Designing and Building
Audio Amplifiers.

Practical Tone Control Circuits.

I have lately found that there is a lot of interest in tone
control circuits. It's beginning to seem as if 2/3 of the earth's surface
is covered by water and the remaining 1/3 is covered with diagrams of tone
controls. Although I have covered this topic under the heading of "Audio
Amplifiers, How they Work" these circuits are not intended to be duplicated
by the reader and there is virtually no information on how and where to
insert them into an amplifier. This page will attempt to rectify that
situation.

Where should tone controls be placed?
The answer to that depends on
whether you are building or modifying an integrated amplifier or a preamp
that is used with a separate power amplifier or monoblock power amplifiers.
Also there is no one correct answer and there can be many variations on the
theme and no one can definitely say that some of them are wrong. However,
there are a few don'ts and they will be pointed out.

Volume and Balance Placement.
Although I have seen it done in
commercial semiconductor amplifiers I don't recommend placing the volume and
balance pots one after the other in a tube amplifier. Figure 1 below shows
what I am talking about.

Figure 1 Diagram Showing Parasitic Capacitance Associated
With
Volume and Balance Controls.

For a verbal description click
here.

In my ill-informed youth I did connect a balance control pot
right after a volume control. The loss of highs was obvious so I never
did that again.
Due to its construction a pot has quite a lot
of stray capacitance to ground. This is indicated by the capacitor in red
in the diagram of Figure 1. The highest impedance is seen at the wiper when
the control is set to half resistance.

Note. An audio taper pot has 10% resistance at half rotation.
The half resistance point for such a pot is near the top, somewhere
between the 3 o'clock and 4 o'clock positions.
When 10 k ohm
pots are used, as in transistor amplifiers, this is not a problem. I
measured a typical pot to have a capacitance to its case as 30 pf. Two pots
together would have 60 pf and add another 90 pf for shielded wiring and tube
capacitance and you have 150 pf. If two 500 k ohm pots are used the corner
frequency is 10,600 cycles. Remember that is the -3 dB point. The response
will be down by 1 dB at 5,500 cycles. That would give it a clearly audible
and objectionable effect. There should always be at least a tube stage
between the volume and balance controls.

Tone and Volume or Balance controls.
Can we place the tone and
another level control pot, volume or balance, next to each other? Well,
that depends. An active tone control, I'll define that later, by definition
includes tube stages so it wouldn't matter. A passive tone control, no
tubes, would be another story. Such a circuit with two pots and wiring to
and from them is going to present a lot of capacitance, easily 150 pf. That
lets out putting a passive tone control after a volume or balance control.
What about placing volume or balance after a passive circuit? Lets take a
look at that.

Figure 2 Passive Tone Control.

For a verbal description click
here.

Whether we can place a pot with
its own capacitance plus wiring capacitance after a passive tone control
depends on the output impedance of the tone circuit. Because of the many
capacitors in the circuit its output impedance is going to change a lot with
frequency. But because high frequency roll-off is what we are concerned
with, the high end is what we need to look at.

Consider the circuit of Figure 2. When the treble control is set flat,
center of rotation it will present 50 k ohms between wiper and CCW end. So
from the output to ground we have 50 k in the pot and a 0.0033 uf capacitor.
The capacitor has 4.8 k ohms of reactance at 10,000 cycles so the
resistance of 50 k ohms is mainly it.

Remember that an audio taper pot has 10% resistance at the
center of rotation.
Over on the bass side there are two
capacitors across the pot. The 0.018 uf across the bottom half of the pot
will effectively short it out at 10,000 cycles. So the impedance to ground
caused by the bass part of the circuit is 27 k ohms in series with 33 k ohms
which adds up to 60 k ohms. The output impedance is the parallel
combination of 60 k and 50 k which is 27.2727 k ohms, lets call it 27 k
ohms. This resistance when combined with 150 pf gives a roll-off frequency
of 39,300 cycles. The amplitude will be 1 dB down at 20,000 cycles. I have
been deliberately pessimistic with the amount of capacitance so the effect
should not be audible even to very young golden ears. Figure 3 is the graph
of the frequency response with 50 picofarads of capacitance connected across
the output. This is a simulation not the measured results from a real
circuit.

Figure 3 Frequency Response of Circuit in Figure 2.

For a verbal description click
here.

