<
Back

John
Broskie's Guide to Tube Circuit Analysis & Design

Next
>

31 July 2012

Our move went fairly smoothly. My back muscles
recovered. My connection to the web restored. My many anchors are safely
stowed—all 1,000 books, 1,000 CDs, 500 LPs, 100 transformers and countless
electronic parts. While hunting through the boxes for needed items, such
as my reading glasses and computer speakers, I discovered many items that
I owned but had forgotten, which where nonetheless quite interesting. "Oh
yeah, I remember the project that I had in mind," repeated in my mind,
along with, "what was I thinking when I bought that?"

Something like this happened when I searched my
computer for last year's electronic schematics that I had drawn, but
hadn't posted here. So, it's time to move forward into the past.

The Howland Current Pump
Circuit
Almost a year has passed and ten posts
since I last wrote about current-output amplifiers (blog
number 218). No doubt, many readers are glad that I left the subject,
as it was just too extreme a notion, even for tube-loving folk. Many
readers, and I mean many, wrote to me, all stating the almost exact same
sentence: "Sure, speakers present a lumpy impedance curve, but they are
still primarily resistive devices; THEREFORE, they should be driven by
voltage-output amplifiers." Even a close friend repeated the same argument
to me, as if he had read it on some billboard. Are they wrong? The answer
is that they are not so much wrong as they are not exclusively right.

If someone had written, which no one did alas,
“Loudspeakers are primarily resistive devices; THEREFORE, they should be
driven by current-output amplifiers," he would be no less right and no
more wrong, as his argument is equally valid. What??

Yes, indeed, loudspeakers are primarily resistive
devices. If they were perfectly resistive devices, with no inductive or
reactive contamination, both voltage-output and current-output amplifiers
would deliver identical results, as both would deliver identical amounts
of voltage and current. For example, if a voltage-output amplifier put out
8Vpk, it would also deliver 1A peak into an 8-ohm resistance; if a
current-output amplifier put out 1A peak, it would also deliver 8Vpk into
an 8-ohm resistance; no difference. As far as the purely resistive
loudspeaker was concerned, both amplifiers would appear the same. Since
loudspeakers present complex impedance plots, both the current-output and
the voltage-output amplifier yield compromises and less than perfect
performance. A voltage-output amplifier produces a fixed output voltage
and varying output current; a current-output amplifier, a fixed current
and varying output voltage.

In other words, being a “primarily resistive device”
does not compel using one amplifier type over another. If any arguments
are going to be made, it should be that tubes, in general, would make
better current-output amplifiers; and transistors, in general, would make
better voltage-output amplifiers. Why? Transistors are primarily
voltage-limited devices, whereas tubes are primarily current-limited
devices. Thus, it should prove easier to have tubes deliver a relatively
low and limited output current, a super-high Zo, and an occasionally
exceedingly high output voltage, just as it should prove easier to have
transistors deliver a relatively low and limited output voltage, a
super-low Zo, and an occasionally exceedingly high output current.

Well, this long preamble brings us back
to the topic of current-output amplifiers. In particular, the Howland
current pump circuit. First, I must mention that the following schematic
is the modified Howland circuit, the original being more complex, as I
remember it. (Sadly, I cannot find the old schematic of the original
circuit.) In addition, the "current-pump" name is unfortunate, as is the
"current-source" name that is often also applied to the circuit. As I see
it, the best name would be the Howland voltage-to-current converter. The
Howland V-to-I circuit maintains a fixed ratio between the input voltage
it sees and its output current.

In the following example, the ratio is equal to 10mA
per input volt. If the 100-ohm series resistor were replaced by a 1-ohm
resistor, the ratio would equal 1A/V. In other words, the input voltage
will be superimposed across this series resistor; the bigger the resistor
value, the smaller the voltage-to-current conversion ratio, as Iout =
Vin/Rs.

Howland Modified Current Pump Circuit

Making things even more confusing, the Howland current
pump circuit comes in many forms, such as the following design that uses
two amplifiers.

Yes, two amplifiers are more expensive and require more
skill in implementing, but they do offer one large advantage: the
signal-source impedance is no longer an important factor, as the critical
four feedback resistors are shielded from the input signal. (The amplifier
within the feedback loop of the other does not need to be a power
amplifier, as it only drives the power amplifier's inverting input and its
own feedback resistors. In other words, an OpAmp could be used with a
power amplifier.)

