[bequietjk]

Hi people! I've yet to read ANY posts really on this thread. But I ordered a Crack and can't wait to join the crew. I've got 647 pages to go through? Oh boy...

Can't wait to pit my Project Horizon up against the Crack.

 

[jamwagon]

You'd better test it asap.

I have a lot of DOA 5998.

I didn't pick that one up. Someone did buy it that same day though! Whoever ended up with it hopefully got a great deal.

My first 5998 was untested and it has worked pretty well so far. Seller did accept returns so it would've went straight back.

Did the getter turn white on the right tube in your picture? Hard to tell but looks like a leak

 

[attmci]

I didn't pick that one up. Someone did buy it that same day though! Whoever ended up with it hopefully got a great deal.

My first 5998 was untested and it has worked pretty well so far. Seller did accept returns so it would've went straight back.

Did the getter turn white on the right tube in your picture? Hard to tell but looks like a leak

All have problems.

 

[SilverEars]

So, I've tried two OTL amps that have different amount of output impedance. One is as low as 3 ohms and high as 10 ohms. The Crack has magnitudes more in ohms being around 120 ohms or so

First of all, dynamic drivers of high ohms (Senny 300 ohms, etc..) reacts really well with OTL. I don't believe the output impedance is why the heaphones react with such level of bass dynamism (hard, quick, and punchy), but the amp being OTL. Something about OTL just really brings out the significant driver air movement. It's like the area of air moved is much greater with OTL.

I don't believe this response is to be consider neutral in terms of amp response. Interms of headphone response on the other hand, OTL may bring out more bass that's needed for the particular headphone, and becomes more balanced in the low-end as result, or just bass fun. Why this type of bass dynamism is fun? It's because bass isn't loose or muddy, but tight. As long as bass is tight, significant level of bass is fine for me.

 

[L0rdGwyn]

So, I've tried two OTL amps that have different amount of output impedance. One is as low as 3 ohms and high as 10 ohms. The Crack has magnatudes more in ohms being around 120 ohms or so

First of all, dynamic drivers of high ohms (Senny 300 ohms, etc..) reacts really well with OTL. I don't believe the output impedance is why the heaphones react with such level of bass dynamism (hard, quick, and punchy), but the amp being OTL. Something about OTL just really brings out the significant driver air movement. It's like the area of air moved is much greater with OTL.

I don't believe this response is to be consider neutral, but if you experience such response, neutral becomes boring.

What OTL are you referring to with an output impedance of 3 Ohms?

 

[SilverEars]

What OTL are you referring to with an output impedance of 3 Ohms?

EC Zana Deux Super.

 

[L0rdGwyn]

EC Zana Deux Super.

Oh I see, they use negative feedback to get the output impedance that low. Interesting!

 

[Maxhawk]

Oh I see, they use negative feedback to get the output impedance that low. Interesting!

I built an SRPP + 6080 headphone amp whose original design had negative feedback. It sounded good but didn't have the holographic imaging of the Crack. When I disconnected the feedback it resulted in the imaging I get with the Crack. I'm certain the distortion is higher but "no feedback" resulted in a much more satisfying experience.

 

[L0rdGwyn]

I built an SRPP + 6080 headphone amp whose original design had negative feedback. It sounded good but didn't have the holographic imaging of the Crack. When I disconnected the feedback it resulted in the imaging I get with the Crack. I'm certain the distortion is higher but "no feedback" resulted in a much more satisfying experience.

It's funny that you say this, as most objective engineering-type folks would say the negative feedback is the better choice: lower distortion, decreased output impedance, etc. but here you say that it has a detrimental effect on the subjective experience.

I'm no engineer, but I have a STEM degree, I try to keep myself educated on the scientific aspects of audio design. I just think it's interesting how what is correct from an engineer's perspective and what measures well may not actually yield the most enjoyable listening experience. How do you measure holographic imaging? This type of debate is dividing a lot of audiophiles these days.

 

[audiowize]

One could certainly put feedback around the Crack circuit and get the output impedance super low, but it does indeed come with some negative side effects. As Gwyn pointed out above, there are many objective benefits of global negative feedback, and it is present in nearly all currently manufactured audio components.

Back in the day of vacuum tube amplifiers, the output transformer put a reasonable limit on the amount of GNFB that could be used before instability occurred, but with solid state amplifiers there is not such a hard limit. Manufacturers learned that saying your amplifier had 0.001% THD at rated power vs. your competition's 0.01% THD drove sales, so this trend continued until enough subjective listening tests were done to determine that minuscule decreases in THD didn't always correlate to a better sounding amplifier. Due to the subjective nature of this research, it took a few decades to be taken seriously, and that is about where we are now.

