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Copyright 2013 by Wayne Stegall
Updated October 26, 2013.  See Document History at end for details.




Voice Control

Circuit enables adjustable euphonic control


Introduction

An euphonic distortion result seems to be an important goal of high end audio.  I understand that purists say that the euphony is a only pleasing side effect of the pursuit of a purity of sound that simple circuits create.  However, some good results have been reported by adding an euphonic component to an otherwise undesirable neutral system.  Here I assert that, with the proper circuit, adjustable euphony can be added to a system.

Circuit

The circuit of figure 1 below aims to meet the specified goal.  A square-law or near square-law buffer is chosen for the basis because of near unity gain under most circumstances.  Then adjustable loading is provided by R4 and R5 through C1.  This allows variation of the magnitude of the output signal current relative to the bias.  Coupling through C1 ensures that the adjustment made does not change the bias of the buffer itself.  Since distortion is proportional to the ratio of the signal to the bias, the amount of distortion can be adjusted with potentiometer R5.

Figure 1:  Source follower buffer with adjustable distortion mechanism
buffer

Initial Design

(1)
R3 =
VT-Q2
ID-J1
 =
0.7V
5mA
 = 140Ω




Round to nearest 5% value:
R3 = 150Ω
R2 of 30kΩ would give somewhat less than 1mA of bias to Q2, choose 27kΩ for a little more.
R227kΩ
R6 = 1MΩ
R4 = 1kΩ
R5 = 100kΩ
(2)
C1 =
1
2πfpoleR4
 =
1
2π 0.1Hz 1kΩ
 = 1.59155mF 




Round up to nearest 10% value:
C1 = 1.8mF
(3)
C2 =
1
2πfpoleRL
 =
1
2π 0.1Hz 10kΩ
 = 159.155F 




Round up to nearest 10% value:
C2 = 180F


SPICE Analysis


Figure 2:  Initial SPICE deck
* source follower buffer
vpos vdd 0 dc 15V
vneg vss 0 dc -15V
v1 vin 0 dc 7 ac 1 sin 0 1.41421V 1kHz
r1 vin 0 47k
j1 vdd vin vsj1 2n3819
r2 vdd vbq1 27k
q1 vsj1 vbq1 veq1 2n3904
r3 veq1 vss 150
q2 vbq1 veq1 vss 2n3904
c1 vsj1 vc1r4 1.8m
* r4 sets maximum distortion limit
r4 vc1r4 vr4r5 1k
* adjust r5 from 0.1 to 100k (zero value resistances may cause simulation problems)
r5 vr4r5 0 100k
c2 vsj1 vout 180u
r6 vout 0 1meg
rl vout 0 47k
.MODEL 2N3819 NJF( VTO=-2.9985 BETA=1.3046M LAMBDA=2.2507M RD=1 RS=1
+      CGD=1.5964P CGS=2.4199P PB=500M IS=33.582F
+      BETATCE=-500M KF=0 AF=1 )
.model 2N3904 NPN (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259
+               Ise=6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1
+               Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75
+               Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10)
.end
.control
* transient analysis for 1k fourier
tran 1u 0.1 0 1u uic
fourier 1k vout
.endc

Initially I wanted a maximum distortion adjustment of ≥ 1%.  My initial chosen value of 1k for R4 did not meet that specification.  A subsequent attempt with 470Ω produced the following distortion result:

Fourier analysis for vout with R4 set at 470Ω R5 set to 0.1Ω:
  No. Harmonics: 10, THD: 1.21397 %, Gridsize: 200, Interpolation Degree: 1

Harmonic  
Frequency  
Magnitude  
Norm.Mag  
Percent  
Decibels











1
1000
1.02541
1
100
0
2
2000
0.0124213
0.0121135
1.21135
-38.3346
3
3000
0.00081424
0.000794063
0.0794063
-62.0029
4
4000
6.98048E-05
6.80751E-05
0.00680751
-83.34023
5
5000
1.51983E-05
1.48217E-05
0.00148217
-96.5820
6
6000
7.35443E-06
7.17219E-06
0.000717219
-102.887
7
7000
6.4829E-06
6.32226E-06
0.000632226
-103.983
8
8000
5.73999E-06
5.59776E-06
0.000559776
-105.04
9
9000
5.25766E-06
5.12738E-06
0.000512738
-105.802

Now to test my initial presumption of a 100kΩ maximum for R5.

Fourier analysis for vout with with R4 set at 470Ω R5 set to 100kΩ:
  No. Harmonics: 10, THD: 0.00232116 %, Gridsize: 200, Interpolation Degree: 1

Harmonic  
Frequency  
Magnitude  
Norm.Mag  
Percent  
Decibels











1
1000
1.40225
1
100
0
2
2000
0.000022202
1.58332E-05
0.00158332
-96.0086
3
3000
1.47504E-05
1.05192E-05
0.00105192
-99.5603
4
4000
1.09449E-05
7.80524E-06
0.000780524
-102.152
5
5000
8.92295E-06
6.36333E-06
0.000636333
-103.93
6
6000
7.22369E-06
5.15151E-06
0.000515151
-105.761
7
7000
6.53083E-06
4.65741E-06
0.000465741
-106.637
8
8000
5.36301E-06
3.82458E-06
0.000382458
-108.348
9
9000
5.08764E-06
3.62821E-06
0.000362821
-108.806

This load produces so little distortion that a value of 100k for R5 may not give any reasonable distortion adjustment range.  Therefore try a value of R5 ten times that of of R4, that is 4.7k. 

Fourier analysis for vout with R5 set at 4.7kΩ:
  No. Harmonics: 10, THD: 0.0273854 %, Gridsize: 200, Interpolation Degree: 1

Harmonic  
Frequency  
Magnitude  
Norm.Mag  
Percent  
Decibels











1
1000
1.35389
1
100
0
2
2000
0.00036985
0.000273176
0.0273176
-71.2711
3
3000
1.92095E-05
1.41884E-05
0.00141884
-96.9613
4
4000
1.05354E-05
7.7816E-06
0.00077816
-102.179
5
5000
8.04703E-06
5.94364E-06
0.000594364
-104.519
6
6000
7.00937E-06
5.17721E-06
0.000517721
-105.718
7
7000
6.25483E-06
4.6199E-06
0.00046199
-106.707
8
8000
4.83021E-06
3.56765E-06
0.000356765
-108.952
9
9000
4.8342E-06
3.5706E-06
0.00035706
-108.945

This minimum distortion result is still in the range considered to be inaudible.  Because it is not a vanishing distortion result, choosing a potentiometer value near 4.7kΩ should give adequate adjustment range.

Design Changes

R4 = 470Ω
R5 = 4.7kΩ
(4)
C1 =
1
2πfpoleR4
 =
1
2π 0.1Hz 470Ω
 = 3.38628mF 




Round up to nearest 10% value:
C1 = 3.9mF

Final SPICE deck.

Final parts list

J1
 
2N3819

C1
 
3.9mF
Q1, Q2

2N3904

R4

470Ω
R1

47kΩ
R5

5kΩ potentiometer
(with log taper?)
R2

27kΩ
C2

180F
R3

150Ω
R6

1MΩ


Application Notes

The circuit is presented in its minimal form.  Therefore the following notes may help in actual application.

Document History
October 26, 2013  Created.
October 26, 2013  Improved formatting of Fourier analysis results.  Correct head calculation of R2 to 27kΩ.