Introduction
After seeing many of the excellent and eye-catching tube-solid state=20 amplifiers of others, we=E2=80=99d like to present a slightly different = topology of=20 hybrid amplifier design, using the same two basic components of a tube = and an=20 opamp. This amp is simple and has less than $50.00USD worth of parts and = no=20 lethally high voltages. It makes an ideal tube headphone amp for those = who like=20 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=20 for his Rio Carbon portable player. He also suspected that others might = like=20 something similar as well. With high bit-rate MP3 or WMA files, the = Carbon can=20 produce excellent sound, but as with many other portable players, = benefits by=20 the addition of a decent headphone amp. He also wanted to use the amp in = his=20 cubicle at work =E2=80=93 requiring good sound without taking up too = much space.
The design of the amp started when Bill suggested that he wanted to =
build one=20
of the YAHA (Yet Another Hybrid Amplifier) amps
Alex went to work taking the design discussions and turning them into = a draft=20 design. Our first name for the amp (as a joke) among the team was = Stoopid Opamp=20 Headphone Amp (SOHA). The name, as so often happens with skunk-works = project=20 names, eventually stuck and finally we are just calling this amplifier = the=20 Stoopid or the SOHA. Another name for the amp, with the same acronym, = might be=20 the Simple Opamp Hybrid Amplifier.
The original prototypes assured us that it is possible to make a = surprisingly=20 good performing amp utilizing a tube at relatively low voltage and while = still=20 keeping the build cheap, easy, and reasonably electric shock-free. After = altering some of the power supply and circuit values and testing the = prototype,=20 we ended up with something that was stable and fun to listen to for = extended=20 periods. It applies some compression but that=E2=80=99s part of its = charm, and it would=20 be a rather sterile sounding amp without the 12AU7/ECC82 altering the = sound the=20 way it does with its 40V plate voltage.
Amplifier Circuit
Most of the hybrid amps that have appeared in HeadWize threads and = elsewhere=20 (such as the Millet hybrid) have used the same B+ for both the tube and = the=20 opamp. Some of these amps are designed to be portable enough to run from = a=20 battery, but most are really constraied to a low voltage DC supply of = some kind=20 plugged into the line and so are not truly portable.
In addition, it is generally true that tubes that are not designed = for low=20 voltage use will not perform well at 12-24V (which is why the Millet = uses=20 special low voltage tubes), so we decided to try to provide the tube = with higher=20 B+ to get better performance, while still keeping the voltages fairly = low. This=20 meant that the amp could be small and portable although requiring AC = power. Like=20 the other hybrid amps, the SOHA is designed to give the sound of tubes = while=20 avoiding the high voltage risk that some builders don=E2=80=99t like. = Still, providing a=20 higher B+ permits us to get excellent sound from a more commonly = available tube=20 like the 12AU7/ECC82, which is in good supply from NOS and current = production=20 sources and which gives a wide variety of choices for tube rolling. = Having this=20 wide selection also makes it easier for the amp to be constructed in any = part of=20 the world.
The way in which most other hybrids use a common B+ for both tube and = opamp=20 has two detrimental effects on a hybrid amp:
- It puts the opamp in a single ended configuration where it needs = an output=20 cap to block half the B+=20
- It forces t\he B+ on the tube to be low so as not to exceed the = maximum=20 opamp rail voltages.
The first design decision was to decouple the B+ for the tube and = opamp.=20 Doing this makes it possible to use a standard bipolar supply for the = opamp,=20 eliminating the large output cap (as in the Chu Moy=E2=80=99s pocket amp = for example)=20 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=20 is partially offset by the high dynamic impedance of the CCS=20
- A CCS has a much better PSRR than a simple resistor making it = possible to=20 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=20 (the BJT opamp section) for a version using BJT input opamps.
After extensive prototyping by Mark and Bill, and after posting the = first=20 design to the HeadWize forums and receiving feedback from several = builders, we=20 modified the original design. The most notable change was to the heater = circuit.=20 Originally the heater voltage was supplied directly from the AC = secondary with=20 voltage dropping resistors. This approach was implemented initially to = maintain=20 simplicity. However, because there is so much variation in transformer=20 regulation, line voltage, and heater characteristics, the simple = resistors were=20 replaced with a regulator circuit. A side-benefit is the elimination of = power=20 wasted as heat since the dropping resistors reached 105=C2=B0C =E2=80=93 = 115=C2=B0C under normal=20 operating conditions.
