The op-amp (operational amplifier) is the chip that does the voltage amplification in the PPA circuit. It has the single biggest effect on the amp’s sound of any component, so it behooves you to pick this part carefully.
Most any op-amp can be made to work in the PPA board, but some are more suitable than others. Unless you’ve got some serious electrical engineering chops, you should only use FET-input op-amps. Bipolar-input op-amps can be made to work with some careful design, but it’s not easy.
The canonical part for this amp is the Analog Devices AD8610. This part isn’t terribly expensive, it sounds good, it has low current draw, and it works well down to fairly low supply voltages (5-6 V) with most headphones. It’s a lively sounding chip, but not harsh. Some prefer the laid-back sound of Burr-Brown chips instead; we recommend the OPA627. For the left and right channels, you can also use the OPA637; you can’t use this in the ground channel because the 637 isn’t unity-gain stable.
There are many other good-sounding chips out there. What you’re looking for are single-channel FET-input low-distortion low-noise op-amps available in DIP-8 or SO-8 packages. (Whew!)
For more details about op-amps, see the companion article, Notes on Audio Op-Amps.
These are the cascode JFETs, used to bias the op-amps into class A.
The standard parts are 2N5486 (IDSS 8-20 mA) and 2N5484 (1-5 mA). There are many other parts that will also work. The 2N5457 is a good substitute for the 2N5484, for instance. In Europe, the BF245 series is more readily available, but beware that you have to turn them 180 degrees from the orientation you’d use for the standard JFETs.
When picking your own JFETs, Q1’s minimum IDSS should probably be around 1 mA, with higher being better. Q2’s minimum IDSS must be higher than Q1’s maximum IDSS. In other words, it must be impossible for the IDSS of any random Q2 to be lower than the IDSS of any Q1. Also, Q2’s input capacitance (CISS) should be low. (Under 10 pF.)
These JFETs provide isolation between the high and low-current sections of the power rails. This means that any ripple put on the rails by the high-current output section will be greatly attenuated by the Q3s so that it doesn’t disturb the op-amps and other delicate input circuitry as much. The Q3s will also remove some power supply noise, too. I have measured isolation ranging from 16 dB to about 30 dB.
In this version of the PPA, the only supported configuration is to populate all 6 positions.
You want each JFET’s IDSS to be at least as high as the op-amp’s current draw, plus enough extra for the cascode JFETs and other small current drains surrounding the op-amps. For instance, let’s say your op-amp has a quiescent current drain of 5 mA, and you’ve biased it into class A with 1 mA through the cascode JFETs. Therefore, you’d need a bit more than 6 mA through each Q3 at absolute minimum. The 2N5486 with a minimum IDSS of 8 mA would do just fine. Or, you could measure the IDSS of some 2N5459s (range: 4-16 mA) to find some that are high enough.
If you want to be able to swap different op-amps in without changing the Q3s, be sure to pick your Q3s such that they will be able to power the hungriest of your op-amps. The highest-drain op-amp you’re likely to use is about 15 mA, so picking Q3s with IDSS higher than this will cover almost any conceivable case.
That’s the simple way to approach the power rail issues. There’s a lot more to this, if you’re interested.
If you can’t get the recommended PN4392 from your parts distributor, look for the MPF4392. It’s the same thing. Failing that, most anything with a minimum IDSS of 30 mA or higher (and with the same pinout, obviously) should work fine.
The recommended 2N5087/5088 transistors aren’t easily available everywhere, but they are highly recommended unless you’re up for some hard-core tweaking. A poor selection here can have drastic consequences to the amp’s sound quality. With an op-amp, swapping out a poor performer is easy. Desoldering 18 TO-92s and putting a different set in is not so easy. (Speaking from experience here!)
With that warning in mind, there are some alternative complementary pairs you can try. We haven’t tried these on a PPAv2 circuit board, and we don’t know of anyone else who has. You’re on your own. In rough order of how suitable we expect them to be, the alternatives are:
The ones marked as having a reversed pinout can simply be installed “backwards” on the board.
These are the buffers’ input transistors. These accept the input signal and drive the signal to the output transistors.