I fear that I am truly jumping too far ahead, but here
is the two-amplifier version with example voltages and resistor values.
The load is 0-ohms, which is ideal for a current-output amplifier,
although it looks wrong, frightfully wrong to our
voltage-amplifier-centric eyes. As a homework assignment, imagine a
100-ohm load and the resulting voltages.

Let's reset our examination of the Howland circuit and
begin with just a simple OpAmp-based example that is designed to drive a
300-ohm headphone element.

The OpAmp that is used is not important. What is
important is the voltage and current relationships within the circuit. The
key point to remember is that an OpAmp will use all its considerable
open-loop gain to keep both its inverting and its non-inverting inputs in
line with each other, i.e. in line in terms of voltage. If the OpAmp's
positive input sees 1V, then its negative input must see 1V as well. If
the input signal is 0V, then the above circuit's output voltage must also
be 0V, as only that voltage will keep the OpAmp's two inputs at the exact
same voltage. If we apply a +1V input signal, the following voltages
result.

If the input voltage had been -1V, then all the other
voltage would be the same, but negative. If the load resistance had been
600 ohms, then the following voltages would result.

The same voltage-to-current ratio holds, but as 600
ohms is twice as large, it must develop twice the voltage across it, as V
= IR. While are at it, note that the 300-ohm requires that the signal
source, which provided the 1V signal, must sink 1/10k amount of current,
whereas the 600-ohm load setup requires 2.5 times more current sinking, as
2.5V/10k. In other words, working out the input impedance takes some
thinking.

Do the four feedback resistors have to be the same
value? No. In the following example, 1k and 10k and 100k resistors are
used, which yield a voltage-to-current ratio of 100mA/V. Note how the
1k/10k pair equals the 10k/100k pair in ratio. (Many of the GainClone
power OpAmps, such as the LM3886, require a final voltage gain of at least
10; thus, the following configuration just might work.)

Okay, I assume everyone is getting how the Howland
circuit works. The question that should arise is, Other than being a
useful current-output-amplifier topology, does the circuit have other
uses? Good question. Indeed, yes.

Here is my idea: what if we combined a standard voltage
amplifier with a Howland V-to-I circuit? The voltage amplifier would be a
high-end affair with a class-A output stage, either single-ended or
push-pull, but very limited output wattage, say 1W. The Howland
current-output amplifier, on the other hand, would be a dirty class-B
design, which could deliver 100W. In other words, we would build a
mixed-mode amplifier that delivered a voltage output with both a low Zo
and low distortion.

Such an amplifier is similar to the old Quad
current-dumping amplifier, but certainly not identical. For example, such
a hybrid amplifier must always be connected to the external load, like a
current-output amplifier and unlike a voltage-output amplifier; but unlike
the current-output amplifier, it cannot have its output shorted to ground.
In addition, we face the problem of the signal source’s output impedance,
which would throw off our exact resistor ratios. The workaround to this
last problem might be as simple as adding a unity-gain buffer ahead of the
mixed-mode amplifier.

Note that the class-A power amplifier is delivering no
current into the load, in the above example, as the current-output
amplifier is delivering 2A of current. Ideally, the high-quality voltage
amplifier will have the relatively small job of cleaning up the output
signal, which should not require much power. (The clean voltage amplifier
would still have to swing the same amount of voltage as the dirty V-to-I
amplifier.) Well, such would be the correct supposition with purely
resistive loudspeakers. With a typical loudspeaker, with its roller
coaster impedance plot, the high-quality small amplifier would have a lot
more work to do.

In fact, the class-A, voltage-out amplifier could
easily be overwhelmed. For example, if a 4-ohm speaker were attached,
rather than the intended 8-ohm loudspeaker, the voltage-output amplifier
would have to deliver just as much current as the current-output
amplifier. If a 16-ohm speaker were attached, rather than the intended
8-ohm loudspeaker, the voltage-output amplifier would have to absorb half
as much current as the current-output amplifier dumped into the load. In
other words, this amplifier would be extremely load-impedance sensitive.
Of course, some loudspeakers, such as the old Apogee ribbon speakers,
present a wonderfully flat impedance plot.

The problem is that few loudspeaker designers care
about the impedance plot of their creations, so it is allowed to run wild.
This is probably a bigger mistake than most would acknowledge. Using a
separate and powered subwoofer could help. If the satellite speaker only
covered frequencies above 100Hz, the impedance plot that it presented to
the amplifier could be made quite flat. Revisiting the transmission-line
enclosure would also be worthwhile, as it, unlike all other enclosure
styles, flattens the impedance hump at the woofer's resonate frequency.

