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The journey to the Pass Labs F4

Having set a path forward with the headphone side of my audio setup, I thought it appropriate time to look back at the stereo setup. I am quite satisfied at the current crop of speakers sitting in boxes, ready to be mounted onto cabinets. With a move to a home with a more fitting stereo room, I think it would make an ideal start in comparison of the speakers.

In this same vein, I have started to review the amps. I have a range of 1W and 8W tube amps ready to be assembled and tested. At the moment, most of my speaker drivers are in the 95 db+ range, and can be considered efficient, with the exception of the Jordan JX92s.

Reading through the various articles about amplifiers in the voltage and current domains, my conclusion (which is very similar to many others) is that tubes are excellent voltage gain devices. They work primarily in the voltage domain. To get them to power speakers, one has to convert some of the voltage into current, hence step down transformers. The output transformer takes, typically, hundreds of volts and milliamps, and transforms that into tens of volts and potentially tens of amps. This is then able to drive a speaker.

Though I have not much been interested in MOSFETS or transistors for amplifying devices, there seems to be a movement towards using them where their strength lies, in the high current domain. In a sense, they operating in tens of volts and tens of amps (or thereabouts). Most popular power amps are of the transistor type, and are able to produce hundreds of watts of power.

The debate between what is better; tubes or transistors, I think will never be concluded, as it is a function of taste. Each to their own. So far, I have experienced primarily tubes, and I like them not only for their sound, but their nostalgia and appearance. For me, a glowing red tube is so much more attractive than a massive heat sink. The fundamental problem, taking aside taste, is one of power. Tubes cannot really go above 100W, and if we limit ourselves to the single ended affair, probably not above 30W, sensibly.

I think what I, and other SET fanatics crave is more power. The only way to achieve that is to operate the tubes at an ever higher voltage and current. I have not found tubes that really work well in 16W plus in SE operation that operates at less than 800V. The 6C33 is close, but operates at a very high current of around 400 mA. The high current represents its own problem in transformers and chokes.

Fundamentally, what I have come to understand is nothing new, or earth shattering. However, it was important that one reach that conclusion in one's own time and way. My tastes at the moment favour; Single tube SE or one tube pair PP, both in Class A. Further, my goal would be 32 Wrms of clean power, irrespective of speaker efficiency.

Now that I am at this point, I find that I have two paths to move forward with. First path is with a high voltage SET, like the GM70, 211, 845 or similar transmitter tube. The other, a hybrid solution mating a tube front end with a transistor output stage.

Since I have ruled out high voltages, that only leaves the second path, one of a hybrid approach. Having only casually explored the hybrid path, I have come across the Moskido design, which is a hybrid of the Aikido design with a MOSFET output stage. The other design is one of a booster amplifier, after a tube output amp. Having further researched this idea, I came across two interesting solutions; DIY Hifi Supply Fusion module and the Pass Labs F4 impedance converting amplifier.

The Fusion module seems to be very easy to implement, with the addition of a low voltage power supply and the insertion of the module. It looks like a current amplifier that is probably in parallel with the speaker. It probably includes some kind of Class D chipamp operating as a current amplifier.

The F4 is more interesting. Now only has it got some really good reviews, it is a similar approach as with Andrea Ciuffoli in his "100W Hi End Hybrid" design. Any which way, they are both interesting approaches and designs. The F4 has a cult following in the DIY Audio community, and there are PCB boards available as well as volumes of forum postings about this. For me, that makes it a safer design, as it can be supported by a range of people who have built it.

Having come to this conclusion, I ordered 2 sets of the PCB boards from the DIY Audio Forum store. Also ordered the matching number of sets of the active devices from Tech DIY. I hope to start the build soon, and share what I have research and progress to date.

Completed the Elekit TU-882

Last night, I had the opportunity and time to fully assemble and complete the last steps of the Elekit TU-882R headphone amp. Firstly, I have to say that the quality of components and fit are top rate. The chassis work is extremely nice, though it may not be your taste, one cannot complain about the quality. I myself, like it, and will most likely leave the tube guard off, as I like to look at the tubes, and also do some tube rolling. More on that later.

Check out the progress build photos below. You will notice that I have put to use the stock 0.1uF polypropylene coupling caps to good use. I have used them as the PSU electrolytic shunting.

I had about one hour last night to listen to the amp before I had to retire for the night. Firstly, it worked! On first impression, the sound is quite relaxed, but what I noticed most was the bass control. A lot firmer than the Ear+. Anyway, more on that later, but I discovered an interesting problem.

