Alternate Design for Crack - New Design?

Here is an idea for a new build of the Crack, with the following parameters.

6080 Stage - CCS (sink) -50V at grid @ 50mA, plate voltage around 120V, bringing B+ to 170V.
5687 Stage - CCS (source) -3.2V at grid @ 20mA, plate voltage around 120V, with swing up to 170V at grid voltage of -5.5V.

Might use K and K Audio's Cascode CCS Kits instead of the Bottlehead Speedball, being cheaper. Going to use a tube rectified choke input filter with 3 stages of LC or RC filtering. With these changes, you really can't call it the Bottlehead Crack anymore, and it becomes another OTL Headphone amp.

Some Crack Measurements and a Surprise!

Took the time to do some measurements on the Crack, and following are the results.

B+ is 166.3 V at the 6080 tube socket pin.
AC Hum at the headphone jack; Right is 0.005 Vac,  Left is 0.013 Vac
85 mA through the first PSU Filter resistor.

After doing this, I thought I give the HD650 a crack before I send them away, and lo and behold, the left channel works again!

However, in the two sessions with the DT880 and the HD650, I found that the right channel is significantly softer then the left channel. I think some is amiss here. Also measuring the voltage across the the large 2W resistor in the driver CCS, there was a big different between the left and right channels, something is not quite right in the amp, that is for sure. Maybe this also has something to do with the imbalance in Vac across the output.

On another note, it might be the HD650 connectors on the left side may be loose, and hence it worked again since I unplugged the cable ready for shipping, and then re-plugged it in after. Again, something is amiss.

I also took the time to plug back in my MiniMax Millet (MMM) and make sure the HD650 were working well and to test out the DT880 on them. I must say, they don't have the life, or the engagement with the music as the Crack. A little dry or lifeless when compared to the crack. The bass is probably better through the MMM than the Crack. However, there was no hum. Dead silence. There was however, noise, which is proportional to the volume, which would suggest that it was source, as this was also present on the Crack. Well, more testing and debugging ahead.

DT880 First Impressions

Without any break in, straight from the box, into the Crack. The Cracks have Sylvania 6189/12AU7WA and Chatham 6AS7, both with around 10 hours on them. The sound is more balanced then the HD650. Less bass (but it is more firm and controlled),  and the vocals are little more recessed but still very articulate and clear. The highs are somewhat louder and more clear. Overall, the presentation is more balanced, but lacking in the mid warmth and tonality that the HD650 has. The fit around my head is much tighter then the HD650 (but them I am comparing a new pair, to a 5 year old pair). The fit around my larger ears is not as god as the HD650, but still very comfortable. At this sage, the sound is definitely not as engaged as the HD650. But, to compare a brand new pair to a very well broken in pair, is not quite fair.  Anyway, more to follow.

DT880 Arrived!

Here are some unboxing photos.

Australian Horn Maker

Azurahorn, local Australian manufacturer of horns, looks like nicely made fibre glass horns.

Speaker Supplier

Stumbled across this speaker supplier, Europe Audio, that stocks Beyma and ships to Australia with reasonable rates.

New Headphones

Took the opportunity to also order the Audio Technica ATH-AD700. These seem to be reasonably cheap at around US$120 a pair and are open back. The only problem being a lower impedance. Reading a post on the Bottlehead forum, it seemed to suggest that the frequency response for f-1dB is still a respectable 30Hz with the standard build, and may get lower with a high coupling cap. Another addition to the arsenal. Made contact with a local Melbourne dealer of the Grado, and have lined myself up for a pair of the SR60i. With three new sets of cans arriving within the next few weeks, it will make an exciting time to compare how they all sound.

Another note is that I am trying to order the Yamamoto HA-02 headphone amp. Many suggest that this was designed for the Audio Tehnica, especially their higher end models (W1000 and W5000). Time will see how that pans out in terms of pricing and availability.

Seas

Here is an interesting driver I found trawling through the net, Seas Exotic X3-06 T35. The webpage is here.

Let's see, it has an Alnico magnet, reasonably smooth response, and decent off-axis response. It has a 94 dB sensitivity and can handle 100W. All this for US$426 each at Madisound!

Alternate Crack Build

Just started to put together an alternate build of Bottlehead's Crack. Enjoying the current stock build of the Crack with the Speedball upgrade has been great. But my itchy building fingers wants to try to build one from scratch. By using the Speedball upgrade, the build becomes incredibly simple. The main component that needs to be thought through is the PSU. Taking advantage of the fact that I have someone working, means that there is more scope for experiementation in the new build.