Source Selector.
Obviously you don't want the tone control to effect
just one source or want to have a separate tone control circuit for each
source. Same goes for volume and balance. The source selector always comes
first.

I can almost hear someone thinking "Hmmm. Separate tone,
volume, and balance for each source. That could have some real
advantages." It would, but you had better design your preamp with a big
panel with lots of room for the controls. My preamp has 12 inputs. If
you take away 2 for internal test signal generator and SCA decoder that
still leaves 10.

Volume and Balance Controls.
Usual practice is to place the volume
control right after the source selector although There is no reason not to
do it differently. The usual rationalization is to be able to reduce the
signal level right away to prevent overloading early stages. This design
procedure was developed more than 50 years ago when input sources might not
have been as well standardized as they are today. I can see definite
advantages in putting the balance and tone controls ahead of the volume
control. Lets look at a few block diagrams shown in Figure 4.

Figure 4 Three Alternative Block Diagrams of Preamp or Integrated
Amplifier.

For a verbal description click
here.

Block diagram (a) is the standard
one and is in fact used in the Harmon Kardon A-300. The tube amplifier
after the balance control is the first tube in the global feed back loop.
This design could also be used in a stand-alone preamp because it ends with
a tube amplifier which could include a cathode follower or two stage circuit
with feedback to obtain a low output impedance.

Diagram (b) has the volume and balance
controls interchanged. The balance control may have anywhere from 0 to 6 dB
of attenuation depending on what balance circuit you plan to use. Look at
balance
circuits to find out more. These circuits are presented with values and
enough information to make them usable in DIY projects. Any tubes that are
between the balance and volume controls, including those in an active tone
control, if used, must be designed so as not to overload at the highest
signal level from any source.

Figure (c) could be used in an integrated amplifier in which the
amplifier right after the tone control would most likely be the first stage
in the global feedback loop. If it were used in a stand-alone preamp the
tone control would have to be an active one or another tube stage would have
to be added.

A preamp has to be connected to the power amplifier with
shielded cables. These have a lot of capacitance to the tune of 30 or 40
pf per foot. A 3 foot cable could have 120 pf of capacitance and this is
not highly pessimistic. To avoid having a corner frequency of less than
100 kc the output impedance would have to be less than 13 k ohms.

Passive Tone Control.
A passive tone control is defined as a network
of resistors, some of them variable, and capacitors, that will function
properly as a tone control in the absence of any amplifying device.
Something else that distinguishes a passive tone control is the use of audio
taper controls. This is very important. If you use linear controls you
will find that the bass and treble are boosted and you will only get it to
flat by setting the controls at 1/10 of their rotation. The cut will be in
the remaining 10% of rotation.

The Baxandall tone control circuit above fits this definition. We have
already established that it's output impedance is low enough to drive a
somewhat capacitive load without audible roll-off. Its input impedance
would be about 190 k ohms, worst possible case at the high end. Any tube
would not have any difficulty driving this impedance.

Modified Harmon Kardon Circuit.
In the original HK amplifier the
circuit diagram shows no resistor between the two wipers of the bass and
treble pots. The circuitry is inside one of those packaged circuits but a
test with an ohmmeter shows zero resistance between the two wipers. This is
what is responsible for a rise in the low band around 200 cycles. I added
the 33 k ohm resistor to bring it into compliance with what most people
expect from this type of circuit. See Figure 2 above.

The passive tone control has very close to 20 dB of attenuation when set
flat. The output voltage will be 1/10 of the input voltage. This is
because resistors and capacitors alone can't provide any bass or treble
boost. When boost is cranked in the bass or treble is attenuated less. The
ideal case would be to precede this tone control with an amplifier that has
exactly 20 dB of gain as shown in Figure 5 below.

Figure 5 Passive Tone Control with Makeup Amplifier.

For a verbal description click
here.

Figure 6 Frequency Response of Circuit of
Figure 5.

For a verbal description click
here.

The simulator steppes the bass
and treble pots together in 10 dB steps. For example in a 500 k ohm audio
taper pot, which most of the circuits use, the tone flat point is with the
wiper at 50 k ohms which is 10% or 20 dB down from fully clockwise. This
setting is always the center one, of the 5 curves on the graph. The one
below flat is 15.8 k ohms and the bottom one is at zero. Above the flat
curve the setting is for 158 k ohms and the top curve is fully clockwise,
500 k ohms.

