Part Selection Guide

Passive Parts

C1 (film)

To understand the issues involved in deciding whether to add C1 or to put a jumper across this position, read my article Input Capacitors for Headphone Amps.

Many types of capacitors will fit here, up to 0.6" pin pitch. You should have no problem finding polypropylene types with large enough values to fit here.

Optional? Yes, jumper across it.

Largest Part Size: 17.5mm × 9mm

C2, C3 (electrolytic)

These are the main power reservoir capacitors.

If you just want me to tell you what will work here, use 330μF or 470μF capacitors with voltage ratings higher than that of your power supply. For example, use a 25V capacitor if your power supply is 24V. I recommend the Panasonic FC and Nichicon PW lines; in the US, they’re available from DigiKey and Mouser, respectively. If your chosen distributor doesn’t carry one of these lines, try to find a cap line that features long life and low ESR. Populate all four positions.

If you want to choose your own power capacitors, there are two main rules to keep in mind:

Sometimes you must compromise on quality (rule 2) to get a sufficient amount of capacitance (rule 1). For a META42, the minimum I recommend is four 220μF caps or two 470μF caps. If you’re looking at caps over 1000μF 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 not to buy that 2200μF capacitor, but instead get the 1000μF one and save some money. I doubt you can hear the difference.

Now to more specific advice.

First, decide on the capacitor’s dimensions. The diameter should be 12.5mm or 10mm, as these fit best on the META42 board. If it’s skinnier, the lead pitch will be too narrow for the cap to securely mount on the board. If it’s too fat you’ll have to use just the C2 or just the C3 positions, or you’ll have to mount the capacitors off the board, neither of which is a great idea. The height will be limited by the amount of space above your board inside your case. You don’t have to use the tallest cap that will fit, you just need to keep this in mind as a limit.

Next, you need to know your power supply voltage. It’s best to use caps with a voltage rating that’s higher than your power supply’s maximum output voltage, but no higher. For instance, if you have a 30V supply, 25V caps could 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.)

Optional? C2, no, C3 yes. (Or vice versa, if you want to be difficult. :) ) Do not jumper.

Largest Part Size: 12.5mm diameter

C4 (film)

These are the high-speed reservoir caps: they provide current briefly while the big and slow electrolytics get around to discharging. They are optional, but adding them can improve the dynamic behavior of the amp. Try to find the largest film type capacitor you can that will fit; the hole spacing allows 0.200" pin spacing, which is a common size for polyester box caps. You should use at least 1.0 μF here, and if you do some hunting you can find caps up to 10 μF that will fit here. You can probably find suitable caps up to about 3.3 μF without too much work. Beyond that, the hunting gets hard.

In my humble opinion, the ideal capacitor for C4 is the Wima MKS 2 series of box caps. Most film capacitors are long and thin, but the Wima MKS 2 series has a squarish footprint which is a perfect match for the boxy section of the PCB given to C4. Another bonus is that larger capacitance parts in this series tend to be taller rather than wider, so they take up very little board space. All of the PCM 5 parts in this series will fit in the C4 position, up to and including the 6.8 μF 50V and 10 μF 16V parts. (These two are the same physical size.)

Optional? Yes, do not jumper.

Largest Part Size: 10mm × 12mm

C5 (film, ceramic, tantalum)

This is a bypass capacitor. You only need it if the op-amp oscillates. Adding a capacitor here can help quell oscillation, but C4 plus the multiloop topology is usually enough to make the op-amp happy. If you’re using multiloop and you get oscillation, first bring the inner loop gain up as high as you dare. If that doesn’t help, put a largish film capacitor in the C4 position. Only if you still get oscillation should you consider putting something in C5.

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. Consensus among everything I’ve read says that C5 should be in the 0.01 to 0.1μF neighborhood. A popular option for bypass is to also parallel a 1 to 10μF tantalum farther away from the op-amp chip in addition to the ceramic or film, because it has synergistic effects. (Different capacitor types have different self-resonances.) If you choose to use a large tantalum, that should go in the C4 position. Using a film cap in C4 is probably a better idea, though. Tantalum is nice only for its compactness.

You should examine your op-amp’s datasheet to see what it advises. Also, there are many web pages and application notes online that discuss the pros and cons. My favorite application notes on stabilizing fast op-amps are:

Op Amps for Everyone by Ron Mancini et al., 2083 KB PDF

High Speed Amplifier Techniques by Jim Williams, 5223 KB PDF

Optional? Yes, do not jumper.

