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Everything posted by Donald North
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I would add as contenders the Sony MDR-F1 and Stax Sigmas, which looked like big boxes around your ears. And then along this theme, for headphone radios there's the Panasonic radio worn by Ben Stiller in "Starsky & Hutch", where he looks like a space martian with these silver bowls over each ear with tall antennas protruding.
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DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
Yes, thank you. I'm happy to be here and always enjoy discussing music and hi-fi -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
On the IEC output setting, I add some series resistance to bring the output impedance up to 120 ohms. Absolutely, providing the proper impedance to the tube is critical. Too many transformer-coupled tube amps provide such low impedance loads at low frequencies (due to low primary inductance), causing the tube to distort. With 32 ohm headphones like the Grados on the IEC output setting, the reflected load impedance to the tube is over 4k ohms, 3x the plate impedance at its operating point. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
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. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
Yes, units with the final chassis & faceplates are on order and will be on display at the show. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
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. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
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. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
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. -
DNA (Donald North Audio) Headphone Amp
Donald North replied to blubliss's topic in Headphone Amplification
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. -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
Thanks for this input. I know my 1W 6DN7 amp isn't powerful enough, while my 7W 300B amp has too much power for my tastes. I anticipate the 2A3 will be the right balance. The 300B can generate enough voltage to take the K1000s to max SPL, however my intention is to maximize sound quality at sane listening levels. From what I've read, the Moth 2A3 is a direct coupled design using the 6SL7. Such a design has its share of challenges. I plan to use a different input/driver tube with capacitor coupling for the first level offering. I custom built a 45 amp for a friend in this configuration and the results are excellent (just 1.75W though). The 2A3 should also work well; we'll see. Donald North -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
Thanks Aura. Nice name - I work full time for Aurasound Will keep you and everyone posted on amp development. Donald North -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
The K1000s sound great with my Audio Note Kit 1 300B SET amp. This combo can play much louder than I listen, and I prefer 2A3s, hence my interest to build a dedicated K1000 2A3 amp. Donald North -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
I wish! Unfortunately it's not possible to do it here in the USA at that price. Donald North -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
Thanks. Yes, that's me It will be a SET tube amp using the 45 or 2A3 on the output with output transformer, no feedback, and tube rectified. I will offer it at a couple levels. Price will be around $2000 for the first level. Donald North -
Tube amp suggestions for AKG K1000
Donald North replied to Blackmore's topic in Headphone Amplification
The best I've heard the K1000s to date is with an Audio Note Kit 1 300B SET amplifier. The sound was so pure and clear. Gain was a bit higher than I need. I'm planning to develop a custom tube amp for the K1000s. Donald North