While reading through these parts lists, you may find it handy to have a copy of the schematic to refer to. (PDF, 4 KB)
These are the parts required to build just the amplifier circuit. Enclosures, optional components, and enhancement components are in the next section.
I’ve been happiest with the parts from Digi-Key and Newark. Mouser sells cheap parts, in both senses of the word, so you do have to be careful when buying from them.
For those not in the US, I’ve added RS Components, who will ship almost anywhere in the world. All the parts numbers are simply a result of me searching through their online catalog. Many times I was able to find the same parts I recommend from the other distributors, but occasionally I had to just guess at a replacement, hoping that it is suitable.
Of the US distributors, Digi-Key is probably the best for people outside the US to use. Other distributors those outside the US might investigate are Farnell and Conrad. Farnell is actually part of the same company as Newark, but their part numbers aren’t unified, and they don’t carry all the same parts.
You may not be able to get all of your parts from a single source. If you are unwilling to order from multiple sources, your best bet is to choose your distributor based on who has the op-amp(s) you want to use in your amplifier first, since that is the part in this amp most likely to push you to one distributor vs another. If you are flexible on your choice of op-amp, you will be more able to single-source your amp’s parts from any of the distributors mentioned. (That said, you may want to read my companion article Notes on Audio Op-Amps before ordering.) For raw beginners, I recommend that you stick with the OPA13x series op-amps, because they’re simply the most forgiving op-amps that also sound good.
The next part most likely to force a choice of distributor is the protoboard. See below for the choices I’m currently aware of.
For several of the parts in this amp, higher quality components will give a noticeably superior build, either in terms of sound quality, durability, or appearance. Examples are the op-amp, the switches, and the enclosure, respectively.
Other parts, primarily the passive components, are commodities, such that many manufacturers provide essentially interchangeable parts. That is not to say that there are not special alternatives, but that you probably can’t tell one generic blue metal film resistor brand from another just by listening to it in this amp. If you’re wondering whether to buy Yageo, BC, or Multicomp resistors, you’re probably obsessing about something that will have zero noticeable effect on the amp’s sound quality. Unless you’re choosing to buy something superior to commodity parts — and for that, see Precision Resistors below — I recommend that you do not obsess over whether to get your resistors from one source versus another. Feel free also to substitute other generics with equivalent specs, if that is convenient for you.
By that, I do not mean to endorse replacing generic components recommended below with nonequivalents. I have seen people build this amp with 5% carbon resistors instead of the 1% metal film resistors I recommend, for example. For just one reason why you should not do that, please read my companion article, Hand-Matching Resistors to Tighter Tolerances.
There is one component that I would not discourage compromises on, and that is the protoboard, because the primary benefit of the expensive stuff is that it will stand up to repeated soldering and desoldering. Since you’ll be building the amp once and probably then leaving it alone, you might as well use cheap protoboard. There are other benefits to FR4 over phenolic paper board, but they are not likely to affect the behavior of this sort of circuit.
Some components are marked “Alt.” These are alternatives for the immediately preceding component.
