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Old 2nd March 2013, 08:11 AM   #1
ja2dhc is offline ja2dhc  Japan
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Default Tube Sound Processor

What is the TUBE SOUND ?
I think the biggest contribution comes from small distortion
such as 0.5% THD.

To generate 0.5% THD, I propose to install a pre processor before
solid state main amp. I did simulation with the attached circuit
schematic using 12B4A on TINA-TI simulator. The results are as follow:

Voltage Gain = 4
THD = 0.512% @0.4Vrms output

Circuit Description:

Power supply: Laptop PC's AC adapter, DC 12V out put for both
plate and heater power supply.

I eliminated the Cathode By-pass capacitor as it affects to
degrade the sonic. 300mA of 12B4A's heater current is
merged into a cathode resistor with the 12B4A's plate current.
Resulting. only 2.2 ohm of the cathode resistor is enough and
the negative feed back due to cathode resistor is minimized

I have not yet assembled the actual amp of this circuitry.
Simulation only at the moment.

'73 de JA2DHC
Attached Images
File Type: gif 12B4A-pre.gif (7.2 KB, 313 views)
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Old 2nd March 2013, 08:28 AM   #2
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does the sim include grid current? some do, some don't. it will affect things at very low voltage. and you've no grid leak resistor, i would sim that, because grid current will put a voltage across it.
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Old 2nd March 2013, 09:04 AM   #3
ja2dhc is offline ja2dhc  Japan
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Hi,Robert Kesh, Thanks for a quick response.

The SPICE model of 12B4A used in my simulation employees
grid current (see quoted SPICE model at the bottom). However
This SPICE model assumes only positive grid domain.
don't care negative domain of the grid (ie; don't care
the thermal velocity effect) .

'73 de JA2DHC

*
* Generic triode model: 12B4A
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Sat Mar 8 22:41:12 2008
* Plate
* | Grid
* | | Cathode
* | | |
.SUBCKT 12B4A A G K
.PARAM X1=1 X2=0.087065329 X3=-1.2036947
.PARAM X4=0.55479636 X5=5.1134435 X6=2.7036947
.PARAM X7=0.0047680512 X8=9.2167935 X9=0.0036584566
.PARAM Y1=0.0023840256 Y2=0.0031806909
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 4.8p
CGK G K 5p
CAK A K 1.5p
.ENDS
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Old 2nd March 2013, 09:20 AM   #4
DF96 is offline DF96  England
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Some people like distortion and may call it 'tube sound'. They often deny this. They may obtain the sound they prefer by misusing valves, but possibly not realising that this is what they are doing.

Others like valves for other reasons, and know that when used properly their distortion is sufficiently low that it will be inaudible and so can be ignored. Mostly, they dislike 'tube sound' as a phrase.
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Old 2nd March 2013, 01:04 PM   #5
12B4A is offline 12B4A  United States
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I think you're going to find the sim is very unpredictive of actual performance. You're going to need 12 volts plate-cathode just to get any conduction going.
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Old 5th March 2013, 06:36 PM   #6
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Actually, "tube sound" means absence of specific distortions generated by poorly designed solid state amplifiers. You can't add absence. But you can design and build solid state gear that does not have that specific distortions, so sounds like tubey.
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Old 6th March 2013, 10:16 AM   #7
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Default tube sound

I think another important aspect of "tube sound" comes from the output transformer drive. Instead of using voltage drive a speaker in solid state design, the transformer acts more like an energy drive. This in turn can achieve better dynamics when driving a nonlinear load such as speaker. So I don't think just introducing the distortion can emulate the "tube sound".
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Old 6th March 2013, 10:36 AM   #8
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by ja2dhc View Post
What is the TUBE SOUND ?
tube sound originates from a time when most speakers lacked midbass BSC, simply because they didnt know better

tube amps could work like magic on that 'problem'
you might say it relates mostly to high output impedance, due to the output transformer
but its only partly true
a very simple tube preamp could also do wonders in this respect

but Im sure its much different today
speakers are much better designed
and knowledge about transistor based design have also improved some
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Old 6th March 2013, 10:59 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by yangshau
I think another important aspect of "tube sound" comes from the output transformer drive. Instead of using voltage drive a speaker in solid state design, the transformer acts more like an energy drive. This in turn can achieve better dynamics when driving a nonlinear load such as speaker. So I don't think just introducing the distortion can emulate the "tube sound".
This appears to be an attempt to turn a weakness into a virtue, by redefining reality. The correct way to deal with this is to deal with the weakness by using feedback; not try to woffle it away.
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Old 28th July 2013, 08:05 AM   #10
ja2dhc is offline ja2dhc  Japan
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I have finished to assemble my sound processor (pre-amp)
to generate "Tube Sound"

The final schematic and external view are shown in the
attached photos.

Focus Points of circuit design

*Single LAP TOP PC use DC15V power supply to cover everything,
Plate Voltage, Heater, Power Amp (Class-D).

*Heater currents are merged into a cathode resister of 1.1 ohm
so that By-Pass capacitor can be eliminated since current feedback
is very low due to low resistance of 1.1 ohm. Channel separation is
about 50dB (measured).

*In order to avoid effect of grid current, I used input
transformer whose resistance of the secondary winding is 240ohm.

* Plate current is about 4mA (measured).

From now on, I will measure. Now I am typing this post with listening
music, good sound !

'73 de JA2DHC
Attached Images
File Type: gif 12B4A-Head-AMP.gif (15.6 KB, 150 views)
File Type: jpg 12B4A1.jpg (77.9 KB, 148 views)
File Type: jpg 12B4A2.jpg (87.8 KB, 144 views)
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