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A Simple Tube/Opamp Hybrid Amplifier

by Alex Cavalli, Mark Lovell and Bill Pasculle

Introduction

After seeing many of the excellent and eye-catching tube-solid state
amplifiers of others, we’d like to present a slightly different topology of
hybrid amplifier design, using the same two basic components of a tube and an
opamp. This amp is simple and has less than $50.00USD worth of parts and no
lethally high voltages. It makes an ideal tube headphone amp for those who like
the sound of tubes but worry about the safety of high voltage equipment.

Bill was looking for an amp that sounded good and also was a good companion
for his Rio Carbon portable player. He also suspected that others might like
something similar as well. With high bit-rate MP3 or WMA files, the Carbon can
produce excellent sound, but as with many other portable players, benefits by
the addition of a decent headphone amp. He also wanted to use the amp in his
cubicle at work – requiring good sound without taking up too much space.

The design of the amp started when Bill suggested that he wanted to build one
of the YAHA (Yet Another Hybrid Amplifier) amps , but
using a different design. Alex, being mostly interested in tube amps, was not
initially interested but as Bill and Mark began to suggest requirements for the
amp and some options for the input tube, it was obvious that there was room to
improve the typical hybrid topology. After more thought, Mark provided a list of
some requirements for the design that he felt were necessary to meet in order to
improve on the hybrid topology.

Alex went to work taking the design discussions and turning them into a draft
design. Our first name for the amp (as a joke) among the team was Stoopid Opamp
Headphone Amp (SOHA). The name, as so often happens with skunk-works project
names, eventually stuck and finally we are just calling this amplifier the
Stoopid or the SOHA. Another name for the amp, with the same acronym, might be
the Simple Opamp Hybrid Amplifier.

The original prototypes assured us that it is possible to make a surprisingly
good performing amp utilizing a tube at relatively low voltage and while still
keeping the build cheap, easy, and reasonably electric shock-free. After
altering some of the power supply and circuit values and testing the prototype,
we ended up with something that was stable and fun to listen to for extended
periods. It applies some compression but that’s part of its charm, and it would
be a rather sterile sounding amp without the 12AU7/ECC82 altering the sound the
way it does with its 40V plate voltage.

Amplifier Circuit

Most of the hybrid amps that have appeared in HeadWize threads and elsewhere
(such as the Millet hybrid) have used the same B+ for both the tube and the
opamp. Some of these amps are designed to be portable enough to run from a
battery, but most are really constraied to a low voltage DC supply of some kind
plugged into the line and so are not truly portable.

In addition, it is generally true that tubes that are not designed for low
voltage use will not perform well at 12-24V (which is why the Millet uses
special low voltage tubes), so we decided to try to provide the tube with higher
B+ to get better performance, while still keeping the voltages fairly low. This
meant that the amp could be small and portable although requiring AC power. Like
the other hybrid amps, the SOHA is designed to give the sound of tubes while
avoiding the high voltage risk that some builders don’t like. Still, providing a
higher B+ permits us to get excellent sound from a more commonly available tube
like the 12AU7/ECC82, which is in good supply from NOS and current production
sources and which gives a wide variety of choices for tube rolling. Having this
wide selection also makes it easier for the amp to be constructed in any part of
the world.

The way in which most other hybrids use a common B+ for both tube and opamp
has two detrimental effects on a hybrid amp:

It puts the opamp in a single ended configuration where it needs an output
cap to block half the B+

It forces t\he B+ on the tube to be low so as not to exceed the maximum
opamp rail voltages.

The first design decision was to decouple the B+ for the tube and opamp.
Doing this makes it possible to use a standard bipolar supply for the opamp,
eliminating the large output cap (as in the Chu Moy’s pocket amp for example)
and requiring only a small coupling cap between stages (see power supply
discussion below).

The tube is loaded with a constant current source (CCS) for two reasons:

The resulting non-linear performance associated with low voltage operation
is partially offset by the high dynamic impedance of the CCS

A CCS has a much better PSRR than a simple resistor making it possible to
have more ripple in the B+ and, therefore, simplifying the B+ PS.

The original amp is designed to run with FET input opamps. See note 1
(the BJT opamp section) for a version using BJT input opamps.