See the table above for alternate part hints.
These are the output transistors. There are many different transistors that will work here. Think seriously before trying different ones than the two recommended types (MJE243/253 and BD139/140). Desoldering six big transistors without damaging the board isn’t nearly as tough as desoldering the buffers’ 18 small-signal transistors, but it’s still a bit of a pain.
Some alternative complementary pairs to look at:
The main thing to look at here is the tradeoff between bandwidth and output current. Higher bandwidth often corresponds to higher fidelity, especially with high-impedance headphones. But higher bandwidth also tends to trade off against output current. Since higher output current tends to help low-impedance headphones sound better, it can be worth it to trade off some of that bandwidth.
Transistors with higher current ratings are also more durable in the face of an accidental output short. Since all headphone jacks I’ve tested have a point where pulling the plug out shorts the right channel to ground, this is not something to take lightly. You’re only likely to be at risk if you unplug the headphones while there is music playing, but it’s easy to forget this risk.
Another nice benefit of lower bandwidth is that a lower-bandwidth amp is harder to make unstable.
The TLEs perform two functions: they set the voltage on the ground plane above the input traces, and they provide low impedance paths for current to flow around the input section in the face of the rail isolation JFETs. If the op-amps will share a single set of rails in your amp, you only need one TLE. If each op-amp has its own rails, you need three of them.
These are the main power reservoir capacitors.
Use 100 µF to 1000 µF capacitors with voltage ratings higher than that of your power supply. For example, use a 25 V capacitor if your power supply is 24 V. 1000 to 2000 µF total C1 capacitance is a good range to stay within.
You can populate only one position if you want, but I think you should populate at least two of them; that is, it’s better to use two 1000 µF C1s than one 2000 µF. The reason there are 9 C1 positions is to account for boutique caps such as Black Gates and Cerafines, which have a low capacitance density. Beware of using the front-most C1 position in smaller cases: you usually need that space for the headphone jack.
If you want to get your capacitors from one of the major distributors, look into the Panasonic FM, Panasonic FC, and Nichicon UPW lines. The Panasonic FC and the Nichicon UPW are identical. The Panasonic FMs are a little nicer than the FCs, but there are fewer values and fewer case size choices in that line. If your chosen distributor doesn’t carry one of these lines, try to find a cap line that features long life and low ESR.
If you want to choose your own power capacitors, there are two main rules to keep in mind:
In the PPA amp, there’s enough room for rail caps that you shouldn’t have to compromise on quality (rule 2) to get a sufficient amount of capacitance (rule 1). If you’re looking at caps totalling much over 2000 µF you’re probably compromising too much on quality; try looking for a line of capacitors that will let you trade some of that excess capacitance for higher quality. If you’re already looking at the best capacitor line available to you, you may simply choose to buy smaller capacitors and save the money. I doubt you can hear the difference.
Now to more specific advice.
First, decide on the capacitor’s dimensions. The diameter should be 12.5 mm, as these fit best on the PPA board. You can use 10 mm caps if you have to, but that’s wasting some of the board’s capacity. Don’t use caps skinnier than this, because the lead pitch will be too narrow for the cap to securely mount on the board. The cap’s height will be limited by the amount of space above your board inside your enclosure. Keep this maximum height restriction in mind as more of a limit than a goal; picking the tallest cap that will fit might give a nasty surprise if you find that your calculations were a little bit off.
Next, you need to know your power supply voltage. Because the C1s in the PPA go from rail to rail, their voltage rating must be higher than your power supply’s output voltage. For instance, if you have a 30 V supply, 25 V caps would be damaged by the power supply, 35 V caps are good, and 50 V caps are wasteful. (For more on this topic, read my article Op-Amp Working Voltage Considerations.)
It turns out that the distance from the V- pad of one of the PPA’s C1s to its neighbor’s V+ pad is equal to the lead spacing of 16 mm and 18 mm diameter caps. The board will accommodate 4 of these larger caps if your C2s are small enough and you’re not using a case where the board has to slide into rails.
Optional? Only 1 cap required. Up to 8 more can be added if you use 10 mm or 12.5 mm caps. Do not jumper.
