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John
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23 June 2013

Missing Sonic Controls

The underlining theme of the Tube CAD Journal since
it was born, even when the topic was auto-bias circuits or I-to-V
converters, has been regaining sonic control. Recapturing (or introducing
influence over over our sound systems has always been implicit and
sometimes explicit in all that I have written, such as in my last
post on harmonic restoration. Just over ten years ago, I wrote a… I am
not sure what to call it, as it was certainly too long and structured to
be a typical blog post and the label "article" just doesn't feel right;
too redolent of the stuff that fills glossy magazines. The label "essay"
is probably best suited to the task, but it has for many readers a fussy
connotation, the still painful memory of school assignments.

Fortunately, extremely fortunately, I began reading
essays before I was forced to do so, which allowed me to sidestep the
usual academic inoculation against essays that most receive in school.
Much like the routine inoculation against poetry and Shakespeare that most
young students receive, a small dosage of essays by Francis Bacon or
Emerson is forced on the unwilling child, so that later in life he has
grown immune to the essay's rewards and pleasures. Indeed, for me, few
things beat a good essay. They are bite-sized and cerebral nourishing.
They are like having a ten minute conversation with someone much smarter
than yourself. In contrast, few things irk me more than full-length books
that should have been only thirty pages of a tightly written essay.

The dictionary tells us that an essay is a short
literary composition on a single subject, usually presenting the personal
view of the author. There were essays long before the name "essay" was
applied to them, just as there were philosophers and scientists before the
words "philosophy" and "science" existed. The altogether brilliant Michel
Montaigne (1533–1592) was the first to label his short, personal,
tentative, probing, reflective writings as "essays."

But my abiding respect and admiration of Montaigne only
makes me more hesitant to categorize my hasty missives as "essays;" thus,
my present preference for the ugly and ill-defined word "blog." Well, the
piece that I wrote over a decade ago should be called an essay, not a blog
post, in my view, as it is pretty darn good. The topic was the missing
sonic controls. Missing from where? Why, from our stereo systems, where
else? If you haven't read the essay, Missing
Sonic Controls, I recommend that you do now, as I don't plan on
restating the same material here. Here is how I put it back then:

"In high-end audio, it is human
beings who were created for audio equipment and it is they who must
conform to its demands. Thus, balance controls are rare; tone controls,
nonexistent; so too, stereo/mono switches and, heaven forbid, equalizers
or loudness controls. It is as if the Puritans' renunciation of shiny
buttons has been one-upped by the elimination of buttons
altogether."

Missing
Sonic Controls 1999

Anyone who has read my audio ramblings and
seen my PCBs that hold dual, separate coupling capacitors and the
accompanying capacitor-selector switches that allow choosing between them,
knows that I promote as much flexibility and choice as possible. Why?
Besides my intrinsic libertarian inclination, I know that musical
recordings and musical tastes and listening moods differ wildly; and that
one size can never truly fit all. I like orange juice in the morning and
port at night—never the reverse. And late at night, smooth sound usually
wins over the bright and detailed, but not always. If the music is simple
and uncluttered, I quite like the burnished softening that some coupling
capacitors impart; but with complex, instrument-filled music, I prefer a
more detailed sounding coupling capacitor.

But the choice of coupling capacitor only imparts
something of a change in texture, not an actual shift spectral balance.
Yes, some coupling capacitors sound bright and other sound dull, but both
will measure flat in frequency plots. Changing the frequency balance
between lows and highs requires more than just part substitutions: it will
require true frequency equalization.

Tilt Control

The tilt control imposes a shelving function, which attenuates half of
frequency band and augments the other half. In other words, it is special
type of a tone control that, unlike the typical tone control that boosts
or cuts just the highs or mids or lows, shifts both highs and lows at
once.

As tone controls are rare and as the tilt control is a
special type of tone control, it is very rare. The only piece of hi-fi
that held one that I know of was the lovely Quad 34.

At the left is a bass boost/cut control; in the center,
a tilt control; and at the right the volume and balanced controls.
Fantastically elegant, don't you think? Here is what Ken
Rockwell had to say about it:

"The Quad 34 preamp excels
in having the world's most useful tone controls which allow a listener
to choose his favourite seat in the hall, regardless of where the
recording's producer thought he should sit.

The Quad 34's most astounding innovation is its
TILT control. The TILT control is an eight-pole filter which allows
subtle optimization of the relative brightness or warmth of the music's
original performance space to your listening room, without adding any
colouration. Unlike conventional tone controls, its effects are very
subtle — and therefore very powerful."