After extensive prototyping by Mark and Bill, and after posting the first
design to the HeadWize forums and receiving feedback from several builders, we
modified the original design. The most notable change was to the heater circuit.
Originally the heater voltage was supplied directly from the AC secondary with
voltage dropping resistors. This approach was implemented initially to maintain
simplicity. However, because there is so much variation in transformer
regulation, line voltage, and heater characteristics, the simple resistors were
replaced with a regulator circuit. A side-benefit is the elimination of power
wasted as heat since the dropping resistors reached 105°C – 115°C under normal
operating conditions.

The Amplifier Circuit

The basic amplifier circuit uses a very small number of components, as
shown:

The topology of the amp is a simple grounded cathode gain stage coupled
through a capacitor to an opamp wired in unity gain mode. The standard SOHA uses
LND150 depletion mode MOSFETs for the CCS for reasons discussed below. The
amplifier circuit is, thus, very simple. Trim pots are provided as part of the
cathode bias resistors to adjust for variations in tubes to set the plate
voltages to ~40V. Each CCS is set to regulate at approximately 1mA.

An advantage of this design over many of the other hybrid designs is that
there is no large coupling electrolytic at the output. The required inter-stage
coupling capacitor is small making it possible to use good quality film/audio
capacitors here at nominal additional cost.

With a 12AU7/ECC82 the input stage has a gain of about 12. This is sufficient
for almost any source driving almost any headphones which is why the opamp is
simply operating as a unity gain current buffer. However, with such high gain it
is possible to exceed the input voltage tolerances for the opamp with just 1Vp
at the input. The data sheet for the OPA2134 (and many similar opamps) indicates
that the maximum input voltage is (V-) – 0.7V to (V+) + 0.7V. This means that
the input swing must not exceed the supply voltage by more than one diode drop.
The diodes ensure that this does not happen.

If the diodes conduct, the excess current passes into or out of the bipolar
power supply. What happens thereafter depends on the ability of the tube to
source/sink current and the ability of the PS to sink/source it. In this case,
the tube will source/sink in the range of hundreds of micro amps which will find
their way to the output caps of the bipolar supply which are in turn supplying
current to the opamp V+ and V-. The output of the regulators will fluctuate
some, but at this point the amp would not be operating properly anyway.

The standard CCS for the basic SOHA uses a single LND150 MOSFET in this
configuration:

For a discussion of why this was selected as the standard CCS, see note
2 (the CCS comparison section) at the end of the article. One limitation on
the standard SOHA CCS is the uneven availability of the LND150 MOSFETs globally.
To ensure that this amp can be built almost anywhere, we have created two
variations that use other devices for the CCSs. The first uses J113 JFETs and
the second uses 1N5297 current regulator (CR) diodes. Here are the schematics
for both variations:

Care should be exercised when building the SOHA with the J113 JFETs. Their
maximum Vdss is 35V. Under normal operating conditions they will see only about
15-20V, but if the plate voltage on the tube is too low it is possible to exceed
this maximum and destroy them. To protect the JFETS, the minimum plate voltage
should never be set below 20V (see below the warning about adjusting the
trimpots). The 1N5297 CRD has a 100V maximum and should withstand all of the
normal voltages in this amp. The J505, noted in parenthesis, will also work but
has only a 50V maximum. This makes the J505 a little more robust in this circuit
than the J113, but less desirable than the 1N5297.

Power Supply Circuit

The key to this amp is the power supply. Initially the amp used an
easy-to-acquire 30VCT/200mA transformer. As noted above, during the development
and testing process, including builds by several HeadWizers, we changed the
heater supply from AC to regulated DC. To accommodate the 150mA DC drawn by the
heater it is necessary to increase the current spec on the secondary to 400mA.
This will also give some headroom for the amp itself. Eventually, we chose the
Amveco TE70053 toroid to replace the original split bobbin transformer. Another
benefit to using the toroid is less EM radiation in the box and, since the SOHA
also designed to be small, this reduces or eliminates problems with PS buzz.
Other transformer possibilities are in the Power Supply section below. You can
use a higher current rating transformer without difficulty, but if you increase
the voltage be careful about not exceeding the maximum input voltage for the
regulators. The bipolar opamp supply is a conventional regulated supply using
78L12/79L12 inexpensive regulators. They have a maximum input voltage of
40V.