Largest Part Size: Designed for 12.5 mm diameter caps, but 10 mm, 16 mm and 18 mm caps can be made to work without lead bending.
These are inter-channel bypassing caps for the output stage. They ensure that the power supply current loop is short between the channels, to avoid instability. They also have a lesser role of providing fast reservoir capacitance near the buffer circuits, lowering the impedance of the power rails you’d get if you only used C1s. Unlike in version 1, the C2s in version 2 are not optional.
The ideal C2 setup is to make the three “odd” capacitors 0.01 µF to 0.022 µF, and the two capacitors between these 0.1 µF to 0.22 µF. You can make them all the same value, but staggering these values gives better bypassing results.
Largest Part Size: 18 mm × 6 mm, 15 mm pin spacing
These are local bulk capacitance for the op-amps. Much of the C1 discussion applies to C4 as well.
The C4s go from ground to rail, so as long as the amp is functioning properly you can get away with caps with half the voltage tolerance as the power supply’s total voltage. However, it’s safer to use caps with a voltage tolerance at least as high as the voltage the op-amp sees. Since you don’t need much capacitance here (100 µF or so per rail), there’s not much point in skimping on voltage tolerances.
The C4s probably shouldn’t be much larger than 220 µF since they must be charged through the current-limiting components Q3s and R8 while the amp is powering up. You want these caps to charge up quickly when the amp turns on. Since the op-amps only “sip” a tiny amount of dynamic current, there’s no benefit to making C4 large.
Largest Part Size: 10 mm diameter
These are bypass caps for the op-amps, and they also act act as high-speed reservoir caps. The minimum useful value here is about 0.1 µF, but there is room here for cheap 1 µF polyester box caps. In version 2, we allow 10 mm and 15 mm pin spacing in addition to the 5 mm pin spacing allowed in previous versions, so you can use polypropylene and other high-end caps here if you want.
These caps are optional. Adding them is a good first step when trying to troubleshoot oscillation.
Optional? Yes. Do not jumper.
Largest Part Size: 0.300" × 0.800"
This is a bandwidth-limiting cap for the ground channel. It is necessary to maintain the stability of the ground channel. Put a tiny high-quality cap here; a 10 to 100 pF silver mica or NP0/C0G ceramic are your best bets.
Largest Part Size: 0.400" × 0.200"
This is for tuning the bass boost. Click that link to learn how changing its value changes the behavior of the bass boost.
In PPA version 2, we made this position much larger, so there’s no longer any excuse for using low-quality capacitors here. It’s directly in the signal path, so you should definitely be using polypropylenes or other high-quality linear types here.
If you’re not very concerned about bass boost quality, you can just use any random metallized polyester box cap here. There’s a reasonable argument for this: bass boost is already a nonlinear adjustment to the sound, so how bad could it be to use a nonlinear capacitor to effect the bass boost? While not wrong, this argument overlooks the fact that good polypropylenes really aren’t all that expensive. Therefore, I don’t see a good reason to give into such hair-splitting.
Optional? Yes. Do not jumper.
Largest Part Size: 16 mm × 8 mm
D1 is in series with the V+ line to the amplifer. Its purpose is to only allow the amp to power up if the power supply’s voltage polarity is correct. Without reverse voltage protection, the op-amps and some of the parts in the buffer circuits will be damaged, or even completely destroyed.
A diode has a small drop across it. If your amp is wall-powered, this voltage drop isn’t a problem because you can pick the wall supply’s voltage to be high enough that this drop is insignificant. If your amp is battery-powered, it’s not so easy to say “use more batteries,” but in fact you have little choice in the matter: D1 on the amp board and D2 on the battery board form a diode OR bridge, which ensures that only the supply with the higher voltage powers the amp. Since the wall supply should always be higher in voltage than the battery pack’s voltage, the amp runs from the wall supply when it’s available and from the batteries otherwise.
Optional? Technically, yes, but you’d better have a very good reason to jumper across it.
This is the power indicator LED. It can be any simple LED.
The holes are large enough to accept 22 gauge wire if you want to mount the LED on long leads for some reason.
