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09 August 2014

Cathode-Coupled Amplifier

Cathode-Coupled
Amplifier
I have been writing about the cathode-coupled
amplifier for a long, long time now and I have been asked for a PCB based
on this circuit many, any times. Well, the wait is over.

1999/tubecircuits/Common-Cathode
Amplifiers

1999/tubecad.com/july99/page16
2005/April/blog
42

2007/05/blog 105

2007/05/blog 106

2007/05/blog 107

2007/05/blog
108

2008/11/blog
152

2010/11/blog
194

2011/11/blog 219

Back in blog
number 245, I revealed a clever variation on the
cathode-coupled-amplifier that fixed three of its problems: dissimilar
cathode-to-grid voltages between the two triodes, very poor PSRR, and a
high output impedance. My remake solved the cathode-to-grid voltage issue,
greatly enhanced the circuit's PSRR, and drastically lowered its output
impedance. In a nutshell, the new circuit consists of a modified
cathode-coupled input stage that uses an extra cathode resistor to
equalize the current drawn by both triodes and eliminate the need for
dissimilar grid voltages; the cathode-coupled input stage's output
terminates into a two-resistor voltage divider, whose output cascades into
a cathode follower's input. The result of these additions is that all the
triode cathodes rest near ground level at idle and the power-supply noise
falls out of the equation and out of the output, which now robustly
presents a low output impedance.

In the above schematic from blog
number 245, 6SN7 triodes are used throughout, but in the new GlassWare
Cathode-Coupled stereo PCB, three noval tubes are used. Why the
change? Two reasons, the noval tubes and their sockets are smaller and a
far greater number of different noval tube types are available. (Who
knows, maybe some time in the distant future, I will create an octal
version, which will be much larger, say 8 inches wide and 6 inches deep.
Do not hold your breath.)

No doubt, the first thought to cross your mind upon
seeing the above PCB layout is "Why are there so many more parts than in
the schematic?" The answer is that the PCB, being a stereo design, holds
two channels of cathode-coupled amplifiers; and I purposely used multiple
resistors in series and in series-parallel in place of single resistors.
Why? Two reasons: greater potential heat dissipation and less
voltage-induced distortion. Obviously, four 10k/1W resistors in
series-parallel will be able to dissipate four times more heat than one
10k/1W resistor, while still presenting 10k of resistance. What isn't
obvious is that they will exhibit less distortion. In general, we tend to
ignore resistor-induced distortion, and with good cause, as it tends to be
very small indeed—but not altogether absent. Under extreme current flow or
high voltage, the resistor's distortion increases. With typical
tube-based, small-signal circuits, which usually run in class-A, it's the
voltage-induced distortion that we need to worry about.

In the above schematic of the new GlassWare
Cathode-Coupled PCB, we see the input stage's and output stage's cathode
resistors replaced with a four resistor combination in series-parallel, R4
- R7 and R16 -R19. In addition, the two-resistor voltage divider is made
up from three resistors, R9, R12, and R13.

Diodes D1 and D2 are protection devices that conduct
only at startup, when the tubes are cold and haven't begun to conduct
current—or when a tube is missing from its socket. Without these diodes,
the cold cathode-coupled input triode would see the full negative rail
voltage on its two cathodes and 0V on its grid; not good. And it would be
even worse for the cathode-follower output tube, as its cathode would be
at the negative rail voltage and its grid at half the positive rail
voltage! One hell of a wake-up call. Imagine a bipolar power supply of
±200V, so the output triode's cathode-to-grid voltage would be 300V! So,
what's the big deal? The big deal is that the electrostatic force between
the cathode and grid, which are nearly touching, would be so great that
chunks of the cathode coating would peel away and fly up to the plate. Not
good. But once the tube are hot and conducting, the diodes become reversed
biased and cease to conduct, thereby falling out of the circuit.

My signature two sets of coupling capacitors are
present on the PCB. Why? I will say it again: I love being able to exert
some sonic control on my music playback. All coupling capacitors exhibit
slightly different sonic overlays. Many recordings are either too bright
or too thin or too dull or too fat sounding. But by using a coupling
capacitor with an opposite sonic character, the result will be closer to
neutral. (In my own cathode-coupled build, I am using old Vitamin Q PIO
and 1kV CDE polypropylene coupling capacitors, but I plan on replacing the
polypropylene capacitors with some American Teflon capacitors, as I want a
bigger sonic contrast between the coupling capacitors.)

