18 December 2004 (continued from 12/10/2004)
Thanks go to Ulrich in Germany for spotting some errors in the
class-AB2 amplifier schematic part values and voltages. The schematic
has been corrected.
For three days last week I was offline due a motherboard failure, which
took with it a CPU and memory. Several capacitors had failed on the
motherboard, swelling and leaking. I have read of this sort of thing,
but it was a first for me. (Actually, the first time I had seen
tantalum capacitors replaced by electrolytic capacitors on
motherboards, I shook my head, and wondered how long before problems
occurred.) Replacing the computer parts was only expensive; getting the
computer up and running again was painful, as a few system files had
been damaged and I desperately wanted not to reinstall XP.
Receive
so many e-mails a day (3,000 to 4,000—99% spam) that if I fail to empty
my mailbox twice daily, my ISP blocks any further e-mail, as the e-mail
will exceed my 10Meg limit. (Yes, I know all about spam killing
software; however, I have had legitimate e-mail killed by the filters
too often.) So, if you have written to me recently, I may not have
gotten the e-mail. As it now stands I have a logjam of about 30 e-mails
to answer. In other words, next week might be a better week to send
e-mail my way. And, please try to follow the one-question-per-e-mail
rule.
Subject: Sandman error takeoff
Thank
you for putting out a wonderful resource for everyone. The Aikido
amplifier looks like my next project and future line stage. And thanks
for resurrecting the Sandman circuit. I am embarrassed to say that in
spite of my 23 years in the analog electronics field and my having
taught electronics for three years in a community college, I have never
read his article or seen his topology used. Yet, I believe that his
approach is pure genius. Twenty years ago I had tried to come up with a
similar circuit/method but I failed because I insisted on using a
non-inverting amplifier as the basis of a composite amplifier.
However, one problem I see with using transformer-coupled amplifiers is
the phase shifts creating distortion that Sandman’s distortion error
take off will not be able to overcome. Therefore, might not a
better--and cheaper--course to follow be to use a cheap but high
quality solid-state amplifier to provide the basis of a composite
amplifier that also used small high quality tube OTL amplifiers. It
seems to me that the “computer OTL” amplifier topology could be used
with a single 6BX7 as output tubes. This amplifier could makeup the
second tier of error take off. The last amplifier could be your Aikido
Amplifier with a 12AX7 input and 6BX7 output. Both of these tube
amplifiers are inverting, so something slightly different will be
needed to get the error take off scheme to work. Just a suggestion and
thanks again.
Bill
USA
Thanks
for the kind words. Yes, a composite mixed-technology amplifier is
possible and it would certainly be cheaper than the all-tube version.
And without causing too much controversy, output transformers are
simply a headache, when used in feedback-based amplifier. But what if
instead of three tube transformer-coupled amplifiers in tandem, three
tube amplifiers in tandem are used with only one common output
transformer. In other words, what if the error-take-off amplifiers used
feedback loops outside of an output transformer. In this configuration,
the output transformer would effectively be the common load. True, the
transformer’s own distortion would lie outside the feedback loop, but
then so would its phase shifts. In such a composite amplifier, each
tube amplifier could be configured in a totem-pole topology, so as to
eliminate the need for a balanced input signal, greatly simplifying the
feedback loops.
Subject: PDFs & DC coupling
Welcome
back to the web. I appreciate all the blog postings, but they are
almost impossible to print. I like to read them away from home and work
and I do not own a laptop. Maybe you could provide a PDF file of each
blog. I would like to know if the [computer] OTL amplifier could be
direct coupled, OCL and OTL. Such an amplifier would require a negative
power supply for the bottom half of the output tubes, but since you
have already provided one, why not use it? And yes I did read your
blog/argument on coupling capacitors, but it takes me a long to change
my mind ;)
Raymond G
Brazil
At work? You are not being paid to read the Tube CAD Journal are you?
One user of my software told me that he often uses my software at work,
designing his own projects, but since he is in electrical engineering,
his boss cannot tell that he is having a good time, as his computer
screen is filled with graphs.
I will think about the
PDFs, as you are at least the third reader to ask for them. I wondered
how long it would take before someone asked for a
coupling-capacitor-less version of the computer amplifier. Below is a
such a remake.