| Description | Qty | ID | Digi-Key | Newark | Mouser | RS Components |
|---|---|---|---|---|---|---|
| 220 µF/35 V electrolytic capacitor, radial leads | 2 | C1 | P5552 | 18C4706 | 647-UVR1V221MPA | 315-0726 |
| Alt. 220 µF/25 V electrolytic cap | 2 | C1 | P5541 | 14M6722 | 75-517D25V220-E3 | 365-4133 |
| Alt. 220 µF/16 V electrolytic cap | 2 | C1 | P5530 | 18C4617 | – | 365-4060 |
| 0.1 µF polypropylene film cap (large!)* | 2 | C2 | P3104 | 89F3466 | 75-715P200V0.1 | 240-5384 |
| Alt. 0.1 µF metallized polyester cap (smallish) | 2 | C2 | E1104 | – | 75-MKT1813410014 | – |
| Alt. 0.1 µF metallized polyester cap (really small) | 2 | C2 | 3013PH | 16M2117 | 505-MKS2.1/63/10 | 179-9880 |
| 10 kΩ 1/4 W metal film resistor | 1 | RLED | 10.0KXBK | 58K3797 | 271-10K-RC | 148-736 |
| 4.7 kΩ 1/4 W metal film resistor | 2 | R1 | 4.75KXBK | 58K3862 | 271-4.7K-RC | 148-663 |
| 100 kΩ 1/4 W metal film resistor | 2 | R2 | 100KXBK | 58K3798 | 271-100K-RC | 148-972 |
| 1 kΩ 1/4 W metal film resistor, gain 11 | 2 | R3 | 1.00KXBK | 58K3796 | 271-1K-RC | 148-506 |
| Alt. gain resistor, 2.0 kΩ, gain 6 | 2 | R3 | 2.00KXBK | 58K3823 | 271-2K-RC | 148-578 |
| Alt. gain resistor, ~2.5 kΩ, gain 5 | 2 | R3 | 2.49KXBK | 58K9120 | 271-2.55K-RC | 148-590 |
| Alt. gain resistor, 3.3 kΩ, gain 4 | 2 | R3 | 3.32KXBK | 58K3844 | 271-3.3K-RC | 148-629 |
| Alt. gain resistor, ~4.7 kΩ, gain 3 | 2 | R3 | 4.75KXBK | 58K3862 | 271-4.7K-RC | 148-663 |
| Alt. gain resistor, 10 kΩ, gain 2 | 2 | R3 | 10.0KXBK | 58K3797 | 271-10K-RC | 148-736 |
| 10 kΩ 1/4 W metal film resistor | 2 | R4 | 10.0KXBK | 58K3797 | 271-10K-RC | 148-736 |
| Dual op-amp OPA2132PA | 1 | OPA | OPA2132PA | 61M2752 | 595-OPA2132PA | 218-8281 |
| Alt. dual op-amp OPA2132P | 1 | OPA | OPA2132P | 35C1844 | 595-OPA2132PA | – |
| Alt. dual op-amp: OPA2134PA | 1 | OPA | OPA2134PA | 74K3897 | 595-OPA2134PA | 285-8069 |
| Power indicator LED, red diffused, 3mm | 1 | D1 | 160-1708 | 14N9386 | 859-LTL-1CHE | 588-386 |
| Alt. power LED, amber/yellow | 1 | D1 | 160-1709 | 14N9405 | 859-LTL-1CHY | 588-392 |
| Alt. power LED, green | 1 | D1 | 160-1710 | 14N9406 | 859-LTL-1CHG | 589-014 |
| Stereo mini jack (3.5mm) | 2 | IN/OUT | CP1-3513 | 96F9608 | 502-35RAPC4BH3 | 476-328 |
| Alt. input jacks (RCA; black, white and red) | 2 | IN | CP-1412 CP-1414 CP-1413 |
26M2031 – 26M2032 |
161-2003 161-1004 161-2002 |
476-586 – 476-592 |
| Alt. output jack (1/4" stereo) | 1 | OUT | SC1125 | 11M0598 | 16PJ509 | 449-348 |
| Protoboard | 1 | BOARD | see discussion below | |||
* These caps are much bigger than the ones in the photos on subsequent pages, and so aren’t great choices for amps in mint tins and other compact enclosures. In exchange for the bulk, you get better sound. The farther down the list of alternatives you go, the worse the sound gets, with the last ones still being okay, but recommended only when space is at a premium. The barrel-shaped axial lead caps in the pictures are the Vishay MKT1813s on the second row, from Mouser.
** If you must use stripboard (such as because you can’t get the recommended perfboard style), you will need to have some way to break the copper strips at strategic points. RS Components sells a stripboard cutter (543-535) for this purpose. Alternately, you could simply use an X-acto knife, which will not produce breaks as definitive as the stripboard cutter, but should suffice.
Here are some parts that you may need, or that are “preference” things: I can’t call them required, since some may disagree.