Sorry, John, but I am an absolute-sound kind of
guy; if a coupling cap isn't absolutely perfect, I don't use it.
Well, even if you have found the perfect, the very absolute, coupling
capacitor, you could still benefit from the two-coupling-capacitor setup.
How so? Use two values of the perfect coupling capacitor, say 1 µF and
0.33µF. Why? With a Select-C rotary switch, you could place both in
parallel for normal listening, but use only the 0.1µF coupling capacitor
for late-night or early-morning listening, when you do not wish to bother
others with deep-bass rumblings.

Or, imagine that some LP is so warped that it is
causing your subwoofers to do push-ups against the back wall; well, you
could choose the coupling capacitor that imposed a 30-Hz cutoff instead of
the bigger coupling capacitor that went down to 3 Hz, say 1µF and 0.1µF,
which would function as a rumble filter of sorts.

What you see above is my own implementation of the new
GlassWare Cathode-Coupled PCB. I drilled, punched, and painted a small
aluminum Bud box, 2in tall, 13in wide and 7in deep. Yes, it's cute—too
cute, alas.

The PCB was assembled with most of the parts on the
topside, such as the tube sockets and resistors, but all the capacitors
were soldered to the bottom side, as I like having the tube protrude from
the enclosure top, like three missiles leaving their silos.

The power supply is a PS-18
high-voltage, bipolar design with a regulated heater power supply.

I configured the heater power supply as a
voltage-doubler circuit, so that a 6.3Vac heater winding could develop a
regulated 12Vdc output voltage for the three 12AU7 tubes. The power
transformer that I used was the tiny Hammond
269AX
type.

Although this transformer is cute, I wish that I had
used a bigger transformer, as it makes too much mechanical noise. (Using
the 125Vac input tap, rather than the 115V tap, helped, and I will add
four long, non-magnetic screws to hold the assembly more tightly together,
but what a pain, as the transformer should have come with six screws, not
just two. I have such mixed results with Hammond transformers.)

As an alternative, Antek offers a 100VA, 400Vac CT
toroidal for only $34.50, an absolute bargain compared to the Hammond
transformer, the AS-1T200,
which also holds two 6.3Vac @3A heater windings, which could be wired in
series, thereby creating a 12.6Vac @3A secondary. The 400Vac CT rectifies
up into ±280Vdc, which the RC filters (C15 & C16 and R7 & R8) on
the PS-18
can drop down to ±200V or ±250V.

Interestingly enough, none of the three knobs on the
front of my design are a volume control. Instead, I used a GlassWare Tilt
Control, a Select
Phase, and a Select-C
switch. So where's the volume control? I didn't need a volume control, as
my DAC can control the volume on its own. I did, however, want more sonic
control. (I wish that I had room to add a stereo-mono blend control, as I
have been listening to many early Beatle recordings. Way too
much  sepa      ration.) Undoubtedly, 99% of
Hi-Fi builders would not use my arrangement, opting instead for an input
selector switch and a volume control.

Since this cathode-coupled line stage was meant to
partner only with my DAC,  only one input is present, the pair of RCA
jacks at the left. So what do the four RCA jacks on the right do? I need
two sets off output jacks, as I run two sub-woofer amplifiers. (I soldered
in place -6dB two-resistor attenuators on the sub outputs.)

The Tilt Control is a gas.
I only use it on one in ten recordings, but it makes all the difference
in my musical enjoyment. Most often, I tilt the sound towards the bass,
particularly with many 1970s recordings; but more than a few times, I have
tilted towards the highs. Well, I was recently reading an excellent
comparative review by John L. Wright of the many classical CD recordings
of Janáček’s Glagolitic Mass at musicweb-international.com.
Deep within the long review, we find this passage:

Ančerl’s comes from 1963 and the one
from Mackerras from twenty years later, yet the sound of the earlier
recording is in no way inferior to the later one. Ančerl is recorded up
close, with the chorus and soloists practically in your face; Mackerras
at a greater distance and at a lower volume level. In fact to make the
Mackerras recording sound as full as the Ančerl requires an upper
adjustment on the volume control and a slight reduction in the treble
and increase in the bass. Once this is done, the performance comes
across just as powerfully as Ančerl’s.

