HA-1A Headphone Amplifier – FAQ


What kind of headphones do you recommend?
The HA-1A is intended to give good performance with all commonly available headphones. Everyone’s ears are different, so if you’ve got some cans you like, stick with them. Personally I’m fond of the Koss PortaPro headphones, an excellent sounding lightweight, portable, over-the-ear model, which sell for around $40; and I also really like my Grado SR80s, which cost a bit more money but sound a bit more even.

What is the best impedance for headphones?

The sweet spot is 50 to 300 ohms. Here’s the full picture:

< 16 ohms: Danger! At high volumes, the output transistors may overheat and suffer damage. 16 — 30 ohms: Not recommended. Distortion will be somewhat higher (but still probably below the audible threshold) and batteries will drain faster. 30 — 50 ohms: Battery life will be slightly shorter than with higher-impedance headphones. 50 — 300 ohms: Ahhhh. Delight. All is well. 300 &mdash 450 ohms: As impedance increases, distortion at normal volumes becomes vanishingly small, but headroom also decreases. > 450 ohms: not ideal The HA-1A may not be able to drive headphones in this range, such as the AKG K240DF, to their full rated power. These headphones require higher voltages than batteries can provide.

What is the best efficiency for headphones?
Efficiency refers to how much sound the headphones produce from a given amount of electrical power. Efficiency is usually measured in decibels of sound pressure level at one milliwatt, although some manufacturers use other scales. I would tend to avoid headphones that are lower than 90 dB/mW, because they drain the batteries fast and they don’t get loud enough. Older AKG 240 headphones are notorious in this regard. On the other end of the scale, headphones or earbuds that are more than about 110dB/mW are so efficient that they make even the slight residual hiss produced by the HA-1A audible. (If you have this problem, it is possible to make an adapter to attenuate the HA-1A’s output.)
There’s some good information about headphone efficiency specs here, on Rane’s web site.

Will earbuds work?
Yes. However, there are two things to be aware of. First, some earbuds, such as the Westone UM-2s, are extremely efficient – see the last FAQ item. Second, the earbuds historically provided with the Apple iPod appear to distort very easily, and overemphasize bass response (perhaps to make up for the anemic bass response of the iPod). Driving these earbuds with the HA-1A can expose these limitations. I have heard reports as of October 2006 that Apple has switched to an improved earbud that does not seem to have these problems, so there’s hope.

AC Adapter

Is it okay to leave the HA-1A plugged into the AC adapter all the time?
Yes it is. When it’s powered by AC, it will not drain the batteries, and leaving it plugged in will not hurt anything or shorten its expected life. However, if you know that you won’t be using it for a while, you might want to unplug it just in case there’s a power surge (which could damage it) or a power outage (which would make it switch to batteries, which would then eventually get drained).

I lost my AC adapter. How do I get a replacement?

For units with serial number 300 or higher, any 12VAC, 500mA adapter with a 2.1mm x 5.5mm coaxial plug will do. For units 299 and lower, you need 12VAC 500mA with a 2.5mm x 5.5mm plug. In the USA, Jameco is one good supplier for these. See the next question for out of the USA. It’s okay for the current rating to be higher than 500mA. However, the 9VDC adapters used for stomp boxes will NOT work. It must be 12V, and it must be AC output, not DC. USA replacements are also available directly from me (but I charge more than Jameco, because I don’t have a warehouse); email audio@cafewalter.com.

I don’t live in the USA. What should I do for an AC adapter?
I haven’t been able to find suppliers within the USA for adapters for other countries’ power systems, so I can’t sell you one. But you can probably find one at an electronics shop where you live. See the previous topic for the specs you need. In the UK, Maplin sell them; as of this writing their part number N57AT (500mA multi-voltage AC/AC mains adaptor) is the right thing to buy. In many Asian countries there are “electronics bazaars” that sell them. I will be happy to help identify the correct adapter for you if I can.

I have an HA-1, not an HA-1A. Do you sell an AC adapter for it?
I can retrofit HA-1s with the AC adapter circuitry, for $59 including the adapter and shipping within the USA. For details email audio@cafewalter.com.


How long do the batteries last?

It depends on a lot of factors: for instance, how efficient your headphones are; how loud you turn up the volume; whether you play fifteen minutes at a time or eight hours at a stretch; whether you’re using the aux input. With Koss PortaPro headphones, playing scales on the bass at moderate volume, the power LED goes out after about 18 hours of continuous play. Even after the LED goes out, the sound continues to be okay for another 4 to 6 hours; but the max volume before distortion goes down, and the sound gradually gets muddier.

Can I use rechargeable batteries?
The HA-1A will work with rechargeable NiCd or NiMH batteries, but not very well and not very long. The problem is that rechargeable “9V” batteries actually only put out about 7.2V even when they’re fresh. The HA-1A power LED is designed to turn off when the batteries reach about 6.6V. As you can imagine, if you start at 7.2V, it doesn’t take very long to run down to 6.6V.

Inputs and Outputs

Is the HA-1A’s aux input stereo?
Yes. The aux input is stereo and sends a stereo signal to the headphones. The instrument input is mono, and gets sent equally to both headphones, so it sounds as if it were being panned to the center of the stereo image.

So, could I use the HA-1A as a headphone amp for a CD/MP3 player?
Indeed you could; it has lower distortion, better frequency response, and more headroom than the headphone outputs of most consumer audio devices. You’d need to plug something into the instrument input to get it to turn on, if you’re running on batteries.

Why is the headphone output 3.5mm instead of 1/4″?

