Download Volume Control in the Hi-Fi/High-End

Volume Control in the Hi-Fi/High-End
© Alex Torres, Netanya.
(part 1, 2004, revised in 2007)
Preamble
This article was written 3 years ago, but only Part I was published.
Unfortunately - the time is always not enough :(, so the second and
the third parts were not finished. Maybe Its even better - during these
3 years some designs were changed, some new ideas were implemented
Now I have decided to finish this work and to change the structure
of this article. The 1st part was totally revised, all information concerning
the “nikitin” volume control was moved to the 2nd part, than now totally
dedicated to this design. The Part III (multi channel application for the
Home Theater) was moved to the Part IV and the new Part III will describe
the advanced tube/solid amplifier’s control (“nikitin” based).
The problems of the Volume control.
In spite of its simplicity, the volume control in the high level audio equipment still performs a
problem. The “short pass” oriented audiophiles sometimes even not want any volume control - the
music should sound “as is”. But in reality - life without volume control is not comfortable.
What the volume control device must to do? The sound pressure level must linearly depend of the
volume knob angle (in order to do this, the volume control element should be logarithmic). It is
not hardly to make, if you have only one channel, but in 2 channel stereo application (and multi
channel Home theater applications too) the big problem is the equality of the channels controllers.
Of course, volume controller is not allowed to produce any noise (as acoustic, as electric). The
good idea is to have the possibility of the remote control.
There are lot of solutions - “audiophile” ALPS potentiometers, simple or with the motor control
(by the way, why Alps? Bourns and Honywell make not worth potentiometers, but they are often
more expansive due to an aerospace orientation of these companies). Gold platted rotary switches,
variable input transformers (well known “Sowter”, transformers from Alex Vorobiev, etc.) and
even undeservedly contemned by audiophiles electronic volume controllers. The last ones
are very ambiguous - the wide range of the existing devices with the real high quality, to the low
end devices, that are suitable only for portable mp3 players, car audio, etc.
In this article I will try to observe some solutions - with electronic attenuators, with relays, with
micro controller control, from the simple up to the more advanced.
Short review of the volume control IC.
Electronic volume control usually based on the voltage control amplifier - VCA (or VGA Variable Gain Amplifier), for example at SSM2160, or switchable resistor network The most
popular IC are digital potentiometers (DS1802 from Dallas-Maxim, etc.), volume control chip
CS3310 from Cirrus Logic (Crystal Semiconductors) compatible with PGA2310 from BurrBrown (now - Texas Instruments), LM1972 from National Semiconductors. One thing - many
efforts to use 3-channel LM1973 fails. I’m sure that LM1972 and LM1973 have the same internal
architecture, but the difference is very enormous.
Last time we have the new TI chips - PGA4311, than have 4 channels and better performance. (2
channel version - PGA2311). The old chip SSM2160 from Analog Devices provides 6 channel
control, but worse performance.
Various IC like LM1036 are out of out discussion due to their abominable sound. Certainly, this
short review does not cover all volume control IC that exists all over the world. We will talk about
some representatives.
Simple LM1972 based volume Control.
LM1972 produce separate volume control of the 2 channels in the range from 0 to --78dB. Step is
0.5dB in the 0 - -48 dB range and 1dB at -48 - --78dB. This chip use simple 3-wire protocol (SPI
bus):
If you need more then 2 channel control, you can use 2 solutions: it is possible to put some
IC in parallel and to control all channels simultaneously (for example - 2 parallel lm1972 are able
to control LF, RF, LS, RS, Center and Sub, but you cannot control all kinds of balance) Or - you
can use daisy-chain connection. The first way is simpler, because you don’t need to make any
changes in the microcontroller’s software. Both two methods are shown at the diagram (right parallel, left - daisy-chain) :.
LEFT_IN
RIGHT_IN
4
20
12
11
9
10
L_BACK_IN
R_BACK_IN
4
20
12
11
9
10
CENTER_IN
SUBW_IN
4
20
DT
CLK
CS
12
11
9
10
IN1
IN2
OUT1
OUT2
2
17
LEFT_OUT
RIGHT_OUT
LEFT_IN
RIGHT_IN
12
11
9
10
DO
DT
CLK
CS
IN1
IN2
OUT1
OUT2
2
17
L_BACK_OUT
R_BACK_OUT
L_BACK_IN
R_BACK_IN
DO
DT
CLK
CS
4
20
12
11
9
10
DO
DT
CLK
CS
IN1
IN2
4
20
OUT1
OUT2
2
17
CENTER_OUT
SUBW_OUT
CENTER_IN
SUBW_IN
4
20
DT
CLK
CS
12
11
9
10
IN1
IN2
OUT1
OUT2
2
17
LEFT_OUT
RIGHT_OUT
2
17
L_BACK_OUT
R_BACK_OUT
DO
DT
CLK
CS
IN1
IN2
OUT1
OUT2
DO
DT
CLK
CS
IN1
IN2
DO
DT
CLK
CS
OUT1
OUT2
2
17
CENTER_OUT
SUBW_OUT
The most simple control device is shown at the diagram - is has no display, control provide by
two potentiometers - not logarithmic, not double way, not “audiophile quality”. Any most
cheapest as you can find, even the value not need to be accurate - use what you have, from 5k to
50k. This controller has also mute button and two LEDs to display mute status ant the balance
control center position.