The 12AX7 stage has a gain of 10 so the overall gain is unity or 0 dB.
The treble range might not seem like much control but the ear is very
sensitive to small changes in the treble band and I seldom set the control
anywhere near its extreme. The only exception is when I play a 78 RPM
record and I turn the treble all the way down. The 270 k ohm resistor
became a 200 k because the tube stage has some resistance in its output. I
know it should be a 220 k ohm but 200 k gives the flattest response when the
bass is set to mid rotation. The simulation was run with 50 pf connected
across the output.

The 500 k ohm load on the output is necessary to give a truly flat
response at center of rotation. If you are sending the signal from the tone
control to an amplifier stage, a 510 k ohm resistor should be placed from
grid to ground. This is necessary, for the above reason, even though the
Baxandall circuit contains a DC return. If you are placing a volume or
balance control after the tone control it should be 500 k ohms and the wiper
should go to the grid of a tube with no additional resistors to ground.

RCA Tube Manual Circuit.

Figure 7 Passive Tone Control From RCA Tube Manual.

For a verbal description click
here.

Figure 8 Frequency Response of Circuit
of Figure 7. For a verbal description click
here.

Figure 7 is also a passive
circuit. The values are quite large and that gives it an increased
sensitivity to capacitive loading. That is why it is preceded by and
followed by a tube amplifier. Even with that, the natural stray capacitance
causes the roll-off you see in the graph below.

If you are incorporating this circuit into an existing preamp you should
modify the existing tube circuits rather than adding these amplifiers after
and before existing stages. If you are designing from scratch the volume or
balance control would go before the first amplifier and the other control
after the output stage. As you can see even with the tube to isolate the
tone control from capacitive loads the flat curve is rolled off to -3 dB at
20,000 cycles. Personally, I would never use this circuit in an audio
project.

Not so many tubes.
If you would like a more robust control than the
HK but not so many tubes as the RCA then you can combine them. Take the
passive part from Figure 7 and drop it in place of the parts in Figure 5.
Now divide all resistor values, pots too, by 2 and multiply all capacitor
values by 2. The value closest to 50 k ohms is 51 k ohms. 5 k ohms, 5.1 k
ohms. The capacitors should be 0.043 if you can get them. If not, 0.047
uf will have to do. This goes for all of the "22" capacitors. You will
have a control with the large boosts and cuts of the RCA circuit but one
that will drive the balance or volume control directly without an additional
tube circuit.

Inserting a tone control between preamp and power amp.
Do you want a
totally passive circuit to connect between an existing setup that is made up
of a preamp and a stereo power amplifier or two monoblock amplifiers?
Someone who wants to do this likely doesn't want any tubes in the tone
control, just a little box with 2 or 4 knobs on it to control the tone.

Many power amplifiers have lots of sensitivity. One I have heard of
needs only 0.15 volts to drive it to full power. However, even if yours is
specified to require 1 volt for full power you still will have plenty of
gain left over. Who plays their amplifiers at full power? Kids in cars do
but at home the adults I know have higher regard for the health of their
inner ears to do that.

Most preamps are designed very conservatively to drive a long shielded
cable. To give it that capability the output impedance is quite low. So it
can drive a relatively low impedance circuit. Knowing that we are going to
connect the tone control box to the power amp with shielded cable we must
give the tone control circuit a low enough impedance so the highs won't be
rolled off by the cable capacitance. Figure 9 is one channel of a circuit.
The other channel looks exactly like it. The pots may be ganged or separate
according to builders taste.

Figure 9 Passive Tone Control for Insertion
Between Preamp and
Power Amp.

For a verbal description click
here.

Figure 10 Top, Frequency Response of
Circuit as Shown. Bottom,
Frequency Response Without Fixed Resistors
in Treble Arm.

For a verbal description click
here.

You will notice the addition
of two resistors. These will ensure that the treble cut will shelve instead
of falling out of site. The top graph in Figure 10 was made with the
resistors in place and the lower one with them set to zero ohms in the
simulation. It may be argued that it really doesn't make any difference
since it all happens above 15 kc. Some people might want to bring the break
points of the treble control down to a lower frequency. If this is done,
shelving could become important. To lower the treble frequencies increase
the values of the capacitors above and below the treble pot. Remember to
keep them in a 10 to 1 ratio.