Largest Part Size: 4mm × 12mm

R1

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.

The main purpose of R1 is to help balance the op-amp’s input impedances. I’m not sure how to calculate a proper value for this resistor; 1 kΩ or less seems to work well, however.

I recommend that you populate this position.

Optional? Yes, jumper across it.

R2

This is the input impedance resistor. It must be significantly higher than the highest resistance of your volume pot, or else your source will see the amp as a varying load impedance as you rotate the volume knob. This may cause it to have different sound characteristics at different volume levels. You should make R2 10× the value of the pot, if you can get away with it. You can go higher, but it won’t improve the performance of the amplifier.

Optional? No.

R3 through R6

These are the feedback resistors, which set the amplifier topology and the gain. Click here for details about how to pick values for these parts.

Optional? In some circumstances, some of these parts are optional, and it’s a mixture of jumpering and leaving parts out entirely. Follow the link for details.

R7

This series resistor decreases ringing on the op-amp’s output at high frequencies. (That’s the electrical engineer’s use of the word ringing, not the bell type of ringing.)

This ringing may or may not be audible, but it is measurable. If you don’t want to measure your particular setup with an oscilloscope to find the precise value required, just put a 47 to 100 Ω resistor here. I recommend you use a 100 Ω resistor unless you don’t have any but you do have some 47’s and you don’t want to buy some 100’s.

If you want to be frugal, you can jumper across R7, but it isn’t recommended.

Optional? Yes, jumper across it.

R8 and R9

If you are getting audible hiss at normal listening volumes with the source disconnected, put a 47 to 100 Ω resistor at R8 or R9. (This mainly happens with low-impedance headphones.) There are frequent arguments in the audio DIY forums about whether it’s better to put the output resistor inside (R8) or outside (R9) the feedback loop, and I don’t intend to try to settle the argument here. I suggest you initially jumper across both positions and only put in an output resistor if you have hiss problems.

Don’t populate both positions — populate one or the other, or neither.

Optional? Yes, jumper across the unused position(s).

R10

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 appreciable 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 need 1K’s for R1 and 100’s for R7, it’s simplest to just double up on some of those.

Optional? Yes. Populate it if you bias the op-amp into class A. Leave it empty otherwise.

RLED

This is the LED current limiting resistor. Use Ohm’s law to figure current given the LED’s voltage drop and the power supply voltage. For example, consider a 1.8V LED with a 15V power supply and a 4.7 kΩ RLED:

	I = V/R
	I = (15 - 1.8) / 4700
	I = 13.2 / 4700
	I = 0.0028
	I = 2.8mA

Most LEDs require 1mA to get a minimal amount of brightness. More current gets you more brightness, but of course uses more power, which mainly matters with battery power supplies.

Typical values are 1 kΩ to 10 kΩ. I personally use 2.2 kΩ and 4.7 kΩ most often.

Optional? Yes. If you don’t use an LED, do not jumper here. If you use an LED with an integral resistor, jumper across this position.

ZNR, CRD

These are an alternative to RLED. The effect is to make the LED stay at a constant brightness until the voltage falls below a particular threshold and then turn the LED off. This is great for battery-powered amps because you can tune the circuit so that it tells you when to change the battery.

A CRD is a Current Regulating Diode, meaning that it passes voltage in only one direction and it maintains a steady current level no matter what voltage level you give it. This performs the same function as RLED, current-limiting, except that it gives a current level independent of supply voltage. Since the current stays constant, the LED’s brightness stays constant until the shutoff feature kicks in. (For what it’s worth, you could use a CRD in place of RLED, if you don’t need the LED shutoff feature, or you can use a resistor in place of CRD if you don’t really care for the constant brightness feature.)

ZNR is a zener diode, which works like a regular diode in that it conducts in one direction given a small voltage difference, but it will also conduct in the other direction if you give it a larger reverse voltage difference. A common zener might conduct in one direction given a 0.7V or greater difference and in the reverse direction given a 5.1V or greater difference. The zener indirectly sets the voltage at which the LED will shut off.

Setting this circuit up correctly requires some work. The main thing you must figure out is the minimum working voltage for your op-amp. Follow the link for the detailed procedure.