The Amplifier Circuit
The basic amplifier circuit uses a very small number of components, = as=20 shown:
The topology of the amp is a simple grounded cathode gain stage = coupled=20 through a capacitor to an opamp wired in unity gain mode. The standard = SOHA uses=20 LND150 depletion mode MOSFETs for the CCS for reasons discussed below. = The=20 amplifier circuit is, thus, very simple. Trim pots are provided as part = of the=20 cathode bias resistors to adjust for variations in tubes to set the = plate=20 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=20 there is no large coupling electrolytic at the output. The required = inter-stage=20 coupling capacitor is small making it possible to use good quality = film/audio=20 capacitors here at nominal additional cost.
With a 12AU7/ECC82 the input stage has a gain of about 12. This is = sufficient=20 for almost any source driving almost any headphones which is why the = opamp is=20 simply operating as a unity gain current buffer. However, with such high = gain it=20 is possible to exceed the input voltage tolerances for the opamp with = just 1Vp=20 at the input. The data sheet for the OPA2134 (and many similar opamps) = indicates=20 that the maximum input voltage is (V-) =E2=80=93 0.7V to (V+) + 0.7V. = This means that=20 the input swing must not exceed the supply voltage by more than one = diode drop.=20 The diodes ensure that this does not happen.
If the diodes conduct, the excess current passes into or out of the = bipolar=20 power supply. What happens thereafter depends on the ability of the tube = to=20 source/sink current and the ability of the PS to sink/source it. In this = case,=20 the tube will source/sink in the range of hundreds of micro amps which = will find=20 their way to the output caps of the bipolar supply which are in turn = supplying=20 current to the opamp V+ and V-. The output of the regulators will = fluctuate=20 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=20
configuration:
For a discussion of why this was selected as the standard CCS, see = note=20 2 (the CCS comparison section) at the end of the article. One = limitation on=20 the standard SOHA CCS is the uneven availability of the LND150 MOSFETs = globally.=20 To ensure that this amp can be built almost anywhere, we have created = two=20 variations that use other devices for the CCSs. The first uses J113 = JFETs and=20 the second uses 1N5297 current regulator (CR) diodes. Here are the = schematics=20 for both variations:
Care should be exercised when building the SOHA with the J113 JFETs. = Their=20 maximum Vdss is 35V. Under normal operating conditions they will see = only about=20 15-20V, but if the plate voltage on the tube is too low it is possible = to exceed=20 this maximum and destroy them. To protect the JFETS, the minimum plate = voltage=20 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=20 normal voltages in this amp. The J505, noted in parenthesis, will also = work but=20 has only a 50V maximum. This makes the J505 a little more robust in this = circuit=20 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=20 easy-to-acquire 30VCT/200mA transformer. As noted above, during the = development=20 and testing process, including builds by several HeadWizers, we changed = the=20 heater supply from AC to regulated DC. To accommodate the 150mA DC drawn = by the=20 heater it is necessary to increase the current spec on the secondary to = 400mA.=20 This will also give some headroom for the amp itself. Eventually, we = chose the=20 Amveco TE70053 toroid to replace the original split bobbin transformer. = Another=20 benefit to using the toroid is less EM radiation in the box and, since = the SOHA=20 also designed to be small, this reduces or eliminates problems with PS = buzz.=20 Other transformer possibilities are in the Power Supply section below. = You can=20 use a higher current rating transformer without difficulty, but if you = increase=20 the voltage be careful about not exceeding the maximum input voltage for = the=20 regulators. The bipolar opamp supply is a conventional regulated supply = using=20 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=20 multiplier to generate the B+ for the tube. To make the voltage = multiplier, the=20 entire secondary of the transformer is rectified through a pair of = coupling=20 capacitors and bootstrapped on top of the V+ of the bipolar supply. With = a=20 typical transformer with 25% regulation and with no load on the B+ for = the tube,=20 this generates over 80V (this is marginally dangerous and will give you = a pretty=20 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=20 voltage is pulled down to between +55-65V. This means that there is = plenty of=20 headroom in the B+ to run the tube at+40V while still leaving space for = driving=20 7-10V into the opamp. And this seems to give very good performance. As = noted=20 above, using a CCS for the plate load relieves ripple requirements on = the B+ so=20 a much simplified and less expensive filter section becomes possible. = For the=20 components as drawn the B+ ripple is about 1mV. The capacitor values are = kept=20 low and, hence, the capacitors are small and inexpensive. Again, for CCS = PSRR=20 comparisons see below.