| Description | Qty | Digi-Key | Newark | Mouser | RS Components |
|---|---|---|---|---|---|
| DIP-8 IC sockets, gold contacts | 1 | ED90032 | 04M0550 | 575-11043308 | 813-115 |
| Power switch (mini SPDT* toggle) | 1 | 360-1788 | 69K7015 | – | 330-840 |
| Alt. power switch (mini SPST toggle) | 1 | – | 13F3970 | – | – |
| Alt. power switch (micro** SPST toggle) | 1 | 360-3420 | 09X8549 | 633-B12A1P | – |
| 5.5/2.5mm DC power jack (standard) | PWR | – | 96C7917 | 163-4024 | – |
| Alt. DC power jack (closed circuit) | PWR | CP-6 | 10X5814 | 163-4305-E | – |
| Volume knob (plastic, single bar indicator) | 1 | 8568K | 57F2374 | 450-2070-GRX | 259-6941 |
| Alt. volume knob (aluminum; black and silver) | 1 | 226-1041 226-2041 |
10M7562 10M7559 |
450-7031 45KN031 |
196-5829 196-5813 |
| Alt. volume knob (aluminum, black and silver) | 1 | 226-1033 226-2033 |
10M7556 10M7558 |
450-7015 450-6015 |
498-845 498-918 |
| R5, 47 Ω 1/4 W metal film resistor | 2 | 47.5XBK | 58K3856 | 271-47-RC | 148-174 |
| Alt. R5, 100 Ω 1/4 W metal film resistor | 2 | 100XBK | 58K3795 | 271-100-RC | 148-269 |
| 9 V battery clip (not needed for Serpac H-65) | 1 | 2238K | 16N942 | 534-2240 | – |
| Enclosure, Serpac H-65-9V (black and bone) | 1 | SRH65-9VB SRH65-9VA |
24K4584 – |
635-H-659V-B 635-H-659V-A |
– |
| Volume control, Alps 10 kΩ (RK097)*** | 1 | – | – | 688-RK0971221Z05 | 729-3521 |
| Alt. volume control, Bourns PTD902 series | 1 | PTD902-2015F-A103 | 75K7874 | 652-PTD902-2015FA103 | – |
* Although you only need an SPST for power, SPDTs are more common, probably because it’s just as easy to make them as SPSTs and you can use them like an SPST by just leaving the second pole disconnected. Since they’re more widely useful than SPSTs, some manufacturers only make SPDT toggles. (Or at least, distributors don’t often carry the SPST variants if the manufacturer does make them.)
** These “micro” switches are the smallest type of toggles you can easily find. Micro toggles tend to be rather delicate, especially the bushing area. I’ve broken these simply by tightening the mounting nut to a tension that is normal for mini toggle switches. If you can afford the space, I recommend that you use standard mini toggles instead. They’re much tougher, and they often look better, too.
*** These pots have a built-in SPST switch, which save you the cost and panel space of a separate power switch. It really saves time on the build, and I think it makes the amp look neater. Naturally, it means you have to readjust the volume every time you turn the amp on. You can get this same pot without the switch. I believe the RS Components part number for that is 249-9159. The equivalent from Mouser is *probably* the 688-RK097122008T, but I haven’t looked too closely into it.
**** Beware, this pot’s body is too big to fit into a mint tin. (Actually, it can be done, but you have to notch the lid to get it to close around the mounting nut.)
All resistors spec’d above are generic 1% metal film resistors. Generic resistors work fine, but many people swear by premium resistors because of their higher inherent accuracy and higher thermal stability. (The latter means that as the resistor heats up due to the power it’s dissipating, it changes value less than generic resistors. All resistors change value as they change temperture; premium resistors simply change value to a lesser degree.)
The most popular brand for this is Vishay-Dale’s CMF (a.k.a. RN) series. Specifically, the RN55x-F (1%) type. (For full details on how to interpret these CMF values, see the CMF data sheet.) Vishay-Dale CMF resistors are more expensive than generics, but in the quantities needed for this amp, the additional cost is negligible.
The Vishay CMF line also has 0.1% resistors, but you do not need that level of precision for this amp. If you do anything to get more accuracy, it should be to use a good multimeter to hand-match your resistors to higher tolerances. That may provide some audible improvement to some ears, but realize that this is relative matching. The point of buying 0.1% resistors is that their absolute values are within 0.1% of nominal. There are very few places in audio where absolute value accuracy is important.
If you can’t get Vishays but still want to try premium resistors, RS Components offers the RC series from Welwyn. I’ve never heard a report about whether these are an improvement over generics, but the specs suggest they’re on par with Vishay-Dale’s CMF series. They’re physically a bit bigger since RS only offers the 1/4 W resistors, but the price is similar to 1/8 W 1% Vishays.
