New top of the line headphone amp

[Doc B.]

Last year we retired the Smack headphone amp kit due to sluggish sales. There were a lot of really good aspects to that kit - adjustable output impedance to handle a wide range of headphones, both single ended and balanced headphone output, a single gain stage circuit with active loading and hybrid shunt regulation, and very good transformers. And very good sound - I thought it was clearly our best sounding headphone amp at the time. Unfortunately for Smack we revamped the S.E.X. kit and in doing so came up with an amp that pretty closely matched Smack sonically and offered enough power (2W!) to run tough cans like LCDs, HEs and K1Ks.

Smack's lower gain and 1/10 the power output was a compromise in the name of greater resolution and speed and more quiet operation. And we somewhat compromised the kit with a very functional but standard stereo volume pot to keep the price down. These things worked against the kit and our customers were buying S.E.X. instead for the same price.

So I decided to rethink the concept of a more premium headphone amp. I wanted to keep the positive aspect mentioned above, and I wanted a bit more gain and more power, and a better attenuator up front. We also had heard a lot of folks ask for extra inputs and express a dislike for our impedance switching setup that required turning the amp off and going inside to adjust output impedance and balanced/SE output.

And so here you see the prototype of Mainline, our new premium headphone amp. It uses 6C45pis as the power tubes and a 12AU7 as the two channel hybrid shunt voltage regulator. The 6C45s and the shunt reg are both loaded with C⁴S active loads. Zero global negative feedback as usual and parafeed transformer output using the same OT-3 we used in Smack. There is a switch which sets the output impedance for low or high impedance headphones, one that sets the output to single ended or balanced, and a source selector for two sets of single ended RCA inputs. Output jacks are both from Neutrik - that kinda obnoxious but very fancy locking TRS jack that tweakers seem to fancy for their Crack kit, and a four pin XLR wired in the standard AKG K1000 pinout for the balanced jack. The attenuator is the same as used in our Submissive 36 step attenuator with six coarse 9dB steps and six fine 1.5dB steps for a total of 56 dB attenuation. Output power is about 600mW. Input impedance is 25Kohms minimum.

I'm using it with HD800s with both balanced and single ended cables. It is clearly more resolving and much more quick and punchy than the very good sounding Smack amp that I kept for myself when we retired that kit. Bass is tighter and the top end is more open while the natural midrange is at least as good.

Kit pricing is expected to be around $1000 - just about what one would have paid for a Smack kit and a Submissive kit if they had coexisted, plus the step up to a 6C45pi configuration.

I'll be showing the prototype at the Seattle Head-Fi meet at the West Seattle public library this Saturday.

[Spinifex]

You really seem to know how to get my hard earn cash. If this does sound better than the Smack, and if I can use it for my planars, I'll be one happy man with a $1000-lighter wallet.

[tdogzthmn]

Very Cool!  Bottlehead has been releasing some great stuff so far this year.  I might need to track down another K1000 again to pair with this amp.

[HF9]

Very nice, are these going to feature Bottlehead output transformers? Any thoughts on how it performs compared to a S.E.X. with stepper and Magnequest iron? That's definitely been my go-to amp for quite a while now.

[Doc B.]

Yes, this uses the OT-3 output transformer that we used in the Smack. That is a transformer that has been tweaked for headphone use. I think the new amp will sound better than S.E.X., with higher resolution and a more dynamic and clean presentation partly because of the single gain stage and partly because of the shunt regulation and active loads. It costs more, and our philosophy is if it costs more it it should sound better. I have not tried it with K1000s. K1000s are ultra nearfield speakers more than headphones, they want speaker type power levels. I think they sound best with Paramounts and very good with S.E.X. I will try my K1Ks at some point - who knows? maybe I will be pleasantly surprised - and I should have a report on how well the amp works with LCD2s and HEs after Saturday's meet.

[xcortes]

Hmmm, these are 8k primaries. I may grab one and build it it with my 8k B7s, just like keto did with a Smack. For the loaded-with-iron office system.

[Paul Joppa]

In this application they are operating as 4K primary impedance. For what it's worth. It doubles the ratio of inductance to resistance giving (in theory!) better bass. The transformer was designed to work well over the range of 4K to 8K.

[Doc B.]

The B7 step down ratio is pretty shallow compared to the OT-3 we designed the amp for. The Mainline is heavily regulated and is just DEAD quiet with HD800s on the high impedance switch setting, that is 64 ohms output impedance. But I don't know if the S/N ratio with a B7, even at the 150 (or is it 125?) ohm tap will be as good. By the way I prefer the OT-2 (4/8/16/32) in the S.E.X. over the OT-3 (16/32/64/128) for the same reason - on the highest impedance configuration (128 ohms) some hum comes through the S.E.X. That tap is hardly necessary with a 2 watt amp anyway, the 32 ohm tap will blow your ears off with 300 ohm cans. If you are using your Sonys I think the OT-3 will be a better match than the B7 anyway.

[xcortes]

I'm thinking of it as a pre. For the Sonys I have the very first Mainline which was modified for 437A, remember that one? The B7 is 600 ohm on the secondary. In any case I have other projects before and I'll just put the B7s in the, since today, idle fp3.

[Doc B.]

My experience indicates that BeePre is a better preamp, and Mainline is a better headphone amp. Headphones require an OT. A preamp does not. If you can avoid a transformer without having to use a cathode follower instead - i. e. you can create a plate follower with low enough output impedance for line level use - that will work a wee bit better. As for the 437A amp, yup, mainline is a close relative. Would be interesting to compare. That amp has some really great parts. Mainline doesn't have the exotic tubes/iron/caps/etc. but has a couple of tricks (balanced output, a very competitive attenuator) that might get it pretty close.

[xcortes]

Oh, I'm sure that the BeePre is a better preamp, just by concept. But for my office I'll be using cds and the more iron I can put in there the better. In my main system I'm enjoying a Beepre already.

I have not used the 437A amp with its matched repro yet as I had some trouble with the Ampex 300. But that deck is working now and I'm sure I'll starting enjoying it a lot very soon!

[Doc B.]

I get the concept you are shooting for. But I don't know that the OT-3 would help smooth the sharp edges of a digital system. They are really open on the top end. Where the 300B as plate follower beats out the transformer output preamps is on the bottom end, it's tighter and more punchy. That's not just vs. the OT-3, every transformer I have tried at line level is a little softer on the bottom end than a plate follower.

BTW if you are going to put 8K B7s in your FP3, why not convert it to a tube that is a better match for them? You don't need the extra triode for the CF, and you really want a tube with a lower plate impedance than a 12AU7. Maybe try 417As?

[4krow]

It's funny, I'm just not a headphone guy. I have tried a few times to get into it, but end up with the regular set up with speakers each time. The odd part is, headphones offer the greatest detail one could ever want, AND there are no room acoustics and such to mess with. When I see designs like the one described here, I get tempted. I mean when it's done right, there really isn't an equal.
  My curiosity would be greatest to hear a headphone system that was balanced, all the way from the phono cartridge to the headphones themselves. That's a little out there, I know, since the resolution is already incredible, but I just can't help the 'what if' gene.

[Chris]

Nice name more "reference" sounding...

[Doc B.]

Yeah, but reference to what?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fphotos.foter.com%2F103%2Fmainlining-cocaine_l.jpg&hash=859943773935294ec1f826bd89c8cd51c56224a6)