This is a review and detailed measurements of a (built) Bottlehead Crack Tube headphone amplifier. It was sold from one member to another and kindly sent to me in between for testing. The version I have has the "speed ball" current source which brings its cost up to US $430.
The original owner did a beautiful job of building this kit:
And here are the internals:
Can't ask more in a DIY build.
I looked thorough the manual and it seems exceptionally well written and annotated.
I liked the feel of the volume control although as you will see later there is significant channel imbalance.
The tubes in the test unit are USA made 6080WC (output) and 12AU7 (driver).
Tube Headphone Amp Measurements
As usual we start with our dashboard:
It is a shock to the system coming from testing many solid state amplifiers to see SINAD (sum of distortion and noise) to be so low. That said, we have seen worse:
Second harmonic dominates the most so audibility should be less than it seems.
Signal to noise ratio is not great due to high mains noise which I could not get rid of by playing with grounding:
Frequency response is very good:
Let's measure power vs distortion+noise into 300 ohm load:
Distortion sets in at just 1/10th of a milliwatt! Since there is no hard clipping my standard method of measuring power at that point is thrown out the window. Using -30 dB as an arbitrary limit gets is 32 milliwatts which is not that high.
Switching to 33 ohm load destroys performance:
The issue here is that without an output transformer, we have a high output impedance of 188 ohm (compared to close to zero for most solid state designs) so a ton of power is lost in the device itself. The high impedance will also impact the frequency response of many headphones as you will see shortly.
Subjective Headphone Amplifier Listening Test
I started testing with the Sennheiser HD-650 and the result was unexpected: almost infinite amount of power with warmer/nicer amount of bass. I switched to my standard amp, the RME ADI-2 DAC and it simply could not keep up with the Crack either in amount of power or bass. Was the "tube glow" finally resulting in "euphoric distortion?" Not so fast...
I have on loan a GRAS 45CA headphone test rig so I decided to measure its response while being driven by a solid state Topping A90 headphone amplifier and then Bottlehead Crack:
The response is almost identical above 500 Hz but below, there is a broad lift to the tune of 1 dB. That means a lot of bass notes are amplified more due to high impedance of the Bottlehead Crack changing the response of HD-650.
It was a good thing I did not start testing with Drop Ether CX as it was a total disaster: almost no output and what was there was severely distorted. The Ether CX has a low impedance of 25 ohm and is very inefficient to boot. The high impedance of Crack meant that hardly any power was delivered to the amplifier and what was there, was at max volume with ton of distortion. Whoever thinks distortion of this type is pleasant should listen to this combo.
I also tested with AKG 371 and Sony MDR-7506 headphones. Neither of these got the beneficial frequency response. While it was hard to tell in my quick testing, I thought the sound was a bit distorted. Available power was not that great but usable.
Conclusions
This is a nice tube headphone amplifier kit and the built version is a good example of how to do it right. Objective performance shows severe problems. However it also points to lots of power availability and potential for modification of frequency response of the headphone. As it turns out, these factors work in favor of Sennheiser HD-650 producing beautiful tonality and music. Other headphones though produced everything from unusable to "what did I buy this tube amp for?"
I hope the new owner has an HD-650 type headphone in which case he will be happy.
Personally I would use EQ to correct the bass response of the HD-650 and use a powerful solid state amplifier to drive it. But if you want to go with the Bottleheack Crack amplifier kit, I understand and won't hound you!
-----------
As always, questions, comments, recommendations, etc. are welcome.
Drove 150 miles roundtrip to dentist appointment and also to pick up a speaker and return one. I spent money out of my own pocket. And no, I don't need you to reimbursement for this since I got a tooth cleaning out of it. Thought about running out to the store and showing off my clean teeth but couldn't figure out how to do that with a mask. But hey, if you insist on putting more money in my pocket, I am not going to complain. You can donate using : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
The original owner did a beautiful job of building this kit:
And here are the internals:
Can't ask more in a DIY build.
I looked thorough the manual and it seems exceptionally well written and annotated.
I liked the feel of the volume control although as you will see later there is significant channel imbalance.
The tubes in the test unit are USA made 6080WC (output) and 12AU7 (driver).
Tube Headphone Amp Measurements
As usual we start with our dashboard:
It is a shock to the system coming from testing many solid state amplifiers to see SINAD (sum of distortion and noise) to be so low. That said, we have seen worse:
Second harmonic dominates the most so audibility should be less than it seems.
Signal to noise ratio is not great due to high mains noise which I could not get rid of by playing with grounding:
Frequency response is very good:
Let's measure power vs distortion+noise into 300 ohm load:
Distortion sets in at just 1/10th of a milliwatt! Since there is no hard clipping my standard method of measuring power at that point is thrown out the window. Using -30 dB as an arbitrary limit gets is 32 milliwatts which is not that high.
Switching to 33 ohm load destroys performance:
The issue here is that without an output transformer, we have a high output impedance of 188 ohm (compared to close to zero for most solid state designs) so a ton of power is lost in the device itself. The high impedance will also impact the frequency response of many headphones as you will see shortly.
Subjective Headphone Amplifier Listening Test
I started testing with the Sennheiser HD-650 and the result was unexpected: almost infinite amount of power with warmer/nicer amount of bass. I switched to my standard amp, the RME ADI-2 DAC and it simply could not keep up with the Crack either in amount of power or bass. Was the "tube glow" finally resulting in "euphoric distortion?" Not so fast...
I have on loan a GRAS 45CA headphone test rig so I decided to measure its response while being driven by a solid state Topping A90 headphone amplifier and then Bottlehead Crack:
The response is almost identical above 500 Hz but below, there is a broad lift to the tune of 1 dB. That means a lot of bass notes are amplified more due to high impedance of the Bottlehead Crack changing the response of HD-650.
It was a good thing I did not start testing with Drop Ether CX as it was a total disaster: almost no output and what was there was severely distorted. The Ether CX has a low impedance of 25 ohm and is very inefficient to boot. The high impedance of Crack meant that hardly any power was delivered to the amplifier and what was there, was at max volume with ton of distortion. Whoever thinks distortion of this type is pleasant should listen to this combo.
I also tested with AKG 371 and Sony MDR-7506 headphones. Neither of these got the beneficial frequency response. While it was hard to tell in my quick testing, I thought the sound was a bit distorted. Available power was not that great but usable.
Conclusions
This is a nice tube headphone amplifier kit and the built version is a good example of how to do it right. Objective performance shows severe problems. However it also points to lots of power availability and potential for modification of frequency response of the headphone. As it turns out, these factors work in favor of Sennheiser HD-650 producing beautiful tonality and music. Other headphones though produced everything from unusable to "what did I buy this tube amp for?"
I hope the new owner has an HD-650 type headphone in which case he will be happy.
Personally I would use EQ to correct the bass response of the HD-650 and use a powerful solid state amplifier to drive it. But if you want to go with the Bottleheack Crack amplifier kit, I understand and won't hound you!
-----------
As always, questions, comments, recommendations, etc. are welcome.
Drove 150 miles roundtrip to dentist appointment and also to pick up a speaker and return one. I spent money out of my own pocket. And no, I don't need you to reimbursement for this since I got a tooth cleaning out of it. Thought about running out to the store and showing off my clean teeth but couldn't figure out how to do that with a mask. But hey, if you insist on putting more money in my pocket, I am not going to complain. You can donate using : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/