Another approach would be to make both current-out and
voltage-out amplifiers equally powerful, but have the voltage-out
amplifier run in rich class-AB and run the current-out amplifier in lean
class-B.

The Future

My own prediction on the subject of current-output amplifiers and their
future is that headphone listeners will be the first to benefit. Why? Most
headphones hold single drivers and no crossovers, which gives them a
flatter impedance plot. The new planer headphones offer a very flat
impedance. For example, here is the impedance plot for the HiFi
Man HE500.

In addition, you can easily buy a headphone jack that
shorts the output to ground, when the headphone plug is pulled out of its
jack. I can also imagine very expensive current-out tube amplifiers being
sold, as there is always a market for insanely expensive stuff.

Very Expensive Balanced Current-Out HPA

But, I also predict that eventually a great
flat-impedance loudspeaker and a great current-output or mixed-mode
amplifier will be created and they will live happily together; perhaps,
they will change the audio industry.

Before leaving this topic, I must mention that the
mix-mode amplifier shown earlier could be made with tubes. It wouldn't be
exactly the same, as I would strive to forgo the voltage amplifier's
negative feedback loop, relying on a triode's own naturally low plate
resistance (rp) to serve in its place. Such an amplifier might use four
KT120 pentodes and one or two 300B triodes. The KT120s would be configured
in a current-output amplifier and the 300B would run in a class-A
single-ended or push-pull voltage amplifier. The resulting amplifier might
put 200W that sound as sweet as 20W class-A amplifier. (It is late,
otherwise I would draw a schematic.)

Another possibility is to build a hybrid mixed-mode
amplifier that used a solid-state V-to-I stage that put out big watts and
a small tube V-to-V amplifier that put out a clean voltage. The problem is
phase shifts due to coupling capacitors and output transformers. Thus, an
OTL tube amplifier might be the best choice. Once again, I can see a tube
hybrid mixed-mode amplifier more easily being made and sold than a speaker
version.

The Howland Circuit and
Precision Resistors
Each amplifier type has its own
its own dangers. A voltage amplifier must never see its output shorted to
ground; a current-output amplifier must never see an open load. The
Howland voltage-to-current converter circuit also must never see an open
load connection.

An easy workaround is to place a large-valued capacitor
in series with the feedback resistor, as shown below.

The real difficulty the Howland circuit faces is
resistor tolerances. Other than RIAA equalization and active crossover
networks, few audio circuits demand tight-tolerance resistors. The Howland
circuit does. Here is an example of why 1% just isn't good enough.

The output voltage should have been 3V, not 2.67V. No
doubt, some are wondering what the dig deal is, as who really cares what
the exact voltage-to-current conversion ratio is? Sure, I can see how a
11% error wouldn't trouble some readers, but what guarantee do you have
that both channels will be off in the same direction? For example, the
other channel might look like this, with a 14.3% error.

Mind you, these two examples were worst-case for 1%
resistors, with 5% or 10% resistors, the results could be insanely off.
This is why 0.1% are so often specified in schematics of Howland current
pump circuits. By the way, it is fairly easy to find matched resistors in
a group of ten 1% resistors. They might be both 9.92k ohms or 10.06k, not
10k.

Next Time
I
hope to give some details on some new PCBs.

//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.

9-Pin & Octal PCBs

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/

Support the Tube CAD
Journal

&

get an extremely powerful
push-pull tube-amplifier simulator for

Only
$19

TCJ Push-Pull
Calculator
Version 2

Click on images to see enlargements

TCJ PPC Version 2
Improvements

Rebuilt simulation engine

Create reports as PDFs*

More Graphs 2D/3D*

Help system added

Target idle current feature

Redesigned array creation

Transformer primary & secondary

RDC inclusion

Save user-defined
transformer

definitions
Enhanced result
display
Added array result
grid

*User definable

TCJ Push-Pull Calculator has but a single purpose: to
evaluate tube-based output stages by simulating eight topologies’ (five
OTL and three transformer-coupled) actual performance with a specified
tube, power supply and bias voltage, and load impedance. The accuracy of
the simulation depends on the accuracy of the tube models used and the
tube math model is the same True Curves™ model used in GlassWare's SE Amp
CAD and Live Curves programs, which is far more accurate than the usual
SPICE tube model.

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

For more information, please visit
our Web site :

www.glass-ware.com

To purchase, please visit our
Yahoo Store:
http://store.yahoo.com/glass-ware

<
Back

www.tubecad.com

Copyright © 1999-2012
GlassWare
All
Rights Reserved

Next
>