Now, if you use two or more different kinds of headphones, the working voltage will be different for each one. In this case, this LED cutoff circuit is only useful if the batteries are fully drained before any of your headphones begins sounding bad. If any of your headphones start sounding bad before the batteries are fully drained, the LED will still be on when it’s time to change the batteries with your voltage-hungry headphones, or it will turn off while there is still usable battery capacity with your voltage-efficient headphones. In either case, you’re probably best off not using this feature.

Once you know your minimum working voltage, you need to look up the voltage drop of your LED. This voltage plus the zener voltage tells you the point that the LED will be completely dark. Unfortunately, this circuit doesn’t have a perfectly sharp cutoff point. Between the cutoff voltage v and about v + 0.5V, the LED brightens rapidly compared to an 0.5V power supply swing when using RLED. Beyond this point, it brightens less rapidly until about v + 1.5V where you get full brightness. This is a lot better than the behavior with RLED, but it’s imperfect. Such is life.

This circuit works best if you’re using rechargeable batteries and a high voltage. The advantage of rechargeables in this situation is that at the end of their usable life, they drop off in voltage quickly, unlike an alkaline battery. This ensures that the cutoff feature works over a matter of minutes instead of an hour or more. The advantage of having a high voltage is that your batteries will be depleted before your op-amp starts sounding bad due to clipping. For example, you might find that the minimum working voltage for your amp with your most difficult headphones is 12V, but if you’ve got 16 AA-style rechargeables powering the circuit, the batteries will be unusable before they hit 12V. With rechargeable AA’s, they stay at 1.2V for most of their working life, so the normal voltage for your amp is 14.4V. If you set this circuit up so that the LED turns off at oh, say, 13V, the LED should turn off as the batteries are giving up the very last of their juice. I’m making these numbers up, so you’ll need to experiment to find the actual numbers.

When picking parts for this circuit, you should know that the holes in the board are the standard size for resistor leads. (29 mils?) There are many zeners with rather thick legs, apparently for high power applications. The zeners that work best are in the DO-35 package. For CRDs, I recommend the 1N5283-1N5314 series, which are also available in the DO-35 package.

Optional? Yes, do not jumper.

D1

This is an optional "crowbar" diode. If you put a diode here, it will normally be unused, since it’s reverse-biased with respect to proper power supply connection. But if the power supply is connected backwards, this diode will short-circuit the power supply so that your amp circuit’s components aren’t damaged. If the power supply is a battery, it will make the battery overheat and possibly leak, but that’s preferrable to frying your op-amps and buffers.

You can use any old diode here, but standard types tend to be rated for high voltage and relatively low amperage. The ideal part for this application is a Schottky diode — these are typically only good for 20-40V, but higher amperage than standard diodes. Perfect. The board is designed to accept diodes in DO-41, DO-201 and TO-220 packages. Readily-available Schottkys that will work here are the 1N5817-5819 (20-40V, 1A), the 1N5820-5822 (20-40V, 3A) and the 10TQxxx (30-45V, 10A). If you only need a 1-amp crowbar, the standard-type 1N400x diodes may be a better alternative since they’re cheap and as common as dirt — even Radio Shack has them.

Those aren’t the only options. Other examples are the 3 amp 1N540x series diodes, and the 5 amp HER50x series. Radio Shack carries all of these, or you can get them at Mouser or other places.

If you’re using AC power, picking the proper diode amperage rating is easy — the power supply will have a maximum amperage spec, and you simply pick a diode based on that. (But see below for a better alternative.)

Batteries are a different matter. A shorted 9V battery will initially put out 4-5A and will take about a minute to ramp down to below 1 ampere. If you use a 1A diode like a 1N400x or a 1N5817-9, you will at least make it get hot enough to burn you, and you may make it fail. If it fails "open", the crowbar will fail to protect your amplifier from the reverse voltage. If you use a 3A diode like a 1N5820-2, that may be safe enough, since a 9V battery obviously drops from 5A down to 3A in much less time than it takes to get from 5A down to 1A. I was able to get a 1N5821 warm, but not hot. If you’re using batteries in parallel, remember that the current increases for each additional battery. Batteries in series have a current rating only as high as a single battery.

Another thing you have to pay attention to is that some Schottkys have a lower reverse voltage spec than their forward voltage spec. It’s reverse voltage you care most about, since that’s how the diode will be used virtually all the time. For example, the 1N5817 has a 20V forward voltage tolerance, but only a 14V reverse voltage tolerance. If you were using a pair of 9V batteries in series, this diode would likely fail in normal operation, since it’s reverse biased except when the batteries are plugged in backwards. You’d need to step up to the 1N5818, which has a 21V reverse voltage tolerance.