The trick to the power supply is the use of a 1.5x full-wave voltage
multiplier to generate the B+ for the tube. To make the voltage multiplier, the
entire secondary of the transformer is rectified through a pair of coupling
capacitors and bootstrapped on top of the V+ of the bipolar supply. With a
typical transformer with 25% regulation and with no load on the B+ for the tube,
this generates over 80V (this is marginally dangerous and will give you a pretty
good sting so be careful).

The power supply, including the heater circuit is shown below:

When the B+ is loaded with the tube, with each triode drawing ~1mA, the
voltage is pulled down to between +55-65V. This means that there is plenty of
headroom in the B+ to run the tube at+40V while still leaving space for driving
7-10V into the opamp. And this seems to give very good performance. As noted
above, using a CCS for the plate load relieves ripple requirements on the B+ so
a much simplified and less expensive filter section becomes possible. For the
components as drawn the B+ ripple is about 1mV. The capacitor values are kept
low and, hence, the capacitors are small and inexpensive. Again, for CCS PSRR
comparisons see below.

The heater supply uses a full-wave rectifier into a negative 12.6V regulated
supply. Pay careful attention to the orientation of the rectifying diodes. The
heater supply is attached to the negative half of the bipolar supply. This was
done because the heater current will pull down the input to the filter section
of whichever half of the bipolar supply to which it is attached. Since we are
using the positive supply to bootstrap the B+ for the tube we don’t want the
heater supply to pull this voltage down. Therefore, it is derived from the
negative supply because if the negative input to filter drops by a volt or two
the regulator will not be affected. Pay careful attention to the orientation of
the rectifier diodes and capacitors in the heater circuit since it is a negative
supply. A power indicator LED can be attached to the heater supply taking care
to note the polarity. The negative regulator should be heatsunk to dissipate
about 2W.

Construction

The full schematic for both channels and the PS is shown below with the
complete parts list less some miscellaneous components such as enclosure, power
switch, etc.

Click here
to see full-size schematic.

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For
the 1N5297 version simply replace the entire CCS with the single diode.

The amp has been built several ways by different HeadWizers [click here to see forum member Neurotica's (Jim Eshleman) SOHA
build narrative]. Mark and Bill initially built the prototypes using point to
point wiring on perfboards and several others did so as well. Bill eventually
also made a homemade PCB while Alex designed a PCB using the commercial package
ExpressPCB (see below). Most builds to date have been like the prototypes with
the PSU and amplifier circuits on the same board. Pictured below is a pictorial
drawing showing how the SOHA can be wired point-to-point on a 4 x 6-inch
perfboard.

Figure
6a – Bill’s SOHA constructed by point to point wiring on a perf board

Click here to see full-size layout.

Part of our purpose with the design and component specs is to keep everything
as small and cheap as possible. The parts list shows the parts from the usual
American suppliers. Mark was able to source similar parts from Farnell and RS in
the UK.

The Amveco toroidal transformer (30VCT/500mA) is available from Digikey.
Remember with 15-0-15 VAC (nominal) secondaries and the poor regulation of these
inexpensive transformers, you will see over 21V with no load at the inputs to
the bipolar power supplies and ~85V with no load for the B+. Because of the poor
regulation make sure to use capacitors with voltage ratings that meet these
off-load conditions. Note that the PS parts table shows 100V capacitors for the
B+ section. If you use a higher voltage transformer make sure that the input to
the regulators does not exceed their maximums (typically about 37V).

Some other possible split bobbin transformers are: Dagnall D3019 (0-240 pri),
D3023 (0-115,0-115 pri). Both are 12VA. Other possible toroids are: MULTICOMP
MCTA015/15 (0-115,0-115 pri), MULTICOMP MCFE015/15 (0-230V pri), or MULTICOMP
MDCG015/15 (0-230V pri).

This design is optimized for 12AU7/ECC82 and its exact equivalents (5963,
6189, and 6680) rather than a close equivalent (or other types of tubes such as
6922).

All three flavors of CCS provide a degree of PSRR and some immunity from
power fluctuations. They differ in availability worldwide and in maximum voltage
ratings. The best overall CCS uses the LND150 MOSFET which is not available
everywhere. The J113 FET is widely available but its maximum DC voltage is only
35 volts. Normally the FET wouldn’t see more than 15-20V unless the plate
voltage gets too low. The 1N5297 CRD has a maximum voltage of 100 VDC but is not
as widely available and is also expensive. Nevertheless, working amps have been
built using all three types of CCS. Trim pots located at the cathodes are used
to adjust the plate voltage. In order to prevent burning out the CCS FETs the
cathode trim pots should always be turned to their highest resistance when
swapping in a new tube.

