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John
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20 August 2011

Aikido Mono Solo

The standard Aikido
mono 9-pin PCB and the power supply from the Aikido
All-in-One stereo PCB have been wedded, creating an All-in-One mono
Aikido board. The new Mono Solo PCB is 6 inches by 6 inches, extra thick,
2-oz copper traces, silk screened on both sides. It holds one channel of
Aikido amplification and the needed B+ and heater power supplies, with the
heater power supply getting a voltage regulator.

Okay, why yet another new PCB?

A super good question, as I have already laid out
many, many Aikido PCBs. Well, I designed this new Aikido board with two
specific types of DIYers in mind: those who want to assemble a monobloc
Moskido power amplifier and those who need a quiet and clean tube preamp
for their microphones. (And let us not forget those few, but brave souls
that run mono HiFi systems. If you find yourself growing weary of HiFi and
audio madness, take a few-week long vacation from stereo, as listening to
mono allows you to forget imaging and to concentrate on the music's
tonality and rhythm. Furthermore, it often allows you hear new things in
old music.)

Thus, combining one channel of Aikido amplification and
its heater and B+ power supplies on one board makes a lot of sense.

Aikido Mic Preamp

The above schematic displays a simple microphone preamp
based on the Aikido Mono Solo. Much more could be easily added, such as
anti-RFI ferrite beads, a resistor-capacitor network shunt for the
transformer's secondary, and, possibly, a phantom power supply. Indeed, a
volume control potentiometer in between the transformer and the Aikido
stage would be truly essential addition. The two output coupling
capacitors can either be tied together or left separated, with one
large-valued and one small-valued capacitor, so that a high-pass cutoff
could be selected when needed.

As a 12AX7-based Aikido stage will yield a gain of only
about +34dB, the step-up transformer must make up the difference. For
example, a ratio of 1:20 equals +26dB, as dB gain = 20 x log(gain). This
gain is added to the the Aikido's +34dB to make a total of +60dB.
(Depending on the microphone, this might be too little or too much gain.)
Moreover, the transformer can readily accept a balanced input signal and
break ground loops and eliminate the need for input coupling capacitors.

Moskido Power
Amplifier
At the other extreme, we see below a hybrid
power amplifier. In other words, a Moskido power amplifier based on the
Aikido Mono Solo PCB. As shown, the MOSFET output stage uses a mono-polar
power supply, but a bipolar power supply can be just as easily be used, by
just lifting resistor R13's connection to the ground and attaching it to
the negative power supply rail.

The above circuit assumes that a 12AX7 is the input
tube and a 6CG7 or 12AU7 or 12BH7 as an output tube. But a much lower mu
input tube can be used, such as the 12AY7 or 12AT7 or, even, a 6N1P.

LT1166
Here is a quick quiz: what does the following
circuit do?

The LT1166 is an interesting device from Linear
Technology. I first read about it in Robert Cordell's must-buy book,
Designing Audio Power
Amplifiers. Thanks Robert. The LT1166 is so interesting that I am
amazed that it was ever created. (I will flatter myself—and I hope not
denigrate Linear Technology—by saying that it looks like something that I
would come up with.)

Okay, just what does it do? It is an auto-bias/driver
circuit for solid state output stages, mostly MOSFET-based output stages.
Here is what the LT1166
datasheet says about it:

The LT1166 is a bias generating
system for controlling class AB output current in high powered
amplifiers. When connected with external transistors, the circuit
becomes a unity-gain voltage follower. The LT1166 is ideally suited for
driving power MOSFET devices because it eliminates all quiescent current
adjustments and critical transistor matching. Multiple output stages
using the LT1166 can be paralleled to obtain higher output current.
Thermal runaway of the quiescent point is eliminated because the bias
system senses the current in each power transistor by using a small
external sense resistor. A high speed regulator loop controls the amount
of drive applied to each power device. The LT1166 can be biased from a
pair of resistors or current sources; and because it operates on the
drive voltage to the output transistors, it operates on any supply
voltage.

Read the last sentence again. That's right it operates
on any supply voltage...well, any positive and negative rail voltage
greater than +/-2Vdc. How is that possible? The LT1166 is safely nested
within the power amplifier, tucked away from huge +/-100Vdc rail voltages.
Its job is to drive and keep the power MOSFETs biased correctly, so it
only sees across its leads the gate-to-gate voltage.

Note the 1-ohm load! Forget about diamond unity-gain
buffers, such as the BUF634
this is a serious buffer. The schematic looks far more confusing than
the circuit actually is. For example, the pair of transistors at the top,
MPS2907 & 2N2907, and the pair at the bottom, MPS2222 & 2N2222,
are arranged as simple constant-current sources. Also note the 4.3K input
resistor. So, is the input impedance 4.3k? No. Its value is closer to 15M.
In other words, this buffer can easily be driven by the wimpiest of tube
circuits. Or, we can drive the MOSFETs directly, letting the LT1166 just
do a lot of housekeeping for us, by attaching two coupling capacitors, one
to Vtop and one to Vbottom on the chip. This means that the MOSFET output
stage's heavy capacitance is now our concern. In other words, our tube
driver stage will have to be fairly robust and must be able to swing
sufficient current to charge and discharge the MOSFET output stage's heavy
capacitance.