If you use the crowbar, you might consider also adding a fuse to the amplifier. If you add, say, a 500 mA fuse, it will only blow if the current in the amp gets extraordinarily high, which should only happen when the amp has been crowbarred. (If you have buffers stacked high on your amp outputs, a high load or an output short could also blow the fuse, but that’s probably a good thing for a headphone amp.)

Optional? Yes, do not jumper.

D2

The crowbar method works best for battery-powered amps. For wall-powered amps, a better alternative is to put a diode in series with one of the power supply wires. This is the purpose of D2.

The disadvantage of D2 is that it will drop the power supply voltage by 0.6 to 0.9V, depending on the diode type. That matters with batteries because it means your battery lifetime is shortened. For wall-powered amps this small voltage drop won’t matter.

D2 has a few advantages: 1) the diode’s current rating can be much lower than if you’d used D1 for power supply reversal protection; and 2) if the power is connected backwards the circuit is simply broken, rather than short-circuited.

If you use D2, you should connect the leg farthest from the power supply wire pads to the pad closest to C2-. That is, you should cross the power switch pads with the diode. Then the power switch must go inline with one of the power supply leads; you can’t use the power switch pads on the board when you use D2.

Optional? Yes, do not jumper.

Are 1/8W Resistors Sufficient?

1/4W resistors are the most readily available sort and the board will accept standard 1/4W resistors, but it takes extraordinary circumstances to make the circuit put more than 1/8W through one of the resistors. If you loaded the amp abnormally, R8 and R9 would be the first to exceed this limit.

R9 shouldn’t ever be lower than 47 Ω. With a dead short on the output, it takes about 2.4V to put 1/8W through a 47 Ω resistor. Almost all headphones reach maximum listening volume with less than 2.4V. If you’re using R9 and one of the few headphones that require more voltage than this, just be careful not to short the amp’s output while playing music. With the headphones plugged in, the headphones' impedance adds to the value of R9, so it requires even higher voltages to risk damage to R9.

R8 is a different story because it doesn’t load the output of the amplifier. If you put a dead short across the output of the amplifier and play music through the amp, you could potentially damage an 1/8W R8. If you go up to 1/4W, you’d need to stack output buffers to have a chance at damaging an 1/8W R8. With headphones plugged in, the headphones limit the current the amp can put out, so R8 can only be damaged if you’re running the headphones so hard that you’re likely to damage the headphones or your ears before R8. If you try to make the amp power speakers directly, you could damage R8; you deserve to smoke some resistors if you try this stunt.

Resistor Sizes

The resistor pads on the META42 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. These are 1/8W, but as I explained above, 1/8W is sufficient. RN60s will not fit in the board without creative mounting.

Single Voltage or Dual Voltage Power Supply?

The META42 board is designed for single-voltage power supplies. These have just a positive and a negative terminal. These are connected with wire to the + and - pads on the board. There’s a "rail splitter" circuit on the board which then splits this single voltage into a virtual dual voltage. For instance, a single 24V supply effectively becomes +/-12V. The TLE2426 is the rail splitter proper, and 2001G is a buffer for the rail splitter.

If you leave the rail splitter and its buffer out, you can instead use a genuine dual voltage power supply. A dual supply has 3 terminals: +, - and ground. You run + and - to the board just as you would for a single-voltage supply, and you run ground to the ground bus running around the edge of the board.

I’ll assume you’re using a single-voltage supply from here on, since that’s the standard configuration. If you want to use a dual supply, you’re on your own.

Choosing a Power Supply

A power supply voltage somewhere in the 9 to 24V range will serve you best. 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, it’s simplest to put them all in series. This gives you a single supply. It’s possible to make a dual supply with batteries, but there are dangers in this; do a search in the Head-Fi DIY forum archives, it’s been discussed before. A third option is to put two or more batteries in parallel, for increased battery life.

If you’re going to use a wall power supply (a.k.a. "wall wart", "AC/DC transformer"), you also need to take care of the type of power supply you use. The two words you need to look for are regulation and isolation.

An unregulated power supply’s output voltage will fluctuate as the wall voltage fluctuates. Also, unregulated power supplies tend to be cheap all around, so they’ll have a lot of ripple and noise on their outputs. The META42 design doesn’t do much to eliminate noise and ripple (N+R) on the power rails; the power caps and the op-amps and buffers will reject some of the N+R, but they won’t get rid of all of it. (See my Op-Amp Power Supply Quality Considerations article for more info.) If there’s enough N+R on the rails, it can get into the amp’s output in audible levels.