This perfectly illustrates the benefit of a tilt
control. Yet, how many high-end stereo systems include one? It might be
rude of me, but I will quote myself from my article on the Missing
Sonic Controls.

Imagine if you encountered a telescope
manufacturer, whose product line embodied a severe minimalism:
telescopes built with the fewest lenses possible, telescopes without eye
adjustment knobs, telescopes without color or polarizing filters,
telescopes without magnification adjustments, telescopes that were
instead built to a single fixed magnification and fixed position,
telescopes that could reveal only a few celestial objects in clear focus
because of the fanatical adherence to preserving all of the purity of
the light entering the telescope; and if you complained about the blurry
image of your favorite star, the manufacturer would superciliously reply
that your favorite star was not worthy of his telescopes and that you
should be looking at other, better stars — if you encountered such a
strange business, then you would not see anything that radically
different from what is practiced in high-end audio today.

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.

The surprisingly effective sonic control is the phase
switch, with a mute position at its center.

I do believe that playback polarity is
important, but I am often too lazy to fiddle with the phase. A mistake.
The difference is subtle and only hearable after a few seconds of silence,
which allows the ear to readjust to the phase reversal. The result is not
a tonal change, but a subtle change in perspective, in sonic presentation,
with one polarity setting just sounding more correct.  (Ideally, I
would like to catalog my preference for each recording and then store that
selection in a database, so when the track is played back via the
computer, the phase selection is made by software rather than by me.) With
the GlassWare Cathode-Coupled PCB, two inputs per channel are offered.
Thus, it is easy to select a phase by choosing an input, while grounding
its partner.

This brings up the topic of balanced-to-unbalanced
conversion. The GlassWare Cathode-Coupled design can accept a balanced
input signal, yet it puts out an unbalanced, single-ended output signal.
Its CMRR is not however, nearly as good as offered by the Unbalancer
circuit, but then you may not need a high CMRR.

At the other extreme, if you are the adventurous type,
you might want to experiment with adding a negative feedback loop to the
Cathode-Coupled Amplifier. All that is needed is to use jumper J12 and
J14, not J13, which causes resistor R21 to become a two-resistor voltage
divider with resistor R11. Of course, in order for negative feedback to do
its magic requires extra gain to fuel the feedback process, which means
that a 6CG7 or 12AU7 would be a poor choice as an input tube, as they
offer too little signal gain. On the other hand, the 12AX7 would be
excessively dragged down by the two-resistor voltage divider that feeds
the cathode follower. Thus, the best choice might be the 12AT7 as the
input tube and an ECC99 as the output tube.

In the above schematic, the rail voltages are +150Vdc
and -150Vdc. This would be a good lower limit; I would be more likely to
use +250Vdc and -250Vdc instead, which is quite hot—but would probably
sound just stellar.

In SPICE simulations, with the +250Vdc and -250Vdc rail
voltages, the output impedance came out at 100 ohms and the gain was
+12dB, with very little distortion; but the key improvement was its fine
PSRR, which in SPICE simulations came in at -60dB. Mind you, the ECC99
will get plenty hot, as it will draw about 10mA with 250V rail voltages.
(The entire stereo line stage will draw 40mA, which against the 500V of
power-supply voltage equals 20W of heat! This is not your father's
cathode-coupled-amplifier (or your brother's grounded-grid amplifier ;).

What do I think of my more modest all-12AU7-based
effort? First of all, you cannot imagine how I wish that I were less
capable.

What!? Really, John!

Let me explain, but in my typically roundabout way. The
greatest English writer ever, William Shakespeare, admitted that he envied
other men's talent, along with their looks, learning, and friendships, in
his Sonnet 29:

When in disgrace with Fortune and men’s eyes,

I all alone beweep my outcast state,
And trouble deaf heaven
with my bootless cries,
And look upon myself and curse my
fate,
Wishing me like to one more rich in hope,
Featured
like him, like him with friends possess’d,
Desiring this man’s
art, and that man’s scope,
With what I most enjoy contented
least.