OPA2134 and its relatives are fairly common opamps for audio. This was a good
place to start. The authors would like to know how other FET input opamps
perform and welcome feedback from builders. The OPA551, for example, is a FET
input opamp that drops right into the Stoopid. However, it only comes in single
packages so you will have to account for this with the build.

FET input opamps are preferred because there is a risk that BJT input opamps
may tend to excessively load the tube and defeat the effect of the CCS. To use
BJT input opamps see the section below for modifications to do this. The authors
welcome feedback on the performance of the SOHA with BJT input opamps.

A BUF634 could easily be put into the unity gain feedback loop of the opamp
to give super high output. One change that might be necessary if really trying
to pull 200mA is to increase the size of the input capacitors in the bipolar PS
to more like 2200uF. Even larger values may be required to get full bass.

Mark added 150 Ohm resistors (R6) at the outputs as this enables the amp to
drive low and high impedance headphones without experiencing a large change in
volume. These can be left out of the circuit at the builder’s discretion,
however, their use is recommended. Likewise the pairs of 1N4148 diodes connected
to the non-inverting inputs of the opamps are optional, but serve to protect the
opamp inputs from overload and their use is recommended.

Here are a few details to pay attention to during construction and double
check before applying power to your SOHA:

Wiring the Triad transformer is not intuitive; the pins are not numbered
consecutively. Study the datasheet carefully.

The 78L12 and 79L12 do not share the same pinout.

The capacitors in the heater supply (as well as those in the negative half
of the bipolar supply) have their positive leads grounded.

Use of a star-ground is highly recommended.

Use of shielded cable from the input jacks to the pot, from the pot to the
tube grids, and from the opamp to the output jack is highly recommended.
Attaching the safety ground to the star ground is optional. Most builds have
worked fine with the star ground floating but an occasional unit has been
quieter with the safety ground connected to the star ground.

Grounding the pot body is usually required to eliminate static/hum.

Wire dress is important in this amp to avoid hum. Keep all signal wires away
from the transformer; keep the filament wires as far away from the audio circuit
as possible.

PC Boards

We’ve created Express PCB boards for the SOHA. These are related to the full
schematics with part numbers shown.

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For
the 1N5297 version simply replace the entire CCS with the single diode.

ExpressPCB and PDF files for both the amp and power supply are included
below. The tube socket on the amp board is in the center of the board. Note that
the tube socket mounts on the foil side of the board. With this configuration
you can easily mark a hole in the center of the standoffs and punch it out to
pass the tube through so that the tube can stick up through the chassis while
the components are sticking downward.

The copper layer in these PDF and ExpressPCB files can be used for home
etched boards.

SOHA Amplifier
Board (PDF)

SOHA Power Supply Board
(PDF)

SOHA
Amp and PS boards (ExpressPCB)

Techniques for making home PCBs were suggested by HeadWizer Bill Blair. Here
are some links that Bill used to make his own SOHA boards using the single layer
PDFs:

EasyPCB
Fabrication

HomeBrew Printed Circuit
Boards

The boards can be jumpered to use all three versions of the CCSs and to
operate as standard plate drive or as source follower drive. This is the
stuffing guide for these possible configurations.

Click here to see full-size stuffing guide.

Figure 6b – Bill’s stuffing guide for the SOHA amplifier
PCB

Setup

Wire everything up but don’t put the tube/opamp in yet. Measure the voltages
at the B+, V+, V-, and heater. They should be >80V, +12V, -12V, and -12.6V
respectively. If they are not then there is a problem that must be fixed before
inserting either the tube or the opamp.

If voltages are good and nothing has fried, power down and then insert the
tube and opamp. Before powering up again, dial your trim pots so that they are
in the maximum resistance position. This will put the maximum bias on the tube.
Measure the voltage at the plates (pins 1 & 6) and adjust the associated
trim pot until the voltage comes down to 40V for each plate. After these
adjustments, measure the B+ again. It should be between 55-65V. Occasionally you
may find a NOS tube does not work well in this circuit. You may need to replace
the tube to get good results. If so, the tube is probably outside of its
published operating characteristics. Each triode of 12AU7/ECC82 draws only 1mA
from the B+ supply and at these low currents there can be a wide variation in
operating characteristics, particularly among tubes that may be marginally
within spec.

