|United States Patent
March 6, 1979
Constant distortion volume control
A device for maintaining a constant distortion output from a musical
instrument amplifier over a volume range of the musical system. The
amplifier is set at a predesired volume setting and volume control is
provided by a potentiometer control interposed between the output of the
amplifier which may, for example, be a guitar amplifier, and the
loudspeaker. The potentiometer is connected so that one end is in series
with the speaker providing the primary means for controlling volume while
the other end is in parallel with the speaker. Because the load that the
amplifier sees is a function of both the speaker impedance and the
potentiometer setting, there is a calibration scale associated with the
control dial of the potentiometer so that the amplifier output impedance
can be matched to the impedance of the speaker being used.
Scholz; Donald T. (13 Rich Valley Rd., Wayland, MA 01778)
March 3, 1978|
|Current U.S. Class:
||381/109; 381/118 |
|Field of Search:
179/1 VL,1 A,1 E
References Cited [Referenced By]
H. Tremain, The Audio Cyclopedia, H. Sams Co., 1969, pp. 1136, 1144.
Primary Examiner: Cooper; William C.
Assistant Examiner: Kemeny; E. S.
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
What is claimed is:
1. For an amplifier and speaker system, a volume control device coupled
between the amplifier and speaker for controlling the output volume level
of the system while maintaining substantially constant distortion from the
amplifier, said volume control device comprising, a variable impedance
means having a first part in series with the speaker and a second part
across the speaker manual control means for controlling the impedance
means to alter the first and second parts in a complementary manner and at
least one calibration display means associated with the manual control
means having at least two segments thereon for indicating proper impedance
match between the amplifier and speaker depending upon the position of the
manual control means.
2. A volume control device as set forth in claim 1 wherein said variable
impedance means comprises a potentiometer and associated volume control
3. A volume control device as set forth in claim 2 wherein said amplifier
has a selector means for selecting one of a plurality of impedance levels,
said control knob for indicating, by reference to the calibration display
means segment, which position to be selected by the selector means.
4. A volume control device as set forth in claim 3 including a plurality of
concentric calibration bands each corresponding to a different impedance
speaker usable with the system.
5. A volume control device as set forth in claim 4 wherein some of said
bands include three segments.
6. A volume control device as set forth in claim 2 wherein said segments
each indicate an impedance value with the lower volume settings of the
control knob corresponding to the higher impedance value.
7. A volume control device as set forth in claim 2 wherein said
potentiometer has ends coupling across the output of the amplifier and a
variable control arm controlled by the control knob coupled to the speaker
whereby a first part impedance on one side of the arm is in series with
the speaker and a second part impedance on the other side of the arm is in
parallel with the speaker.
BACKGROUND OF THE INVENTION
The present invention relates in general to a musical amplifier and speaker
system, and pertains, more particularly, to a volume control device
associated with the system for maintaining a constant distortion output
over the normal volume range of the system. In accordance with the
invention the device is for preserving the same quality of distortion at
all output levels of the amplifier and speaker system.
Some amplifier systems are constructed to minimize distortion. However,
modern music seeks to inject distortion into the musical tones with the
distortion being considered as part of the musical composition. With
modern music systems, the nature and quality of the distortion is created
by the amplifier itself. However, one of the difficulties with an existing
system is that the nature, character, quality and sound of the distortion
changes with different amplifier output levels. At a high gain level, the
distortion has a different quality than at a low gain level. When one
simply turns up the volume of the amplifier there is a change in the
quality of the distortion, and similarly there is a change when the gain
of the amplifier is reduced. This is an undesirable characteristic of
existing systems wherein the distortion changes with volume.
Accordingly, one object of the present invention is to provide an amplifier
and speaker system wherein the distortion is maintained substantially
constant over the output volume range of the system.
Another object of the present invention is to provide an indicator scale in
association with the device of this invention indicating to the user of
the system the appropriate impedance match between amplifier and speaker.
SUMMARY OF THE INVENTION
To accomplish the foregoing and other objects of this invention there is
provided a volume control device for an amplifier and speaker system,
which device is coupled between the amplifier and speaker for controlling
the output volume level of the system while maintaining substantially
constant distortion from the amplifier. The volume control device
comprises a variable impedance means having a first part in series with
the speaker and a second part in parallel with the speaker and further
comprising a manual control means for controlling the impedance means to
alter the first and second parts in a complementary manner so that as the
first part impedance increases the second part impedance concurrently
decreases and vice versa. The total impedance across the variable
impedance means is maintained constant.
In the preferred embodiment of the present invention the device comprises a
variable resistor or potentiometer intercoupled between the amplifier and
speaker with one part of the potentiometer in series with the speaker and
another part in parallel with the speaker. In the embodiment disclosed
hereinafter a 15 ohm potentiometer is used. The volume of sound from the
speaker is controlled by the setting of the potentiometer and is a
function of the series resistance and the resistance in parallel with the
speaker. The parallel resistance functions as a variable attenuator of
volume particularly at its lower resistance setting which occurs when the
series resistance is at its higher relative resistance. Thus, the
potentiometer setting affects volume via both legs thereof. When the
series resistance is at the maximum setting (parallel leg at minimum
setting) the speaker is effectively cut-out. Alternatively, the full power
from the amplifier (at that particular amplifier gain setting) is applied
to the speaker and the parallel leg when the potentiometer is at its
minimum series resistance position (parallel leg at maximum setting).