There is sever noise or ground contamination. This is the situation. When the volume is all the way down, I can hear hum on both channels. Relatively loud, and doesn't go away when I connect or disconnect the source, or when I touch the chassis. When I turn the volume 2 clicks on, I can hear some strange buzzing on the left channel, with the hum still on in the background. As I turn the volume up, the buzzing doesn't get any louder. Until about the top two clicks, the buzzing disappears immediately, and is replaced with noise on both channels (the white random kind). I can still hear the buzzing at the top three clicks, but it is getting progressively softer. Meanwhile, the noise is also getting softer the further away from full volume, within the first 2 or 3 clicks, it is gone.

The strange buzzing is heard through the music, quite annoying. Now, I did not use the stock supplied PCB for the volume control, and this may be attributed to that, but I am sure it has something more to do with the ground scheme. Perhaps, I have to get the saw out earlier to get the IEC with a proper ground connection done sooner. Anyway, will be speaking with Victor about this, and see if he has come across this before, and find a solution.

Otherwise, this has spoilt a good and fun kit to build. And whilst the music is playing, but minus the buzz or hum, it is quite enjoyable and pleasant. More to come soon!

Progress on the TU882

After having received the resistors earlier today, I actually found some time to progress the build further. Check out the photos below. I found that it was best to pre-tin all the wires before installing them. I also note that there wasn't an Australian lead supplied, and it has no earth connection. This is something that does concern me, as I want to make sure the equipment is safe, with kids around. I might look into modifying the kit once complete to add a proper IEC three terminal connection to allow for a ground connection. Otherwise everything went very smoothly.

Final Resistors Arrived!

Very excited, the final missing resistors for the TU882R build arrived this morning. Hopefully time for the build later today, and maybe finish by the weekend!

Progress Build of TU882R

The main board, without the missing 4 resistors, tube
sockets and the wiring. 

Have had an opportunity to start the build of the TU-882R kit. The first thing I did was to clean the PCB with alcohol to make sure that the through hole enlargement and my handling didn't leave any foreign matter that may cause any poor solder joint. I was going to try the Mundorf M-Solder silver. This has 95.5% tin, 0.7% copper and 3.8% silver. Up until now, I was using the old Welborne Labs Silver solder, which had 2% silver. Initial impression was that I needed more time with the solder iron to get a good flow with the Mundorf. Setting at around 280 C, I needed around 5 seconds for the Mundorf to flow well. On the same component type, I only needed 3 seconds on the Welborne. I reverted back to the Welborne solder.

During the build, as per normal, I realised that I ordered the replacement resistors four less then what was needed. I was short 220k and the 3k3. So, immediately placed another order for these missing four resistors, but continued to build.

Close up of the signal part of the PCB. Note the
Mundorf caps and the Tantalum resistors 

Close up of the main PCB signal section, note that both
the Mundord caps and the Tantalum resistors stand off
the PCB by a bit, as they were larger than what was supplied

Have a look at the photos below of the progress. The instructions was simple to follow and straight forward. The PCB has good room, and the solder pads weren't too small. Everything fit and as you can see, I am just waiting on the replacement resistors before I do the assembly, wiring and chassis part of the kit.

The front panel sockets PCB.

Front Panbel sockets PCB showing the header pins.

The replacement Noble pot. This will be direct wired from
the pot to the PCB, not requiring PCB 'B'.

The PSU/Transformer PCB.

Additional technical data on Beyma 15XA38Nd

Having requested a few bits of additional information from Beyma regarding the compression driver unit, they kindly responded quickly and I post this information that has been kindly provided by Beyma. They are all associated with the HF unit (or compression driver) on the 15XA38Nd. Also of note, I received a confirmation from Europe Audio regarding the shipping of the drivers, and I look forward to their arrival next week via FedEx.

DC Resistance (Re) of the HF Unit = 10.05 R


Impedance vs Frequency



Lvc vs Frequency

Disappointment

Have been thinking about whether I wanted to write this post or not. I am not wanting to spread false rumours, but want to share with you the fact, and what happened in my experience. Now, I have been buying items from the Internet for over 10 years, I have generally had very good service from all the various vendors. In particular, I have had great experiences from online DIY suppliers such as (in no particular order); Welborne Labs, Bottlehead, Maple Tree Audio, DIY Hifi Supply, Sowter, K and K Audio, Chipamps, eBay, Hifi Collective, Tube Depot, Audio Tubes, Tango Tubes, Angela, Beezar, Front Panel Express, Digikey, Mouser, Allied Electrical and Farnell (now Element 14). Partsconnexion would've been a part of that list, until my last order.

This particular order included most of the upgrade parts for the Elekit, which included the 3k3 Tantalum resistors. For whatever reason, after I selected this correctly in the online shopping cart, they did not transpose that correct to their invoice. Instead, they put in 4k3. And with them, they send you the invoice to confirm first before they take payment and ship. I opted for the UPS option, which for a bunch of resistors and a pot, cost around US$60. Well, the time between when they sent me the invoice, and the shipping confirmation, was around one hour, and it occurred in Australia time, around 4am in the morning. Not much time for someone to respond.

