DNA (Donald North Audio) Headphone Amp

[blubliss]

I met Donald North over at an audio dealer's house a few months ago. Nice guy. That's the extent of my involvement with DNA. I guess this is his first headphone amp. I'll copy what he wrote me:

When we met I talked about developing my own tube headphone amp. The design is now done and operational. I'm calling it the DNA Sonett. It uses the 6H30 dual triode with transformer loading for the audio amplification. Power supply is tube rectified using the 5AR4 and choke filtered. I played the prototype for Elliot and thought it sounded very good. I have also played it for some friends who liked it and placed orders.

The audio circuitry uses no coupling capacitors or feedback. Measured frequency response is very flat and extended, while distortion is very low. Triodes are naturally low distortion voltage amplifiers when loaded by a high impedance, which transformers do very well. Selectable on the output is 2 different output impedances: 120 ohms following the IEC standard and a low 28 ohms. This affects the tonal balance, allowing you to tailor the sound to your particular headphones and achieve the best sound. It's very quiet with no hum or buzz and has just the right amount of gain.

I will offer it in 2 versions: Single ended output using standard 1/4" stereo jack and Balanced output using dual XLR jacks.

Attached is a photo of the prototype. The production unit will have a nice brushed aluminum faceplate.

MSRP is $1200. I will be exhibiting at the upcoming Can Jam show at LAX and will offer a special price of $1000 to everyone who places an order at the show.

The 2 transformers at the bottom are the custom output transformers made to my specification. The 6H30 is DC heated (production unit will have professional pcb for DC supply) and has its own power transformer (at the top of the photo).

Anyway, I may have a chance to go over to the dealer's house this weekend (in Studio City) and hear this amp. I can take 1 or 2 people with me. If anyone is interested, please PM me.

[HeadphoneAddict]

Nice. I look forward to seeing it at CanJam and meeting DNA.

[dsavitsk]

You don't get a lot for $1200 these days. It is the single feed version of the now apparently discontinued parafeed Hagtech headphone amp (which I seem to recall sold for $1200 ... or rather, didn't sell for $1200), with a more rudimentary power supply. It's as basic as can be. I am all in favor of supporting local small shops, but they've got to try a little harder. There are a bunch of things that could be done with this circuit to improve it for only marginally more money. Also, either this is only meant for high impedance phones, or the load on the tube is awfully low.

[Donald North]

Thank you for sharing these comments. I would first like to say, the unit in the photos is the prototype. Final production units will have a milled aluminum faceplate with silkscreening in addition to text on the chassis.

My design philosophy is direct, elegant solutions without needless complexity. For headphone listening, a tube of the appropriate parameters with proper transformer loading provides the right balance of gain, current drive, wide bandwidth, and low distortion without any feedback.

Production units will feature custom-made chassis and output transformers - not off-the-shelf components. Manufacturing custom, small-batch specialty products in the United States is not inexpensive. I can assure you, no one is getting rich here

The custom output transformers (which are not the same as in the Castanet you reference) feature 2 taps: One providing 6dB input-output gain with 120 ohm output impedance, in accordance with the IEC spec. The other tap provides less gain and a low 28 ohm output impedance. Many headphones are designed and tuned around the IEC spec - now you can hear them as they were intended; others are tuned for low impedance drive. This amp will worth well with both high and low impedance headphones and gives the listener the option to select the output impedance which sounds best to them with their particular headphones.

2 versions will be offered for the same price: One with 1/4" stereo single ended output; the other with dual XLRs for true balanced output.

[n_maher]

Donald,

Thank you for sharing the design and your comments with us, I have a few comments.

1. I can assure you that Doug (dsavitsk) was not commenting on the aesthetics of your amplifier. He and I both have a high level of appreciate for clean, straightforward designs both from a circuit and aesthetic perspective. You'll also find that there is a good sized group of us around here who have more than just a passing acquaintance with both design and manufacturing costs.

2. Could you provide a link or further explain the IEC standard for output impedance? All I could find was reference to "IEC 60268-7 Headphones and earphones" which I was not able to access. The 120ohm output impedance sounds prohibitively high to me perhaps even with 300ohm headphones and will almost certainly sound pretty bad with some of the lower impedance headphones we tend to use around here. My own personal amplifier has an output impedance much closer to your 28ohm tap, I suspect that this is what people will end up using exclusively unless they own some of the 600ohm Beyer cans.

