DNA (Donald North Audio) Headphone Amp

[dsavitsk]

Theoretically, is there a lower bound on how low one can (reasonably) lower the impedance on either OTL or transformer-based designs

I'm not much of an OTL guy, so I really know very little about how to optimize a circuit. But, I guess you can keep paralleling tubes and keep adding feedback and whatnot to get the Zout sufficiently low.

As for transformer coupling, there are probably practical limits as to the load that can be presented as too high of an impedance leads to high frequency issues. But again, you can keep paralleling tubes to lower the Z out or use very low rp tubes. You might need very high mu tubes to get sufficient voltage gain to overcome the impedance ration, which means using the super high Gm stuff like the 7788 or 437a, or a driver. Also, an autoformer can change that a little as the winding is not as capacitive. As an example, the L'espressivo uses a 71a which has a plate resistance of a little under 2K. On the lowest setting (which is actually plenty loud in most cases) the impedance ratio is 1:16000 which results in an impedance of about 1/8th of an ohm. If you used a 300B it would be less than half of that. One might use a similar design but with a fixed autotransformer, a standard pot and a driver tube to drive higher Z phones, or to get the same damping with higher output. The particular autoformers I used are limited in voltage handling due to their small size, so you'd need a bigger core.

[dsavitsk]

I prefer not to disclose all the intimate details of my output transformer specifications

I think that's too bad. All someone will need is a sine wave and a multimeter to uncover this great secret, and it would be easier if you just said what you used.

the 120 ohm output as an input-output gain around 6dB; the low 28 ohm output as an input-output gain around 1dB.

I guess the issue is that, unless you have tremendous core and copper losses, the numbers just don't seem to add up. But, maybe I'm missing something -- wouldn't be the first time

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.

I know you are a speaker designer, so the emphasis on matching to the speaker is reasonable. Maybe you are on to something. But, matching the load to the tube also matters, and with what little we know, this seems to not be optimal here.

[Donald North]

I think that's too bad. All someone will need is a sine wave and a multimeter to uncover this great secret, and it would be easier if you just said what you used.

I guess the issue is that, unless you have tremendous core and copper losses, the numbers just don't seem to add up. But, maybe I'm missing something -- wouldn't be the first time

I know you are a speaker designer, so the emphasis on matching to the speaker is reasonable. Maybe you are on to something. But, matching the load to the tube also matters, and with what little we know, this seems to not be optimal here.

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.

[boomana]

Donald, did anyone welcome you to head-case? If not, this thread serves. Welcome. See you at CanJam.

[Dusty Chalk]

Donald, did anyone welcome you to head-case? If not, this thread serves. Welcome. See you at CanJam.

Oui

That said, this thread is more along the lines of the sort of welcome one should expect at Head-Case.

[Donald North]

Oui

That said, this thread is more along the lines of the sort of welcome one should expect at Head-Case.

Yes, thank you. I'm happy to be here and always enjoy discussing music and hi-fi

[mike1127]

Hi, I tend to post more on head-fi.org but wanted to chime in on this thread. I'm one of the people who got to hear Don's prototype. Here are my impressions:

I have heard several SET amps before, including the Decware MLB and the Wheatfield HA-2 (1?) I never liked them---they seemed too aggressive and forward. That doesn't mean I like a "laid-back" presentation, but I definitely don't like a forward presentation. Why? With a neutral presentation, I feel that my attention is free to move as I wish it to move... certainly musical events grab and hold my attention, but not so much that I feel a kind of "locked-in" feeling where I can hear only what the amp wants me to hear.

Because I didn't have a lot of money and time to explore, at the time I settled on the ASL MG-HEAD. It's okay, but not terrific.

Don brought his amp over. I got out my K601's and the source was a NAIM CD5x. My first impressions of Don's amp, listening to classical music, were

- Sweetness on massed strings. That's a good thing!

- Lovely beauty in the ebb and flow of phrasing.

Secondly, listening to Mozart's Requiem, I realized that the amp was fantastically high-resolution compared to the ASL. I always prefer the term "high-resolution" to "detailed" because it suggests that the "detail" is honest and in service of the music.

