[eric343]

*cue cheesy music and opening animation*
*background announcher*
Live, from the studios of HNN* in Bozeman Montana, we bring you the finest news availible this side of the ski lift...

*anchor-guy*(jude?) Good evening ladies and gentlefish. We've just recieved reports from our reporters in the field of a gathering in Kent, Washington of tube audio enthusiasts, apparently centering around a figure known as "Doc Bottlehead." I take you now to our reporter on the scene.

*me* Thanks, Jude. The Bottlehead meeting, as it's now known, was attended by well over thirty people, who in a bizarre twist on the "potluck" tradition brought various tubed amplifiers and speakers to the house behind me. Among these amplifers were a unique pair of integrated monoblocks (monoblock preamp *and* power amp!) that ran 30B tubes as 300Bs and used a beautiful chassis manufactured by the chassis-design champion of last year's Valve State of the Art Conference and a prototype of the Bottlehead Seduction headphone amplifier. The headphone amplifier was set up in the kitchen, next to an MG Head (Rayethon 5751/Ei Yugo EL84), ETA42, and an unnamed, unknown tube amplifier capable of accepting two different driver tubes.

Attendees were able to listen to the headphone amplifiers using either Etymotic 4Ses, two pairs of HD600s (Clou Blues both of 'em), and a set of Audio Technica W100s currently on loan. To my ears the Bottlehead Seduction was far and away the winner; making the MG Head sound weak and laid-back in comparison. Fans of tube-audio DIY will rejoice when they build this kit, it has incredible bass kick, fantastic and clear treble, and fantastic mids. In fact, Jude, that was one of the best tube headphone amplifiers I've heard yet.

Doc Bottlehead was there also, listening to the other amplifiers and expounding at length in the kitchen (next to his Seduction) on topics ranging from vibration absorption to green LEDs mounted in a CD player transport mechanism. (apparently they do the same thing as Auric Illuminator)

*anchor cuts in*
Well, thanks for the report, but I'm going to have to cut you off there because we just got a report that mousepads are great at vibration absorption, and I want to go try it out right now.

* Headphone News Network

 

[timoteus]

Thanks so much for the creative reporting eric343. More encouraging words about the upcoming Bottlehead headphone amp. I'm sure this is going to be a winner once it hits the market. You're lucky to be one of the first to actually hear it.

*Seduction is the name of the soon to be released phono preamp. I don't think the headphone amp has been named yet.

*Seduction (phono preamp) before Foreplay (preamp) get it?

Was this headphone amp the all out original version he designed for the recording industry or the prototype of the version that he will be kitting? Thanks for being our eyes and ears.

 

[eric343]

Well, the one he had there he said he'd be putting into a rackmount chassis, so...

 

[timoteus]

I think that's the $2000 giant slayer that Dan H reported on and said uses 6CK4s. I guess we'll have to wait a little longer for info on the kit version.

 

[tone]

Arrghh i wish i would have heard about this meeting earlier! i would have definetly showed up. a tube audio gear meet in washington and i missed it.

any idea on what the bottlehead diy kit will cost?

 

[Nick Dangerous]

I think the price of the headphone kit is hovering in the $200's. No word yet on what "the headphone amp for the peasants" will sound like. If the typical performance of Doc's other kits are any indication, it will sound terrific for the money.

I'm glad to hear that the amp is a winner, but it isn't very fair to compare Doc's expensive, totally tricked out, price no object, professional headphone amp against the MG Head...

 

[Doc B.]

Thanks, Eric, for the nice report on the meeting, and the kind words about the headphone amp.

To clarify a couple of things, Seduction is indeed a new phono preamp kit we are releasing. The headphone amp you heard doesn't have a name yet.

The one you were listening to is a proof of concept prototype of a studio grade headphone amp I will be building. It will be available on a custom order basis under the Schmalle name. Price will probably running in the $2K to $4K range depending on how it's set up. The first production unit is going to Paul Stubblebine Mastering.

The circuit uses a DC heater supply, SS rectified B+ with some additional line and reverse recovery spike filtering, and an active loaded hybrid shunt regulator ( to which those 6CK4s belong) feeding active loaded 6C45pi tubes running as single gain stages, parallel feed coupled to MagneQuest TL-404 C-core headphone autoformers with 600 ohm, 200ohm and 50 ohm taps. More, lower impedance taps may be available in the production model

There's a lot of attention paid to RFI attenuation and grounding, and the production model will probably sport Jensen input transformers running into 500 ohm stepped attenuators, although the prototype uses some 10K series attenuators straight off the inputs. The production model will be a dual mono unit, with a separate power supply for each channel and current regulation of the heaters as well as shunt regulation of the B+ voltage. The one you heard uses a single power trans with a separate shunt regulator for each channel and filtered DC.

We'll do a kit using some of the same ideas at some point , but right now we have several projects ahead of that, so maybe it will come about next year sometime. No idea on price yet, it will depend a lot on what people want in terms of features and what they want to spend.

 

[timoteus]

Hi Doc-

Thanks for dropping by and filling us in. And also welcome to Head-Fi.

I'm looking forward to the Seduction and also the headphone amp kit. Will the headphone kit use the same tubes as the studio amp or is that still undetermined? Thanks.

 

[eric343]

Sorry about the muckup with the name, I'd jsut remembered that the Seduction was your next product and assumed that what you brought would be your next product. Funny how the mind works

So you won't be releasing schematics for us DIYers to DIY a studio version, under the condition that anything we build not be sold or used for studio work?