Click on schematic to see
closeup

What we see above is a Moskido based on the LT1166. No
doubt, a few more parts might be needed, but you get the idea. The aspect
that does trouble me is DC offset at the the output. Adding a DC servo
would be easy enough.

Click on schematic to see closeup

The meta-view of the LT1166 is that it makes using IRF
MOSFET output stages much easier. But why use IRF MOSFETs? First, they are
readily available and relatively cheap. Second they are sturdy,
dependable, and powerful. My own prejudice has always been in favor of
"audio" type MOSFETs, such as the those lateral MOSFETs from Hitachi,
Toshiba, and Magnatek. At the same time, I know that many good-sounding
MOSFET-based amplifiers have been built.

SO must the LT1166 only be used with IRF MOSFETs? No.
In fact, the LT1166 can be used with bipolar transistors, but the circuit
will require more devices, as the transistor's small base-to-emitter
voltage is too small to create a useable window of voltage for the LT1166
t operate within. No doubt, the same circuit configuration will be
required when using the BUZ901/BUZ906 lateral MOSFETs, as their 1.2V
gate-to-source voltage will also choke the the LT1166.

Back to the Aikido Mono Solo
PCB
Of course, these two audio applications for the
new Mono Solo PCB are not the only ones possible, as a dual-mono stereo
line amplifier or a tube-based mixer or a tube headphone amplifier could
also be made from this new PCB.

In fact, I plan on using two of the boards myself, as I
plan on building two monobloc single-ended power amplifiers using the new
PCB. By using a 12AX7 as the input tube, the AIkido stage will yield a
gain of 50 (+34dB), which would be plenty for driving a single KT120 tube
to full output. On the other hand, if three or five EL84 tubes were used
in parallel, then a 6CG7 or 6DJ8 or 12AU7 would offer sufficient gain (I
assume that no negative feedback loop would be used).

The Aikido Mono Solo printed circuit board holds two
9-pin triodes, such as the 6AQ8, 6BQ7, 6DJ8, 6CG7, 6FQ7, 6GM8, 6H30,
12AT7, 12AU7, 12AV7, 12AX7, 12BH7, 5751, 6922, ECC99…as the input or the
output tube; and the 5687, 6900, 7044, 7119, 7370, and E182CC can be used
as the output tube, but not as the input tube.

By the way, I often am asked how to use a negative
feedback loop with an Aikido input stage. Assuming that he amplifier is
non-inverting, the following circuit shows how to attach the feedback
resistor.

The 12AX7-based in pout stage uses a 1k cathode
resistance for both the top and bottom triodes, as the bottom 1.1k
resistor in parallel with the 10k feedback resistor equals 1k. (In
parallel with, whose symbol is ||, follows this formula

R = R1 || R2
or     R = (R1 x R2) / (R1 + R2)      or
R = 1 / (1/R1 +1/ R2)

By the way, the Aikido Mono Solo PCB can be used upside
down; all the parts other than the two tube sockets can be soldered in the
bottom sid of the board, so the PCB can be mounted close to the chassis's
top plate with the tubes protruding through two holes. (The heater
regulator has a second set of solder pads on the bottom side of the
board.)

The Aikido Mono Solo PCB and kit is available at the GlassWare-Yahoo
store. (I only have 18 PCBs, so it's first come, first serve...maybe,
only serve.)

VIAS

I found the followingwebsite by accident and I
am glad that I did. These seem to be my kind of people, as my motto is
conserve and improve; conserve everything worth keeping and improve all
that you can. In the website's own words:

The Virtual Institute of
Applied Science (VIAS) has been founded by Hans Lohninger to
supply high-quality learning materials at virtually no costs. Thus, the
VIAS Web site is dedicated to the enhancement of learning and the
understanding of science concepts.

The VIAS group consists of a few private and
institutional partners led by Epina GmbH, and is supported by the following
institutions and companies (in alphabetical order):

Electronic Media Group, Vienna University of
Technology, Vienna, Austria

Epina
GmbH, Pressbaum, Austria

Springer
Verlag, Heidelberg, Germany

As far as I can tell, what they are doing is finding
old, classic—out of copyright—technical books and scanning and OCRing
them. A great public service, in other words. For example, we can read and
download Norman H. Crowhurst's three volume set, Basic
Audio. In addition, Reuben Lee's masterly tome, Electronic
Transformers and Circuits, is found here as well. (I love this book
and I used to own a copy, but I lent it out one too many times.) Check it
out.