At this point, I was wondering if the amp was a bit too "tender" and "sweet", perhaps imposing its character on the music, so I put on a recording of a trombone quartet that I knew had some rockin' attacks. It was amazing how well differentiated the legato and stacco parts of the music were.

I have decided to buy this amp. Honestly, I can't claim to be familiar with a broad range of amps and designs, but I feel this is a big step up from my ASL, and the first headphone SET I've heard that does it for me.

The K601's sounded way better on the 120 ohm tap. The bass was not underdamped at all. I also listened to my DT880s and preferred the 28 ohm tap with those, at least in initial listening. However, this amp really made me appreciate the good qualities of the K601, such as the wide "headstage" and abundant "air".

[dsavitsk]

I have heard several SET amps before, including the Decware MLB and the Wheatfield HA-2 (1?) I never liked them---they seemed too aggressive and forward.

Just for the record, these are not topologically similar to the DNA, nor to each other. The DNA is a standard transformer coupled single ended design. The Wheatfield is a cathode follower that is cap coupled on the output. The Decware has a single ended driver stage and a push-pull output cap coupled to an impedance matching transformer.

I've no doubt that the DNA sounds better than either of those based on just a peek at the schematics.

[Donald North]

Here are photos of the first production units

Both will be on display for demonstration at the show.

[Dusty Chalk]

Very nice.

[Hopstretch]

Very nice.

... but can you get it in black?

Looking forward to hearing it.

[Spiug31]

please post a pic of the rear panel.

[Voltron]

... but can you get it in black?

Looking forward to hearing it.

Black?! But the blue ones are Smurfect!

[Kees]

Very nice. I like the blue.

Just one detail: I think the XLR jacks on the balanced amp are not really blending in so to say.

I definitely think you are on to something with a balanced tube amp like this though.

[n_maher]

Another front panel butchered by fugly XLR jacks, I have a strong dislike for the P-series jacks. Otherwise I like it, clean and straightforward.

[grawk]

I'd personally put combo jacks on it

[aardvark baguette]

meh, the green goes away once the phones are plugged in. I think it looks alright. I'm sure there's a market for 'fun' colors on Headfi.

[luvdunhill]

Another front panel butchered by fugly XLR jacks, I have a strong dislike for the P-series jacks. Otherwise I like it, clean and straightforward.

agreed. rear mount is the only way to go for XLR jacks.

[Donald North]

Thanks for the comments. Regarding the XLRs there is no invisible way to fit them. I investigated the offerings for Neutrik and Switchcraft with solder cup termination and felt these blended-in the best. Overall I was going for an art deco look mixing modern and retro.

[n_maher]

agreed. rear mount is the only way to go for XLR jacks.

I can understand the desire front mount them if you're going to hand-fab the panels but if you're going to go to the trouble of having a front panel made there's not much of a reason not to rear mount them. And if you are going to front mount them at least use something that looks decent.

.

And yes, I know, I'm a fussy bitch.

[n_maher]

Thanks for the comments. Regarding the XLRs there is no invisible way to fit them. I investigated the offerings for Neutrik and Switchcraft with solder cup termination and felt these blended-in the best. Overall I was going for an art deco look mixing modern and retro.

Uh, Donald, no offense, but some of us have been doing it for years.

Yes, you can still see the fasteners but it's a lot less obtrusive looking in my book.

[blubliss]

IMO, I prefer the way Donald did it. To each his own.

[Donald North]

Thanks for your comments. I considered the Neutrik you show but preferred the appearance of the Switchcraft. The Switchcraft's flange is beveled, looking more flush and color better matches the aluminum front panel than the Neutrik. Of course it's all a matter of personal preference.

Here's a photo of the rear.

[n_maher]

Donald,

One other thing to consider, I see that you are using a JJ GZ34 for the rectifier. I have been told by multiple sources that there was a rather large batch of bad tubes produced last year that tend to arc and fail in a rather catastrophic manner. If you are running the rectifier very hard you might consider using another brand of rectifier. I had one of the bad JJ's in my personal headphone amplifier (which runs the rectifiers quite hard) and thankfully its failure didn't do any damage down stream.

[atothex]

Can't comment on anything but the looks... so yeah. I like the color. I'm sorry, but black/silver is boring as hell.