REVIEW: DIYRE HC1 Headphone Amplifier

Most audio interfaces have a built-in headphone amplifier, but they tend to be a bit noisy and under powered. Like speakers, headphones sound their best when they are powered optimally, and I suspect that audio engineers who might dismiss headphones as reference tools simply don’t know how good a set of quality open-back set of headphones, driven by a better headphone amp sounds like.

The headphone market has grown a lot in the last few years and there are now many dedicated headphone amps to choose from, but it’s still unusual to have really good headphone amps implemented in pro audio gear. Some Mastering-grade DACs do, like my Crane Song Avocet. In the manual, all that is said about it is that “the headphone audio path is done with high quality integrated circuits”. No one is buying an Avocet specifically for its headphone amp, but I like it! It drives my 600 ohm AKG 240DF headphones really well and it sounds cleaner (more neutral, accurate) than any other solid state amp I’ve tried them with.

Right now there are many solid state, tube and output transformerless headphone amps that are available for purchase, but if you know how to use a soldering iron, the DIY community also has some really good designs that you could build for a fraction of the cost of a high-end amp and you might end up with something that rivals or surpasses amplifiers costing much more. There’s also the added benefit of being able to modify and alter the circuits to further customize the sound of these DIY amps to your liking.

Recently DIYRE, a company known for their awesome-sounding and easy to build Pro Audio DIY kits for the 500 series format added their own headphone amp kit, the HC1. This amp offers something most DAW interface headphone amps don’t: power, and a lot of it to drive most headphones with ease. They saw the need for a high current headphone amp for recording musicians who sometimes have a hard time hearing cue mixes clearly through their built-in interface headphone amps during recording. You can daisy chain up to four HC1 amp boards for a multi-channel headphone amp (they provide detailed instructions on how to build it). The amp promises low noise, and the ability to drive a wide variety of headphones, from low to high impedance. Everyone has their preferences for the types of headphones and IEMs they use in the studio, and in the spirit of DIY audio – let’s also try to keep the cost as low as possible.

Hold on a sec. Low noise, lots of power, transparency and the ability to drive headphones of varying impedance is what you pay hundreds of dollars for in the headphone HiFi market, where they’ll use catchy marketing phrases such as “exceptionally low noise”, “amazingly versatile”, etc. The HC1 is just $25. In the process of designing a powerful, flexible DIY headphone amp for recording musicians, these guys also created a very desirable, low-cost, reference grade headphone amp.

The origins of the HC1 are rooted in DIY audio, where maximum performance at the lowest possible cost is often a desirable goal. Douglas Self, a renowned electrical engineer and amplifier designer published an article in Elektor magazine back in 2010 titled “The 5532 OpAmplifier” complete with a schematic on how to build a stereo power amplifier using a whopping thirty-two 5532 op amps in parallel. The idea is to use parallel operational amplifiers in a simple circuit, taking advantage of the chips’ low noise, linearity and power supply rejection. While doing a little bit of research for this article, I found a thread on diyaudio.com soon after the article was published. A few people were skeptical at first, and doubted the performance of the amplifier apparently because the promised performance and low noise seemed too good to be true on paper. Douglas Self replied to the thread, and among other things had this to say about the design:

“I’d like to say that this is a wholly serious project, that does exactly what it says in terms of performance, but it also has its light-hearted aspect. I hardly think it is going to revolutionise the whole field of audio amplifier design, but I do believe it at least provides a new way to look at the subject, and if it triggers off some other interesting designs based on the same concept, I shall be more than happy.”

The comments that follow in that thread are reports of people building the amp and saying that it sounds as good as promised (clean, without noise or a sonic footprint – again, a highly desirable feature for a reference amp). I read comments such as “you don’t hear the amp” and “the measured THD + Noise performance is better than one might expect.” The HC1 follows this parallel op amp concept, using one op amp per channel.

