its lowest in the midrange, then slowly rises in the high frequency range, it would seem that a tube amp will produce a little more bass and a little more sparkle in the treble region compared to a solid state amp, which produces lower power at these same frequencies.

This would indicate that the preamp may not really contribute anything to the tube sound after all. It also says that it isn't distortion as well because the distortion components (if the amp is operated within its design limits) should be below the threshold of detection.

The debate that a tube preamp versus a solid state preamp produces different sound may be simply listener bias if both preamps have the same specifications.

Nevertheless, my preamp and power amp are both tubes and I am happy with the sound I get from them and regardless of the technical performance specs it is my happiness that is important to me.




Quote:
Originally Posted by ja2dhc View Post
Hello from Japan !

Based on my experiences, one of the reasons of tube sounds comes
from 2nd order distortion as is far different from solid state amps.

In the sense, I did a circuit simulation of low supply voltage
pre-amp as follows:

Supply Plate Voltage = 34V
(consists of 19V + 15V by Laptop PC Power Supplies)
Used tube = 6N6P, Russian Low rp tube to work under low voltage
Voltage Gain of Pre-amp = 2.6 (approx.)

In order to consist power amp, I added a power booster for
simulation purpose whose spec is as follows:

*Simulator = TINA-TI version 9
*power amp = Ideal OP amp
*GV = 6.2
* Simulation circuit schematic and result are shown in the attached photos.

As the simulation result, THD= 0.4% @1W and 1.3% @10W which are
near same as 211 SET.

For the actual amp, I am going to use Class-D amp as the power
booster.
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Old 12th July 2012, 02:21 PM   #10
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Quote:
Originally Posted by disco View Post
THD does not make the characteristic of a tube amplifier. When looking at distortion only, it's important to maintain the relation of higher uneven order harmonics with lower even harmonics.
No, it's the opposite. What the listeners commonly characterize as "tube" sound is the dominance of second order harmonics followed by much weaker third order harmonics and then diminishing harmonics beyond those. It's the typical traits of SE design.

The other "tube" sound that listeners describe is the high frequency roll-off of some tube amps. You can make class AB transistor amp to sound like this by using equalizer.
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