The main purpose of R1 is to help balance the op-amp’s input impedances. You want it to be equal to R3 + R5.
R1 interacts with R2 to form a voltage divider. If R1 is much smaller than R2, this effect is negligible, which is the way you’ll almost certainly want it. I imagine someone might choose to configure this to divide the voltage down by a significant amount on purpose, but that’s not the intent of this layout.
Optional? Technically yes, you can jumper it, but you should put a resistor here.
This is the input grounding resistor. Without this resistor, the amp can misbehave if the volume control ever becomes an open circuit. Potentiometer wipers can briefly lift off the tracking surface as they age, for instance.
This resistor should be at least 10× the value of your volume control, or else your source will see the amp as a significantly varying load impedance as you change the volume setting. This may cause it to have different sound characteristics at different volume levels. You shouldn’t make it higher than 1 MΩ, because this will raise the amp’s noise floor.
Together, these rules mean that a 100 kΩ volume control is the highest value you should tolerate. See below for more information on choosing a volume control.
These are the feedback resistors, which set the amplifier topology and the gain. Because the PPA is a high-end amp, the only topology we support is the Jung multiloop topology, which uses all four resistors.
The values given on the schematic are good for most purposes.
The only value you’re likely to need to change is R4, to adjust the gain. You could instead adjust R3, but this would upset the impedance balance at the inputs of the op-amp, increasing distortion.
If you were to use a slow or cheap op-amp, you might want to change R5 and R6, but that goes against the nature of the PPA; you should build a PIMETA if you want to use such op-amps.
Optional? If you want a “true” PPA, no.
This resistor is for tuning the bass boost. It is an alternative to using a bass boost adjustment potentiometer. Click that link to learn how changing its value changes the behavior of the bass boost.
This resistor forms an RC-low pass filter for the op-amp power rails in conjunction with C4||C5. This attenuates any high-frequency noise on the rails, which increases stability of the op-amps. This filter opposes the drop in the op-amp’s PSRR as frequency goes up, giving an overall better PSRR to the amplifier.
The value of this resistor is not critical, but 10 Ω is a good value for most purposes. This gives a corner frequency of 16 kHz with a 1 µF C5. This counteracts the op-amp’s falling PSRR vs. frequency. If you want a lower corner frequency, you must raise the value of C5. Theoretically you could instead raise the value of R8, but as its value rises, so also does the current-modulated rail ripple due to the op-amp’s varying current draw. We decided during development that 10 Ω was the best trade-off here.
Optional? Yes. You can jumper it.
This is the source resistor for the op-amp’s class A biasing cascode. See this section for details of how this works.
R9 is a multiturn trim pot configured as a variable resistor. A trimmer in the 1 kΩ to 10 kΩ range will be about right. The right value depends on how much adjustment range you need, and how fine your control in setting particular values needs to be. If you don’t want to experiment with this, 2 kΩ is a reasonable value.
Optional? Yes. Leave it out if you’re not biasing the op-amps.
If you bias the op-amp into class A with JFET cascodes, you should also add R10. JFETs have an input capacitance, and you don’t want a capacitor across the output of the op-amp. R10 is a kind of insulation between the two, so they don’t affect each other as much.
The exact value of this resistor is not critical, but it does have to fall within a certain range. If it’s too low, it doesn’t do its job very well; 100 Ω is probably the smallest value that will provide sufficient benefit. If it’s too high, it will cause the cascode to fall out of regulation during parts of the swing of the op-amp’s output, negating its advantages; 1 kΩ is about the largest you can get away with. Since you probably are using 1K’s elsewhere in the circuit, it’s simplest to just double up on some of those when ordering parts.
Optional? Populate it if you bias the op-amp into class A. Leave it empty otherwise.
This resistor isolates the input capacitance of the buffer stage from the output stage of the op-amp. Without this resistor, you’re likely to get electrical ringing, or even instability.
There is some wiggle room on the value of this resistor, but for most purposes 1 kΩ is a good value.
For most purposes, 2 kΩ will be fine. You might choose to go with a 5 kΩ unit instead if you need extreme adjustments, such as when you are experimenting with the buffer.
