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JDS Labs

cMoyBB™ v2.02 Frequently Asked Questions

When will you have more amps for sale?

The cMoyBB is always in stock. However, I only post eBay listings a few times each month.

While you wait for my next listing, I invite you to check out cMoyBB.com for more product information and news.

Do you offer customization?

All amplifiers I sell on eBay are built identically. You may optionally mention your headphones during checkout for free gain customization. Alternative enclosures are occasionally available.

The following cannot be customized:

Input Jack
Output Jack
DC Jack
Bass Boost Toggle Switch

Will the cMoyBB work well with my [insert model] headphones?

Having sold nearly 2000 cMoyBBs, I have received positive reviews from owners of every brand and most models of headphones on the market. Chances are, someone has tried my amplifier with your headphones and enjoyed it.

I personally use Sennheiser HD-280 Pros, Grado SR-60s, Beyerdynamic DT-880s, and a variety of earbuds. The cMoyBB sounds great with them all.

What kind of cable do I need? What do you recommend?

The cMoyBB uses standard 3.5mm (1/8") stereo input and output jacks. You will need a male-to-male 3.5mm interconnect cable (also known as a mini-to-mini cable) to connect your audio player to the amplifier.

I have no specific brand recommendations. Instead, I offer the following advice:

Choose a heavily shielded cable. Look for terms such as "shielded," "noise-rejecting," or "braided." These cables use an additional grounding wire and and/or shielding sheath to avert electromagnetic interference. Additionally, every cheap interconnect cable I have used has worn out in under a month, causing static, popping sounds, or a broken signal when the cable is wiggled. Invest in a rugged cable! Finally, shop wisely: A very fine cable can be purchased for under $30 USD. The cable pictured at right cost $25 USD. For the most part, highly priced cables offer no audible advantages (in other words, don't waste your money).

Are kits available?

I am highly supportive of the do-it-yourself audio community and have created a complete DIY section at the cMoyBB website. There, you will find a list of required components (including the cMoyBB circuit board), a quick assembly guide, and other information that will aid you in building your own, customized cMoyBB v2.02.

I do not sell DIY kits. Instead, the bare cMoyBB v2.02 printed circuit board is available so that do-it-yourselfers can select their own combination of parts and features. Offering kits would defeat the point the project.

Misconceptions of Headphone Amplifiers

Unlike most consumer electronics, headphone amplifiers are frequently designed, assembled, and sold by individuals, rather than large companies. As such, it is common to find false information being published by these sellers. Much of this misinformation is naive ignorance from amateurs who lack of a formal electrical engineering education, or from those who are working outside of their professional field (only some electrical engineers specialize in signal integrity!). But, it is most disappointing to see bright engineers write blatant lies in their eBay listings. Whatever the case may be, the information that follows is an attempt to rectify several of these misconceptions.

To the best of my electrical engineering knowledge, the following clarifications are true and accurate. Contact me should you believe otherwise.

Myth: "Micron-thin" PCB traces hinder signal transfer and circuit reliability. Wide traces are best because they reduce resistance and therefore maintain the best signal.
Fact: These ideas are purported by builders unwilling to pay for professionally manufactured PCBs. There are absolutely no physical reliability issues with thin traces. Have you ever heard of a broken or failed trace in your cell phone or computer?! With that myth out of the way, let's consider the physics behind trace width.

The first obvious limitation of trace width is the trace's ability to carry enough current. Signal current in a cMoy headphone amplifier is low enough that even narrow 10mil traces are more than adequate (assuming a typical, small board). Basically, width is not a factor for current requirements.

Wide traces do reduce trace resistance, however, this is BAD! When resistance decreases, capacitance and inductance of the trace dominate. Capacitance wishes to raise or maintain voltage, while inductance wishes to reduce voltage. Consequently, the signal voltage ripples up and down like a wave in the ocean. This is known as ringing, and it is Very Bad. Ringing can render a high frequency circuit unoperational; the same behavior also applies to low frequency audio amplifier circuits (generally in the form of oscillation), but most cMoys are simple and stable enough to still function. However, sound quality suffers.