The tilt control used in the Quad 34 preamp was an active affair that
relied on a negative feedback loop and a high-gain amplifier to make the
frequency-selective tilt possible.

When the potentiometer is centered, no tilt occurs. As the
potentiometer is adjusted to the right, the highs go up and the lows go
down. As the potentiometer is adjusted to the left, the lows go up and the
highs go down. The math behind selecting the resistor and capacitor values
is scary looking but it's not all that difficult. The EE web site,
www.edn.com, has a good article on designing
a Quad tilt control by Mr. Francesco Balena.

Most would use an OpAmp or a discrete-parts, solid-state amplifier, but
a tube-based Quad tilt control could be made with either a CCDA or Aikido
gain stage. What about the grounded-cathode amplifier or an SRPP circuit?
I wouldn't use them, as their output impedance is too high, which will
interfere with the tilt control's network of capacitors and resistors,
throwing off the results.

In fact, the Quad tilt control circuit must be driven by a
low-impedance signal source, such as a CD player or stand-alone DAC, not a
volume potentiometer or a high output impedance tube circuit.

One problem with the above circuit is that it offers
too much boost and cut; and our goal is subtle shifts in tonal character.
The following variation uses two extra resistors (37.4k) to limit the
maximum boost and cut to about 2.5dB.

An even better solution is to not use a potentiometer
at all. Why not? Potentiometers, even the best of them, stink. They become
noisy; they break contact; they don't track between sections, they don't
center. The alternative is to use a many-position rotary switch. A near
perfect choice would be the Elma 11-position, dual pole, hard-gold rotary
switch that I used in my A4
attenuator and in my stereo
11-position series stepped attenuator. Sadly, I haven't had the
courage to buy more Elma switches, as I would be required to purchase
thousands of dollars' worth of them.

The switch holds two poles, so only one is required for
stereo use. With the following resistor values, 0.5dB steps are possible.
The 37k resistor at the right must be set off the PCB, as the PCB only
holds 22 resistors (11 per channel).

Another possibility is using the GlassWare
Trim-1 control, which is a mono series stepped attenuator, with 11
positions. Because it is a mono design, one per channel would be needed,
but then it cost a
fraction of what the Elma-based attenuator costs.

Using either switch in this setup yields the following
graph. The center position is wonderfully flat. In addition, the output is
unity gain, but inverted.

Now, if no gain is offered by this circuit, would this
make a good line-stage amplifier? Probably not. To be immodest, the Tetra phono
stage might make a wonderful test bed for this circuit, as the four
CCDA gain stages could be put to good use. For example, one stage, the
last stage, can be used as the tilt control; and the other stage can be
used as a gain amplifier, with the added chance to impose more sonic
control.

In the Missing
Sonic Control essay and in blog
number 33, I covered simple sonic width control that would allow
adjustment of the perceived stereo image width, making it either wider or
narrower.

One complaint that I often hear about headphone
listening is that it offers too much stereo separation. True enough. But
then, some recordings could benefit with more separation, even with
headphones.

If the input triodes' plates are tied together through
a resistor, the two channels will blend together, creating a narrower
stereo image. Conversely, if the input triodes' cathodes are tied together
through a resistor, the two channels will pull apart, creating a wider
stereo image. Once again, a rotary switch would be ideal, say one with
five positions and four poles. At the center position, neither plates nor
cathodes combine. As we click clockwise, the cathodes would find less and
less resistance between them. As we click counterclockwise, the plates
would find less and less resistance between them. Why four poles and not
just two? Two cathodes and two plates equals four connections.

Putting it altogether and adding an isolation
transformer at the input, just after the selector switch, would allow
picking the phase and create something like the following, which is more
than just a line-stage amplifier, being closer to an audio palette.

Click on image for closeup

Next Time
I will
write about the new passive GlassWare Tilt control. Why not now. There's
that trip I am going on and I haven't finished the user guide for it yet.
But here is a sneak peak.

//JRB

Kit User Guide PDFs
Click
image to download

E-mail from GlassWare Customers

Hi John,

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shipping speed.
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I'm sure you could charge double and still have happy customers.

Looking forward to building the Aikido, will
send some comments when I'm done!
Thank you, regards

Gary

Mr Broskie,

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Firstly, I'm extremely impressed at the quality of what I've been sent.
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you.

Sean H.

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