Bottlehead Crack Headphone Amplifier Kit Review

[solderdude]

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.

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.

This is exactly what Marv's (SBAF) beef is with 'measurements extremists', and it seems in particular with Amir .
Obvious amounts of distortion reaching over 1% (bottom of the SINAD chart), having channel imbalance and an output resistance of about 1/2 (breaking the 1/8th rule even with a headphone showing impedance swings) doesn't lead to 'poor sound quality' but rather can give a pleasant and impressive change to certain headphones.

 

[Doodski]

It appears this amp is ripe for a mod or two. I specialized mostly in mechatronics and know next to nothing about tubes but somebody might have a mod or two.

 

[solderdude]

It needs silver wiring inside... and rainbow foil stickers, and maybe some chakra stones or quantum dots for sure.
And a DACT, audiophile grade capacitors, a tube rectifier, special audiophile resistors and .....

 

[wwenze]

Here you go:
View attachment 80850

Somebody remind me if it is legal to tie the secondary ground to earth.

 

[milosz]

Somebody remind me if it is legal to tie the secondary ground to earth.

Only if it consents to being tied. And it's important to establish a "safe word."

 

[amirm]

This is exactly what Marv's (SBAF) beef is with 'measurements extremists', and it seems in particular with Amir .
Obvious amounts of distortion reaching over 1% (bottom of the SINAD chart), having channel imbalance and an output resistance of about 1/2 (breaking the 1/8th rule even with a headphone showing impedance swings) doesn't lead to 'poor sound quality' but rather can give a pleasant and impressive change to certain headphones.

Well, he doesn't know the plot. The measurements predicted the change in frequency response. And the high distortion killed the audible performance with a number of headphones. So the issues were real and audible.

It just happens that EQ effect it provided through high impedance was a positive one in one instance. This too was predicted in the measurements of the amp. And confirmed through headphone measurements.

Were it up to him, he would have falsely assumed that tube = warm sound and distortion = inaudible. Both are wrong. Measurements prove it. And my proper listening tests demonstrate it.

All of this aside, I provide listening tests for when it matters. So his assertion that we only go by measurements is absolutely false in these scenarios. When there is a potential for subjective testing adding value, that is what I provide.

 

[bobbooo]

This is exactly what Marv's (SBAF) beef is with 'measurements extremists', and it seems in particular with Amir .
Obvious amounts of distortion reaching over 1% (bottom of the SINAD chart), having channel imbalance and an output resistance of about 1/2 (breaking the 1/8th rule even with a headphone showing impedance swings) doesn't lead to 'poor sound quality' but rather can give a pleasant and impressive change to certain headphones.

It does lead to poor sound quality (reproduction). But it doesn't necessarily lead to low sound preference...with one particular pair of headphones (with its particular impedance curve), according to this particular listener, on this particular day, listening to their particular music, at their preferred volume.

 

[Mad_Economist]

Well, he doesn't know the plot. The measurements predicted the change in frequency response. And the high distortion killed the audible performance with a number of headphones. So the issues were real and audible.

It just happens that EQ effect it provided through high impedance was a positive one in one instance. This too was predicted in the measurements of the amp. And confirmed through headphone measurements.

Were it up to him, he would have falsely assumed that tube = warm sound and distortion = inaudible. Both are wrong. Measurements prove it. And my proper listening tests demonstrate it.

All of this aside, I provide listening tests for when it matters. So his assertion that we only go by measurements is absolutely false in these scenarios. When there is a potential for subjective testing adding value, that is what I provide.

For what it's worth, my read of Marv's position is actually weirder (and, to me, more offensive) than "ASR only goes by measurements". Rather, in some of his posts he seems to contend that a forum with a scientific bent and fondness for measurements shouldn't include subjective preference calls. Examples would be his complaints about the recommended/not commentary, the panthers, the colour commentary on your reviews, etc.

Edit: Re Zout interactions, it's probably worthy of note that in the "genre" of open, higher-impedance electrodynamic headphones, you pretty much always see an impedance characteristic like the HD6x0 has, so in the Crack's defense, it's not pure chance that it's a subjectively preferable effect to many people.

 

[solderdude]

Well, he doesn't know the plot. The measurements predicted the change in frequency response. And the high distortion killed the audible performance with a number of headphones. So the issues were real and audible.

It just happens that EQ effect it provided through high impedance was a positive one in one instance. This too was predicted in the measurements of the amp. And confirmed through headphone measurements.

Were it up to him, he would have falsely assumed that tube = warm sound and distortion = inaudible. Both are wrong. Measurements prove it. And my proper listening tests demonstrate it.

All of this aside, I provide listening tests for when it matters. So his assertion that we only go by measurements is absolutely false in these scenarios. When there is a potential for subjective testing adding value, that is what I provide.