This resistor simply sets a minimum on the buffer bias adjustment. You want some minimum amount of resistance here for best current regulation through Q4.
These resistors should be left at their standard values unless you have some specific reason to change them.
These resistors control the current drive to the output transistors.
Unless you are playing around with the output stage parts, leave these at their standard values. As a rule, lower is better here, but there is some minimum value for stable operation with any particular configuration.
These are the output resistors. Their purpose is to help keep the buffer stable.
Lower values translate into lower distortion, up to the point where the buffer becomes unstable. The default values were determined to be the best balance between low distortion and acceptable stability during development.
Because these resistors can experience high power dissipation at times, you should use at least 1 W resistors here. In many lines of resistors, you can find 2 W resistors that will fit here, if you bend the leads very close to the resistor’s body.
This is the power indicator LED’s current limiting resistor. You use it instead of the LED cut-off circuit if your amp doesn’t use batteries, or you just want a simpler LED configuration. It can be a 5% carbon type; the exact value isn’t at all critical.
Vs is the power supply voltage,
rail to rail
Vf is the LED’s forward voltage drop
If is the desired current through the LED
1 mA gives enough brightness for a power indicator with most LEDs, but some may require a bit more. The limit for most LEDs is 20 to 40 mA, but even approaching that much current makes the LED too bright for my taste.
Typical values for RLED are 1 kΩ to 10 kΩ, depending on the power supply voltage and the LED being used.
Optional? Add RLED if you don’t use the LED cut-off circuit. Leave it out otherwise.
This subcircuit is an alternative to RLED. It works best with battery powered amps that have enough supply voltage that they are able to completely drain the battery pack before they start clipping with your headphones. If this is not your situation, using RLED probably makes more sense. (If you’re not sure which situation pertains, read the article Op-Amp Working Voltage Considerations.)
Rechargeable batteries drop quickly in voltage from their fully-charged state, then stay near 1.2 V per cell for most of their useful life, then drop off in voltage quickly as they deplete the last of their charge. If you use RLED in this situation, the LED will vary in brightness as the batteries drop in voltage. You might be able to look at the LED and make a guess at how much longer the amp will run, but it would be better if the LED directly indicated remaining battery life. That is the purpose of this circuit.
FET and RFET make a constant current source, keeping the LED at a constant brightness even as the battery voltage drops. You will need to try various values for RFET to get a given current through the FET, since each FET is different. I just touch values into the position with the amp’s power on until the LED’s brightness is acceptable. Alternatively, you can pick JFETs by testing them for IDSS and jumpering across RLED. A third option is to use a CRD instead of FET and RFET.
ZNR is a reverse-biased zener diode. In this circuit, it indirectly sets the voltage at which the LED will shut off. NiMH cells are spent at about 0.9 V. To give some warning between the time the LED turns off and the amp starts sounding bad, let’s make the LED turn off when the cells are at 1.0 V apiece instead. Let’s say you have a 12-cell battery pack and a 5.1 V LED. A common zener voltage is 6.8 V; if you add that to the LED voltage drop, you get 11.9 V, close enough. The batteries are dead at about 10.8 V, which should give us enough warning before the amp starts sounding bad. If not, you can swap out the zener for one with a higher voltage drop until you get the behavior you want.
The holes for the zener aren’t big enough to accommodate big power zeners. You should only use the DO-35 type, as their legs are thin enough to fit in the holes.
Optional? Yes. Do not jumper.
This is a pad near RLED. You put a CRD (Current Regulating Diode) across RLED to this pad instead of using FET and RFET. A CRD is in fact a FET with a source resistor, trimmed to a specific current value and packaged as a small 2-lead device. They’re much more convenient than making your own current source with a FET and resistor, and accordingly they’re more expensive. If your time is valuable, they’re a good deal.
I recommend the 1N5283-1N5314 series CRDs, which are available in the DO-35 package.
Optional? Yes. Do not jumper.
Except for the output resistors, the resistor pads on the PPA board are only 300 mils apart, which limits the size of the resistors you can use. Standard 1/4 W metal film and carbon resistors will fit in the board without a problem.