3. Your statement "Many headphones are designed and tuned around the IEC spec - now you can hear them as they were intended" doesn't sit particularly well with me. Could you please share whatever research you did to back up that claim? What headphones in particular are designed and "tuned" around this spec? I would have expected that given the proliferation of SS devices that most headphones produced these days (outside the professional studio ranks) would have been designed around a very low output impedance.

Regards,

Nate

[recstar24]

Good questions Nate. I cannot speak for DNA, but that standard of 120 ohms is pretty old school. The older version Beyer DT880, which I believe is 600 ohms, is an example where if driven by a more modern near zero ohm output, sounds really really bad. That was my 1st review, and looking back at it, I wish I knew then what I knew now. However, the fact that Beyer released a 32 ohm version I believe shows that even Beyer understands that the 120 ohm output spec might be a little outdated. The older Meier amps also had a 120 ohm output, I believe Jan once talked about in Europe especially Germany their studio standard for headphone output was for a very long time 120 ohms, hence the "Standard" spec. But even with the Sennheisers, I still prefer it well damped versus the decreased damping when that output gets high, as the lower damping factor can really make the senns sound sloppy. Just my personal experience, and would be interested in what DNA has to say.

[Looser101]

Meier-Audio

^ Left pane navigation "Tips & Tricks" has a bit of info on the 120ohm standard.

[Donald North]

Thanks for these questions and your interest guys.

I've been a headphone enthusiast since 1991 when I bought my first pair of beyer DT990s. In college at Caltech I developed my own dummy heads and binaural microphones to make recordings.

A couple years ago I started investigating headphone amps to power some beyer DT931s. A friend who listens to headphones more often than I had already started such in investigation and owned a few different amps. Among them were a Singlepower amp and an Antique Sound Lab MG Head DT OTL MkIII. When I was trying his AKG and my beyer DT931 headphones with these amps, I was surprised to find they produced very different tonal balances. With the ASL, it gives you the option of capacitor (high output impedance) and transformer (low output impedance) coupling on the output. I found my DT931s sounded lean and bright with no little bass on the transformer-coupled setting. With the cap coupled (high output impedance) setting, there now was plenty of deep, extended punchy bass along with good mids and highs. My friend's AKGs, however, were much less sensitive to the setting, sounding similar in both.

My older brother used to complain about the sound of my beyers, saying they're only mids and highs with no bass. My experience thought me that this might be a function of the amplifier's output impedance which is powering them.

Intrigued by this experience, I researched around and found there is an international standard for headphone outputs, IEC 61938 spec, calls for 5V output with 120 ohm internal impedance. One of the goals is to provide similar power into both low and high impedance headphones.

From my professional experience, I know companies like Sennheiser, beyerdynamic, and AKG tend to follow these standards. I wanted an amp which met it, allowing the headphones to sound as they were designed.

I then searched around to see which headphone amps comply with this spec. With the exception of the Meier and a Bottlehead kit, I couldn't find any, let alone one which is a series-feed single ended triode without feedback - my amplification topology of choice.

So I set out to design my own. I like to do "blank sheet" design approaches, first outlining all the goals and specifications for a product. In this process, I determined the 6H30 is the perfect tube for this application, having the just the right balance of gain, plate impedance, and current drive when transformer loaded. I then drew up the required specs for the output transformers and had them made.

When you listen to the amp with the 2 different output impedance settings, you'll get to hear how this affects the tonal balance on your headphones. I've evaluated it with beyer DT931s, DT880s, AKG K601s, AKG K701s, AKG K240M & DFs, JVC HP-DX1000s. So far everyone who has tried it has preferred the IEC setting, but I'm sure some will prefer the low 28 ohm setting.

Another benefit of transformer loading, is it allows for true balanced output. I will offer both standard 1/4" stereo single ended and dual XLR balanced output versions. They have to be different models due to slight differences in the output transformer design and safety grounding.

[Icarium]

It sounds like, though, only the 600 ohm Beyers would benefit from this standard and the market that owns those cans is absurdly small.

By including the feature as standard you risk newbs using it and feeling your amp sucks with their less demanding phones.

I'm not saying this will happen for sure or anything... just mentioning the possibility.

[Donald North]

The selectable output impedance provides benefit to lower impedance headphones too. I found 250 ohm beyer DT931s and DT880s sounded better with the IEC output. AKG 601s (which are 120 ohm) sounded better too with the 120 ohm output impedance. With the 64 ohm JVC HP-DX1000s, I haven't found a preferred setting yet.