If you find that the bass and treble boost and cut are indeed more than
you need you could increase the input impedance by increasing the four fixed
resistors that are above and below the bass pot and above and below the
treble pot. Make the 10 k a 22 k and the 1 k a 2.2 k ohm. This will reduce
the bass boost and cut by 6 dB but it will raise the input impedance to 24 k
ohms.

At low frequencies where the value of the output coupling capacitor may
become important the tone control will present a higher impedance load
because the reactance of the two capacitors across the bass control has
increased. The 0.022 uf capacitor has a reactance of 145 k ohms at 50
cycles. The impedance will decrease some as the bass is boosted.

The simulation was run with a capacitive load of 105 pf which would be
the approximate capacitance of a 3 foot shielded cable.

Active Tone Controls.
Now we are up to active tone controls. They
are defined as needing an amplifier to work properly. They also use linear
controls. The advantage of linear controls is they always have half
resistance at half rotation. Some audio taper controls do not have 10%
resistance at half rotation. With active controls it is easier to be sure
that setting the controls to center of rotation will give tone flat. It is
still the basic Baxandall circuit but it is in the feedback loop of an
amplifier. Transistor designers jumped on this one early on because gain is
so easy to come by in a small space as compared to tubes. It is doable with
tubes but might require one extra 12AX7 per channel as compared to passive
circuits. Many people might say that the improved symmetry of response
curves is worth it. I would be inclined to agree.

The Brimar Circuit.
Brimar was the name of a tube manufacturer in
England who came up with the circuit shown in Figure 11.

Figure 11 Brimar Tone Control Circuit.

For a verbal description click
here.

Figure 12 Frequency Response Showing a
Flaw.

For a verbal description click
here.

This circuit looks quite
elegant on paper, or computer screen, but the frequency response shows a
real problem. Look at the flat gain in the bass portion and compare it with
the same curve in the treble region. I imagine this would give the
amplifier a rather dim sound, the opposite of bright. The tube on the right
has a lot of feedback around it but the one on the left is operating
completely open loop and at full gain with a bypassed cathode resistor. The
output resistance of this stage is the 100 k ohm load in parallel with the
plate resistance of the tube (12AX7) which is 80 k ohms. That works out to
44 k ohms. The graph in Figure 15 below shows how the impedance of a
similar circuit varies with frequency. This problem could be cured by
adding a cathode follower to each amplifier stage.

In spite of its other shortcomings you will note a much improved symmetry
of the frequency response over the open-loop Baxandall tone control. This
makes for a much smoother cut and boost as well as identical spectra for
both.

The circuit has 33 dB of gain at tone flat and 100 cycles. Removing the
cathode bypass capacitor only lowers this figure to 25 dB. That's still a
lot of gain to stick into the middle of a preamp. I have never owned a
commercial preamp but my design approach would be to give it 10 dB of gain
with the volume control set to maximum. The argument being, the output
stage should not be driven into clipping even at maximum volume and tone
setting. Most sources deliver about 0.7 volts RMS which is about 2 volts
peak to peak. 10 dB will increase this to 6.3 volts peak to peak. Granted
that's not much for a tube amplifier but if the owner decides to crank in
full bass boost at 15 dB more we are up to 35 volts P-P. That's certainly
at a level which could increase distortion if not taking it to the point of
clipping. So dropping this control into the middle of a preamp would
increase the output to a whopping 623 volts peak to peak. If you are still
awake you will remember that I put in an initial gain of 10 dB. If we take
that out and just have the tone control with the cathode bypass removed, 25
dB of gain from the starting point of 2 volts P-P gives us 36 volts and then
the 15 dB of bass boost puts the overall gain at 40 dB which will give 200
volts P-P from our original 2 volts P-P. No matter how you slice it, it
seems to be too much gain. A figure of 10 dB gain plus 15 dB of bass boost
will give 36 volts P-P from our original 2 v P-P. So I think I'll stick to
my 10 dB overall gain figure.

Putting the Baxandall Inside a Feedback Loop.
The curves of the open
loop Baxandall circuit may seem a little less than perfect. I would be the
first to grant that it probably would be impossible to hear the difference.
However, I am somewhat of a perfectionist so it would be nice if a circuit
could be developed that behaved better than the passive circuits above. It
seems that if we put the passive circuit inside of the feedback loop of an
amplifier it will give curves that look much better.