It’s a tradeoff. Most small headphones come with 3.5mm plugs and 1/4″ adapters, so either kind of jack will work. The 1/4″ jacks are more rugged and make a better contact, so I would prefer to use them. But I’m afraid that then people would plug mono plugs in (in order to use the output to drive a power amp or something), which would short out one channel of the amplifier and probably fry it. When I come up with some protection circuitry that doesn’t cost too much or take too much space or impair the audio performance, then I’ll switch to 1/4″ jacks, I expect.

Can I use the HA-1A as a bass preamp, to drive a power amp?
Not very well. You don’t want to plug a mono plug into the headphone output (see previous FAQ); and the aux output doesn’t put out enough voltage to drive most power amps to full power. It is possible to modify an HA-1A for this purpose if desired, although one consequence is that the aux out signal will then be too hot for some tuners.

Can the HA-1A drive speakers?
No. Most speakers have impedances around 4 to 8 ohms, and they require much more power than headphones do. The HA-1A is not intended to drive anything under 16 ohms, and its maximum power output is only a few hundred milliwatts. Trying to power speakers from the HA-1A will probably damage it.

Is the HA-1A only for basses?
No. The HA-1A was designed with bass in mind, but it does a great job with other electric instruments, such as electric guitar. Because the instrument input is mono, it may not be appropriate for stereo keyboards.


What is your warrantee?
If it breaks, and you didn’t abuse it, I’ll do my best to fix it, for free. If you did abuse it, well, lemme take a look at it and figure out what I can do for you. I’d love to make a stronger warrantee, but I can’t promise I’m not going to leave the country / join a Buddhist monastery / get run over by a truck.

What are the specs?
I think specs mostly lie: there are just too many ways to define the terms. But since you ask: the latest revision has frequency response +/- 1dB from 10Hz to 25kHz, and THD+N under .005% when driving 1V rms at 1kHz into a 30 ohm load.

How big and heavy is it?
The HA-1A is 4.6″ x 3.6″ x 2.5″, which is 11.7cm x 9.2cm x 6.4cm according to my calculator. It weighs 18 ounces including batteries, or 510g. If the US ounce keeps getting weaker, pretty soon it will weigh nothing at all in grams.

It would be really cool if the HA-1A also had tone controls, a built-in metronome, a balanced DI output, two headphone outputs, stereo inputs, and fins on the side.
There are other things out there that do that. I feel that there is a place in the world for something that does what it needs to, and no more; that reflects my philosophy about playing bass.

Well, okay, but it would be really cool if I could pan the inputs, so that I could practice to Aebersold tapes.
Yeah, that I agree with. It’s on the top of the feature list for the HA-2.

Why does the HA-1A have that weird wire-brush finish on the sides?
I don’t know either. The enclosures come that way when I buy them. Maybe it’s so that paint would stick better, if I painted them, which I don’t.

What’s the difference between an HA-1 and an HA-1A?
The HA-1 was the predecessor of the HA-1A. The difference is that the HA-1A can be powered either by an AC adapter or by batteries. The HA-1 only took batteries. Also, there are a few small improvements in the circuitry.

About Cafe Walter Audio

Do you ship outside of the USA?
Actually, I don’t ship anywhere at all. As of 2015 the HA-1A is no longer available for sale. I am working on the next version. It will be available globally. Note that the AC adapter that is included with the HA-1A is for 120V mains, USA-style plug, so if your country uses something different you’ll need to use a voltage transformer or else buy a different adapter. If you need, let me know and I can try to help you find the right adapter at a supplier in your country.

How come you don’t charge more for these and manufacture them in bigger quantities and get rich?
Because I like thinking about music and electronics, not money. I’m a lousy businessman. My eventual goal is to make other products in the future and charge more for them. Plus, if I grew the market I figure someone would just steal the design, make them in China for $25, and take away all my business. Which would actually be fine with me, if the quality and support were good, but they probably wouldn’t be.

I run a music store. Can I be a Cafe Walter distributor?

I’m not currently looking for more distributors in the US. I would consider taking on a distributor outside of the US. My criteria for distributors are idiosyncratic, picayune, and unreasonably stringent, especially given the small amount of money you’d stand to make, but don’t let that discourage you. I’m just sayin’.

I’m an industry figure. Will you give me a discount or a free unit in exchange for publicity?
Generally, no. I’ve tried loaning a unit out for reviews, but I’m not going to do that again unless the review has already been approved for publication. If you’re one of my favorite bass players, I might give you one for free, just because it would be an honor for me to return in a small way the gift you’ve already given me (Tony Levin, are you reading this?). But I don’t generally buy gear just because someone famous uses it; I don’t have enough headphone amps or enough profit margin to give them to everyone influential in the industry; and if I ever did get a big publicity surge I don’t think I’d be able to handle the volume. By manufacturing in small quantities I can keep a tight handle on quality and I can offer superb customer service, and those both matter more to me than being able to “move a lot of units.”

Do you really make these all by hand?
Originally we did all the work except for etching the printed circuit board right here in our house. But we weren’t able to make them fast enough to keep our distributors in stock, plus I felt that we could get higher quality by automating some of the operations (if you’ve ever hand-soldered surface mount components, you’ll agree!). Our machining is now done by LiveWire Prototyping; our printing is done by Superior Imprints; and the components are soldered onto the printed circuit boards by Schippers and Crew. I’m happy to be able to support these locally-owned small businesses in my community.

Why does this exist anyway?

The HA-1A grew out of a musician friend’s request to design him a circuit he could build to try to learn about electronics. I never managed to honor that request, but I started using the prototype I built for my own practice, and eventually decided it was useful enough that others might like one too.

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