The heater supply uses a full-wave rectifier into a negative 12.6V = regulated=20 supply. Pay careful attention to the orientation of the rectifying = diodes. The=20 heater supply is attached to the negative half of the bipolar supply. = This was=20 done because the heater current will pull down the input to the filter = section=20 of whichever half of the bipolar supply to which it is attached. Since = we are=20 using the positive supply to bootstrap the B+ for the tube we = don=E2=80=99t want the=20 heater supply to pull this voltage down. Therefore, it is derived from = the=20 negative supply because if the negative input to filter drops by a volt = or two=20 the regulator will not be affected. Pay careful attention to the = orientation of=20 the rectifier diodes and capacitors in the heater circuit since it is a = negative=20 supply. A power indicator LED can be attached to the heater supply = taking care=20 to note the polarity. The negative regulator should be heatsunk to = dissipate=20 about 2W.
Construction
The full schematic for both channels and the PS is shown below with = the=20 complete parts list less some miscellaneous components such as = enclosure, power=20 switch, etc.
Click here=20
to see full-size schematic.
For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 = 1/8W. For=20 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=20 build narrative]. Mark and Bill initially built the prototypes using = point to=20 point wiring on perfboards and several others did so as well. Bill = eventually=20 also made a homemade PCB while Alex designed a PCB using the commercial = package=20 ExpressPCB (see below). Most builds to date have been like the = prototypes with=20 the PSU and amplifier circuits on the same board. Pictured below is a = pictorial=20 drawing showing how the SOHA can be wired point-to-point on a 4 x 6-inch = perfboard.
Figure=20
6a =E2=80=93 Bill=E2=80=99s 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=20 as small and cheap as possible. The parts list shows the parts from the = usual=20 American suppliers. Mark was able to source similar parts from Farnell = and RS in=20 the UK.
The Amveco toroidal transformer (30VCT/500mA) is available from = Digikey.=20 Remember with 15-0-15 VAC (nominal) secondaries and the poor regulation = of these=20 inexpensive transformers, you will see over 21V with no load at the = inputs to=20 the bipolar power supplies and ~85V with no load for the B+. Because of = the poor=20 regulation make sure to use capacitors with voltage ratings that meet = these=20 off-load conditions. Note that the PS parts table shows 100V capacitors = for the=20 B+ section. If you use a higher voltage transformer make sure that the = input to=20 the regulators does not exceed their maximums (typically about 37V).
Some other possible split bobbin transformers are: Dagnall D3019 = (0-240 pri),=20 D3023 (0-115,0-115 pri). Both are 12VA. Other possible toroids are: = MULTICOMP=20 MCTA015/15 (0-115,0-115 pri), MULTICOMP MCFE015/15 (0-230V pri), or = MULTICOMP=20 MDCG015/15 (0-230V pri).
This design is optimized for 12AU7/ECC82 and its exact equivalents = (5963,=20 6189, and 6680) rather than a close equivalent (or other types of tubes = such as=20 6922).
All three flavors of CCS provide a degree of PSRR and some immunity = from=20 power fluctuations. They differ in availability worldwide and in maximum = voltage=20 ratings. The best overall CCS uses the LND150 MOSFET which is not = available=20 everywhere. The J113 FET is widely available but its maximum DC voltage = is only=20 35 volts. Normally the FET wouldn=E2=80=99t see more than 15-20V unless = the plate=20 voltage gets too low. The 1N5297 CRD has a maximum voltage of 100 VDC = but is not=20 as widely available and is also expensive. Nevertheless, working amps = have been=20 built using all three types of CCS. Trim pots located at the cathodes = are used=20 to adjust the plate voltage. In order to prevent burning out the CCS = FETs the=20 cathode trim pots should always be turned to their highest resistance = when=20 swapping in a new tube.