A regulated power supply’s output will not fluctuate as the wall power fluctuates, and they generally have much better N+R behavior than cheap unregulated wall warts. Most power supplies that are simply called "regulated" are in fact switching power supplies. These can have more noise than unregulated supplies, because of the way they work. The better type of regulation (for audio, anyway) is linear regulation. If it doesn’t say "linear regulated", assume it’s a switcher.

Your power supply also needs to be isolated, which means that the output leads are not directly connected to any of the input leads. It’s common for non-isolated single-voltage supplies to tie their V- on the DC output side to the earth ground connection on the AC side. Since the META42 uses a "virtual ground", tying V- to earth ground can cause problems, since this places the virtual ground several volts above earth ground. If you plug the amp into a source component that uses earth ground for its outputs, the amp’s virtual ground will sit there fighting against the source’s true earth ground. The amp probably won’t win that fight.

If you use an isolated supply, the amp’s virtual ground can "float" to whatever level is required by the source, and V+ and V- will float right along with the virtual ground. Any supply with a transformer directly between the AC side and the DC output (i.e. most linears and unregulated supplies) is isolated. There are ways to make a switching power supply isolated, but check to be sure: most switchers are not isolated, in my experience. Because it’s uncommon, if you have a switching power supply that doesn’t say that it’s isolated, assume that it isn’t.

Bottom line: use linear power supplies if you can afford them, or follow an unregulated supply with a linear regulation stage, or use a really good, isolated switching power supply. Or use batteries, and avoid this whole rigamarole. :)

Choosing an Op-Amp

The op-amp (operational amplifier) is the chip that does the actual amplification in the META42 circuit. It has the single biggest effect on sound and power draw of any component, so it behooves you to pick this part carefully.

Most any op-amp can be made to work in the META42 board, but some are more suitable than others. The simplest to use are FET-input op-amps. If you use an op-amp with bipolar inputs, you will have to do a lot of careful design to minimize DC offsets on the op-amp’s output.

The canonical part for this amp is the Analog Devices AD823. This part is inexpensive, easily available, sounds good, is a dual-channel chip, has low current draw, and works well down to very low supply voltages (3-5V). This part has a bit of an aggressive sound, which may mate well with your headphones and music, or it may cause problems. If you have laid-back headphones and music, this chip may give your system the "snap" it needs to sound better. If your headphones or music are already aggressive, the combination may be annoying.

There are better, more balanced chips in Analog Devices’ line, such as the AD843, the AD825, and the AD8610. These are all single-channel chips, so you need two of them and a BrownDog adapter: single DIP-8 to dual DIP-8 for the AD843 and single SO-8 to dual DIP-8 for the AD825 and the AD8610. BrownDog also has a simple SO-8 to DIP-8 adapter that you can use for converting SO-8-only dual-channel op-amps like the AD8512 to DIP-8, but you don’t strictly need this one since the META42 v2 board has an SO-8 pad for dual-channel chips on the bottom of the board. You would use this adapter if you wanted to socket the chip using a DIP-8 socket.

The AD8620 deserves special mention. This is the dual version of the AD8610 mentioned above, whicn means that it can be soldered directly to the META42 board, or you can use the simpler SO-8 to DIP-8 BrownDog adapter if you want to socket it. The AD8620 has power specs nearly as good as AD823’s and it sounds better. It’s quite a bit more expensive, though: $12 for the AD8620 chip vs. $6.77 (Newark) for the AD823. It’s also harder to find, so I’m offering it along with the META42 boards for your convenience.

You can get Analog Devices chips through Newark, RS Components, or direct from Analog’s web store.

Burr-Brown also makes some nice chips. A cheap and workable chip for the design is the OPA2132PA. A better-sounding chip with the same sonic signature (and a higher price tag!) is the OPA627AP. Both of these chips are available in higher grades, but I personally can’t tell them apart from the lower grade chips, sonically. You should also consider the OPA637, which is the same as the OPA627 but with an internal limiting capacitor removed which allows it to sound better, but it requires a higher gain to be stable. This is not a problem if you use the Jung multiloop topology; I typically use an inner loop gain of 200 and an outer loop gain of more like 5 with this chip. Burr-Brown chips are available from DigiKey and RS Components.