Yes, measurements show and predicts the changes to the sound.
The manual states that this amp can only be used with high impedance headphones (> 200 Ohm) so Marv would claim you would have tested it with not suited equipment.

I don't think he would have assumed warm sound and distortion = inaudible, in fact rather the opposite and would say for some folks it means more sonic bliss.

The listening test you did confirms his line of thinking is what I was pointing out. The listening test made his point.

Of course the notion that tubes posses a form of 'magic' is utter nonsense which is what is the general belief for many.

But let's not make this yet another SBAF thread

It does lead to poor sound quality (reproduction). But it doesn't necessarily lead to low sound preference...with one particular pair of headphones, according to this particular listener, on this particular day, listening to their particular music, at their preferred volume.

It does lead to inaccurate reproduction of the input signal. Nobody (also not Marv) will dispute this. It would be very wrong to say this amp has better fidelity and thus is the 'better' amp. It is not. Technically it is garbage. Sonically, when used with suitable headphones it may well be a more pleasant listening experience. That's what it is all about for those that care less about measurements.

You say it would only lead to a preference with one particular headphone/listener on one particular day with particular music at their preferred volume.

This is complete nonsense. It has been proven a zillion times that the sound signature Amir described is reported and verified by measurements that this is not a 1x lucky shot that only works with HD650 using the 'Crack' happening to Amir on that day with his preferred music and loudness.

 

[milosz]

I have a Bottlehead Crack + Speedball. I love the way my 600 ohm Beyer DT 880's sound with this amp. I also like the way my HD-600's sound with it. I had HD-800's but had to sell them, the treble gave me headache, but when I had them, the Crack was my favorite amp to use them with.

I don't listen to it much. (I actually don't listen to headphones of any description much these days.) But it is a "FUN" headphone amp especially with my 600 ohm 'phones.

I equipped mine with a switch to allow going back and forth between the stock 100 uF electrolytic and a 100 uF mylar film capacitor for coupling the amplifier to the load. I purposely did not pay attention to which position was which on that switch when I assembled the thing. I A/B'd the sound using a number of different headphones, and the difference between the two positions was NOT subtle. There was an IMMEDIATELY audible difference between the two switch positions- one sounded kind of "raspy" in the highs - as though there was some distortion of the highs, or some ringing or noise bursts kind of thing. The other position the treble sounded quite smooth. I did not know which was the electrolytic and which was the film cap, but I knew almost instantly that I prefered one switch position over the other. After a few weeks I looked under the chassis top and saw that it was the film cap that I preferred. Make of this small sample size, not exactly double blind, no control group used, study what you will. But the conclusion I draw is that film caps sound better when they are in series with the audio signal than polarized electrolytics. Maybe a non polarized electrolytic would sound as good if you could find one with a high enough voltage rating, I don't know.

 

[Mad_Economist]

It does lead to inaccurate reproduction of the input signal. Nobody (also not Marv) will dispute this. It would be very wrong to say this amp has better fidelity and thus is the 'better' amp. It is not. Technically it is garbage. Sonically, when used with suitable headphones it may well be a more pleasant listening experience. That's what it is all about for those that care less about measurements.

Ultimately, a fairly large share of the conflict on the objective-subjective split comes down to what we mean when we say "better". Objectively, almost all amplifiers are better in terms of their linear amplification and output of the input signal than the Crack is - it is very low fidelity. If your criteria for "better" is being high fidelity, it's pretty clearly not better. For a lot of folks, that's better. But for a lot of folks, "better" requires "sounds better", and what people perceive in sound is quite annoying to pin down unless you happen to be in their head or have significant sway over testing parameters.

I generally try to be specific about what I mean when I make comparative statements like "x is better than y" for this reason - it's part of why I'm a little longwinded, but saying "the Topping A90 has far lower noise and distortion than the Schiit Magni 3" is less likely to be misinterpreted as attacking someone else's perceptions of sound quality than saying "the Topping A90 is far better than the Schiit Magni 3".

 

[solderdude]

Yes, the definition of better is what I always point at in my replies when someone claims this or that is better because.....
We need to differentiate between better signal integrity/quality and preference of sound as in both cases the word 'better' is used.

Instead we could say 'preferred' for perceived sound quality and 'accurate signal' for better measuring devices and describe which parts are 'better' and why.

 

[Mad_Economist]

As an alternative to your pitch, the entire world could unanimously adopt my personal hierarchy of values, and then there would be no more ambiguity when people said "better" either

 

[solderdude]

I feel better now...

Sorry for the derailment.. back to crack.