Then, hundreds of years later, in 1846, Soren
Kierkegaard wrote about modern envy in his book, The Present Age,
where he writes about an age when other men's gifts are not envied:

Men do not envy the gifts of others, their skill,
or the love of their women; they only envy each others' money. . . .
These men would die with nothing to repent of, believing that if only
they had the money, they might have truly lived and truly achieved
something.

Well, I envy those who cannot fix broken cars, cannot
assemble stereo gear, cannot cook, cannot kill zombie hoards... as they
just toss or give to others to repair their irreparable items,
irreparable by them that is. In contrast, I can do neither, neither throw
away or handoff to others to repair, as I find very little to be
irreparable or un-assemblable by me. So, much in the same way that car
mechanics always own cars that do not run and hair stylist often have bad
hair cuts, my basement is filled with electronic parts and stereo
equipment that I could repair or assemble—if I had the time and the will.

Well, that cute little enclosure was far too small. It
would be easier to vanquish a hoard cannibalistic zombies than it was to
assemble this cute piece of tube gear. (By the way, are there any vegan
zombies?) Yes, I did get everything to fit, but just barely. Had the Bud
box been three inches deeper and four inches wider, the assembly would
have been ten times easier.

Here is my recommendation, which few will follow: buy a
cheap, but sturdy cooking pan, the type you would bake brownies in.

Next, assemble your tube project in it. Quick and dirty
is the goal. Forget about looks, but not safety. Fiddle with the design,
adjust positions, take notes, spot potential problems, measure
performance, make upgrades as needed... Then, when you are absolutely sure
and satisfied, build the real project within a fancy enclosure, using all
your parts. When you are done making the transition, like a butterfly's
old cocoon, toss the hole-filled cooking pan in the recycle bin. This is
the smart path to follow, but even I—and I do know better—fail to do so.

In short, it was a pain to put this cute project
together, which ill disposed me to like the resulting sound. Second, the
Hammond transformer disappointed me by making so loud a mechanical buzz.
Yet, several hours of listening (and placing a heavy power transformer
atop the Hammond transformer) have erased most of my ill will towards the
cute project. Indeed, the more I listen, the more I discover that I like.
The GlassWare Cathode-Coupled line amplifier produces clean and quiet
gain, allowing many subtle pieces of sonic information to pour forth,
especially sonic imaging information.

So far, I am quite pleased with the JJ long-plate
ECC802 (12AU7) tubes, but at some point I will dig out some RCA NOS clear
tops and some very old NOS Amperex, Telefunken, and Tungsol 12AU7 tubes.

One design issue we encounter with any
cathode-coupled-amplifier is the triode's maximum plate voltage, as both
one input triode and the cathode-follower triode will see the full
positive rail voltage. The 12AT7
(6201, CV10662, CV455, CV8154, CV9859, ECC81) limit is 300Vdc, as it is
for the 12AU7 and 12BH7; the JJ
ECC99 plate voltage limit is a whopping 400Vdc. For some twin triode
tubes, however, the limit is 250Vdc. The JJ
E88CC's plate-voltage limit is confusing, as its data-sheet list
several maximum plate voltages, with the lowest being 220Vdc. Other
data-sheets specify a 130V plate-voltage limit, which I am sure applies
only to cascode operation, as both triodes share the same heater
connection, as to high a cathode-to-plate voltage for the bottom triode
will result in too great cathode-t0-heater voltage for the top triode.

For most setups, a 6CG7 or 12AU7 will be chosen as both
the input and output tubes. A 12BH7 or JJ ECC99, however, would make a
better output tube. Sadly, the excellent 5687 cannot be used, because of
its oddball pin-out. By the way, the heater setup is quite flexible on the
PCB.

As you can see, the two input tube heaters can be
placed either in parallel or in series, so two 6CG7 input tubes and one
ECC99 output tube can share either a 6.3V or 12V heater power supply. But
three 6CG7 tubes could not be used with a 12V heater power supply.