Herewith, part numbers for Vishay-Dale RN55C/D resistors in all the values given above:
| Description | Newark | Mouser |
|---|---|---|
| 47.5 Ω | 03F7910 | 71-RN55D-F-47.5 |
| 100 Ω | 83F1209 | 71-RN55D-F-100 |
| 1 kΩ | 83F1211 | 71-RN55D-F-1.0K |
| 2 kΩ | 83F1230 | 71-RN55D-F-2.0K |
| 2.49 kΩ | 83F1402 | 71-RN55D-F-2.49K |
| 3.32 kΩ | 83F1256 | 71-RN55D-F-3.32K |
| 4.75 kΩ | 83F1272 | 71-RN55D-F-4.75K |
| 10 kΩ | 03F9465 | 71-RN55D-F-10K |
| 100 kΩ | 83F1502 | 71-RN55D-F-100K |
I spec 1/4 W resistors for the generics above simply because they’re the most readily available sort, not because the CMoy amp really dissipates 1/4 W anywhere in the circuit. The resistors in a CMoy amp that see the highest load are the power supply splitter resistors. With the highest supply voltage you’re likely to use — 30 V — these resistors only dissipate about 50 mW. Therefore, even 1/8 W resistors are adequate for all aspects of the CMoy design.
This tutorial was written around the RadioShack 276-0150 patterned perfboard. Now that RadioShack is out of the retail electronics business, that product is no longer available. However, there are many places to get compatible patterned protoyping boards via mail order.
In the US, RadioShack still offers this product through their web site. (If that link breaks, search for “2760150”, without dashes.)
Another option in the US is Newark part #38C9100. It is basically identical to the 276-0150 board from RadioShack.
For those outside the US, the most readily available alternative is RS Components part number 631-3223. A spot check of several of their country-specific sites doesn’t turn it up, but it is on their international and UK sites.
That same board (the Sunhayato ICB-86) is also available from other sources, mainly on Asian electronics parts supply sites. You might also be able to find an alternate version of the board, part number ICB-86G, which uses a higher-quality epoxy-fiberglass board, rather than the cheaper phenolic paper substrate. RS Components sells it as part number 631-3239. The only reason to get this version is if you plan on doing a lot of rework to the board, as it is more likely to withstand repeated soldering.
If none of those options work for you, there are a number of patterned perfboard designs on the market that are close enough that adapting the layout presented here is straightforward.
Many items in the SB series from BusBoard Prototype Systems are suitable, including the SB300, SB400, and SB404. Their POW3U board is a similar design, though much larger than necessary here, so you’d have to cut it down if you can’t find one of the SB series. You can find BusBoard products many places online, including Amazon.com. I haven’t studied these board layouts in detail, but it looks like you might even be able to get away without adding the M jumpers needed to make the RadioShack board work, because the 5-hole pads surrounding the central bus strips should provide sufficient tie points for this circuit.
In much the same vein as the BusBoard POW3U is the Roth Elektronik RE315-LF, available from the Farnell/Newark behemoth. Farnell lists it as part number 1172142 from Farnell, while Newark calls it part number 26M2205. This is a much bigger board than the RadioShack board, but if you study its layout, you can see that there are several ways to cut this board up to create something similar to the half-2760150 board used in the in the tutorial’s standard board layout. As with the BusBoard offerings, it looks like fewer jumpers would be needed to use this board than with the RadioShack board. It uses 4-hole pads around the central bus strips rather than the 5-hole pads used by the BusBoard products, so it might require a few jumpers where the BusBoard offerings do not.
If you must use a much different type of protoboard, I recommend against trying to use the same parts layout I came up with for the RadioShack board. Instead, create a new layout tuned to that pattern. If your board has a very generic pattern, such as stripboard or pad-per-hole board, I’d probably just go with a point-to-point layout: all the connections are made direct from one part lead to another on the bottom side of the board, instead of going through copper foils on the board itself.
You can read more about different types of prototyping boards here.
The op-amp (operational amplifier) is the chip that does the actual amplification in the CMoy circuit. It has the single biggest effect on sound and power draw of any component, so it behooves you to pick this part carefully. It’s also the part most likely to fail if your implementation is imperfect; a tolerant op-amp will reduce the chances of failure.