However, throughout the entire range of the potentiometer settings, the
signal applied to the speaker is of the same quality insofar as distortion
is concerned. Thus, the output sound level of the speaker may be varied
but without varying the distortion quality of the sound from the system.
With the inclusion of the volume control of this invention coupled between
the amplifier and speaker, the load which the amplifier sees is a function
of both the potentiometer setting and the particular speaker impedance.
The load may vary from a maximum equal to or greater than the value of the
potentiometer resistance, to a minimum on the order of 4 ohms or less.
This load that the amplifier sees at its output depends both on the
setting of the potentiometer and the impedance of the speaker and thus the
load is a function of the volume setting that is chosen. The amplifier is
preferably provided with a number of output taps or a tap switch for
matching to a 4 ohm, 8 ohm or 16 ohm speaker and with the use of the
volume control of this invention it is desired that the output impedance
of the amplifier be closely coupled to the load which the amplifier drives
in order for there to be maximum efficiency in power transfer from the
amplifier to the speaker as well as to insure that neither the speaker nor
amplifier are damaged by overloading because of any mismatching. Thus, in
accordance with the invention, the volume control device includes a
calibration scale which is associated with the control knob for the
potentiometer. The scale includes three concentric circular bands, each of
the bands intended to correspond to a particular one of the typical 4, 8,
or 16 ohm speakers. Each of the bands is subdivided into arcuate segments
which provide an indication from the pointer on the potentiometer knob as
to what the impedance load of the speaker circuit is. Thus, for a
particular impedance speaker and for a particular volume setting, the user
can set the switch on the back of the amplifier to correspond to the
proper load actually seen by the amplifier thus providing proper matching
and reducing the chance of damage to the amplifier or speaker.
BRIEF DESCRIPTION OF THE DRAWINGS
Numerous other objects, features and advantages of the invention should now
become apparent upon a reading of the following detailed description taken
in conjunction with the accompanying drawings, in which:
FIG. 1 is a perspective view of the volume control device of the present
FIG. 2 is a front elevation view of the device shown in FIG. 1;
FIG. 3 is a side elevation view of the device shown in FIG. 1;
FIG. 4 is a schematic diagram showing the control device of FIGS. 1 and 2
in an amplifier and speaker system;
FIG. 5 shows a version of the invention employing a potentiometer for
volume control; and
FIG. 6 depicts an equivalent circuit for the volume control device and
FIGS. 1-3 show one embodiment of the volume control device 10. This device
comprises a housing 12 having side walls, a top wall, a bottom wall, and a
back wall. The top has a series of open vents to permit heat transfer away
from the device. The housing 12 also has a front wall 14 for supporting an
input receptacle 16 and a pair of output receptacles 18. The receptacle 16
receives a connecting cable from the amplifier and the output receptacles
18 receive one or more cables for connecting to one or more speakers
associated with the musical system. FIG. 4 shows the amplifier 20 with a
cable 21 connecting to the volume control device 10, and also a speaker 24
connecting by means of cable 25 to the device 10.
A potentiometer 28 is mounted within the housing 12 from the front wall 14.
This mounting may be accomplished in a conventional manner. The
potentiometer 28 has a control knob 30 extending through the wall 14 and
has associated therewith a pointer 31 which is in turn usable with the
volume scale 34 and the calibration scale 36 discussed in more detail
When the device 10 of this invention is employed, the amplifier 20 is
preferably set at a predetermined output gain and it is left at that
particular setting. In this way, the volume of the system is not
controlled by the volume control on the amplifier, but is instead
preferably totally controlled by the volume control device 10. In this
way, the output distortion from the amplifier 20 is maintained constant
and the device 10, being purely resistive, effects only volume and does
not alter the distortion from the amplifier.
FIG. 5 shows the connection of the potentiometer 28 between the amplifier
20 and the speaker 24. The two fixed ends of the potentiometer 28 couple
to line 21 and the common line 29. The wiper arm 35 of the potentiometer
28 couples by way of cable 25 to the speaker 24. FIG. 6 shows the
equivalent circuit with a portion of the potentiometer being depicted by
the series resistor R1 and the other portion of the potentiometer being
depicted by the parallel resistor R2. In FIG. 6 the speaker is depicted by
the resistor RS and the wiper arm is depicted by the node 35'. FIG. 6 also
shows the dotted line 42 depicting the common complementary control
between the values of the resistors R1 and R2 as the knob is turned to
move the wiper arm 35 along the potentiometer. The values of the resistors
R1 and R2 are always the same total value which in the preferred
embodiment is a total resistance of 15 ohms. However, by control of the
knob 30 the specific values of each of the resistive parts R1 and R2
changes. For example, at a mid-setting of the knob 30 the resistor values
are equal each to 7.5 ohms.