If you use Vishay Dale CMF series resistors, use the RN55 series. These are specified as 1/8 W, but at the temperatures your amp will see, they’re actually good for 1/4 W.
1/4 W resistors are the most readily available sort and the board will accept standard 1/4 W resistors, but I can’t think of a situation right now where the amp could put more than 1/8 W through any of its resistors.
The exception is the output resistors, of course, but they’re big enough for 2 W resistors.
The PPA will only work with single-voltage power supplies unless you modifiy the circuit. If you have a dual-voltage power supply, it’s probably better to run the amp from the outer terminals and ignore the ground lead than to try and hook up the ground lead.
If you wanted to try it anyway, you could run the ground lead from the power supply to the ground plane, and jumper across the TLE positions. This should work, but it is doubtful that it will give improved performance relative to the standard power supply configuration. Since a dual supply costs more than a single, the only reason to try this is out of curiosity.
A power supply voltage somewhere in the 10 to 30 V range will serve you best with this amp. More voltage will probably hurt more than it helps, and lower voltage will require very careful part choices to make a workable amp. For the full ugly details on how to measure and calculate your way to the ideal power supply voltage level for your situation, see my article Op-Amp Working Voltage Considerations. A good “default” value for a wall supply is 24 V, or 12 to 18 V for batteries. There’s not anything special about these values; they just seem to be good all-round practical values.
If you’re going to use batteries, you must use rechargeables; alkalines are not capable of continuously putting out the high current level that this amp requires. There was a companion battery board for the PPA amp, but it is now discontinued. Nevertheless, the pages are still there, complete with details of how it works. It’s simple enough that you could build up something similar on perfboard.
The wall power supply recommended in the parts list is simply one of the few audiophile grade power supplies sold by the major distributors. If you search the forums, you’ll find other recommendations.
Personally, I no longer use off-the-shelf power supplies, since there’s nothing truly great in the general commercial market, just several decent choices. You can do better by DIYing a power supply. I no longer offer PCBs for DIY power supplies, but there are others out there. Plus, there’s always the option of DIYing from scratch. Check out the LM317 datasheet for some ideas.
Whatever power supply you use, it must be an isolated type: i.e. none of the output leads can be electrically connected to the input leads. If it isn’t isolated, the virtual ground setup of the amp will often interfere with the grounding setup of other equipment plugged into the amp. Also, if you use a metal case, virtual ground will probably be tied to the case through the pot chassis and/or the input jacks. In order to avoid the problem of tying V- to virtual ground through the case, the DC input jack needs to be isolated from the case. The recommended DC input jack is plastic for this reason.
The board is designed to accept an ALPS RK27 potentiometer — called by some the Blue Velvet. This is not the only thing that will work, but unless your choice of volume control shares the RK27’s footprint, you’ll have to hand-wire it to the board.
Stepped attenuators can be better than potentiometers for reasons described in my article, Notes on Audio Attenuators. There is a stepped attenuator built as an ALPS RK27 clone, but I tried it once and I didn’t like its performance. If you go with a stepped attenuator, it’ll most likely be bigger than an RK27, and you’ll have to hand-wire it to the board. Keep this in mind when selecting your amp’s enclosure.
The most useful value for a headphone amplifier’s volume control is 50 kΩ. If you choose a lower value, the source can have a significantly harder time driving the amp, in which case the source will sound worse. If you choose a higher value, the amp’s noise floor will rise, possibly audibly. Some people choose 10 kΩ to lower the noise floor when they know their source is strong enough to drive it, but we haven’t found the noise from a 50 kΩ volume control to be a problem. (See the benchmarks.)
It’s not obvious from studying the PPAv2’s circuit that if you’re using a metal case, your output jack needs to be an isolated type. A non-isolated jack connects the ground connection to the chassis; since the chassis is probably tied to input ground by way of the pot or the input jacks, this will short out the ground channel. At best, shorting out the ground channel makes it useless, and at worst it will cause it to become unstable.