Click on image to see closeup
The amplifier’s topology has been slightly altered (or drastically,
depending on your perspective). The output stage now functions as
grounded-cathode amplifiers rather than cathode followers. Why? The
negative power supply brings with a new noise source, which the
previous version sidestepped. The bottom output tubes’s cathodes attach
directly to the negative 80-volt power supply rail, which holds a good
deal of power supply ripple, as the power supply is not regulated. On
the other hand, the grid of these bottom output tubes do not lug any of
this noise. The result is that the power supply noise would be
amplified, if we stuck to the old topology. By shifting the driver
stage’s connection to the amplifier’s output from the top triodes to
the bottom triodes, we can also shift the driver stage’s connection
that had gone to ground to the negative power supply rail instead. (For
much more complete analysis see my article on Totem-Pole Output Stage PSRR)
The DC offset becomes much more important without the coupling
capacitor, so I would place a voltmeter on the front panel to read the
offset each time the amplifier is turned on. Alternatively, a DC servo
loop can be added. In the schematic below, we see a simple DC servo
that uses a modified heater power supply to supply its power supply
rails. The OpAmp reads the amplifier's DC offset and offers a
correcting reference voltage to bring the output in line with ground.
The OpAmp’s inputs are centered at ground and its output is DC shifted
negatively—via the two-resistor voltage divider—to suit the output
tubes.

The
entire power supply is shown below. Note the lowered B+ for the input
and driver stages. Also note the new capacitor ratings.
//JRB
10 December 2004
(continued from 12/09/2004)
The power supply schematic posted yesterday held a miswiring: the
negative power supply rail would put out –160 volts, not the desired
–80 volts. The schematic has been corrected.
Before anyone asks, let me say that the amplifier can be restored to a
higher output wattage by three means. The first is to use a much high
power supply voltage for the output stage. By returning to the original
160 volts of B+ voltage, the amplifier can force a lot more current out
of the output tubes before hitting positive grid current conduction.
The distortion will be quite a bit higher, as the idle current will
have to be reduced to keep within the 6AS7’s plate dissipation limit,
but this may not be a liability in all situations, as musicians may
prefer a little grunge in the mix.
In
fact, one reader from Brazil, George, wrote to explain that music
loudspeaker enclosures often hold four 12-inch or 15-inch fullrange
drivers, wired in series/parallel for an 8-ohm total impedance. Thus,
rewiring the four drivers into a 32-ohm load would be easy. Below is an
example of how to hookup a toggle switch to allow easy switching from 8
to 32 ohms.
The second path to more watts is to increase the number of output
tubes. While more tubes will mean more watts, the price is a lot more
heat and heavier power supply transformers. Below is a graph from TCJ Push-Pull Calculator that plots output wattage against the number of triode pairs used.
Click on screen capture to see enlargement
The third way to up the wattage, but without upping the B+ voltage, is
to allow the output tubes to run in positive grid current (class-AB2).
Adding a DC-coupled cathode followers to the output stage will isolate
the 6AS7’s grids from the 12AX7’s plates. More importantly, each 6AS7
triode can put out 400mA with 55 volts on its plate and +9 volts on its
grid. This much current almost doubles the zero-grid-current limit,
which in turn more than doubles the potential output into 32 ohms to
10W.
Click on image to see enlargement
Above
is the revised amplifier schematic. Each channel now holds an extra
tube, a 5687 in this case, but any 12-volt-heater tube would work
(although probably not as well); for example the 12AU7, 12BH7, 12SN7,
5963, 5965, or E182CC. The power supply’s negative rail must be
augmented to –160 volts (as shown below), so that the cathode followers
have more room to work in. Note the two capacitors that enclose the
cathode follower that feeds the top output tubes. If they look
suspiciously like the Transcendent OTL magic zeners, look again. The
drive equalization occurs in the driver/phase-splitter stage, not at
the cathode follower (if you think otherwise, just remove the
capacitors and see what happens). These capacitors merely work to
ensure that both cathode followers operate under similar conditions.
The
power supply has the same number of parts as in the original version,
but a slightly different layout to create the negative power supply
rail. Compare the schematic below to the power supply schematic above;
note where the negative rail diodes attach.
 Click on image to see enlargement
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13 types of tube circuits, each one divided into four variations: 52
circuits in all. Tube CAD calculates the noteworthy results, such as
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gain? Tube CAD's scenario comparison feature shows which tube wins.
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