Bottlehead Crack Headphone Amplifier Kit Review

[amirm]

Hey! What’s the big deal? This is just proper component matching and balancing of the system through careful assembly of the various parts!! An audiophile never ending quest that objectivists just simply do not appreciate!! LOL

The heck you are telling me that for? I tested using listening and gave a much higher review score because of that.

 

[SIY]

The heck you are telling me that for? I tested using listening and gave a much higher review score because of that.

Poe's Law.

 

[amirm]

Here is the multitone with unity gain:

 

[Mad_Economist]

Here is the multitone with unity gain:

View attachment 80983

Thanks! What's the load? 300Ω?

 

[amirm]

Thanks! What's the load? 300Ω?

It was unloaded (100k). It is a bit worse at 300 ohm:

 

[JohnYang1997]

Thanks! What's the load? 300Ω?

I assume no load.

 

[Blumlein 88]

With a 32 ohm load, the bass rolloff frequency is 50 Hz. I'm always amused to see some of my design features pop up a few years later in their products.

Mods to improve this are easy. First, a safety resistor from the grid of the 12AU7 to ground. Second, bias up the heaters to 50V above ground and bypass each side to ground to eliminate common mode heater noise. Third, either improve the plate loading of the 12AU7 or (best yet) replace it with something more linear, making sure to re-bias it. 90V on the anode is far too low for it to run linearly. The output stage is a disaster, which would require some pretty extensive redesign to fix, but getting some feedback in there is bound to help- there's already excessive gain, so why not use it?.

I had some of the same thoughts looking at the schematic. Almost like the designer didn't understand how to design with tubes.

 

[solderdude]

This design should not be seen as a serious attempt to make a good all tube headphone amplifier.

Like the NP ACA 'class-A' amplifier it is intended as an entry level DIY amplifier with some typical characteristics.
Of course one can also build a C'Moy amp and have something that measures better or go for the O2, or when you want to DIY a tube amp that isn't as lethal, there are other DIY options so to say.
Some just want to dabble in all tube amps and perhaps some of the easiest ways to start this, without starting from scratch, is Bottlehead amps (they have better ones though).
It is what it is.

 

[Robbo99999]

Here is the multitone with unity gain:

View attachment 80983

Thanks! What's the load? 300Ω?

What's this test illustrating again, I think I remembered from an earlier post.....it's showing that the 2nd Harmonics introduced in the amplifier are destructive when it comes to real music rather than synergistic....something along those lines?

 

[304290]

This amp brings back memories of my days spending time over at headfi. There was a time when the masses had a particular headphone that matched a particular headphone amp. There were constant debates(probably still is) of which amp to get with what headphones. That led to many wasting tens of thousands on buying several tube amps to go with each individual headphone they owned instead of just getting one properly designed amp that was great with any type of headphone. I'm glad I never fell into that trap. Though my knowledge many years ago, wasn't what it is now, I was never that stupid. Though I did enjoy the distorted sound of a couple of tube amps I did own. Each one was a compromise compared the the solid state headphone amps I've owned over the years.

 

[Mad_Economist]

What's this test illustrating again, I think I remembered from an earlier post.....it's showing that the 2nd Harmonics introduced in the amplifier are destructive when it comes to real music rather than synergistic....something along those lines?

The nonlinearity shouldn't be conflated with its symptoms, basically. 2nd harmonic distortion of a sine wave is a manifestation of 2nd order nonlinearity in the playback system...but that isn't necessarily going to just produce innocuous and easily ignored harmonics when fed a different signal.

More broadly, you could easily argue against single sine testing on this basis, in fact, unless the desire is to know the nonlinear behavior of the system at a specific level and frequency alone.

 

[solderdude]

A really interesting test would be to null the amps output (at 'normal' listening level) and listen to the null that is not amplified.
Oh wait... you can test this yourself using @pkane software.

Measurements do say something but only to those that understand the test and know how to interpret all the test results.
Measurements are easily compared when the needed info is also present about test conditions but by itself will not tell the whole story.

 

[solderdude]

To claim that all high impedance headphones will sound good (even for a subset of people) using this tube amp is ludicrous, because as shown above the changed sound will be different for each headphone, depending on its impedance curve and frequency response.

Of course all headphones will change their sound differently. Of course they will all raise different parts of the frequency range in a different way.
Stating this only pans out 'positive' (and to whom, I prefer less midbass hump) for the HD650 is ludicrous. The Focal example you shifted in there to make your point is NOT compatible with the Crack because it is way below 200 Ohm. The other headphones all clearly showed a small bump in the lows. Those that prefer 'thick' 'tubey' sound will prefer this small boost in general.
If I would have to guess you haven't experimented with different output resistances and a whole bunch of headphones, tubes and different designs.
Can I ask whether or not you actually played with this amp and several high impedance headphones, what's your practical experience and electronics knowledge/build level ?