OPA2134 and its relatives are fairly common opamps for audio. This = was a good=20 place to start. The authors would like to know how other FET input = opamps=20 perform and welcome feedback from builders. The OPA551, for example, is = a FET=20 input opamp that drops right into the Stoopid. However, it only comes in = single=20 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=20 may tend to excessively load the tube and defeat the effect of the CCS. = To use=20 BJT input opamps see the section below for modifications to do this. The = authors=20 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=20 to give super high output. One change that might be necessary if really = trying=20 to pull 200mA is to increase the size of the input capacitors in the = bipolar PS=20 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=20 drive low and high impedance headphones without experiencing a large = change in=20 volume. These can be left out of the circuit at the builder=E2=80=99s = discretion,=20 however, their use is recommended. Likewise the pairs of 1N4148 diodes = connected=20 to the non-inverting inputs of the opamps are optional, but serve to = protect the=20 opamp inputs from overload and their use is recommended.
Here are a few details to pay attention to during construction and = double=20 check before applying power to your SOHA:
- Wiring the Triad transformer is not intuitive; the pins are not = numbered=20 consecutively. Study the datasheet carefully.=20
- The 78L12 and 79L12 do not share the same pinout.=20
- The capacitors in the heater supply (as well as those in the = negative half=20 of the bipolar supply) have their positive leads grounded.=20
- Use of a star-ground is highly recommended.=20
- Use of shielded cable from the input jacks to the pot, from the = pot to the=20 tube grids, and from the opamp to the output jack is highly = recommended.=20 Attaching the safety ground to the star ground is optional. Most = builds have=20 worked fine with the star ground floating but an occasional unit has = been=20 quieter with the safety ground connected to the star ground.=20
- Grounding the pot body is usually required to eliminate = static/hum.=20
Wire dress is important in this amp to avoid hum. Keep all signal = wires away=20 from the transformer; keep the filament wires as far away from the audio = circuit=20 as possible.
PC Boards
We=E2=80=99ve created Express PCB boards for the SOHA. These are = related to the full=20 schematics with part numbers shown.
For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 = 1/8W. For=20 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=20 below. The tube socket on the amp board is in the center of the board. = Note that=20 the tube socket mounts on the foil side of the board. With this = configuration=20 you can easily mark a hole in the center of the standoffs and punch it = out to=20 pass the tube through so that the tube can stick up through the chassis = while=20 the components are sticking downward.
The copper layer in these PDF and ExpressPCB files can be used for = home=20 etched boards.
SOHA =
Amplifier=20
Board (PDF)
SOHA Power =
Supply Board=20
(PDF)
SOHA=20
Amp and PS boards (ExpressPCB)
Techniques for making home PCBs were suggested by HeadWizer Bill = Blair. Here=20 are some links that Bill used to make his own SOHA boards using the = single layer=20 PDFs:
EasyPCB=20
Fabrication
HomeBrew Printed =
Circuit=20
Boards
The boards can be jumpered to use all three versions of the CCSs and = to=20 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 =E2=80=93 Bill=E2=80=99s stuffing guide = for the SOHA amplifier=20 PCB
Setup
Wire everything up but don=E2=80=99t put the tube/opamp in yet. = Measure the voltages=20 at the B+, V+, V-, and heater. They should be >80V, +12V, -12V, and = -12.6V=20 respectively. If they are not then there is a problem that must be fixed = before=20 inserting either the tube or the opamp.
If voltages are good and nothing has fried, power down and then = insert the=20 tube and opamp. Before powering up again, dial your trim pots so that = they are=20 in the maximum resistance position. This will put the maximum bias on = the tube.=20 Measure the voltage at the plates (pins 1 & 6) and adjust the = associated=20 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=20 may find a NOS tube does not work well in this circuit. You may need to = replace=20 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=20 from the B+ supply and at these low currents there can be a wide = variation in=20 operating characteristics, particularly among tubes that may be = marginally=20 within spec.
Results
OK, how does it sound? Well, in short, stoopidly good. When first = powered up,=20 the prototype plate voltage was only 17V and the amp sounded decidedly = solid=20 state. Very =E2=80=9Csteely=E2=80=9D and just tonally = =E2=80=9Coff=E2=80=9D. As the plate voltage was raised the=20 sound became more lush and tube-like. At 40V the amp began to perform = extremely=20 well. The SOHA easily rivals the Cavalli-Jones/Morgan Jones which costs = over=20 three times more to build! It=E2=80=99s got decent amounts of bass, = classic sweet tube=20 midrange and plenty of top end extension. Also the soundstage is = extremely wide=20 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=20 without problems, which covers most that are currently available.