The volume of the sound generated by the speaker is controlled by the
combination resistor R1 which is in series with the speaker and reisistor
R2 which is in parallel with the speaker operating together in a
complementing fashion. The speaker may be reduced to zero volume by having
the entire resistance in series therewith so that the resistor R1 is
approximately 15 ohms and the resistor R2 is approximately 0 ohms. On the
other hand, the full power from the amplifier at a predetermined amplifier
gain setting can be applied to the speaker and the resistor R2 by setting
the potentiometer so that the series resistance R1 is at a minimum or very
small resistance, even as low as zero ohms. Throughout the entire range of
the potentiometer, the signal applied to the speaker is of the same
quality insofar as distortion is concerned. Thus, the output sound level
of the speaker may be varied but without varying the distortion quality
from the amplifier 20.
As previously indicated, the load that the amplifier drives depends both
upon the setting of the potentiometer and the impedance of the speaker.
Thus, the load that the amplifier drives will vary with variations in the
volume that is selected by the knob 30. In order to assure a proper match
to the particular speaker that is used the calibration scale 36 is
employed in association with the pointer 31 of the control knob. The
calibration scale 36 includes three concentric circular bands 36A, 36B,
and 36C which correspond, respectively, with 4 ohm, 8 ohm and 16 ohm
speakers that are used with the system. Each of the bands is further
subdivided into arcuate segments 37. In the drawing, each of the segments
37 is identified by a 4 ohm, 8 ohm or 16 ohm designation corresponding to
the proper setting that is to be selected by the tap switch 50 depicted
schematically in FIG. 4. The bands 36A and 36B include all three impedance
positions while the band 36C includes only the settings for 8 ohms and 16
For example, the 4 ohm speaker band 36A is divided into three segments 37
and depending upon the position of the knob 30, the user is able to
determine the proper setting of the switch 50 by reference to the
particular segment 37 that is being pointed out. Thus, at the lower volume
settings of the potentiometer the resistor R1 may, for example, be 11 ohms
and the resistor R2 is 4 ohms. In this example, the speaker impedance is
also 4 ohms and thus the parallel combination of resistors R2 and RS
provide a total impedance of 2 ohms which in series with the impedance of
resistor R1 provides a total impedance seen by the amplifier of 13 ohms.
For that particular impedance, the pointer 31 points to the 16 ohm segment
37 because that setting of the switch 50 is the closest to the total
impedance seen by the amplifier. As the volume is increased, the load seen
by the amplifier decreases and at a predetermined point the pointer will
then be disposed in the relatively wide 8 ohm segment 37. It should be
noted that as the volume is increased the effective circuit is one in
which the first part of the potentiometer is in series and the second part
is in parallel with the speaker. The effective resistance of the parallel
speaker and second part of the potentiometer is less than either of them
although it will have an additive effect to the series resistance R1. At
the high volume end with a 4 ohm speaker, the resistor R1 may be 1 ohm and
the series resistance of resistors R2 and RS may be on the order of 2-4
ohms thus making the total impedance on the order of 4 ohms. The drawing
shows this 4 ohm segment of band 36A at the high volume end which
indicates that the switch 50 should then be selected to the 4 ohm
position. Thus, there is a correspondence between the setting of the
switch 50 and the particular speaker used with regard to a 4 ohm speaker,
only at the high volume setting.
With regard to the band 36B the operation is quite similar but it is noted
that the segments are essentially shifted because of the higher impedance
of the speaker that is now used. The band 36B corresponds to an 8 ohm
speaker. For this band, at low volumes, the switch 50 is at its 16 ohm
position and at very high volumes the switch 50 is at its 4 ohm position.
At the midband volumes the switch 50 is at its 8 ohm setting corresponding
to the impedance of the speaker.
The last band 36C has associated therewith only two segments corresponding
to 8 ohm and 16 ohm positions of the switch 50. This band corresponds to a
16 ohm speaker that is used with the system. When the volume is turned all
the way up, it is desirable to use the 8 ohm setting of the switch 50. At
higher volumes the resistor R2 is greater than the resistor R1 and the
total impedance is more nearly matched when the 8 ohm setting is used on
the amplifier switch 50.
Having described one embodiment of the present invention, it should now be
apparent to those skilled in the art, that numerous other embodiments are
contemplated as falling within the scope of this invention. For example, a
potentiometer has been shown as a preferred form of control device.
However, it is understood that other impedance devices may be used having
individual impedance parts as illustratively depicted by the resistors R1
and R2 of FIG. 5. Also, although conventional speaker impedances have been
disclosed, it is understood that any other combination of speaker
impedances may be employed and that the invention may be practiced with
more or less than three calibration bands depending upon the number of
different types of speakers that may be employed.
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