Passive Line Stage Vs
Unbalancer
I finally got around to wiring
up my passive line
stage that is based on two M1 mono stepped attenuators and one
Select-1 switches and ten RCA jacks.

Vs

I had just been listening to my Unbalancer
circuit, which I like a great deal. After swapping out the active line
stage for the passive one, I immediately longed for the Unbalancer's
return. Less was not more this time. How strange. I have often noticed
that active can sound better than passive, in spite of all the extra stuff
in the signal path, but this listening event seemed different. Just in
case the passive line stage needed breaking in, which makes little sense
to me, as I don't see how resistors, wire , and switches need any breaking
in, but I let the music play for the rest of the day nonetheless. (I once
built a special box for breaking in speaker and line cables; it held a
wall socket and a power cord that plugged into the wall outlet, and four
RCA jacks and eight binding posts. With this box I could run 120Vac at 1A
through either two patch cords or two speaker cables at once, as they
completed a circuit with a floor lamp. With the speaker cables, I often
plugged in the TV as well. After a few days of this serious voltage and
current, I figured the cables had to be broken in.)

I then held a second shootout and, once again, the
Unbalancer clearly won again. Weird, very weird. I checked all the wiring
in the passive unit and everything was correct. Now understand, I wasn't
actually comparing apples to apples, as the Unbalancer used XLR terminated
balanced cables and the passive line stage used unbalanced, RCA-terminated
cables. But here was the odd thing, the RCA line cords cost a bundle $$$,
whereas the XLR cables cost $20 the pair. I thought about it and then it
hit me, hit me hard: the Unbalancer was getting its balanced input signal
in front of the passive line stage, as the following schematic
illustrates.

Click on Schematic to see
closeup

In other words, the passive line stage was seeing an
additional OpAmp and a bunch of resistors and capacitors in its signal
path, so I was comparing one apple to two apples (or more apples). What to
do? The interesting thing about the Unbalancer circuit is that it can
readily accept an unbalanced input signal. Just feed either its inverting
or its non-inverting input an input signal and leave the other input
grounded.

This is exactly what I did, feeding both the passive
line stage and the Unbalancer the same double-OpAmp signal, using the same
expensive RCA-terminated cables. Now it was one apple against one apple.
So, who won this time? Well, it was no longer decisive. In fact, it was a
complex mix of results. At times I preferred the passive line stage, as it
seemed to offer more subtle details; but, at other times, I preferred the
Unbalancer, as it seemed to present a wider, more solid and believable
sound stage, depending on the music playing. Sometimes it was not a fair
fight, as the CD was recorded at such a low volume that the passive line
stage could not fully power the amplifiers.

The conclusion that I walked away with was that the
Unbalancer is a great-sounding circuit. On the other hand, the passive
line stage is wonderfully simple, inexpensive, and "green" as any audio
component can be, as it draws no power from the wall outlet. (Of course,
if the exact same attenuators were sold in a finished box, complete with a
thick front faceplate and huge expensive knobs, built by a high-end
company, the passive line stage would cost well over $1,000; with fancy
resistors, several thousands of dollars. And, of course, for many
audiophiles, it would—perforce—sound much more exalted, as befits a new,
more dignified price tag.)

Next Time

I will think that I will return to power supplies and regulators.

//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.

And

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.

High-quality, double-sided, extra thick, 2-oz traces,
plated-through holes, dual sets of resistor pads and pads for two coupling
capacitors. Stereo and mono, octal and 9-pin printed circuit boards
available.

Designed by John Broskie & Made
in USA

Aikido PCBs for as little as
$24

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

Only
$9.95
to start
designing
tube-based
crossovers
and much more...

TCJ Filter Design

The Tube CAD Journal's first companion program, TCJ
Filter Design lets you design a filter or crossover (passive, OpAmp or tube) without having to check out thick
textbooks from the library and without having to breakout the scientific
calculator. This program's goal is to provide a quick and easy display not
only of the frequency response, but also of the resistor and capacitor
values for a passive and active filters and crossovers.

TCJ Filter Design is easy to use, but not lightweight, holding
over 60 different filter topologies and up to four filter
alignments:

Bessel,

Butterworth,

Gaussian,

Linkwitz-Riley.

While the program’s main concern is active filters, solid-state
and tube, it also does passive filters. In fact, it can be used to
calculate passive crossovers for use with speakers by entering 8 ohms as
the terminating resistance. Click on the image below to see the full
screen capture.

Tube crossovers are a major part of this program; both buffered
and un-buffered tube based filters along with mono-polar and bipolar power
supply topologies are covered. Available on a CD-ROM and a downloadable
version (4 Megabytes).

Download or CD ROM
Windows
95/98/Me/NT/2000/XP

For more information...

To
purchase , please visit our Yahoo Store:

http://store.yahoo.com/glass-ware

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