My basic design premise is to stick to the basic design, with a potential substitute for the driver tube. At the moment, the choices are: 5687 family, 12SN7/12SX7 (I have a stash), 6n6p and 12BH7 in the dual triodes. But more experimentation to try to use a single triode or a single triode strapped pentode. This could definetly open up more options, but this will have to be made soon, as heaters and bias points are a consideration.

The PSU will be a choke input in common mode, followed by a LC stage and then a seperate RC stage for each channel. But again, I might decide later on to do a LC instead for the last stage, to provide that inductive seperation between channels. The transformer should be a 240VCT to produce around 170V at the B+.

So far, I have committed to parts for the resistor side only and had the various PSU caps already on hand. Going to try the Charcroft metal foil for the shunting resistors at the output and using Kiwame 5W (in parallel) for the bleeder and RC filters. Also got some Amtrans carbon for the grid stoppers if I use a higher Gm tube. The PSU caps will be Obbligato oils, and tenatively using 70uF for the common stages and 30uF on the individual channel stages.

More details to follow.

Beyerdynamic DT880

After the failure of my HD650, I have ordered a pair of Beyerdynamic DT880 in the 600 Ohm version as a replacement. I still will be getting my HD650 fixed, but is an excuse to make an earlier purchase on the DT880. It will be fun to compare the two through the Crack amp. Hopefully they will arrive by the weekend, so that I have some time to try them out. Probably end up burning them in for 10 or so hours first before I give them a good listen.

During the research for different headphones, I decided that I will try to get a pair of Grado SR60i, they seem fun and cheap and should be a better match for the computer outputs direct and iPad. Another few headphones I'd like to try eventually are the AKG K702, the Denons and the Audio Technika. But, they all seem to be lower impedance headphones, and Id prefer to stick with the higher impedance ones so they can all be listened through the crack.

The wait begins.

Problem Found!

After checking through my wiring again on the Crack, I found one old solder joint, which was fixed. However, there was still no sound on the left channel, after some more checking, I decided to use a different am, and to my surprise, the left was still not working! Some more checking and swapping of the headphone connectors, confirmed my fears, the left headphone speakers is not working!

Well, problem found and solved. But the problem changes to getting them fixed. But for the mean time, I will be without any cans until they are fixed. Perhaps this will be a good time to purchase my next pair of cans... Not good.

Workmanship Issue

Just started this evening session, and to my horror, only the right channel is working. The left channel was completely silent, no hum, nothing. I immediate thought it might be the tubes, so I replaced both tubes with new ones, and same problem. I then proceeded to carefully check the wiring. I know that by substituting the larger Aeon caps, the soldering and mechanical support for those large caps was not optimal. Trying to gently tap the left output caps did get the sound to come back sporadically. Something strange I also noticed that when I turned the power off, the left channel would come back!

Well, all of this points to a bad solder joint in the left capacitors somewhere. I think I will need to re-solder and recheck that all the parts of still good. But after a modest bump to the sides of the chassis, the left channel was coaxed back into action. But not the ideal situation. Guess this is another joy of DIY, some measure of risk to tame the rewards that it brings.

Joy of Music

Well, it has been almost three months since I had to pack up my equipment and move house, and unpack. Finally, I got my butt into gear, and set up the Crack on my new Mac Pro. Am now listening to Missy Higgins as I am writing this, and I almost forgot how much music relaxes me, and helps me unwind at the end of a long week.

Without being too critical, the sound is as good as I remember it. It does sound like the whole band is in my head, with excellent presentation of the music. Highs are crisp, bass articulate and vocals so wonderful! I must say, that Higgin's voice through the headphones are very addictive, and does put you in that place.... Especially with one of my favourite tracks, 'Sound of White' from her first album.

The only issue with the crack that is troubling is the hum. I can hear it, its not too loud, but it is there. Especially when you take one headphone off, and only have the other to your ear.

Moving onto something quite different with The Muse, and their latest album, Resistance, and the first track, 'Uprising', the bass line is so powerful, even through the headphones. Ballamy's dark and penetrating vocals to this track really hits you, along with the strong drums, the slam really do hit you and make a great contrast to the rest of the track. The guitar rift/solos have a real sense of presence and definition. Definitely nothing to complain about the setup.

Anyway, time to stop writing, and enjoy the rest of the session!

Alternate Load Lines for F2a

Here are the two alternate options using a 2k6 and 5k loadlines. The 5k seems to be a good compromise between power and apparent linearity. Here is the 2k6 loadline. For some reason, I wanted to put in a 3k5, but ended up with 2k6. The power out is approximately 8.6 Wrms which is significantly better than the 7k5 scenario.

 The one below shows the 5k loadline. The power output is around 5 Wrms, which is decent.

I think the last option would be to prepare a 3k5 line as well, as it opens up various options for output transformers. At the higher powers, between 5 to 8w, it might be powerful enough to power up a full range speaker without the need for a seperate subwoofer.