No wonder headphone amps sound so different from one another - they all have differing output impedances among other attributes. I want to give the listener the option of listening as per the IEC spec or with low source impedance.

[Icarium]

Yeah the impedance thing seems to make quite a bit of audible difference it is too bad that things aren't more standard.

Well best of luck!

[n_maher]

No wonder headphone amps sound so different from one another - they all have differing output impedances among other attributes. I want to give the listener the option of listening as per the IEC spec or with low source impedance.

I think what we're trying to get across (at least I was) is that your 28ohm "low impedance" output is what most of us would consider quite high. I can say first hand that I have heard amplifiers with an output impedance of ~100ohm and it sounded quite back with the 32ohm Grados that I was using at the time. I now use an amplifier with an output impedance of 27ohm and it sounds quite good so I suspect your amplifier on the lower setting will have the potential to sound good. As with many things in this hobby/market the proof will be in the listening. I'll definitely try to give it a listen at CanJam.

[morphsci]

Dam, I'm now up to three headphones I need to bring to CanJam. I am looking forward to hearing the balanced version of this amp.

[Donald North]

I think what we're trying to get across (at least I was) is that your 28ohm "low impedance" output is what most of us would consider quite high. I can say first hand that I have heard amplifiers with an output impedance of ~100ohm and it sounded quite back with the 32ohm Grados that I was using at the time. I now use an amplifier with an output impedance of 27ohm and it sounds quite good so I suspect your amplifier on the lower setting will have the potential to sound good. As with many things in this hobby/market the proof will be in the listening. I'll definitely try to give it a listen at CanJam.

I'm surprised 28 ohms is considered high For comparison, I suspect the OTL setting on the ASL MG Head amplifier is much higher than 120 ohms because the bass is more pronounced (thicker) than through my amp.

The Sennheiser HD600 and HD650s are 300 ohm and might be influenced by amplifier output impedance.

Absolutely, the proof is in the listening and I look forward to meeting all of you at the show.

[grawk]

There are a lot of options that are regularly discussed here and the other site that aren't widely known outside of our circles...

[luvdunhill]

Production units will feature custom-made chassis and output transformers - not off-the-shelf components. Manufacturing custom, small-batch specialty products in the United States is not inexpensive. I can assure you, no one is getting rich here

if you're expecting pre-orders, you better bring pictures of the new chassis at a minimum.

[Donald North]

Yes, units with the final chassis & faceplates are on order and will be on display at the show.

[dsavitsk]

My primary concern is 32 ohm Grados which sound best with a large damping factor. But, be that as it may, I am not sure your numbers add up here. With an rp of about 1500 and a mu of 20, what possible transformer ratio gets you less than 6db of gain and a Zout of 28?

As for the overall design, I've built that amp (down to the JJ cap, the cerafine bypasses, the Edcor OPTs and the 5AR4 -- I think I used 6N6p's instead of the 6H30 ... basically the same tube) and it is fine, but there is plenty of room for easy improvement.

I found my DT931s sounded lean and bright with no little bass on the transformer-coupled setting.

I wouldn't take much from this one data point. Sounds more like a transformer with a too-low primary inductance for the load. ASL is not known for their parts quality, and it is doubtful that they invested in high quality OPTs for this.

[nikongod]

My primary concern is 32 ohm Grados which sound best with a large damping factor. But, be that as it may, I am not sure your numbers add up here. With an rp of about 1500 and a mu of 20, what possible transformer ratio gets you less than 6db of gain and a Zout of 28?

Unless I read it wrong, 6dB is the gain on the high-impedance output.

An 8:1 OPT with a center tapped secondary looks good crunching the numbers, also allows for the balanced outputs conveniently.

[dsavitsk]

An 8:1 OPT with a center tapped secondary looks good crunching the numbers, also allows for the balanced outputs conveniently.

The 120 is supposed to be 6db, the 28 less. An 8:1 is an impedance ratio of 64:1. This gets you a Z out in about the right range but a voltage gain that is a bit high. The problem here is that with an 8:1, the reflected load from low Z phones is too low for the tube -- you'll get more power but you'll also get higher distortion. For the 120, a 4:1 is about right, but again, the load on the tube is too low.