The Principle of an Inverting Amplifier.
To understand how an active
tone control works we must digress a little to study inverting amplifiers.
The inverting amplifier has several unique features but the one that is
important to us is the ability to employ feedback to set it to less than
unity gain. The noninverting amplifier can be set to unity by strapping the
output to the inverting input. The gain can't be set any lower than that.
The inverting amplifier, in Figure 13 (a) below, does not have this
limitation.

Figure 13 (a) Showing How Feedback Sets Gain.
(b) Showing
Tone Control in Feedback Loop.

For a verbal description click
here.

In the circuit of Figure 13 (a) ,
a generalized amplifier has feedback applied from output to inverting input
through R2. The input signal is applied through R1.
If the gain of the amplifier is large the voltage from the inverting input
to ground will be small. That means the currents through R1 and
R2 are set mainly by the magnitude of the input and output
voltages. If the input impedance of the amplifier is large the current
flowing through the input terminal will be very small. Thus we can say that
the current in R1 is the same as the current in R2.
We can write the equations,

VIN = I1 x R1     and
VOUT = I2 x R2

But,

I1 = I2

Because the currents in R1 and R2 are
equal and in the same direction and VIN is on the left end of
R1 and VOUT is on the right end of R2 the
input and output voltages have to be of opposite signs.

Lets try some simple logic on that one. Now, we must be sure to say
exactly what we mean and mean exactly what we say. In part A of Figure 13
above, the term, input refers to the left end of R1. Output
refers to the right end of R2 which is also the amplifier output.
Inverting input refers to the point where R1 and R2
connect to the amplifier. Remember that the voltage at the inverting input
of the amplifier is very small. Lets call it zero for the sake of argument.
If you place a positive voltage on the input the only way to get zero at
the inverting input is for the output to be negative. The inverting input
is a tube grid so it is an open circuit. You can't have both ends of the
resistor string be negative and have the middle be zero. They must always
be opposites. Now, if the input signal is AC the output will be AC of the
opposite phase. And that, Virginia, is why its called an inverting
amplifier.

We are going to solve the two equations above for I, since they are
equal, and set them equal to each other. But because of the sign difference
between input and output we have to sneak in a minus sign.

VIN / R1 = -VOUT / R2

Rearranging gives,

VOUT / VIN = -R2 / R1

Of course VOUT / VIN is the gain. If
R2 is larger than R1 the gain will be greater than
unity. If R2 is less than R1 the gain will be less
than unity.

One more little fact about an inverting
amplifier is that the inverting input acts like a low impedance point. If
the gain of the amplifier is very high, say one million, the input will act
like a dead short to ground. The reason for this is, what ever the input
voltage does the output voltage will be adjusted to the opposite sign and
the necessary magnitude to keep the inverting input at zero. There is
current flowing but no, or very little, voltage and that spells low
impedance. In practical operational amplifiers the inverting input point,
or summing node, is often referred to as a virtual ground. For more
information on the subject of feedback amplifiers refer to Chapters 5 and 6
of my textbook Electronics
for Physicists. The book is on another web site so use your back button
to return here.

Now look at the schematic of Figure 13 (b) above, in which the Baxandall
circuit has taken the place of R1 and R2. Strange as
it may seem there is virtually no current flowing in the 15 k ohm resistor
or the 6.8 k ohm resistor.* Looking back at part A, the current due to
signal is flowing through R1 and R2. There is
essentially no current flowing in the vertical line connecting the junction
of the two resistors to the inverting input. A resistor could be placed in
this line with no measurable effect on the performance of the amplifier.
The one restriction would be that the added resistor must be much less than
the input impedance of the amplifier. In part B, the input resistance of
the amplifier is much greater than the 15 k ohm resistor. At frequencies
where the reactance of the 0.001 uf capacitor becomes large the treble
control is no longer having any effect so this is of no consequence.

* This statement is true only if the two controls are set to
the same point. If there is a difference the circuit becomes more
difficult to analyze and we will leave such analysis for another time and
place when and where everyone understands how to use the j operator.