 

[PeteL]

I get that tube amp by design makes it difficult if not impossible to have a low impedance output, but I have seen mentioned in the reviews and the comments that it don't have output transformers, but in real life, do we know ANY any headphone amps with output transformers? This would make it complicated since transformers don't work by bridging impedance, no notion of damping factor. They transfer power by impedance matching. For speaker, it can be simple, you need a tap at 4 ohm, and one at 8. But wondering how it could be of any use in a headphone amp, with load impedance all over the place. unless the amp is designed specifically for a particular set. or have multiple taps AND multiple headphone jacks, which would be a bit odd.

 

[Veri]

I feel better now...

Sorry for the derailment.. back to crack.

The only crack I'd want to get hooked on lol

I get that tube amp by design makes it difficult if not impossible to have a low impedance output, but I have seen mentioned in the reviews and the comments that it don't have output transformers, but in real life, do we know ANY any headphone amps with output transformers? This would make it complicated since transformers don't work by bridging impedance, no notion of damping factor. They transfer power by impedance matching. For speaker, it can be simple, you need a tap at 4 ohm, and one at 8. But wondering how it could be of any use in a headphone amp, with load impedance all over the place. unless the amp is designed specifically for a particular set. or have multiple taps AND multiple headphone jacks, which would be a bit odd.

Umm there's plenty of transformer (so not OTL) coupled tube amps, ECP uses expensive Lundahl output transformers. Tor Audio produces affordable transformer coupled tube amps. Just to name a few..

 

[Mad_Economist]

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.

Ceasing to derail for a moment, if viewers at home would like to produce an equivalent to this plot looking at Zout behavior with a given headphone, note that you do not need a HATS or other ear simulator, you can do it with any ADC! All you need is a splitter that lets you have an amp's output connected to your (presumably very high Z) interface/ADC input and your headphone simultaneously. An example with a very cheap USB dongle (Creative Play! 3, which has high Zout):

The blue trace is the output of the Play 3 looped back into its input - as you can see, the FR isn't very flat, but that's fine for our purposes. The gold trace with an average level about 11dB lower is the same loopback with a 33 ohm resistor in parallel, the pink trace is a Koss KSC75.

Since the Play! 3 isn't very flat, we can subtract its loopback from the measurements to look directly at the losses from voltage drop on its Zout as a function of frequency here:

This gives us the grey trace (the resistor), which is essentially flat minus a little noise, and the orange trace (the KSC75), which corresponds with the impedance magnitude by frequency measured by an independent source.

This is, indeed, essentially how we measure impedance as a function of frequency for headphones in most cases - comparing the loss by frequency with a known resistance.

Edit: People who've been in the audio hobby for a while may remember NwAvGuy's article on Zout - all of his plots were acquired by this method.

 

[3125b]

So this is an effects machine with a limited use case.
If one buys it for that, it's well and good.
But a serious and universal hi-fi product, this is not.

And there lies the problem I think.
One can appreciate it for what it is, but not claim that it is something it just isn't.
Unfortunately, many audiophiles like to do the latter.

 

[thehun]

it measures more like a "bottleneck" .

 

[xarkkon]

Love my bottlehead crack!

I like that despite the ****** SINAD, Amir came out to give a pretty fair subjective review with the HD 650. On the others, well Bottlehead does set it out right from the get go that the amp should be used only with high impedance headphones. Even with my HD 600, I do have some of the issues Amir alludes to (channel imbalance for one), but the sound is something I can get behind for fun listening (even with the distortion!).

I have a Bottlehead Crack + Speedball. I love the way my 600 ohm Beyer DT 880's sound with this amp. I also like the way my HD-600's sound with it. I had HD-800's but had to sell them, the treble gave me headache, but when I had them, the Crack was my favorite amp to use them with.

I don't listen to it much. (I actually don't listen to headphones of any description much these days.) But it is a "FUN" headphone amp especially with my 600 ohm 'phones.

I equipped mine with a switch to allow going back and forth between the stock 100 uF electrolytic and a 100 uF mylar film capacitor for coupling the amplifier to the load. I purposely did not pay attention to which position was which on that switch when I assembled the thing. I A/B'd the sound using a number of different headphones, and the difference between the two positions was NOT subtle. There was an IMMEDIATELY audible difference between the two switch positions- one sounded kind of "raspy" in the highs - as though there was some distortion of the highs, or some ringing or noise bursts kind of thing. The other position the treble sounded quite smooth. I did not know which was the electrolytic and which was the film cap, but I knew almost instantly that I prefered one switch position over the other. After a few weeks I looked under the chassis top and saw that it was the film cap that I preferred. Make of this small sample size, not exactly double blind, no control group used, study what you will. But the conclusion I draw is that film caps sound better when they are in series with the audio signal than polarized electrolytics. Maybe a non polarized electrolytic would sound as good if you could find one with a high enough voltage rating, I don't know.View attachment 80856

What a beautiful build! Congrats on a job well done!