Part Selection Guide

Active Parts

For more details about op-amps, see the companion article, Notes on Audio Op-Amps.

These are the output buffers; they do the heavy work of driving the headphones. You only have to use one per channel, along with the corresponding buffer input resistor.

Using just one buffer per channel keeps costs down, and keeps current draw reasonably low for battery powered amps. There are many advantages to adding more buffers per channel, though, so see this section for more information.

Within North America, the best places to get this buffer are Newark and Arrow; they usually keep some stock, though sometimes they run out. There are a couple of other places that can order it, but they almost never have stock.

If you are elsewhere in the world, there are several places you can try. Farnell is probably your best bet. I have also been told that AITEC in Belgium has them.

These are the cascode JFETs, used to bias the op-amps into class A.

The standard parts are 2N5486 (IDSS 8-20mA) and 2N5484 (1-5mA). 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 1mA, 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 10pF.)

You want the JFETs to be able to provide enough current for the op-amp you want to use, 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 5mA, and you've biased it into class A with 1mA through the cascode JFETs. Therefore, you'd need a bit more than 6mA through the Q3s at absolute minimum. The 2N5486 with a minimum IDSS of 8mA would do just fine. Or, you could measure the IDSS of some 2N5459s (range: 4-16mA) 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 find is about 15mA, so setting the Q3s to provide 20mA per channel 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.

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.

Passive Parts

Next, you need to know your power supply voltage. Because the C1's 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 30V supply, 25V caps would be damaged by the power supply, 35V caps are good, and 50V 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 16mm and 18mm diameter caps. The board will accommodate probably 4 of these larger caps, maybe 5 depending on various factors. This isn't a standard configuration, so you'll have to experiment to find out how many caps you can fit in, which will depend on the physical parts you use in the amp's output stage. This won't work in enclosures where the board slides into slots on the inside walls of the case.

Optional? 1 cap required, up to 8 more can be added if you use 12.5mm caps. Do not jumper.

Largest Part Size: Designed for 12.5mm diameter caps, but 10mm, 16mm and 18mm 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 buffers, lowering the impedance of the power rails you'd get if you only used C1s. They are optional, but adding them can improve the dynamic behavior and stability of the amp. If you have a stability problem, add C3 before adding C2, as it is more likely to help.

The value of C2 isn't critical. The 6.8 µF ones used with the PIMETA board will work, or you can just get some smaller generics. The hole spacing allows 0.200" pin spacing, which is a common size for polyester box caps. You should use at least 1 µF here, and if you do some hunting you can find caps up to 10 µF that will fit here. There are suitable caps up to about 3.3 µF from many manufacturers. Beyond that, about the only thing available seems to be the Wima MKS-2s.

Optional? Yes. Do not jumper.

Largest Part Size: 0.400" × 0.400"

These are purely bypass capacitors. If your amp is stable without it, just leave it out. If your amp is oscillating, adding this is a good first troubleshooting step. If it doesn't help, you'll need to add C2 and/or C5 as well.

The proper type and size for the bypass capacitor are a matter of much debate. Some people like ceramics in this position, and others use film caps. The right answer depends on what the amp is doing. Sometimes tuning the cap size and type will let you get an abnormally low ESR right at the oscillation frequency, providing maximal bypassing. Other times, you just need some nearby fast capacitance to quell oscillation. Whatever type you pick, something in the 0.01 to 0.1 µF neighborhood will probably be the most helpful.

Notice that C3 will accept both SMT chip capacitors and leaded capacitors. The SMT pads are spaced to allow PPS film caps, and should also accept some of the larger ceramic types. The pads are sized for 1210 package caps; smaller caps may also fit.

Optional? Yes. Do not jumper.

Largest Part Size: 0.400" × 0.100"

These are local bulk capacitance for the op-amps. Much of the C1 discussion applies to C4 as well.

Unless you leave out the Q3s, C4 is mandatory. (And why would you leave the Q3s out? They're cheap and very helpful.) If you don't add the C4s, the op-amps will almost certainly become unstable.

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 Q3s, which act as current limiters 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.

Optional? Technically, yes, but in practice, no.

Largest Part Size: 10mm diameter in PPA v1.1, 8mm in PPA v1.0

This is for tuning the bass boost. Click that link to learn how changing its value changes the behavior of the bass boost.

This cap has some very uncommon requirements: it should be a high quality linear type (it's directly in the signal path), there isn't much space alotted to it on the board, and the cap line must have many different values in the 0.01 to 0.1 µF range to allow reasonably precise tuning. So far, the only cap we've found with all of these attributes is the BC 416 line, available from DigiKey. The Xicon PF series from Mouser comes close, but the quality isn't as high.

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...after all, 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? Personally, I think there's some merit to this argument, but since the BC 416s (metallized polypropylene) are available, I don't see why one should bother with metallized polyester, unless you just can't get the BC's where you live.

Optional? Yes. Do not jumper.

Largest Part Size: 0.300" × 0.300"

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.

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.

Optional? No.

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 to 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. Click that link to learn how changing its value changes the behavior of the bass boost.