The wipers of the pots are at zero potential so if they are set
at center, the connection points on the left and right are at the same
potential and opposite phase. This is equivalent to R1 =
R2 in part A. The gain is unity for all frequencies. If the
wipers are moved to the left this is equivalent to R1 growing
smaller and R2 growing larger. This will increase the gain but
only for lows and highs. The mid range, around 1,000 cycles will be little
effected. Moving the wipers to the right of center will cause the gain to
be less than unity and the highs and lows will be cut.

If the bass control is moved to the left while the treble control is left
at the center, the unbalance will drive current through the 15 k ohm
resistor at low frequencies. The effect will be that of decreasing
R1 and increasing R2. The gain will be increased for
low frequencies. At mid and high frequencies the two 0.1 uf capacitors will
keep the circuit balanced, R1 = R2, and the gain for
these frequencies will remain at unity.

When the bass pot is returned to center and the treble pot is moved to
the left the wiper becomes unbalanced for all frequencies but only the highs
are coupled to the inverting input through the 0.001 uf capacitor. A
circuit with two capacitors, one on each side of the treble pot performs
better and is easier to understand. I have not run a simulation on the
circuit in part B so I can't say how well it performs. It could be used
with op amps or transistorized gain blocks but it's impedance is too low to
be used with tubes. Well, maybe with cathode followers.

A Tone Control You Can Love.
Figure 14 shows the tone control that I
think has the best compromise between performance and tube count. Yes
Virginia, everything is a compromise. It uses only two triodes per channel,
one 12AX7 or your choice of other hi mu triodes, and it has a nice
symmetrical looking frequency response curve. Its gain when set to flat is
about -1 dB which is close enough to unity to not cause overload problems in
a preamp or integrated amplifier. You could transplant the tube stages from
Figure 18 below if inclined. The performance would be slightly improved.

Figure 14 Practical Tone Circuit Using One 12AX7 Per Channel.

For a verbal description click
here.

The pots are linear taper and the
center of rotation is tone flat. Moving the wipers to the right on the
diagram is cut, which would be the counter clockwise position, and to the
left is boost, the clockwise end of the control. All resistors are 1/4 watt
although you may use 1/2 watt if you prefer. The 100 k ohm resistor from
pin 6 of the 12AX7 should be a 1/2 watt in any case. The 0.47 uf capacitor
on the left may have a voltage rating as low as 200 volts but the one on the
right should be a 630 volt. The voltage rating of the 0.047 uf depends on
what is connected to the input of the circuit. If it is the plate of a tube
the voltage should be 630 volt. If the wiper of a pot a 200 volt would be
fine. The three capacitors in the tone circuit don't need to be anymore
than 50 volts although there is no harm in going much higher if they are all
you can get.

Unlike the passive circuit this one uses a single capacitor across the
bass pot instead of two from each end to wiper. If this were to be done in
a passive circuit setting of the bass control would alter the amount of
treble boost or cut. The virtual ground of the amplifier grid effectively
isolates the bass and treble circuits and the interaction amounts to about
1/100 of a dB.

This was not done to save capacitors. Connecting two capacitors from
wiper to each end causes the resistance in parallel with the capacitor to
change as the bass pot is turned. This causes the corner frequency of the
bass boost and cut to change depending on the setting of the pot. This
effect is clearly visible in Figure 10. Figure 15 looks much better as the
breakpoints on the bass curves remain constant regardless of the setting of
the control.

Figure 15 Graph Showing Frequency Response, and Drive Impedance
of Circuit of Figure 14.

For a verbal description click
here.

The graph at the bottom in
green is k ohms of input impedance of the tone circuit. The measurement was
made by inserting a current controlled voltage source in series with the
0.47 uf capacitor on the left. The bottom line is full boost, the next one
up is half boost and the highest one to be completely on the graph is flat.
Now you can see the reason for the gain variation in the Brimar circuit
above. With an impedance of 80 k ohms at 100 cycles and 45 k ohms at 10 kc
it's no wonder why a cathode follower is required to drive the circuit. The
amplifier on the right would have the same trouble if it didn't have so much
feedback around it. It's open-loop gain does change but not enough to cause
any significant problems.

Three Band Tone Control? Maybe.
The circuit of Figure 16 was sent
to me by a gentleman who asked me to evaluate it for him. It turns out to
be useless as a tone control but I am posting it for what it has the
potential to become, which is a 10 band graphic equalizer using tubes. Each
channel would require 6 and 1/2 12AX7s which may or may not look feasible to
you.