For this project, I was contemplating using a permalloy output transformer, and Tamura makes them in the 3k, 3k5 and 5k, which are ideal for this project. More details to come later.

F2a Operating Point

Have been looking at using an European power tube to build a SE amp. Trying to find a tube different to the usual pentodes (KT88, El34, 6550, etc), and steering away from the very expensive triodes (AD1, PX4, PX25, DA30, etc), and came across this tube, the Siemens F2a. Touted at a high quality tube lasting 10,000 hours, build only by Siemens, and used by some high end audio amps then, and now. The Shindo Cortese. There are actually 2 tubes, the F2a and F2a11. From reading through various datasheets and info, both are identical except that the F2a11 has a different basing. Having bought 4 samples of this tube, calculating that I listen to around 250 hours of music a year at present, maybe growing to 500 hours in the future, 2 sets would last me around 20 to 40 years! But I'll probably invest in another 2 pairs, just in case...

Looking at a basic design for the amp, I started with the F2a operating points. See the curves below. The loadline is at 7k5, operating point at 375V on the plate, 70mA bias current and grid bias at -16V. The approximate power is around 3W. This requires a driver stage that can swing 32 V peak to peak. Not a bad set of parameters.

The interesting item with this setup is if you use the estimation tools at Steve Bench's site, with the following parameters:

Va = 120V, Vb = 375V, Vc = 600V

Ia = 95mA, Ib = 85mA, Ic = 70mA, Id = 60mA, Ie = 45mA

Power is: 3.84W

2nd Harmonic Distortion is: 75*(95+ 45- 2*70)/(95+ 85- 60- 45) = 0%

3rd Harmonix Distortion is: 50*(95- 170+ 120- 45)/(95+ 85- 60- 45)= 0%
 
Now, I wouldn't read too much into this, as the formulas are only approximations and the curves are only approximations and the determination of the various points are only approximations. But, it does give some confirmation that this is a good starting point.

One might be thinking that it is a pretty expensive way to get 3Wrms of power with the plate disspation at 26W. Perhaps a different load line is required, this might warrant a 5k or 3.5k loadline to get more juice out of this tube. More to follow.

Russian Capacitors KBG

Here are two big Russian NOS KBG capacitors I piece up for my projects. Rated at 1000V, these should be plenty safe to use on supplies as high as 800V. These will be the input cap for any power supplies I will be building in the future. They are heavy, and feel very robust. Not the CF card in comparison for size.

From what I can gather on the Internet, they are Paper-in-oils, and have an aluminum foil (not metallized). They are glass sealed and enclosed in a metal body. These have the markings OCKBG-MN. Not 100% sure what the OC and the MN means.

Exotic Connectors

Have been recently thinking about connectors and plugs. In the Hi-fi world, the standard is for the use of; RCA, Binding Posts, Spades and Bananas. However, moving the other way, I was thinking about the recording and live performance industry. In that world, Neutrik/Amphenol type rules king. And when you think about it, all the recording studios use XLR connectors for almost all their equipment. Connecting long cables, in noisy environments. Same thing for live performance. How come no one ini that industry has insisted on the use of amorphous cryogenically treated machined virgin silver billet with rhodium plated contacts? I think this is possibly where snake oil is really at its best. From this thought point, there are two conclusions I can make. One, reduce the number of connections as much as possible. Two, use reliable and industrial quality connectors. To this end, I am now slowly replacing all my connectors with Neutrik types. Using XLR for signal level, and Speak CON for the speakers. With the XLR, I am using 3-pin for mono line, and using 5 pin for any stereo (or Phono) connections. I think this would result in robust connections, that can be made quickly, safely and less damage to the connectors. This is because they were designed to be plug, un-plugged thousands of times. Further, using the Speak CON connectors removes the possible exposure of lethal voltages between the speaker binding posts. Couple this with solid silver braid as the cable medium, sheaved in various combinations of natural cotton and teflon, should make a robust and high quality cables. More build photos and experiences to come.

D3a Headphone Amp

Here is the schematic of the D3a amp mentioned in the previous post. Nothing too fancy, but using some nice ideas in the PSU that others have tried before.


Using the CCS to feed the VR Tube ensures that during start up, the in-rush current is limited, and provides a stable operating point for the VR. a further refinement would be use some form of LED or CCS for the tube biasing, to ensure that the operating points remain stable. as suggested on the epc.cc site, tying the ground of the VR to the top of the cathode, may achieve is, and then the resistor below the cathode, simply sets e same current as the CCS. This will eliminate the cathode bypass resistor, and give the bias point of the tube a stable bias.