 

[Mad_Economist]

If I would have to guess you haven't experimented with different output resistances and a whole bunch of headphones, tubes and different designs.
Can I ask whether or not you actually played with this amp and several high impedance headphones, what's your practical experience and electronics knowledge/build level ?

Questions framed in this way rarely precede enjoyable discussions, in my experience.

 

[solderdude]

Don't worry, he is not going to answer my rhetorical question anyway and will only address what I wrote and disagree with it by default.

 

[Veri]

Here is the multitone with unity gain:

View attachment 80983

Wow that is a lot worse than I expected! Thanks Amir for doing this additional test.

 

[Jimbob54]

Questions framed in this way rarely precede enjoyable discussions, in my experience.

He might as well have spilled his pint and goosed his girl

 

[DualTriode]

That THD graph looks really good for a tube amp. Can you add a link or some extra info?

Over several iterations I have learned a few things and a couple of biases to go along.

First for tubes, noise especially hum and buzz is the worst.

In terms of noise and distortion tubes have a sweet spot or preferred operating voltage and bias current. How do you know where that sweet spot is?

Using a Keysight variable voltage DC Supply, a resistor switch box and an AP Analyzer I hook up the vacuum tube bread board and start cranking up the voltage, adjusting the cathode resistor value then watching the continuous FFT display of the analyzer. It is pretty straight up selecting the optimal voltage and current operating point for the vacuum tube under test. As the voltage goes up the noise maybe goes up and the measured distortion goes down. If you set the test band to eliminate the power supply hum and buzz the SINAD peaks at the sweet spot. If you look at the plot you can see rising 1/f noise with decreasing frequency. I used a TT-106-OT transformer from https://www.transcendar.com/single-ended-transformer/5-watt-se/

Once you have the voltage and current bias point selected then you put together the power supplies. Both the B+ and heater supplies need to be really clean. For the plot, the B+ was a shunt regulated thing from KandKaudio.com and the heater supply is something I cobbled together. Using a voltage divider the 12 volt Heater is referenced to ¼ of the B+ supply voltage. If the heater supply has ripple the ripple leaks into the B+ supply and shows up as power supply hum and buzz on the FFT.

In terms of tube distortion the headphone driver distortion is greater in magnitude making the discussion of tube amplifier distortion only academic.

Thanks DT

 

[Veri]

Over several iterations I have learned a few things and a couple of biases to go along.

First for tubes, noise especially hum and buzz is the worst.

In terms of noise and distortion tubes have a sweet spot or preferred operating voltage and bias current. How do you know where that sweet spot is?

Using a Keysight variable voltage DC Supply, a resistor switch box and an AP Analyzer I hook up the vacuum tube bread board and start cranking up the voltage, adjusting the cathode resistor value then watching the continuous FFT display of the analyzer. It is pretty straight up selecting the optimal voltage and current operating point for the vacuum tube under test. As the voltage goes up the noise maybe goes up and the measured distortion goes down. If you set the test band to eliminate the power supply hum and buzz the SINAD peaks at the sweet spot. If you look at the plot you can see rising 1/f noise with decreasing frequency. I used a TT-106-OT transformer from https://www.transcendar.com/single-ended-transformer/5-watt-se/

Once you have the voltage and current bias point selected then you put together the power supplies. Both the B+ and heater supplies need to be really clean. For the plot, the B+ was a shunt regulated thing from KandKaudio.com and the heater supply is something I cobbled together. Using a voltage divider the 12 volt Heater is referenced to ¼ of the B+ supply voltage. If the heater supply has ripple the ripple leaks into the B+ supply and shows up as power supply hum and buzz on the FFT.

In terms of tube distortion the headphone driver distortion is greater in magnitude making the discussion of tube amplifier distortion only academic.

Thanks DT

I hate tubey gear because of said hum and noise. Like, why would you insist on noisier sound! Anyway, with high impedance phones it's less audible so less of an issue.
Schiit's Saga has very little noise which I respect, but it's likely so hybrid it barely qualifies heh

 

[solderdude]

In terms of tube distortion the headphone driver distortion is greater in magnitude making the discussion of tube amplifier distortion only academic.

A HD650 easily get -70dB distortion levels at 1kHz whereas the Crack manages -45dB with 300 Ohm load.
At lower voltage levels the distortion may be a bit better for the crack.
Frequencies between 200Hz and 8kHz is where distortion matters the most.