Figure 16 Three Band Tone Control? Or Unfinished 10 Band Equalizer?

For a verbal description click
here.

Moving the wipers to the bottom in
the diagram gives full cut and to the top full boost. The 110 k ohm and 120
k ohm resistors should definitely be 1 watt. The two 100 k ohm resistors in
the plate of one of the 12AX7s should be 1/2 watt and all other resistors
may be as small as 1/4 watt. All capacitors should be at least 200 volt.
The frequency determining capacitors aren't exposed to any appreciable
voltage during operation but if B- comes up before the tube cathodes are
hot, these capacitors would feel 150 volts.

Figure 17 Frequency Response of Each Band and Then All Three.

For a verbal description click
here.

The first graph shows the
effect of setting the low band pot to full cut, half cut, flat, half boost,
and full boost. The other two are not changed but left set flat. The
second graph shows the effect of the mid band pot with all others held
constant and the third shows the effect of the high band pot. The fourth
shows all three being changed simultaneously. The equation for the
frequency is,

f = 1 / (6 Pi R C)

Where R is the 10 k ohm
resistor and C is the value of the capacitors connected to it. The values
of capacitors for a 10 band EQ would be as listed below.

Frequency (cycles)

Capacitor
Standard Value

Capacitor
Calculated

31.5

0.18 uf

0.168 uf

63

0.082 uf

0.0842 uf

125

0.039 uf

0.0424 uf

250

0.022 uf

0.0212 uf

500

0.01 uf

0.0106 uf

1,000

0.0056 uf

0.00530 uf

2,000

0.0027 uf

0.00265 uf

4,000

0.0012 uf

0.00133 uf

8,000

680 pf

663 pf

16,000

330 pf

332 pf

A 3 Band Tone Control You Can Love.
The circuit shown in Figure 18
originally came from a book of op amp circuits. I increased the impedance
level to make it suitable for tubes and did quite a lot of reengineering on
it. The pin numbers on the left hand triode are for half of a 12AU7. If
you elect to use a 6C4 the pin numbers will have to be looked
up on line or in a tube manual. If you would like to use one less
triode per channel you could transplant the amplifiers from Figure 14 above.
You might notice a slight reduction in performance. All resistors are 1/4
watt unless otherwise noted. All the capacitors that are associated with
tubes are 200 volt. The exception is the 50 microfarad which is a 25 or
15 volt unit. All others can be as low as 50 volt if you can find them
that low. The two 1 uf caps and the 0.047 uf cap are the only ones that
will have any DC voltage across them. The 0.047 uf cap may need to be a 630
volt if there is another tube stage connected to the input.

Figure 18 Practical 3 Band Tone Control.

For a verbal description click
here.

Figure 19 Frequency Response of 3 Band
Circuit with Controls Set In Various Combinations.

For a verbal description click
here.

The first of the four graphs
above shows the mid band control being adjusted while the bass and treble
are left set flat. The second shows the bass and treble being adjusted
while the mid band is left flat. The third shows the bass and treble being
adjusted while the mid band is set at full boost. The fourth shows the bass
and treble being adjusted while the mid band is at full cut. The mid band
may not seem to be having much effect but the ear is so sensitive in this
frequency range that a little goes a long way. At some time in the past I
owned a receiver that had a three band tone control of this type. A friend
who owned the same model plotted the frequency response curves of the tone
control and they looked very much like these. The mid band control had a
clearly audible effect.

Alternative Amplifier Circuit.
As stated above the amplifier
circuits can be switched around from one control to another depending on how
much of a perfectionist you are. Figure 20 shows an amplifier circuit
contributed by Mike from the Fun with Tubes email list.

Figure 20 Another Set of Amplifiers for Figures 14 and 18.

For a verbal description click
here.

The output stage is borrowed from
the phase inverter circuit used in a series of amplifiers found in RCA tube
manual RC-28. This gives higher gain than a triode along with low output
impedance. The cathode follower keeps the tone circuit from lowering the
gain of the amplifier unlike the one used in Figure 14. Using a pentode as
the input cathode follower will give a lower output impedance because the
tube has a higher transconductance than a triode.

Well, that's about it. I hope this gives you something new to have fun
with.

Contrary to popular belief, a cold 807 is not a burned out tube.

Home.

This page last updated January 23, 2007.