Headphone Amps - A Review

Over the course of my audiophile life, or around five years, I have amassed (like most others) a stash of components, schematics, ideas and projects (mostly half built...). To date, I can happily say that I have 4 working headphone amps, and 2 headphones. Learning my lessons from previous hobbies, I shelled out the Sennheiser HD650 from the start. I didn't want to buy cheap and upgrade later. For me, that has always cost me more time, pain and money. The second set of cans are my travel set of Shure SE210 in-ear monitors. Whilst I love the size and compactness of the SE210, I do not really like the way they sit in my ear. Though they are a good fit and comfortable to an extent, having these things in your ears for long periods is not the best feeling.

With the headphone amps, my first build was a PCB (original version) of Peter Millett's Hybrid 12AE6A amp. This was a great project. Easy to build, fun to use and good sound. It was my default amp for a few years. Using pretty much the standard parts, it has proved to be a reliable and enjoyable amp.

The second venture was with a full point to point kit, in Maple Tree Audio's Ear+. This was the version with the line stage. The build was done with the Blackgate, Tantalum and Noble pot upgrades. This was really a step up for me in terms of building and testing. Being a full point to point kit, it proved to me that good solid experience is required to ensure a smooth build. Overall, I thought the kit was well designed, laid out and provided good to very good sound. My main issues with this amp are:

1. There was hum. (Bad soldering and wiring??)
2. Left channel was always buzzy and intermittent. (Bad tube socket??)
3. Sound could be considered a little flat or laid back.

Whilst items 1 and 2 can most likely be attributed to bad construction on my side, I could not find a way forward with item 3. Hence, my quest continued in search of a better headphone amp. With this design, I have always mooted about doing a full re-build, with the courtesy of a few years of experience. This would include taking out the line stage, full rewire and layout, replacing the cathode coupling capacitors and general parts tweak and upgrade. Also would try to get rid of the 12AX7 as the input tube and replace with something more interesting. This I will leave for later, as there are more exciting projects in my mind.

After a few years, I decided that the next upgrade would be in the MiniMax Millett. I chose the PCB version and it basically replace the OPAMP buffer on the original with a discrete diamond buffer, and it replaces the current sources in the original with more robust IC versions. This time around, I also tried to build an enclosure of timber sides and metal top panel. The net result was a great sounding amp, that has been really fun to use. The build was fairly straight-forward, with the only parts tweaking being the coupling capacitors. I added a Russian K40 and Vishay MKP in parallel with the Nichicon Muse electrolytic.

During this period, life caught up, and babies were now in the picture. Headphone hifi seemed to be the most logical method, as speakers would inevitably get damaged or disturb the family. Though loud singing when listening to headphones could still be a source of nuisance. By this time, I had built a transformer coupled (cathode) amp, two hybrids, and wanted to try an OTL. At the same time, Bottlehead released their Crack. $200 and a few months later, the kit was delivered.

First, I can say that the value for money, from the Bottlehead kit is far above the Maple Tree Audio. Though the latter did have better components, it was not worth THAT much more. The kit was built with the Speedball upgrade at the same time, to avoid having to build and test twice. The end result was a great amp. The sonics seem to combine the best the amps to date. From this point, I can see that there are a few items that can be upgrade and improved.

1. Volume control, replace with nice GoldPT or DACT attenuator.
2. PUS Caps, replace with Oils (means a new enclosure).
3. Input and output jacks replace with quality Neutrik ones.
4. Potentially adding a choke to the PSU.
5. New Tube sockets.

A lot to upgrade, but I believe that they will bring it amp from a 90/100 to a 98/100. But right now, I am just enjoying the amp for all its worth.

Where to from here? Well, there are two upgrade/rebuild paths for my existing amp. But I have always wanted to build a standard SE Transformer coupled headphone amp. And to that extent, I have taken a few designs from the net, and made a few changes, and made my own. For me, I have coined it my final headphone amp, the one amp to rule them all! Perhaps not. But, the basic design premise is similar to the designs by Andrea Ciuffoli, Lynn Olson, epc.cc and a few others out there. Nothing unique.

The design is a D3a feeding into a Amorphous core Lundahl transformer. The PSU is glow tube VR150 regulated and the PSU itself is a choke loaded in common mode, followed by one shared LC stage and one channel specific LC stage. More details and schematics to follow. But this should be an interesting build.

Having looked back on all of this, I think after completing the D3a headphone, I will go back and rebuild the EAR+ and the Crack and then have on hand 5 quality tube based headphone amps. This will I think give me a good overall impression of tube headphone amps, and probably serve as a foundation for any future headphone amps. Finally, considering an upgrade to my HD650, and the two choices so far are the Beyers T1 and the HD800. But that, will have to wait for some time.

Anyway, will be posting more info and build notes on the D3a amp, and I will hope to write sensible and proper reviews on the other amps later this year.