The GlassWare Cathode-Coupled PCB and kit are available
now at the GlassWare-Yahoo
store. The PCB comes with a 16-page user guide and is only $29. Dirt
cheap for such a high-quality, USA-made board, in other words. The chef's
12 by 9 inch cooking pan, on the other hand, will set you back $18 at
Target.

More 3-Technologies
Hybrid
Remember from my last
post, long-time reader, Kerry, asked, "Why not use FETs in the input
stage of a power amplifier, as FETs are known for their low noise and low
distortion; followed by a tube-based driver stage, as tubes are known for
their ability to swing cleanly huge voltage swings; then, terminate the
signal chain with a MOSFET-based, unity-gain output stage, as these
devices are known for offering wide bandwidth and a high input impedance?"
My answer sidestepped his question, offering an easier solution: use an
OpAmp and Aikido gain stage with a MOSFET-based, unity-gain output stage.

While this alternate suggestion offers much to like,
its glibness bothered me. Well, after more thought, the following design
overview came to me.

The two FETs define a bastode circuit, a vertical
differential amplifier in other words. The two transistors cascode the
FETs, which protects the FETs from the relatively high voltages. The
leftmost triode is configured as a grounded-grid amplifier, which receives
its input signal at its cathode. The rightmost triode is configured in a
cathode-follower circuit, which both shields the grounded-grid amplifier's
output from the output stage and offers a low output impedance. The
MOSFET-based output stage delivers lots of current without creating any
voltage gain.

The lowly 1N4007 diode is the hero of the day, as
protects the triodes and FETs at start-up. The MOSFET-based output stage
would provide about 36W into 8-ohm loads. (I will put up with less power
output, if I can get away with only two output devices.) The two triodes
form a CCDA circuit and would, unlike the typical CCDA setup, exhibit a
stellar PSRR, as first tube stage will split the two power-supply rail
noises, which should null, as they should be be equal in magnitude, but
out of phase.

The DC servo at the input does not correct the DC
offset voltage at the amplifier's output, but at the cathode-follower's
output; much in the same way, the negative feedback loop only extends up
to the cathode-follower's output. So, yes, this hybrid power amplifier
does use a feedback loop, but not a global one. Thus, this amplifier is a
double hybrid of sorts, which uses FETs, transistors, tubes, and MOSFETs;
feedback and no global feedback.

The power supply would be easy to assemble, as the four
rail voltages can be had from one center-tapped secondary:

If more voltage is needed for the tube stage, then the
following variation would work well.

Note where capacitors C3 & C4 terminate and how the
bridge rectifiers are attached. Also note the change in parts values,
particularly the voltage ratings.

Next Time
More
designs, of course.

For those of you who still have old computers running
Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the
download availability of my old old standards: Tube
CAD, SE
Amp CAD, and Audio
Gadgets. The downloads are at the GlassWare-Yahoo store and the price
is only $9.95 for each program.

http://glass-ware.stores.yahoo.net/adsoffromgla.html

So many have asked that I had to do it.

WARNING: THESE THREE PROGRAMS WILL
NOT RUN UNDER VISTA 64-Bit or WINDOWS 7 & 8 or any other 64-bit
OS.

I do plan on remaking all of these programs into 64-bit
versions, but it will be a huge ordeal, as programming requires vast
chunks of noise-free time, something very rare with children running
about. Ideally, I would love to come out with versions that run on iPads
and Android-OS tablets.

//JRB

Kit User Guide PDFs
Click image
to download

E-mail from GlassWare Customers

Hi John,

I received the Aikido PCB today - thank you for the first rate
shipping speed.
Wanted to let you know that this
is simply the best PCB I have had in my hands, bar none. The quality is
fabulous, and your documentation is superb. I know you do this because
you love audio, but I think your price of $39 is a bit of a giveaway!
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,

I bought an Aikido stereo linestage kit from you some days ago, and I
received it just this Monday. I have a few things to say about it.
Firstly, I'm extremely impressed at the quality of what I've been sent.
In fact, this is the highest quality kit I've seen anywhere, of
anything. I have no idea how you managed to fit all this stuff in under
what I paid for it. Second, your shipping was lightning-quick. Just more
satisfaction in the bag, there. I wish everyone did business like
you.

Sean H.

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& Octal PCBs

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TCJ Push-Pull Calculator has but a single purpose: to
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