The easiest way to deal with this is to use a fully isolated jack, like the Neutrik NJ3FP6C or the Switchcraft N112B. (Part numbers for both are given in the parts list.) Another common type has plastic mounting threads but there’s a metal contact that goes to ground and is meant to touch the inside of the panel when you mount it. You simply have to slip a plastic washer onto the jack’s snout before mounting it to make it fully isolated. The Switchcraft RN112BPC jacks mentioned in the parts list are this way. Keystone has a line of nylon washers that you can use to isolate this sort of jack.
The output buffers in PPAv2 do not have output current limiting. That means that it is theoretically possible for an output short to destroy either the output resistors or the output transistors, or both. Since the tip-ring-sleeve plugs used on headphones have a design flaw that shorts the right channel to ground within the jack while pulling the headphone plug out, it’s worth considering the consequences of an output short.
The output resistors are the easiest to deal with. We made space for up to 2 W resistors here. To push 2 W through 2.2 Ω, you need about 2 Vrms. That much voltage is definitely excessive with a lot of headphones, but it’s only pushing the safety limit a little bit with some. Raising the output resistor to 4.7 Ω pushes the voltage requirement up to about 3 Vrms, a comfortable safety margin. The highest value you should use is 10 Ω — distortion gets too high otherwise — which allows you to get away with putting over 4 Vrms across this resistor.
There’s another aspect to the output resistors: their presence means the output is never actually shorted: there’s always at least 2R in series with any output short, where R is the value of R24/34. Henceforth, we will use this value, rather than considering true short circuits.
As for the output transistors, there is no simple answer here: one has to do the engineering.
Let’s try to get a handle on what it takes to get into trouble with the MJE243/253. We’ll say that the maximum output voltage will be 6 Vrms, which is about 8.5 V, peak. This transistor’s safe operating area curve tells us that the current limit is about 2A with that much voltage across it. To force that much current through two 2.2 Ω output resistors, we’d have to develop 8.8 V across them. So indeed, we are barely able to exceed the limits here. It’s a bit of a stretch, though, because 6 Vrms is excessively loud in all headphones I’m aware of. For practical purposes, then, it doesn’t seem likely that you can kill MJE243/253s with 2.2 Ω output resistors.
Just looking at the front page of the BD139/140 datasheet tells us we’re not going to be so lucky with this transistor. It has a 1.5 A nominal maximum, and with our self-imposed voltage limit, the SOA curve tells us that the real current limit is more like 1 A. We only need about 3 Vrms to push that kind of current through 4.4 Ω. But again, this is still louder than anyone should be listening. We’re on much less firm ground here, but it still seems reasonable to say that we’re not likely to blow up the output transistors.
There’s another side issue that isn’t terribly relevant, but it’s worth examining it to see why because so many people get caught by it: the power supply. A simplistic view is that a wall power supply is likely limited to something on the order of an amp or two. That’s within even the pessimistic limit we set for ourselves with the MJE243/253 transistors. The thing is, the rail cap bank has to discharge before that limit matters. The rail cap bank impedance probably isn’t even 10 mΩ. You can get stupid-high currents with even low voltages with that little impedance in the way. And if your power supply is a battery pack, it’s going to have a continuous current ability of several amps and a flash current ability up in the tens of amps. My advice is, don’t depend on power supply current limiting to save you. Add a fuse at the amp’s DC input if you wish, but it’s the downstream components where you should concentrate your efforts.
In conclusion, there are two practical things you should consider if the lack of output current limiting worries you. First, use MJE243/253 output transistors, or something else equally studly. Second, consider raising the output resistor, possibly as high as 10 Ω.
The PPA board is designed to fit into the Hammond 1455N16 series cases. Lansing also has some suitable cases in their C and E series. We haven’t tried those yet, though, so there may be gotchas we’re not aware of at the moment.
You can also choose some custom case, as long as it will hold a 6.3" × 3.925" board. There are screw holes in the corners of the board suitable for imperial #4 or metric M3 size machine screws, for mounting the board to the bottom of your case.
||Step-by-Step Assembly Guide >>|
|Updated Sun Jan 18 2015 04:24 MST||Go back to The PPA Project||Go to my home page|