The HC1 implemented into my Rapaudiology VU Meters prototype

So, why am I so hyped up about the HC1? About seven years ago, I designed a VU metering box that allows me to quickly calibrate three positions to have 0 VU represent whatever I want (yes, this is totally off spec). I use them daily in my mastering work for various loudness targets. Since then, I’ve sold a couple of them to other audio engineers and around a year ago, I thought it would be cool to add a (good) headphone amp to future units, providing something that is typically lacking for other audio engineers since most of the headphone amps found in typical DAW interfaces just aren’t good enough. Since the meters are designed to take signal directly from the main outputs, it’s an ideal place to have a reference-quality headphone amp. The meters use a stereo VU buffer amp from JLM audio, which in addition to supplying voltage and signal to the meters, prevents noise generated by the meters’ diodes from coming back into the output signal. Additionally, the HC1’s circuit design inherently does a good job of rejecting noise from the power source, and after building the HC1 into my “Rapaudiology VUs” prototype, I was happy to hear that the signal was very clean considering the unit is powered by an inexpensive 12v DC “wall wart” power supply.

At first, I was thinking of implementing one of two low cost, legendary DIY headphone amp designs that I’m very familiar with, having built both of them in the past, Pow Chu Moy’s Cmoy or NwAvGuy’s Objective2. To give you an idea of how good the O2 is in case you’ve never heard of these, in a blind test it was found to sound as good, or better than the headphone amp found in a very popular audiophile DAC that I won’t name in this article. The Cmoy still has a reputation for being one of the best-sounding (in terms of clean power) portable amps under $100. Both the Cmoy and O2 are fairly inexpensive to build, and really good in terms of audio transparency. The idea that low-cost, well-implemented DIY audio gear can rival gear costing much more isn’t new (in case you haven’t heard of the infamous $5 DIY mic preamp, look it up).

I’ve been a fan of DIYRE for a while now, having bought some of their 500 kits which I use on a regular basis. A few months ago, I got a subscription alert from YouTube telling me that they had added a new video for their “From The Bench” series on a high current headphone amp, where Peterson Goodwin of DIYRE mentions how underpowered and noisy typical interface headphone amps can be. I was just about to add a Cmoy circuit to my VU brainchild when I got that alert, and after watching the video I decided to wait until the kits were ready for purchase and so I ordered one. I really liked the idea that the amp’s power requirement is flexible (9v – 44v DC) so it was going to be easy to implement it by splitting the rails of the inexpensive 12v wall wart power supply that already powers the VUs. Initially, I wanted to implement an O2, but it wasn’t going to work off the 12v power supply that was already powering the meters.

After building the amp, I wanted to hear some of the headphones I own that have varying impedances. I started with my Grado SR80s (32 ohms). My first impression was that the amp is nearly silent, I can hear only a tiny bit of noise when the volume is turned up all the way without anything playing. Later, I discovered that the amp is effectively silent at full volume with cans that have impedances greater than 200 ohms, and although you can hear a tiny bit of noise with low impedance headphones when the amp is turned all the way up, the amp is silent at those lower impedance headphones’ maximum listening levels, plus 10-20% above that range for my own listening preferences (much like speaker levels, I stick to listening to headphones at approximately 85-90 dB SPL). Grados are a bit bright in the mids and highs, and I found that the HC1 accentuated this, but without hyping up the upper high frequencies too much or sounding harsh; the tiny amount of noise I heard with the volume turned all the way up, with nothing playing in the background registers mostly in the low to mid frequencies, it wasn’t a high frequency hiss like I’ve heard with lower quality amps. The low end sounded tight, fast and overall the soundstage was great, with excellent instrument separation. It didn’t take much for the amp to drive the SR80s, I didn’t even go half way up on the volume knob for them to sound loud enough to listen comfortably with material that’s mastered to current loudness levels. When playing back some of my vinyl transfers, I dialed up the level close to 50% and it still sounded clear and punchy.

Next, I listened to my Sennheiser HD650s through it. I was surprised to hear that the HC1 opens up the HD650s in a similar way that my Bottlehead Crack amp (with the Speedball upgrade) does around the mid frequencies. The low end was clear and punchy, but not as extended. It didn’t have the same depth and soundstage that I hear with the Crack, but keep in mind that OTL amps like the Crack are considered optimal pairings with the Sennheiser 650s. What surprised me while listening to the 650s on the HC1 is that the frequency response was very similar in that it opened up the mids in a way that most solid state amplifiers don’t. That’s crazy for a $25 amp! As mentioned earlier, with higher impedance headphones like the 650s (300 ohms) the amp is completely silent when turned all the way up. I went back and did more listening on the 650s days later and still felt the same way about their response with the amp. If you’ve recently purchased a Massdrop 6xx (known as Drop now) and your wallet isn’t ready for the Crack with Speedball, you should definitely check out the HC1.