 

[L0rdGwyn]

One could certainly put feedback around the Crack circuit and get the output impedance super low, but it does indeed come with some negative side effects. As Gwyn pointed out above, there are many objective benefits of global negative feedback, and it is present in nearly all currently manufactured audio components.

Back in the day of vacuum tube amplifiers, the output transformer put a reasonable limit on the amount of GNFB that could be used before instability occurred, but with solid state amplifiers there is not such a hard limit. Manufacturers learned that saying your amplifier had 0.001% THD at rated power vs. your competition's 0.01% THD drove sales, so this trend continued until enough subjective listening tests were done to determine that minuscule decreases in THD didn't always correlate to a better sounding amplifier. Due to the subjective nature of this research, it took a few decades to be taken seriously, and that is about where we are now.

Very interesting, this is a topic I am going to read into more thoroughly. I'd love to read some actual funded studies that do blind tests with GNFB vs no GNFB, tube vs. solid-state, etc. I know the GNFB topic has been discussed ad nauseum. This type of subjective, blinded research might shed some light. It seems to me there is a lack of measurable parameters to fully describe the subjective experience. Distortion alone is not telling the whole story.

I have owned a few solid-state amps, most recently the Arcam rHead and Violectric V100. These amps never performed as well for me compared to say, the Crack, Crackatwoa, or my Glenn OTL. I am certain if you measured them both with an audio analyzer, the solid-state amps would come out on top, especially when it comes to distortion and output impedance, but it just does not translate to the actual listening experience for me. Maybe I am smitten by shiny, glowing orbs and it is feeding a placebo effect, but I doubt it. There is a realism and three-dimensional listening experience you get with these tube amps, a sort of haze that is lifted compared to solid-state, for me. The Violectric V100 is also more expensive that the Bottlehead amps and even the base model GOTL, so it is not a matter of dollars spent. To be fair though, I have not listened to the absolute TOTL solid-state amps. Even if they matched or exceeded the tube OTL listening experience, the tube amps would then be the smarter choice from a financial perspective.

Do you have an engineering background, audiowize? You seem very knowledgeable on the technicalities.

 

[Maxhawk]

I suspect one reason that a lot of people like tubes, especially circuits without negative feedback, is the presence of even order harmonics. I know there have been attempts to add these harmonics in solid-state circuits, but I don't recall any formal conclusions. I used to fuss over distortion and frequency response measurements, but now I use those only as a guide when fine tuning circuits. If you care, you might check out some measurements I did years ago with the Crack and different tubes: https://imgur.com/a/dRFeJ

 

[L0rdGwyn]

I suspect one reason that a lot of people like tubes, especially circuits without negative feedback, is the presence of even order harmonics. I know there have been attempts to add these harmonics in solid-state circuits, but I don't recall any formal conclusions. I used to fuss over distortion and frequency response measurements, but now I use those only as a guide when fine tuning circuits. If you care, you might check out some measurements I did years ago with the Crack and different tubes: https://imgur.com/a/dRFeJ

Aha! Those are your measurements? I have referenced them many times, thank you for posting them! To my knowledge, they are the only measurements of the Crack done using different input and output tubes.

If I am not mistaken, they were done without the Speedball installed. Would be very interested to see how they compare with it.

A hobby project I would like to pursue in the near future is to build an affordable, at home audio analyzer setup, most likely using the QuantAsylum QA401. I think it would be fun to measure the distortion profiles of my tube amplifiers and try to match my subjective preferences to a specific profile.

 

[Maxhawk]

If I am not mistaken, they were done without the Speedball installed. Would be very interested to see how they compare with it.

The measurements were taken with Speedball. It was also my first experience with tubes thus my fascination with taking measurements and trying to correlate them with my favorite tube combos. At the time my favorite output tube was the slotted graphite 6080, which happened to be the one with the highest measured distortion. Perhaps if I need to go back and do an FFT measurement with different output tubes....

 

[Toad_of_Toad_Hall]

I highly recommend this small change to the Crack if you are having trouble with noise from a digital source:

https://forum.bottlehead.com/index.php?topic=11676.0

For those of you who are too lazy to click the link, this is what Paul Birkeland says:

"For those who can hear their computer working away through their Crack or who are using a powerline ethernet kit and hear lots of clicking and static, a ground breaker installed in place of the buss wire between the chassis and the ground tab of the IEC power entry module will very likely solve this issue. I recommend using a diode that's rated for at least 2A, and the voltage isn't particularly important. Part number STTH2R06 looks to be a solid choice. I do not recommend using a Schottky diode here."

I used 2 x 1N5408 diodes instead and they work well. Best mod you can make: Obvious and beneficial results and it only costs a couple of bucks.

Image is Paul's and added for convenience.