So, to my surprise when I opened the package, I did not find my 3k3, but instead found the 4k3. I emailed them immediately, along with the web invoice email confirmation that I got first. They were very quick and nice to respond. Long story short, after a few emails, they were happy to send me the replacement resistors, but only after I have sent them back and they received it. Also, they were silent on the shipping method they would use.

To me, that is where they lost me. Firstly, it was their mistake, and secondly, why should I send back something before I got what I paid for? Well, I have encountered this scenario with a few other suppliers mentioned above, and in ALL instances, they were willingly to ship the correct goods first. Not after.

Well, that is my rant for the week. Take from it what you will. I am not warning anyone to stay away from Partsconnexion, but just sharing my experience. They do have an excellent range of components and are very well priced. But what about the service?

Beyma Coaxial 15XA38Nd and some Visaton Kits

Following on my experience last weekend at the audio shop, I decided that I needed another speaker. Have been looking at the Beyma range of speakers over the last few months, and the specifications seems to be generally impressive. I couldn't find a reliable source of supply in Australia, and following my previous post, I ordered a pair of the 15XA38Nd from Europe Audio the all inclusive price was around AU$1000 including FedEx delivery, which seems reasonable.

With this speaker, I plan to use a 1st or 2nd order passive crossover with a Zobel network on the tweeter to equalise the impedance. For the moment, I plan this to be as simple as possible, and will be mounting this in an open baffle box, which may become an Onken style box later. With different sensitivities between the 15" and the compression driver, I may need to introduce a L-pad to bring down the HF unit a little to match better. The Beyma datasheet shows a frequency response for the combined units, and appears to be relatively smooth. They also produce a special crossover just for this unit, which has a 1.5dB attenuation, and 2 inductors, 3 capacitors and 2 resistors. This arrangement seems to be similar what I am proposing, 2nd order with Zobel. They have crossed at 1800 Hz.




This now makes 5 speakers that are on the shelf, yet without a home! As always, it is much easier to buy it, then to build it... However, when I do finally get around to it, it will be fun times!

Also, looking around at speakers found this most interesting design from Visaton, the Pentaton kit. Using the very popular B200, mated with 4 cheaper BG20 speakers, it specs up at 99 dB sensitivity and flat from 70 Hz to 10 kHz. Interesting. Also stumbled on the beast, Grand Orgue, 81 dB, but FLAT from 25 Hz to 20 kHz. Crazy looking speaker too, especially with the Oval shaped drivers. Final kit of interest I found was the VIB 70 BP. 85 dB and flat from 40 Hz to 20 kHz.



Pentaton



Grand Orgue



Start of the TU882R Kit and a visit to the audio shop

The pin vise (small hand drill) arrived from a eBay purchase last night, and I started to enlarge the through holes in the Elekit PCB to accept the larger resistors and capacitors. I must say for AU$20, this has got to be the crappiest pin vise or hand machine tool, I have used. In addition to leaving grey machine grease all over my hand, it did drill and work well, and now, all the resistor and capacitor holes are nominally 1mm in size. They are now able to fit the standard resistors and caps.

I drilled relatively slowly, and started from the pad/trace side and drilled through to the top size. It was pretty easy to drill, just making sure I wasn't creating too many burrs on the edges. After this, I will give the board a good clean with alcohol to remove any grease and 'sawdust' prior to the soldering.

Have a look at the photos. You can see that R7 has not had the holes enlarged, and can compare the difference in size. Now that this process has completed, I will slowly, but surely move towards the build process. If this can be completed and running by Easter, I'd be very happy.

Over the weekend, I had time to drop by the local Audio Note Kits distributor/retailer. First of all, he was a very nice salesman. Very friendly and helpful. He had the 300B Kit 1 and the 300B parallel interstage kit setup and ready to play. The SET was hooked up to a single drive full range in a Onken style box with a ribbon tweeter. The parallel 300B was connected to some $15k audiophile multi-way speaker. It was good to sit, listen and compare between the two setups.

The soundstage with the parallel 300B was much wider, and it felt that the musicians had room to move between them. The SET on the other hand, had beaming, and the sound stage felt as if it was between the speakers. So to my ear, the FR had the music confimed between the speakers and with the conventional multi-way, the music was between and beyond both speakers.

But the one constant that was confirmed to me, and my wife, was that both amps was really easy to listen to. The tube sound is so much more easy on the ears, and there was no fatigue, it was effortless to listen to the music being played. The way that the music sounds through speakers is quite different to that of headphones, obviously, but having tasted the speakers again, has spurred me on to making sure I can get a speaker set back up as quickly as possible. But, given I am in-between houses and with kids, this might be a tall order!

The Headphones

I think it is time fir a headphone stand. You can see the purple glow of the AD700 making the DT880 also look purple!

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