Results

OK, how does it sound? Well, in short, stoopidly good. When first powered up,
the prototype plate voltage was only 17V and the amp sounded decidedly solid
state. Very “steely” and just tonally “off”. As the plate voltage was raised the
sound became more lush and tube-like. At 40V the amp began to perform extremely
well. The SOHA easily rivals the Cavalli-Jones/Morgan Jones which costs over
three times more to build! It’s got decent amounts of bass, classic sweet tube
midrange and plenty of top end extension. Also the soundstage is extremely wide
and respectably deep. This thing is just plain stoopid fun to listen to!

The amp drives headphones of any impedance between 16 Ohms and 300 Ohms
without problems, which covers most that are currently available.

The compression applied by running the 12AU7 with 40V at the plate allows an
unexpectedly refined sound with no sharp edges, yet without being mellow. It
will reproduce transients when required and has a respectable dynamic range. The
overall result is something than can be listened to for extended periods with no
“listening fatigue” and providing a pleasingly wide and reasonably deep sound
stage.

As is the case with tube amps, a warm up time is required and in this respect
the authors agree 20 minutes is required for it to sound its absolute best, but
of course it’s up and running after 30 seconds.

Mark has compared this amp to three other headphone amplifier designs
available at HeadWize having built them: namely the CJ, the CL MkII, and the BCJ
MkI, (the BCJ MkII was unavailable). All of the alternative designs used for
comparison tests are more expensive to build, all require potentially lethal
voltages, and all are optimized to ensure the tubes are working at their
optimum.

Clearly, the SOHA would be the worst of the bunch? Not so. It proved itself
to equal the CJ and gets closer than expected to the CL MkII. That’s pretty
impressive stuff for a tube amp deliberately designed to be cheap and not use
lethal voltages.

Tube rolling in this amp is also a lot of fun. Mark and Bill, who listen
mostly through Sennheiser HD-600’s, found that grey-plate 5963’s from GE and RCA
and Brimar sounded better than other tubes. Some other Headwizers with low-Z
cans seemed to prefer black-plate versions of these tubes. Among the new
production tubes, the Electro-Harmonix 12AU7 seemed to approach (but not exceed)
the performance of the NOS tubes while the JJ 12AU7 was a somewhat distant
second. Differences between tubes seemed to be in the clarity of the top end and
the amount and quality of the bass.

Here are some photos of Bill’s SOHA in its final home.

Figure
7 – Bill’s SOHA Top Side and Figure 34 – Bill’s SOHA The Guts

Note 1: BJT-Input Opamps

As noted above the SOHA was designed to use FET input opamps. However, to
permit opamp rolling, we’ve created two minor variations that permit the use of
BJT input opamps.

Bipolar-input opamps like the TSH22IN, NE5532, or NE5534 can substitute for
the 2134. But bipolar opamps will have lower input impedance than the FET input
opamps. This will increase the loading on the tube and increase the
distortion.

One way around the increased loading is to configure the CCS as an active
load source follower. This variation requires only one change in wiring at the
CCS and will only work for the LND150 and the J113 versions. An active load
source follower is a variant of a well-known tube topology where the CCS that is
acting as a plate load is also utilized as the output device in a follower
configuration. With this topology the output impedance of the gain stage drops
considerably and its ability to supply current increases in proportion. With
both FET topologies we can wire the FETs as source followers to make a hybrid
follower configuration for the first stage.

If you’re using the LND150 CCS you can convert the CCS into a source follower
by simply changing the point where the coupling capacitor is connected. The FET
then becomes a source follower with low output impedance and the ability to
drive higher currents into the load.

Figure
8 – Changing the LND150 CCS for BJT-input Opamps

If you’re using the J113 CCS you can covert it to a source follower using the
same technique:

Figure
9 – Changing the J113 CCS for BJT-input Opamps

Although the 1N5297 CRD is actually a JFET wired as a CCS we cannot access
the source of the device so the CRD cannot be used when driving BJT opamps.