Per unit resistance is actually a component of characteristic impedance--a significant design aspect. Each trace, wire, connector, pin, and via (hole in a PCB) has a characteristic impedance. You have probably heard of "75 ohm" cables at one time or another. This quantity refers to the cable's characteristic impedance. Mismatched characteristic impedances generate signal reflections and refractions, meaning non-ideal signal behavior. For PCB traces, characteristic impedance is set by width, according to copper weight and other PCB stackup parameters. Matching the characteristic impedances of each element in a high-speed circuit is crucial to achieving good signal integrity. But again, headphone amplifiers operate at such low frequencies that signal integrity is not critical to overall functionality. As a result, headphone cables and 3.5mm input cables were never designed for a specific characteristic impedance. However, it is still ideal to at least maintain a constant characteristic impedance within the headphone amplifier. When all traces are matched, signal reflections and refractions are approximately eliminated, thereby ensuring the most ideal signal transmission. Cardas Audio, Ltd. has put together a number of videos demonstrating the effects of cable signals (see "Signal in an Audio Cable").
Myth: Large power supply capacitors produce "strong, powerful bass".
Fact: This claim is true to an extent, but many sellers contort this fact and build their amplifiers with massive capacitors to make them more visually appealing. The fact is, power supply capacitors have no direct effect on bass response; their role is entirely indirect. The chief purpose of a power supply capacitor is to maintain a steady power supply voltage. When the supply voltage is well regulated, an operational amplifier is better able to perform. The effect of a stable power supply on an opamp is most noticeable in bass frequencies, hence this frequent claim.

Some sellers describe capacitors as having a low "ESR" (equivalent series resistance). This simply means the capacitor has a low internal resistance, which allows it to deliver charge more rapidly than a capacitor with a higher ESR. A less often mentioned specification is "ESL" (equivalent series inductance). Similarly, low ESL is ideal. Large capacitors tend to have low ESR and ESL ratings. However, many cMoy builders fail to realize that ESR and ESL are inversely proportional to a capacitor's voltage rating. For instance, a 16V capacitor will have a worse ESR than a 25V part. That said, it takes a well-trained ear to distinguish the difference between a 2200uF capacitor and a 220uF component. Many people will never hear the difference.

The greatest bit of misinformation pertaining to capacitors is in regards to decoupling. While those large electrolytic capacitors are important, they should be thought of as the last reservoir of charge. All high-performance analog circuits rely on four stages of capacitance for power supply buffering. In order of decreasing speed:

1) Embedded capacitance of the power/ground plane pair (very small amount of charge, very fast charge delivery)
2) Decoupling capacitors (usually 0.1uF or smaller ceramic capacitors)
3) Bypass capacitors (usually 10uF+ tantalum capacitors)
4) Bulk capacitors (the huge 100uF+ electrolytics--the slowest)

Embedded power/ground plane capacitance is only possible in 3+ layer circuit boards, since two layers must be reserved for the power and ground planes. Most headphone amplifiers are built on 2-layer PCBs, so the best reservoir of charge is absent. Decoupling capacitors are the next line of defense against charge fluctuations. Surprisingly, decoupling is often neglected in simple amplifiers. This is comparable to driving a car with low tire pressure. Sure, it works, but is it ideal? Certainly not.
Myth: Single sided, homemade circuit boards are superior to 2-layer PCBs because dual layer traces "criss-cross" and suffer severe crosstalk.
Fact: Homemade circuit boards are cheap. This is the only reason they are promoted! With each additional layer, the cost of a PCB increases substantially. Extra layers allow for power and/or ground planes, which further enable proper trace characteristic impedance, lower ground impedance, and optimal trace routing. Designers of single-layer PCBs are often forced to create long, awkward trace paths around the circuit board. Unfortunately, the path of a trace can be the difference between a typical conductor and an antenna. Additionally, it is difficult to avoid the creation of trace loops on single-layer boards; loops are a source of unwanted inductance. As discussed above, such inductance is terribly detrimental to a circuit's behavior.

Last, crosstalk is a function of many variables. At audio frequencies (below 20kHz), crosstalk is hardly a concern on a PCB. In fact, the thickness of a standard PCB (0.062 inches) is enough that crosstalk between the top layer and bottom layers of a 2-layer PCB can nearly be ignored.

In summary, the more layers, the better (assuming a competent designer is in charge...). For a more in-depth discussion, Texas Instruments Incorporated has provided an excellent paper to the public, "The PCB is a component of op amp design".

Copyright © 2009 John Seaber. All Rights Reserved.
cMoyBB™ is a trademark of JDS Labs. All other trademarks are the property of their respective owners. Commercial usage of the cMoyBB is strictly prohibited without explicit written authorization.

The above page is maintained by: three3three3( 612)

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