The compression applied by running the 12AU7 with 40V at the plate = allows an=20 unexpectedly refined sound with no sharp edges, yet without being = mellow. It=20 will reproduce transients when required and has a respectable dynamic = range. The=20 overall result is something than can be listened to for extended periods = with no=20 =E2=80=9Clistening fatigue=E2=80=9D and providing a pleasingly wide and = reasonably deep sound=20 stage.
As is the case with tube amps, a warm up time is required and in this = respect=20 the authors agree 20 minutes is required for it to sound its absolute = best, but=20 of course it=E2=80=99s 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=20 MkI, (the BCJ MkII was unavailable). All of the alternative designs used = for=20 comparison tests are more expensive to build, all require potentially = lethal=20 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=20 to equal the CJ and gets closer than expected to the CL MkII. = That=E2=80=99s pretty=20 impressive stuff for a tube amp deliberately designed to be cheap and = not use=20 lethal voltages.
Tube rolling in this amp is also a lot of fun. Mark and Bill, who = listen=20 mostly through Sennheiser HD-600=E2=80=99s, found that grey-plate = 5963=E2=80=99s from GE and RCA=20 and Brimar sounded better than other tubes. Some other Headwizers with = low-Z=20 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)=20 the performance of the NOS tubes while the JJ 12AU7 was a somewhat = distant=20 second. Differences between tubes seemed to be in the clarity of the top = end and=20 the amount and quality of the bass.
Here are some photos of Bill=E2=80=99s SOHA in its final home.
Figure=20
7 =E2=80=93 Bill=E2=80=99s SOHA Top Side and Figure 34 =E2=80=93 =
Bill=E2=80=99s SOHA The Guts
Note 1: BJT-Input Opamps
As noted above the SOHA was designed to use FET input opamps. = However, to=20 permit opamp rolling, we=E2=80=99ve created two minor variations that = permit the use of=20 BJT input opamps.
Bipolar-input opamps like the TSH22IN, NE5532, or NE5534 can = substitute for=20 the 2134. But bipolar opamps will have lower input impedance than the = FET input=20 opamps. This will increase the loading on the tube and increase the=20 distortion.
One way around the increased loading is to configure the CCS as an = active=20 load source follower. This variation requires only one change in wiring = at the=20 CCS and will only work for the LND150 and the J113 versions. An active = load=20 source follower is a variant of a well-known tube topology where the CCS = that is=20 acting as a plate load is also utilized as the output device in a = follower=20 configuration. With this topology the output impedance of the gain stage = drops=20 considerably and its ability to supply current increases in proportion. = With=20 both FET topologies we can wire the FETs as source followers to make a = hybrid=20 follower configuration for the first stage.
If you=E2=80=99re using the LND150 CCS you can convert the CCS into a = source follower=20 by simply changing the point where the coupling capacitor is connected. = The FET=20 then becomes a source follower with low output impedance and the ability = to=20 drive higher currents into the load.
Figure=20
8 =E2=80=93 Changing the LND150 CCS for BJT-input Opamps
If you=E2=80=99re using the J113 CCS you can covert it to a source = follower using the=20 same technique:
Figure=20
9 =E2=80=93 Changing the J113 CCS for BJT-input Opamps
Although the 1N5297 CRD is actually a JFET wired as a CCS we cannot = access=20 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=20 opamp is:
Figure=20
10 =E2=80=93 Driving BJT input opamps
If your amplifier exhibits high DC offset with BJT opamps, you can = decrease=20 the value of R5 from 1M to 100k or even 50k without overloading the gain = stage.=20 Note that decreasing the value of R5 while leaving C2 at 100nF also = reduces the=20 low frequency response of the amplifier. To correct for this, increase = the value=20 of C2 so that the product of R5 x C2 is the same as 1M x 100nF. For = example, if=20 you decrease R5 to 100k, then to maintain the same low frequency = response,=20 increase C2 to 1uF.
For BJT input opamps, the full schematic is this:
Click here=20
to see full-size schematic.
Note the part changes shown in red. These are the only changes = necessary to=20 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=20 criteria:
- maximum breakdown voltage of the CCS=20
- current regulating ability=20
- PSRR
These comparisons were done using PSpice simulations. These = simulations are=20 not likely to give absolute accuracy, but they are good at providing a = relative=20 comparison among the various CCSs.