My guess would be that the changing sound with different Z settings has more to do with the load the tube is seeing than the interaction of impedance and phones. Maybe the distortion is all 2nd harmonic, and maybe the designer likes this. Or, maybe I am totally wrong that damping factor matters and I have been barking up the wrong tree for years. But, when someone shows up and says that, notwithstanding that fact that in nearly every case nearly everybody thinks lower Zout means better sound (with the exception noted above), that he thinks the opposite is true, I think there is reason to be skeptical. This is easy enough to test, or course, with an opamp and some resistors. I'd also like to see some distortion numbers from his amp with various loads.

[aerius]

dsavitisk's numbers look right. The Rp and Zout of the tubes & amp would imply a stepdown ratio of around 7.3:1, which gives a gain of 9dB or so (voltage gain being 20log[Vin/Vout]).

[luvdunhill]

nearly every case nearly everybody thinks lower Zout means better sound

Doug:

Theoretically, is there a lower bound on how low one can (reasonably) lower the impedance on either OTL or transformer-based designs (two different estimates, obviously... also, without resorting to a hybrid amplifier)? I agree with the comment that lower impedance is pretty much always better... I haven't heard a case where this isn't true, let's put it that way.

[recstar24]

Doug:

Theoretically, is there a lower bound on how low one can (reasonably) lower the impedance on either OTL or transformer-based designs (two different estimates, obviously... also, without resorting to a hybrid amplifier)? I agree with the comment that lower impedance is pretty much always better... I haven't heard a case where this isn't true, let's put it that way.

My only thoughts, based purely on listening impressions, is that with grados, that is definitely correct, that the lower z out the better sonically (however Doug touches on an interesting notion that it may not necessarily be the zout but the loading of the tube that has more of an effect).

I prefer the senns with a lower output too, but I can see how someone might enjoy them more from a much higher z out. To my ears you get a "sloppier" sound, there is more bass but it is wobbly, and the highs have that rolled off character to them. Now those 600 ohm beyers DT 880 i believe they sound horrific on a low zout solid state type amp, very thin, brittle metallic sound with no bass, yet what happens if you plug it into a high zout like 120 ohms, voila, bass is back, not as much sharpness up top (I believe the meier link explains this stuff too in a much better way than I am).

[recstar24]

I think what we're trying to get across (at least I was) is that your 28ohm "low impedance" output is what most of us would consider quite high. I can say first hand that I have heard amplifiers with an output impedance of ~100ohm and it sounded quite back with the 32ohm Grados that I was using at the time. I now use an amplifier with an output impedance of 27ohm and it sounds quite good so I suspect your amplifier on the lower setting will have the potential to sound good. As with many things in this hobby/market the proof will be in the listening. I'll definitely try to give it a listen at CanJam.

Just to add a comparison amplifier, the Zana Deux new edition will have a low/high switch, with its low zout being 3 ohms, and high zout being 12 ohms. You guys can factor in the price difference if you wish.

[Donald North]

The 120 is supposed to be 6db, the 28 less. An 8:1 is an impedance ratio of 64:1. This gets you a Z out in about the right range but a voltage gain that is a bit high. The problem here is that with an 8:1, the reflected load from low Z phones is too low for the tube -- you'll get more power but you'll also get higher distortion. For the 120, a 4:1 is about right, but again, the load on the tube is too low.

My guess would be that the changing sound with different Z settings has more to do with the load the tube is seeing than the interaction of impedance and phones. Maybe the distortion is all 2nd harmonic, and maybe the designer likes this. Or, maybe I am totally wrong that damping factor matters and I have been barking up the wrong tree for years. But, when someone shows up and says that, notwithstanding that fact that in nearly every case nearly everybody thinks lower Zout means better sound (with the exception noted above), that he thinks the opposite is true, I think there is reason to be skeptical. This is easy enough to test, or course, with an opamp and some resistors. I'd also like to see some distortion numbers from his amp with various loads.

I prefer not to disclose all the intimate details of my output transformer specifications, the 120 ohm output as an input-output gain around 6dB; the low 28 ohm output as an input-output gain around 1dB.

People can discuss damping factor all they want. Some headphones are designed with low source impedance in mind; others with the 120 ohm IEC spec. If a headphone was designed to see a 120 ohm source impedance but is used with much lower impedance amplifier, the bass may then be overdamped and not necessarily as the designers intended. My desire is to offer both and let the listener choose.