I then plugged my 240DFs into it, and I was surprised to hear that it has enough power to drive these 600 Ohm cans (it has roughly the same amount of power to drive these as my Avocet’s headphone amp) but I felt that it made the 240DFs sound a little bit bright in the mid frequencies. I was not surprised by this at all, since the 240DFs are the most accurate cans I have heard in the mid/upper frequency range and my ears are used to listening to these over my Avocet’s neutral DAC/amp for about 10 years now. The HC1 makes all the headphones I have sound a tiny bit brighter, from the low mids through the highs but without making things sound harsh; there is no “graininess” here. I believe this is why it sounds really good with the HD650s.

This prompted me to try my Beyerdynamic DT 770 (80 ohms) with it, since I’ve always felt that the 770s have a bit of a “smiley curve” response to them (boosted lows and highs, slightly dipped mids) and I immediately noticed a more natural-sounding mid range, without loss of punch in the lows. The little bit of brightness that is added by the HC1 isn’t excessive, and I heard some mid frequency detail that I hadn’t heard before with some familiar mixes. If you own a pair of 770s and feel the same way I do about their response, I think you will be really happy pairing them with the HC1 also. Expecting a bit of noise without anything playing and turning the attenuator all the way up, I was happy to hear just a small amount of noise with the 770s at full volume that barely registered.

Next, I tried my everyday Shure SE215 IEMs. I use these with my phone to work out or while commuting. They’re also popular entry-level IEMs for recording/performing musicians, so I was curious to hear how these performed through the HC1. They’re only 17 Ohms and it didn’t take much to drive them to comfortable levels. I heard fast, accurate bass and everything sounded better than it does on my phone, but to be fair, the signal coming into the HC1 is directly from the unbuffered output of my Avocet, which is much better (cleaner) than my phone’s DAC. It definitely didn’t take much for the HC1 to power these headphones. I was actually a little scared to plug them in, because I remember blowing out some Klipsch IEMs I used to have when I built and tested my Objective 2 amp a few years ago, so it’s great to see (hear) that the HC1 doesn’t kill low impedance headphones/IEMs.

Overall, I am very impressed by the HC1. The fact that it drives both low impedance IEMs and headphones like the Grado 80s and 300 ohm headphones like the Sennheiser 650 well enough really makes this inexpensive amp a big winner in my book. It has become my favorite amp to use with my Beyerdynamic 770s, and I plan on building this amp into future Rapaudiology VU metering boxes that I plan on making available for sale. The HC1 is definitely reference grade; the mid range has a subtle “bump”, but it’s not harsh or fatiguing. I’m used to hearing some amount of graininess on many headphone amps that come built into many audio interfaces and there is none of that on the HC1. It is very detailed and I think anyone who uses headphones for mixing and mastering audio would definitely appreciate having a headphone amp that provides enough power to drive most headphones with ease.

The kit comes with a detent potentiometer, and I’m not sure if this type of potentiometer is better at L/R channel tracking than a smooth pot. I can see more skilled DIYers replacing it with a better quality stepped attenuator. My impressions when listening at lower levels is that the included potentiometer tracks both channels evenly. The kit also costs less than a Cmoy or an Objective 2 kit from reputable manufacturers (and those tend to come with smooth potentiometers for volume control). After doing some listening comparisons with one of my Cmoy amps and an O2, I’m glad I waited for DIYRE to make these available. I truly believe the quality of this amp is equal to, if not better than some “audiophile” headphone amps that cost a helluva lot more, although it’s up to you to worry about the case and power supply.

Build it or have someone to build one for you, especially if you don’t have a mastering grade DAC with a nice headphone section built in, or a dedicated headphone amp that drives your headphones efficiently. Put it inside a small case and power it with one or two 9V batteries and you’ll have a very decent, portable reference headphone amp for those of you who like to work on audio on the go. If you’re mostly concerned about the sound of the amp and not a shiny case and maybe a nice LED display that does nothing for its sound, this amp is truly another awesome story of low-cost DIY audio sounding just as good, if not better than other headphone amps that you would pay a lot more for.