Simulations were done for the following CCS types:
- Single LND150=20
- Single J113=20
- Single PN2907A=20
- Single 1N5297=20
- Single PN2907A with CRD bias string=20
- Cascoded J113=20
- Cascoded PN2907A=20
- Cascoded PN2907A with CRD bias string
This table shows the current variation, PSRR, and breakdown voltage = (BV) for=20 these various configurations:
The cascoded JFETs have the best current regulation, followed by the = MOSFET.=20 The cascoded BJTs with CRD and without CRD have the next best current=20 regulation. This might make these the next best choices. But, we must = look at=20 the PSRR and BV tables too.
The cascoded JFETs also have the best PSRR but they have a low BV. = The LND150=20 has nearly the same PSRR (indistinguishable as a simulation result) but = a very=20 high BV. The LND150=E2=80=99s current variation comes in fourth behind = the cascoded=20 BJTs. However, the PSRR for the cascoded BJTs is 12db and 27db less than = the=20 LND150. Furthermore the BV for the BJTs is on the margin of where the = voltages=20 in the amp may be, and the BJT CCSs require many more parts than the = either the=20 JFETs or the MOSFET.
Taking all of these results together, the LND150 rises to the top for = the=20 standard SOHA because of its good current regulation, excellent PSRR, = very high=20 BV, and low parts count. The cascoded JFETs come in second because of = their=20 excellent regulation, PSRR, and low parts count. The CRD comes in third = because=20 of its good regulation, high BV and extreme simplicity (only one = part).
Appendix: Simulating the = Amplifier in=20 OrCAD PSpice
Alex Cavalli has provided the project files for simulating this = amplifier=20 using OrCAD Lite circuit simulation software. The simulations will run = in OrCAD=20 Lite 9.1 or 9.2 only (later versions of OrCAD Lite and OrCAD Demo are = more=20 restrictive and will not run the simulations). The installation files = for OrCAD=20 Lite 9.1 or 9.2 can be downloaded from various educational sites on the=20 internet. Search for them using the keywords OrCAD or = Pspice and=20 9.1 or 9.2. OrCAD 9.1 is the smaller download (27MB). If = you have=20 trouble finding these files, email a HeadWize administrator for = help.
There are 4 programs in OrCAD Lite suite: Capture, Capture CIS, = PSpice and=20 Layout. The minimum installation to run the amplifier simulations is = Capture=20 (the schematic drawing program) and PSpice (the circuit simulation = program).
Download = Simulation Files=20 for SOHA Headphone Amplifier
After downloading cavalli2_soha_sim.zip, create a project directory = and unzip=20 the contents of the cavalli2_soha_sim.zip archive into that directory. = Move the=20 .lib and .olb files into the <install=20 path>\OrcadLite\Capture\Library\PSpice directory. These are the = component=20 libraries containing the SPICE models for the vacuum tubes, MOSFETs and = opamps=20 used in the SOHA. (Note: heater connections are not required for any of = the=20 triode models.) In OrCAD=E2=80=99s Capture program, open the stoopid.opj = project=20 file.
The two basic types of simulation included are frequency response (AC = sweep)=20 and time domain. The time domain analysis shows the shape of the output = waveform=20 and can be used to determine the amplifier=E2=80=99s harmonic = distortion. They both run=20 from the same schematic, but the input sources are different. For the = frequency=20 response simulation, the audio input is a VAC (AC voltage source). The = time=20 domain simulation requires a VSIN (sine wave generator) input. Before = running a=20 simulation, make sure that the correct AC source is connected to the = amp=E2=80=99s input=20 on the schematic.