If you’re a raw newbie, I recommend that you try one of the Burr-Brown OPA132/134 series op-amps first because they have low voltage requirements, they don’t oscillate easily (see below), the lowest grade is adequate for this project and is quite reasonably priced, and they’re very popular so a lot of your fellow DIYers are familiar with them and so can help you more easily. I won’t say they’re easy to find; you’ll probably have to mail-order one. Take my word: these are very good chips for the price. Any replacement you try and source locally will likely not perform as well.
“Oscillation” is a condition where the chip develops periodic or constant noise due to problems in the surrounding circuit. If you keep at this hobby, you will eventually develop the necessary knowledge and experience to avoid oscillation in your circuits. Until then, stick with tolerant op-amps.
The specific chip I recommend for beginners is the OPA2132PA. Under ideal conditions, the cheaper OPA2134PA performs just as well, but your first DIY headphone amp probably won’t be flawless. I’ve had circuits where the OPA2134PA would distort or oscillate, but popping in a 2132PA or 2132P would fix the problem. As a beginner, you don’t need problems like this. You’ll have enough difficulties just assembling and testing the thing without the op-amp acting up as well! If you later want to build another amp, go ahead and try the 2134PA. If you run into problems, you can pull the 2132 out of your first amp temporarily and try that in the new amp to see if it fixes the problem.
As for the OPA2132P, it’s about twice the price of the 2132PA, and I can’t hear a difference relative to the 2132PA. This isn’t surprising, since the only specs that are different between the two chips are the DC specs, and there is no DC in audio.
If you think you might want to try different op-amps in your headphone amp, it’s all but mandatory that you use DIP sockets in your design so you don’t have to desolder the chip to try a new one. Sockets are useful even if you never change the chip since they prevent damage to the sensitive op-amp chip during soldering. However, you should only use high-quality machined sockets. If you can only find cheap sockets, you’re better off just soldering the op-amp to the board, because a cheap socket will result in weak connections, which is a serious risk factor for bad sound.
If for some reason you want to try another op-amp, first give my companion article “Notes on Audio Op-Amps” a read. There’s a fair chance I’ve already reviewed the chip you’re looking at. My test circuit is a lightly-modified CMoy pocket amplifier, so if I give a chip you want to use a bad review, it maens it doesn’t work well in a CMoy. If I haven’t reviewed the chip or my review is sketchy, it may be because it is not a good fit for a CMoy. Generally speaking, you want to use chips similar to the OPA134/132 family: JFET-input, middlin’ bandwidth, capable of driving low impedances from low supply voltages. That knocks out a huge number of op-amps on the market right there.
There are two main kinds of enclosures used for pocket amps: plastic enclosures designed to house electronics, and various types of boxes designed for other uses and converted to hold an amp. The latter includes the popular mint tins, plus other found objects.
For general portable use, I like the Serpac H series cases. The H659V (1.0 × 2.75 × 4.95 in.) has plenty of space inside for a CMoy circuit, its rounded corners and slim body give it a sleek look, and this particular variant includes a nice 9 V battery compartment with fixed contacts instead of the more common floppy battery snap. The overall impression is of a pocket cigarillo case; it slips into a pocket very nicely. There is a taller version called the H-67. While the 9 V version of the H-67 only holds one battery, the “AC” version has a battery door without a battery compartment behind it; you can stuff two 9 V batteries in there. You’ll have to add some padding and such to keep the batteries from rattling around inside the case if you go this route.
If you’re thinking of going with a much larger case but still want to be able to run from batteries, I recommend adding a Bulgin dual-9 V battery drawer to one of the larger cases (Mouser part #122-BX0026).
Your choices in this area go much deeper than “one 9 V battery or two,” and “batteries or wall power?” so the topic is covered elsewhere:
This is a deep topic, so each of those link’s targets then links out to further information.
If you just want an easy answer, put 2 × 9 V batteries in series and call it good. That’s not the best method on several axes, but it is the overall easiest for this circuit design.
There are differences among the recommended audio jacks above that make some of them unsuitable for some configurations. The biggest thing to look out for is that if you’re going to use a metal enclosure and are going to also use a wall power supply that either the audio jacks or the DC inlet jack needs to be insulated, else you’ll end up tying V- to virtual ground through the case, causing the amp to fail when plugged in.
For more on this topic, please read my companion article, How to Wire Panel Components.
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