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

Choosing a Volume Control

First you need to decide which model you’re going to use. The default choice is the cheap but good and small Panasonic EVJ-C20 series pot. This part is an excellent value, it works well, and you can get it quickly from DigiKey. The main alternative is the big, nice and expensive ALPS RK27112 pot, which some call the Blue Velvet. This part is expensive as pots go, and you have to pay special attention to grounding when using it. Your third choice is to use something else entirely. There are two other footprints on the board for Bourns 51 and Noble XVB93 pots, but neither of these are readily available at this time; however, they may become available in the future, and in the meantime, these are common pin layouts so you may find that your pot of choice fits these footprints already. If you can’t board-mount your pot for some reason, see the pot wire pad labeling section of this document for info on how to wire your pot to the board.

There’s a new and strange fourth choice now. Apparently some company has built a stepped attenuator into a package similar to that of the ALPS Blue Velvet. It’s marked as an ALPS pot, but the compnay claims not to know about this part (labeled RH2702) and the web site doesn’t have any information on it. One guess is that it’s made by a Chinese company called Soundwell. I’ve measured one, and it’s got excellent channel matching. It’s a bit tough to turn the knob, and there’s a click in between settings unless you turn it quickly, but it fits the board and it’s inexpensive. You can buy them for $6 each with a minimum order of 4 by contacting Wai Kei Leung of Wai Yip Electronics, Hong Kong. You should read the Head-Fi thread about these parts before buying.

Next you need to decide on a pot value. The maximum reasonable range is 10K to 100K. If you use a pot lower than 10K, you put too much load on the source, which can cause it to sound bad. 10K is high enough that you shouldn’t have any problems with source loading, though some people say there are exceptionally weak sources that will have problems even driving a 10K load. 50K is said to be a well-established input impedance. It’s a good middle ground, and so if you can find the pot you want in this value, it’s probably as close to ideal as you can get. 100K and above have their own problems. If the input impedance is too high, you can get noise and such. The only reason to go this high is if the pot you want is only available in these higher values.

By the way, if you go with a Panasonic EVJ series pot, make sure you get the C20 type, which is the horizontal mount version. (i.e. For horizontal boards if the shaft is to be horizontal.) There’s also a Y10 version for vertical boards and horizontal shafts. That part might be used on a stereo receiver where there’s a large PCB behind the front panel. Also, be sure to get the "D" taper. There are several other tapers, and since the curves aren’t given in the datasheet, I can’t say in what situations these others would be useful. I’ve only used the D curve ones, and so those are the only ones I can recommend.

Sources for ALPS Blue Velvet pots:

Angela Instruments — USA. Rather expensive, but reliable and quick to deliver.

Percy Audio — USA. He sells the motorized version only, and has a reputation for being slow to deliver.

RS Components — Basically worldwide, except USA. (Part# 236-9604 for 50 kΩ version) Reasonable price, and a reputation for great service.

Audiograde — UK. Cheaper than Angela, but it’s just one guy with a personal web site. The guy’s honest, but slow to bill customers and ship product.

THLAudio — Taiwan. Their price is quite low, but probably the slowest shipping option for most people. Also, they make you order via fax, which is a pain.

Octave Electronics — Malaysia. Very slow to respond, at least to quotes for people in the US. Prices in the midrange of others I’ve received.

Choosing an Enclosure

The META42 board will just barely fit in PacTec HML-9V and Serpac H-65-9V series cases. You can build a workable amplifier in either of these cases, but going with a larger case will allow more flexibility.

If you go with the Serpac H-65, you will have to use a Panasonic EVJ series pot or something even smaller. Also, you can’t board-mount the pot, because it’s taller than it is wide — you would have to turn it on its side and hand-wire it to the board. If you go up to the H-67 case, that adds enough case height that you can board-mount a Panasonic EVJ pot. You won’t be able to use the ALPS Blue pot with that case, though, because that’s a very large part. (It’s roughly a 25mm cube, not counting the shaft parts.) The H-67 is also not available in the clear colored varieties that the H-65 comes in.

The PacTec HML case (note: no "-ET") looks just barely tall enough to allow a board-mounted Panasonic EVJ pot. The HML-ET will definitely allow that, and also the ALPS Blue pot.

If you want to add crossfeed, large caps, and other large components, you will probably have to go with yet larger cases, or pack a small case tightly with parts.


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