NOTE: This part had no equivalent in PPA v1.0. R8 in PPA v1.0 is now known as R9.

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 ~70Hz with 220 µF on the op-amps' power rails, which is plenty low since all op-amps have excellent low-frequency PSRR. Indeed, you might choose to use a lower value to trade off less PSRR improvement at low frequency for reduced current-modulated ripple on the rails. If you don't understand all that, stay with 10 Ω.

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.

On the PPA v1.1 board, 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, 2K is a reasonable value.

In the PPA v1.0, this part was known as R8, and it was a fixed resistor, not a trim pot. The R9 position on the v1.1 board also allows for the use of a fixed resistor. If you're building a v1.0 PPA or you want to futz with fixed resistors on a PPA v1.1 board, you will need an assortment of resistors on hand in the 10 Ω to 1 KΩ range. You don't need to have every value in this range. It might be simplest to get a 5% carbon film resistor selection kit, which many manufacturers offer. Follow the link above for information on matching the bias point of the channels using fixed resistors.

Optional? Yes. You can jumper it in some situations if you're using the cascodes, and if you're not using the cascodes you should leave it empty.

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 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.

	RLED = (V+ - Vf) / If

where:

V₊ is the power supply voltage, rail to rail
V_f is the LED's forward voltage drop
I_f 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. You don't want it to be too bright, or it's annoying. 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.

FET and RFET make a constant current source, keeping the LED at a constant brightness no matter what the battery voltage is. 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 zener diode arranged so that it will cut off power to the LED when the voltage across it drops below its zener voltage (Vz). The voltage across ZNR is the supply voltage minus the voltage drop across the LED minus 0.7V for D1.

All together, these pieces keep the LED at a constant brightness until the voltage drops to a certain point, and then it cuts the LED off, to warn that the battery voltage is low. Assuming you want to drain your battery pack fully, calculate its minimum useful voltage by multiplying the number of cells by 0.9V. Then, subtract the forward voltage drops of LED and D1 from that, and you will have the ideal Vz. You should pick a higher Vz rather than lower, given the choice, in order to give you some warning.

Let's work through an example. Let's say you have a 12-cell battery pack and a 3.6V LED. Minimum pack voltage is 10.8V, and subtracting 3.6V and 0.7V gives 6.5V. The closest two common Vz values to that are 6.2V and 6.8V, so we'll pick 6.8V. Thus, the LED should be off when each cell has 0.925V across it.

There is one caveat with this circuit: because a real zener doesn't have an infinitely sharp 'knee' in its voltage curve, the LED doesn't shut off quickly unless the voltage is dropping rather fast. If you pick Vz such that the per-cell voltage is more like 1.1V or higher, the pack voltage won't be dropping all that fast, so it'll take a while to shut off. Even down around 0.9 to 1.0V, where the voltage of a NiMH cell is dropping pretty quickly, it can take a short while for the LED to dim and go out. A tension against this fact is that you often want a fair amount of warning, so you might be tempted to pick a higher Vz value than calculated above. It's up to you whether a slowly dimming LED is good enough for your purposes.

The holes in the PCB for the zener are only large enough for the leads of small DO-35 zeners, not the bigger DO-41 types with their thicker leads.

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.

Resistor Sizes

The resistor pads on the PPA board are only 300 mils apart, which limits the size of the resistors you can use. Standard 1/4W 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, not the RN60s. The RN55s are specified as 1/8W, but at the temperatures your amp will see, they're actually good for 1/4W.

A power supply voltage somewhere in the 10 to 30V 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.

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 is a companion battery board which includes a charging circuit so you never have to remove the batteries from the amp until they're completely dead. See that section for info on how using the battery board affects the wall supply voltage you need.

We will also have a companion wall power supply at some point. Until we get that sub-project finished, you will have to use a standard wall wart or build your own power supply. While the PPA is very tolerant of power supply noise, it is best to use a regulated power supply with this amp.

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.

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.)

Choosing an Output Jack

There are two considerations for the output jack that might not be obvious from studying the circuit design.

First, 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 virtual ground (i.e. input ground) through 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 instability in the ground channel.

The easiest type of jack to deal with is fully isolated, like the Neutrik NJ3FP6C and Switchcraft N112B, mentioned in the parts list. (The Neutrik jack has a metal body, but the ground connection isn't tied to it.) 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.

The second consideration is that the design of the 1/4" and 1/8" plugs have a weakness: as you insert it or pull it out, there's a point in most jacks where the right channel connection on the plug will short out between the right and ground contacts inside the jack. If the amp is putting out a significant voltage, Ohm's law tells us that current becomes infinite. Since the PPA's output buffers don't have output protection, this momentary short will likely fry the buffers. Nothing but luck will save you if you plug the headphones in or remove them while music is playing, but the recommended Neutrik NJ3FP6C helps here because it's a locking type. Because it isn't easy to remove the plug once it locks down, it reminds you to think before you unplug the headphones. The superior friction of all 1/4" types does help with this to some extent. As a result of this issue, I wouldn't recommend 1/8" output jacks at all.