For example, a full amplifier schematic for the standard LND150 CCS with BJT
opamp is:

Figure
10 – Driving BJT input opamps

If your amplifier exhibits high DC offset with BJT opamps, you can decrease
the value of R5 from 1M to 100k or even 50k without overloading the gain stage.
Note that decreasing the value of R5 while leaving C2 at 100nF also reduces the
low frequency response of the amplifier. To correct for this, increase the value
of C2 so that the product of R5 x C2 is the same as 1M x 100nF. For example, if
you decrease R5 to 100k, then to maintain the same low frequency response,
increase C2 to 1uF.

For BJT input opamps, the full schematic is this:

Click here
to see full-size schematic.

Note the part changes shown in red. These are the only changes necessary to
use BJT opamps in the SOHA. The layout of amp does not change.

Note 2: CCS Comparisons

The choices for standard CCS and acceptable variations are derived from three
criteria:

maximum breakdown voltage of the CCS

current regulating ability

PSRR

These comparisons were done using PSpice simulations. These simulations are
not likely to give absolute accuracy, but they are good at providing a relative
comparison among the various CCSs.

Simulations were done for the following CCS types:

Single LND150

Single J113

Single PN2907A

Single 1N5297

Single PN2907A with CRD bias string

Cascoded J113

Cascoded PN2907A

Cascoded PN2907A with CRD bias string

This table shows the current variation, PSRR, and breakdown voltage (BV) for
these various configurations:

The cascoded JFETs have the best current regulation, followed by the MOSFET.
The cascoded BJTs with CRD and without CRD have the next best current
regulation. This might make these the next best choices. But, we must look at
the PSRR and BV tables too.

The cascoded JFETs also have the best PSRR but they have a low BV. The LND150
has nearly the same PSRR (indistinguishable as a simulation result) but a very
high BV. The LND150’s current variation comes in fourth behind the cascoded
BJTs. However, the PSRR for the cascoded BJTs is 12db and 27db less than the
LND150. Furthermore the BV for the BJTs is on the margin of where the voltages
in the amp may be, and the BJT CCSs require many more parts than the either the
JFETs or the MOSFET.

Taking all of these results together, the LND150 rises to the top for the
standard SOHA because of its good current regulation, excellent PSRR, very high
BV, and low parts count. The cascoded JFETs come in second because of their
excellent regulation, PSRR, and low parts count. The CRD comes in third because
of its good regulation, high BV and extreme simplicity (only one part).

Appendix: Simulating the Amplifier in
OrCAD PSpice

Alex Cavalli has provided the project files for simulating this amplifier
using OrCAD Lite circuit simulation software. The simulations will run in OrCAD
Lite 9.1 or 9.2 only (later versions of OrCAD Lite and OrCAD Demo are more
restrictive and will not run the simulations). The installation files for OrCAD
Lite 9.1 or 9.2 can be downloaded from various educational sites on the
internet. Search for them using the keywords OrCAD or Pspice and
9.1 or 9.2. OrCAD 9.1 is the smaller download (27MB). If you have
trouble finding these files, email a HeadWize administrator for help.

There are 4 programs in OrCAD Lite suite: Capture, Capture CIS, PSpice and
Layout. The minimum installation to run the amplifier simulations is Capture
(the schematic drawing program) and PSpice (the circuit simulation program).

Download Simulation Files
for SOHA Headphone Amplifier

After downloading cavalli2_soha_sim.zip, create a project directory and unzip
the contents of the cavalli2_soha_sim.zip archive into that directory. Move the
.lib and .olb files into the <install
path>\OrcadLite\Capture\Library\PSpice directory. These are the component
libraries containing the SPICE models for the vacuum tubes, MOSFETs and opamps
used in the SOHA. (Note: heater connections are not required for any of the
triode models.) In OrCAD’s Capture program, open the stoopid.opj project
file.

The two basic types of simulation included are frequency response (AC sweep)
and time domain. The time domain analysis shows the shape of the output waveform
and can be used to determine the amplifier’s harmonic distortion. They both run
from the same schematic, but the input sources are different. For the frequency
response simulation, the audio input is a VAC (AC voltage source). The time
domain simulation requires a VSIN (sine wave generator) input. Before running a
simulation, make sure that the correct AC source is connected to the amp’s input
on the schematic.

The following instructions for using the simulation files are not a complete
tutorial for OrCAD. The OrCAD HELP files and online manuals include tutorials
for those who want to learn more about OrCAD.