Maybe you have a question: “Why we need all this, to use potentiometer?”?” The answer is
simple, and I give is above -this potentiometers has no influence to the audio signal. Both volume
and balance potentiometers change the voltage at microcontroller’s ADC inputs. Micro measures
the voltage and produce the necessary signals to the volume control chip. The IC’s control line
used also for LEDs, as these lines can be in any position in inactive time.
C2
1
MUTE
PIC12C671
Remark:
D2
CENTEr
SW1
D1
MUTE
3
IN
+
+9..+15v
C4
1
5
6
14
16
C3
100.0
C7
100n
1
+
3
C6
100n
100n
GND
-5VA
U3
1
OUT
2
VDD
VDD
GNDAC
GNDAC
GNDAC
GNDAC
GNDAC
2
17
OUT
GND
R5
2k
U4
L78L06
13
15
R4
2k
VSS
VSS
DO
DT
CLK
CS
DGND
OUT1
OUT2
7
18
12
11
9
10
8
5
3
2
IN1
IN2
GND
GND
4
20
3
19
8
GP3/MCLR
GP2
GP4
GP5
U2
100n
+5VA
R2
2k
1
BAL
4
GP0/AN0
GP1/AN1
GND VCC
7
6
R3
25k
OUT
C1
100n
R1
25k
VOL
IN
2
R6
10k
L78L05
GND
U1
3
L_OUT
R_OUT
GND
L_IN
R_IN
IN
2
-9..-15v
C5
100.0
LM1972
U5
L79L06
PIC12C671/672, PIC12F675
For the best performance, it should be connected to big input resistance. If you amplifier have low
- you should use the OP buffer
(ОРА2132, ОРА2134 и т.п.). Take care about ground wiring - IC has the special “digital” and
“analog” ground pins... Input signal can be up to 3V RMS. level. Input resistance - 40kOhm.
Remark: I received many complains than PIC12C671 is not widely available, and because using
of the One Time Programming micro is not a good idea for the D.I.Y, I make the software also for
the flash micro - PIC12F675.
PGA2310 based Volume Control
Not all the people are able to make the changes in the software for LM1972, to work with
PGA2310. Both chips have the same SPI interface, but different internal structure. This device is
for those peoples.
At 1st, PGA2310 can not only attenuate the signal, but also to amplify! The control range is from
-95.5 dB up to +31.5dB wit the 0.5 dB step. The powerful output can be loaded with 600 Ohm
directly, without any buffers! But using of the full PGA’s range looks very plenty and I decided to
make 3 software versions:
1. Full – original control range, from-95.5 dB to +31.5 dB, but with 1dB step.
2. NoGain – without amplification, i.e. from -63 dB to 0 dB, with 0.5 dB step
3. Gain3 – with limited amplification, from -53 dB to +10dB with 0.5 dB step.
Balance control in all versions is +/-8 dB with 0.5 dB step.
With the “full version” the hardware limits are also possible (as will be described below).
L_IN
R_IN
AGND
PIC12C671/672, PIC12F675
5
3
2
R3
2k
7
3
6
2
DO
DT
CLK
CS
OUT1
AGND
OUT2
14
10
11
C2
100n
+ C3
100.0
AGND
VSS
100n
GP2
GP4
GP5
DGND
GP3/MCLR
8
R2
25k
VCC
BAL
4
GP0/AN0
GP1/AN1
GND
VOL
7
6
IN1
AGND
IN2
5
C1
1
R1
25k
16
15
9
VCC
8
1
4
PIC12C671
-MUTE
ZCEN
VDD
R8
10k
12
+15v
U2
C4
100n
13
Remark:
L_OUT
R_OUT
AGND
C5
+ 100.0
R4
2k
D2
CENTER
D1
MUTE
MUTE
-15v
SW1
U1
PGA2310
+5V
+ C8
100.0
GND
As you can see - the schematic was not changed dramatically (power supplies regulators are not
shown). The major differences are in the software. Like LM1972 it is possible to use some PGA
in parallel to obtain the multi channel volume control.
The possibility to operate with the low impedance (600 Ohm, 1000pF) load (output current of
the PGA2310 – 35мА, and 2311 – 50мА) give us the elegant opportunity to use PGA as
preamplifier, or to get other benefits as the buffer. Below you can see some examples of these
solutions.