The following instructions for using the simulation files are not a = complete=20 tutorial for OrCAD. The OrCAD HELP files and online manuals include = tutorials=20 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=20 open.=20
- In the Project Manager window, expand the =E2=80=9CPSPICE = Resources|Simulation=20 Profiles=E2=80=9D folder. Right click on = =E2=80=9CSchematic1-ac=E2=80=9D and select =E2=80=9CMake = Active.=E2=80=9D=20
- In the Project Manager window, expand the =E2=80=9CDesign=20 Resources|.\cavalli.dsn|SCHEMATIC1=E2=80=B3 folder and double click on = =E2=80=9CPAGE1=E2=80=B3.=20
- On the schematic, make sure that the input of the amp is connected = to the=20 V4 AC voltage source. If it is connected to V3, drag the connection to = V4.=20
- To add the triode library to the Capture: click the Place Part =
toolbar=20
button (
). = The Place Part dialog appears. Click the Add Library button. Navigate = to the=20 triode.olb file and click Open. Make sure that the analog.olb and = source.olb=20 libraries are also listed in the dialog. Click the Cancel button to = close the=20 Place Part dialog.=20
- From the menu, select PSpice|Edit Simulation Profile. The =
Simulation=20
Settings dialog appears. The settings should be as follows:=20
- Analysis Type: AC Sweep/Noise
AC Sweep Type: Logarithmic = (Decade),=20 Start Freq =3D 10, End Freq =3D 300K, Points/Decade =3D 100 - To add the triode library to PSpice: Click the = =E2=80=9CLibraries=E2=80=9D tab. Click the=20 Browse button and navigate to the the triode.lib file. Click the Add = To Design=20 button. If the nom.lib file is not already listed in the dialog list, = add it=20 now. Then close the Simulation Settings dialog.=20
- To display the input and output frequency responses on a single =
graph,=20
voltage probes must be placed on the input and output points of the =
schematic.=20
Click the Voltage/Level Marker (
) = on the=20 toolbar and place a marker at grid of U6. Place another marker above = R9 at the=20 amp=E2=80=99s output.=20
- To run the frequency response simulation, click the Run PSpice =
button on=20
the toolbar (
). = When=20 the simulation finishes, the PSpice graphing window appears. The input = and=20 output curves should be in different colors with a key at the bottom = of the=20 graph.=20
- The PSpice simulation has computed the bias voltages and currents =
in the=20
circuit. To see the bias voltages displayed on the schematic, press =
the Enable=20
Bias Voltage Display toolbar button (
). = To see=20 the bias currents displayed on the schematic, press the Enable Bias = Current=20 Display toolbar button (
). =
Time Domain (Transient) Analysis
- On the Capture schematic, make sure that the input of the amp is = connected=20 to the V4 sinewave source (VAMPL=3D0.4, Freq. =3D 1K, VOFF =3D 0). If = it is=20 connected to V3, drag the connection to V4.=20
- In the Project Manager window, expand the =E2=80=9CPSPICE = Resources|Simulation=20 Profiles=E2=80=9D folder. Right click on = =E2=80=9CSchematic1-signal=E2=80=9D and select =E2=80=9CMake = Active=E2=80=9D=20
- From the menu, select PSpice|Edit Simulation Profile. The =
Simulation=20
Settings dialog appears. The settings should be as follows:=20
- Analysis Type: Time Domain(Transient)
Transient Options: Run to = time=20 =3D 80ms, Start saving data after =3D 40ms, Max. step size =3D = 0.001ms - To display the input and output waveforms on a single graph, =
voltage=20
probes must be placed on the input and output points of the schematic. =
Click=20
the Voltage/Level Marker (
) on the toolbar and place a marker at grid of U6. Place = another=20 marker above R9 at the amp=E2=80=99s output.=20
- To run the time domain simulation, click the Run PSpice button on =
the=20
toolbar (
).= =20 When the simulation finishes, the PSpice graphing window appears. The = input=20 and output curves should be in different colors with a key at the = bottom of=20 the graph.=20
- To determine the harmonic distortion at 1KHz (the sine wave =
frequency),=20
harmonics in the output waveform must be separated out through a =
Fourier=20
Transform. In the PSpice window, press the FFT toolbar button (
). = The=20 PSpice graph changes to show the harmonics for the input and output = waveforms.=20 The input and output curves should be in different colors with a key = at the=20 bottom of the graph.=20
- The fundamental frequency at 1KHz will have the largest spike. The =
other=20
harmonics are too small to be seen at the default magnification. In =
the PSpice=20
window, press the Zoom Area toolbar button (
) = and drag=20 a small rectangle in the lower left corner of the FFT graph. The graph = now=20 displays a magnified view of the selected area. Continue zooming in = until the=20 harmonic spikes at 2KHz, 3KHz, etc. are visible.=20
- Harmonic spikes should exist for the output waveform only. The = input is an=20 ideal sine wave generator and has no distortion. To calculate total = harmonic=20 distortion, add up the spike values (voltages) at frequencies above = 1KHz and=20 divide by the voltage at 1KHz (the fundamental).
Note: simulations only approximate the performance of a = circuit. The=20 actual performance may vary considerably from the simulation as = determined by a=20 number of factors, including the accuracy of the component models, and = layout=20 and construction techniques.
c. 2006 Alex =
Cavalli, Mark Lovell and Bill=20
Pasculle (remove =
_nospam_).