Frequency Response (AC Sweep) Analysis

Run OrCAD Capture and open the project file stoopid.opj, if not already
open.

In the Project Manager window, expand the “PSPICE Resources|Simulation
Profiles” folder. Right click on “Schematic1-ac” and select “Make Active.”

In the Project Manager window, expand the “Design
Resources|.\cavalli.dsn|SCHEMATIC1″ folder and double click on “PAGE1″.

On the schematic, make sure that the input of the amp is connected to the
V4 AC voltage source. If it is connected to V3, drag the connection to V4.

To add the triode library to the Capture: click the Place Part toolbar
button ().
The Place Part dialog appears. Click the Add Library button. Navigate to the
triode.olb file and click Open. Make sure that the analog.olb and source.olb
libraries are also listed in the dialog. Click the Cancel button to close the
Place Part dialog.

From the menu, select PSpice|Edit Simulation Profile. The Simulation
Settings dialog appears. The settings should be as follows:

Analysis Type: AC Sweep/Noise
AC Sweep Type: Logarithmic (Decade),
Start Freq = 10, End Freq = 300K, Points/Decade = 100

To add the triode library to PSpice: Click the “Libraries” tab. Click the
Browse button and navigate to the the triode.lib file. Click the Add To Design
button. If the nom.lib file is not already listed in the dialog list, add it
now. Then close the Simulation Settings dialog.

To display the input and output frequency responses on a single graph,
voltage probes must be placed on the input and output points of the schematic.
Click the Voltage/Level Marker () on the
toolbar and place a marker at grid of U6. Place another marker above R9 at the
amp’s output.

To run the frequency response simulation, click the Run PSpice button on
the toolbar (). When
the simulation finishes, the PSpice graphing window appears. The input and
output curves should be in different colors with a key at the bottom of the
graph.

The PSpice simulation has computed the bias voltages and currents in the
circuit. To see the bias voltages displayed on the schematic, press the Enable
Bias Voltage Display toolbar button (). To see
the bias currents displayed on the schematic, press the Enable Bias Current
Display toolbar button ().

Time Domain (Transient) Analysis

On the Capture schematic, make sure that the input of the amp is connected
to the V4 sinewave source (VAMPL=0.4, Freq. = 1K, VOFF = 0). If it is
connected to V3, drag the connection to V4.

In the Project Manager window, expand the “PSPICE Resources|Simulation
Profiles” folder. Right click on “Schematic1-signal” and select “Make Active”

From the menu, select PSpice|Edit Simulation Profile. The Simulation
Settings dialog appears. The settings should be as follows:

Analysis Type: Time Domain(Transient)
Transient Options: Run to time
= 80ms, Start saving data after = 40ms, Max. step size = 0.001ms

To display the input and output waveforms on a single graph, voltage
probes must be placed on the input and output points of the schematic. Click
the Voltage/Level Marker () on the toolbar and place a marker at grid of U6. Place another
marker above R9 at the amp’s output.

To run the time domain simulation, click the Run PSpice button on the
toolbar ().
When the simulation finishes, the PSpice graphing window appears. The input
and output curves should be in different colors with a key at the bottom of
the graph.

To determine the harmonic distortion at 1KHz (the sine wave frequency),
harmonics in the output waveform must be separated out through a Fourier
Transform. In the PSpice window, press the FFT toolbar button (). The
PSpice graph changes to show the harmonics for the input and output waveforms.
The input and output curves should be in different colors with a key at the
bottom of the graph.

The fundamental frequency at 1KHz will have the largest spike. The other
harmonics are too small to be seen at the default magnification. In the PSpice
window, press the Zoom Area toolbar button () and drag
a small rectangle in the lower left corner of the FFT graph. The graph now
displays a magnified view of the selected area. Continue zooming in until the
harmonic spikes at 2KHz, 3KHz, etc. are visible.

Harmonic spikes should exist for the output waveform only. The input is an
ideal sine wave generator and has no distortion. To calculate total harmonic
distortion, add up the spike values (voltages) at frequencies above 1KHz and
divide by the voltage at 1KHz (the fundamental).

Note: simulations only approximate the performance of a circuit. The
actual performance may vary considerably from the simulation as determined by a
number of factors, including the accuracy of the component models, and layout
and construction techniques.

c. 2006 Alex Cavalli, Mark Lovell and Bill
Pasculle (remove _nospam_).

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