LM4780 (LM3886) inverting
Inverting schematic of the LM3886 (double version – LM4780) have better sound then widely
used non-inverting. In order to solve the stability problem, gain must be more than 10 (some
people said even 20-30), and feedback resistor should be not more then 50 kOhm. This determine
the input resistor (input impedance) not more then 3-5 kOhm. With its low output impedance,
PGA2310/2311 can be buffers for inverting power amplifier. You must remember than input
impedance of the PGA is also not very big - 10 kOhm, so sometimes PGA need the buffer at the
input.
L_IN
R_IN
AGND
+15v
PIC12C671
+VCC
+VCC
+VCC
+VCC
+VCC
U3
5
8
9
17
27
+U
Remark:
VCC
12
C2
100n
OUT1
AGND
OUT2
14
10
11
R2
4k7
R4
4k3
R5
4k3
16
22
21
-INA
OUTA
INA
MUTEA
INB
OUTB
-INB
VSS
MUTEB
7
14
OUTPUT
25
20
-VCC
-VCC
-VCC
-VCC
-VCC
-15v
U2
PGA2310
R9
47k
-U
GNDA
+5V
+ C6
100.0
GND
R10
?
2
4
10
11
18
100n
C4
+ 100.0
GND
GND
C3
12
19
R12
2k
4k7
15
R1
47k
AGND
13
DO
DT
CLK
CS
DGND
R7
5
VCC
8
R11
2k
C5
100n
SW1
MUTE
7
3
6
2
5
3
2
IN1
AGND
IN2
D2 CENTER
GP3/MCLR
GP2
GP4
GP5
D1 MUTE
4
GP0/AN0
GP1/AN1
BAl
R8
25k
7
6
GND
25k
R6
16
15
9
1
VOL
R3
10k
-MUTE
ZCEN
VDD
8
1
4
+ C1
100.0
U1
LM4780
PIC12C671/672, PIC12F675
There are no capacitor between the volume control and the power amplifier, and PGA’s output
offset voltage will be amplified by the LM. That’s why the output stage gain should be as small as
possible. The same reason - to use PGA2311 instead of PGA2310 due to it's half offset. We not
discuss LM3886/4780 in this article, you can pass to the data sheets and to the various FAQs in
the Internet. Bypass capacitors and Bushero output circuit re not shown at the diagram above.
SE Headphone Amplifier.
In opposite to the previous schematic, in this design more preferable is PGA2310, because the +/5v power supply limitation of the PGA2311. PGA2310 have up to +/-15v working range, with
shown at the diagram below +/-12v power supply, PGA2310 with the emitter follower is able to
provide output voltage 3.2v @ 32 Ohm load and 6.5v @ 300 Ohm, that is quite enough!
This design enjoys the “Full” control version with the hardware limit adjustment. With the help of
two internal trimmers you can limit the maximum and minimum volume, and to get the most
comfortable control according to you headphones.
Remarks: p-n-p transistors MJE170 can be changed to BD140, etc., or mirroring the schematic
and use n-p-n transistors like MJE180, BD139, etc. The sound quality depends of the transistors MJE170 from Motorola sound very nice, also BD140 from Philips. But MJE170 from ST–
sounds worse... Transistors idle current is about 220 mA and can be adjusted by R1R2.
L_IN
+12V
R1
51R
R_IN
R2
51R
HEAD OUTPUT
AGND
LINE INPUT
C1
2200.0
12
8
4
1
DO
DT
CLK
CS
+
VCC
14
10
11
100n
CENTER
PIC12F675
C3
D1
U2
C2
2200.0
Q2
MJE170
R6 1k
J1
R9 1k
8
5
R8 2k
OUT1
AGND
OUT2
VSS
7
3
6
2
DGND
GP2
GP4
GP5
5
3
2
Q1
MJE170
13
GP0/AN0
VCC
1
GP1/AN1
VOL
7
GND
6
GP3/MCLR
IN1
AGND
IN2
-MUTE
VDD
ZCEN
10k
BAL
R5
10k
R3
2k2
R4
10k
R7
4
R10
2k2
16
15
9
+
+5V
1
2
3
U1
CON3
-12V
PGA2310
+/-12V_GND
5V_GND
Remember - the efficiency of the “Class-A” amplifiers is like the efficiency of the steam train. All
class-A amplifiers are extremely HOT - be careful, use the heatsink. With 220 mA idle current,
about 11W going to the heat on the two output transistors and their resistors. If you will use only
low impedance headphones (32 Ohm) - its better to drop the power supply voltages from +/-12v
down to +/-9v or even to +/-5v (using 7809/7909 and 7805/7905 regulators). If in opposite - you
will use only high impedance headphones (300, 600 Ohm) – it is practical to reduce idle current
by increasing the emitter resistor.
-- end of part 1 --