$2-$4k is a bit steep for most of us, I think, since that's the range of the HeadRoom Blockhead...

(this one *will* have the capability to run in dual-monoblock balanced mode, won't it?)

 

[Doc B.]

The basic circuit was published several months ago in VALVE, and has been reprinted in the current issue, an article called the Kugelis.

http://www.bottlehead.com/valve/valve.html

The main difference from the production circuit is that the Kugelis shunt regulator is a gaseous regulator. Also the input attenuator design as published is bogus, a paper experiment gone awry in the rush to complete the design. But a 10K series attenuator works fine in its place.

As to the tubes used in the kit, the 6C45s are a little bit spendy to work with if folks want a truly inexpensive kit. We will no doubt examine several options before we decide what we want to use. Although the typical internet discussion about tubes might have one believing that the tube type is the first decision the designer makes, the truth is it's a ways down my list. There are a lot of other factors to consider along the way, price point, transformer availability and cost, choice of circuit topology, resultant limitations in the allowable operating points and loads, etc.

 

[TimSchirmer]

eric, when it comes out... you will build one for me right?

 

[Nick Dangerous]

Doc: Would the 6N1P be a suitable valve for a headphone amp kit? I'm about to receive John's upgrade kit for the Paraglows. Everything I've read about the change from the 12AT7 is overwhelmingly positive. It gives me hope for a high performance/low cost tube that is still in production.

 

[eric343]

Tim: I still haven't decided if I'd ever do tube amps... I've got a Foreplay kit on the way, so we'll see after that

 

[Doc B.]

Nick,

Yeah one could use a 6N1P with a suitable step down method to get to your load impedance - depending on the sensitivity of your headphones and how you implement the output loading of the 6N1P. My Exemplar Exception transformer output 6N1P preamp has a headphone jack on the front.

My take on the whole tube headphone amp thing is that a lot of folks don't really grasp the idea that varying impedances of various headphones is easily dealt with when using tubes because you have the ability to tailor your output impedance to most effectively use that high voltage swing (relative to SS devices) available from a tube through the use of a tapped output transformer.

The most telling indication of this misunderstanding is the homily that, "all 600 ohm headphones are inefficient and require lots of power". Not necessarily true, they require some voltage swing, but you have only to look at the sensitivity figure to determine how much power you need once you have the ability to match the impedance of the phones. As an example I have an old pair of AKG K240s. They are rated 88dB/mW. So to get to 110 dB I will need to deliver only 128mW at 600 ohms output impedance. That's inefficient? Heck no. We only need to swing about 8.8V RMS (about 12V peak) across that 600 ohm load. A 6N1P would want to see maybe a 15K:500 ohm reflected load, so the turns ratio of our transformer is 5:1 and thus we only need to get about 60V peak out of our 6N1P to crank those AKG 240s LOUD. No problem as long as the 6N1P sees at least 120V on it's plate, which is in fact a rather low operating voltage.

The root of the misunderstanding lies in the fact that a SS amp with a low output impedance may have a tough time delivering the voltage swing necessary because it's typical 8 ohm power rating drops like a rock into higher impedance loads. Say you build a SS amp rated for 1 watt output into 8 ohms. Well, if you guess that maybe output power cuts in half everytime the load impedance doubles (not necessarily the case but a decent rough estimate), you would be able to put out only about 15mW. That will only get us to about 100dB peaks. Note that 1W at 600 ohms output impedance will give us an estimated 120dB and probably smoke those tatty old 240s...and also requires about 24.5V rms of drive, which would require 1.4 x five times that swing from our 6N1P, or 171.5V peak, not a likely scenario as the 6N1P is going be strapped putting out much more than 2/3 of a watt and can only take about 250V on the plate.

Once we get past this misunderstanding we can begin to address the finer points and look at the different tubes that will let us play with a given headphone. One interesting design exercise is an amp for the AKG K1000. This is a very insensitive headphone, at 74dB/mW. To get to 110 dB we will need about 4W (not sure why the reviews claim 7W is needed, I guess they like about 114 dB peaks). 4W at 120 ohms requires a rms drive of 22V, which is about 31V peak. At the roughly 11:1 turns ratio required for our 6N1P to see a 15K load reflected from 120 ohms that means we need to get 341V peak drive into our transformer primary. Not gonna happen.

So we need to look at another tube with a lower plate impedance that doesn't require such a high step down ratio. How about a tube that will work into a 5k:120ohm transformer? This is about a 6.5:1 turns ratio, so now we only need to see about 200V peak swing from our tube. Still too big a job for a lot of small signal tubes like the 6C45 or 417A, which will work very nicely into a 5K load, but we can see that something along the lines of a 300B running at 400V might do the trick.

For a last quick exercise, the HD600, which after a few listening sessions I am leaning towards purchasing-

97dB/mW at 300 ohms.
OK, to get to 110dB we only need 16mW! That's 2.2Vrms or about 3V peak. Damn near anything will run these cans, even a 1W SS amp. A 6N1P with a 200-300 ohm tap on it's 15K primary will be loafing.

 

[Nick Dangerous]

Good explanation... now I can delete that old AKG=Add Kilowatts Greatly post. Oops, too late.

Granted, as a brute force approach, it works... but I prefer your method of dealing with it.