[DeweyCH]

https://bottlehead.com/product/stat-electrostatic-headphone-energizer/

I just ordered one of these to drive my L700 Mk2s, since I've been using a Woo Wee for them since buying. Anyone have any impressions? What I've read so far is pretty good, and I really appreciate Bottlehead giving me the chance to build a proper OTL amp from their kit.

Interestingly, they're already assembled - not a DIY kit like most Bottlehead stuff (kinda disappointed, kinda not as a 4-year-old leaves me with precious little time for amp-building).

From the site:

"The Bottlehead Stat is a zero global feedback, class A tube electrostatic headphone amplifier. It features both 580V Stax Pro Bias and 230V Stax Normal (legacy) bias jacks. There is a single input, a balance control, and level control. It has a universal power transformer. Because of the high voltages present inside the chassis this product is only offered in assembled form.

The circuit is rather unique as far as electrostatic headphone amplifiers are concerned, as it is a simple cascade of two differential amplifiers per channel, with the second differential amplifier directly coupled to the headphones themselves. This amplifier is a tube roller’s dream! A short list of tubes that can be used in the first diff amp position are: 12AT7, 12AU7, 12BH7, 6211, 6201, 5965, 7062, 6829, 6679, 7728, CV4024, 7062, E180CC, 7728, and many others. There are fewer tubes that can be used in the second diff amp, as the plate voltage is twice as high as the first stage and the operating current is also higher. We recommend either 5965s or 12BH7s in this position, with the 5965s providing more gain and the 12BH7s providing more bandwidth. 12AU7 and compatible substitutes will work in this position, as will 6SN7GTA and 6SN7GTB used in adapters.

Towards the end of 2019 Sajeev Ranasinghe of Nectar Sound contacted me asking if we had plans to do an ES headphone amp, and after some conversation he sent a demo pair of his very interesting headphones for us to listen to. Once our development backlog was clear PB set about building up a direct drive headphone amp design.

No global feedback is used in this design, which provides a smooth, open sound. We have listened to Nectar Sound, Stax and Koss ES headphones and they all work as expected.To further enhance this design, each differential stage uses our Bottlehead C4S constant current source. These provide a very high impedance under the cathodes of each differential amplifier, providing very tight AC balance in each stage.

Quality polypropylene capacitors are used in the signal path, long life high temperature electrolytic capacitors are used in the power supply, and a very generously over-sized power transformer was chosen to ensure a long lifetime. The headphone output jack is custom made and very gentle with both stock Stax plugs and the Moon Audio 5 pin plug. The jack pin contacts are recessed for added safety when plugging and unplugging headphones.

The bias supply for this amplifier is of the Cockroft Walton voltage multiplier type with additional series resistance between each output of the supply and the headphones themselves. This supply is unregulated, as it is designed as an ultra low current power supply for safe headphone operation. A 12AL5 diode sits at the bottom of the voltage multiplier and slowly turns on as the amplifier warms up.

The high voltage power supply in this amplifier is a bridge rectified split rail type. This provides a positive and a negative voltage rail that allow for an enormous swing at the output stage without voltages that are any higher than what you would find in a 300B amplifier. The positive side of the high voltage rail is soft started by a 6CG3 damper diode that provides a controlled application of the positive rail voltage to the amplifier to further reduce undesirable behaviors during startup. There is an additional small 12V DC power supply to heat the 12AL5 and the first gain stage tubes to maintain proper heater cathode voltages."

Really looking forward to getting a chance to try this out, and hopefully get more out of the L700s. $1500 is a solid price for a full-on electrostatic amp, considering decent-to-good energizers run that much (looking at you iESL).

 

[k2doggo]

hi, so what was your experience with this amp?

 

[DeweyCH]

Amp took a while to get to me, but it slaps. Mine was built by Paul Birkeland. The ordering experience was nice - I got an email a couple days after ordering from Paul asking me what color I wanted, so I just went with a simple black finish (he says he can apply most of what's available in the Prismatic Powders catalog, which is a really cool option for customization). I didn't do the pot upgrade - they offer a PEC hot molded carbon pot for a moderate upcharge, but I just stuck with the original.

I've been listening for a little over a month, somewhat sporadically as I haven't had a ton of time to just sit and listen to music, but the whole six-mile-long STAX cable thing lets me listen while I solder, so that's nice. I have 2 sets of STAX - the Lambda Pro in the pic (recently purchased from @Strayngs , thank you again) and a L700 MkII. I don't have any headphones with normal bias, so everything has been through the pro bias jack.

It came with hand-selected tubes - 4x GE/Jan 5965, a GE 6CG3 and a GE 12AL5. I haven't tried rolling tubes yet, and Bottlehead says the 12AL5 and 6CG3 aren't sonically critical. You can roll a ton of amp tubes in the front - there's a list on the Stat's product page, but nearly any 12xx7, 6211, 6201, 5965, E180CC, etc. etc. etc. The rear two are the second-stage amplifying tubes with fewer options (plate voltage is 2x that of the first-stage amplifying devices). BH recommends 5965 or 12BH7 back there.

As for the sound, it's frankly fantastic. As a disclaimer, my only estat option prior to this was a Woo WEE driven by a Luxman R-117, so I can't say how it compares to a proper estat amp (or even a "proper" energizer driven off the Luxman), but damn. It sounds fantastic to me. Both headphone options I have sing with it. Soundstage is impressive, there's a real ethereal quality to the music. I've been using a FiiO M11s as my source. Of course it blows the WEE to hell and back.

I kept thinking about getting something from Mjolnir, but couldn't bring myself to swing the $$. The Stat isn't cheap ($1699), but it did come in lower than most of the well-regarded options I was considering, and I figured it would be fun to try something that not a lot of people have heard. I'm also a Bottlehead supporter, since building a Crack and later adding the Speedball was a great experience.

Anyways, I'm happy with the purchase. I really like this amp. Sorry about the dust, New England is just a hurricane of pollen lately.

ETA: At first I thought the two pots were separate volume controllers for the two channels, but one is volume and the other is balance.

 

[bagwell359]

I read up on this with interest a year ot two ago, but, lost interest when a user or two said it didn't push enough juice for my favorite stat - the DCA Voce. Glad your 700 sounds good on yours. That's a fine can.

 

[DeweyCH]

I read up on this with interest a year ot two ago, but, lost interest when a user or two said it didn't push enough juice for my favorite stat - the DCA Voce. Glad your 700 sounds good on yours. That's a fine can.

I heard people asking about whether it pushes enough gain but I’ve had no problem whatsoever. Does the Voce have the same voltage as Stax Pro?

 

[bagwell359]

I heard people asking about whether it pushes enough gain but I’ve had no problem whatsoever. Does the Voce have the same voltage as Stax Pro?

It runs at Stax Pro 580v - which is the bias voltage. Current is what determines the volume - the Koss is 2mA/channel and the Blue Hawaii is 36mA/channel. Can't find a number for the BH. I have not heard it myself.

I do know that the HFM Shangri-La Jr does fine with the Voce, but again hidden stats on mA, and a $5k price tag.

Head-Case.org has some comments from Gilmore on the BH which are somewhat arch.