Mini Max Millet Part 3

Completion!
After a a few day's worth of soldering and assembly, the amp is finally working! As usual, the assembly work took longer than the PCB assembly itself. See some of the photos below for the progress builds. The biggest mistake I made was to solder the fuse clips on the wrong side of the board. Now I have to remove the PCB fully to be able to change the fuse. On hindsight, that may be a good thing, as a blow fuse would usually mean more of a problem down stream. So having to remove the PCB also forces me to have a quick check of the PCB itself.
The wiring for the volume control, crossfeed, etc was all done with solid silver wire and the use of tabs. Although this took a very long time, the results are well worthwhile. Using 24 AWG meant that I had to strip the teflon insulation off very carefully, otherwise any slight cut/scratch of the wire will cause it to break off when it is bent.

Biasing and setting up the amp wasn't too difficult. The B+ adjustment was straight forward, and I biased it to 25.6V, and it is now hovering around 25.58V. The diamond buffer bias is set at 100mA at present. Reading through the advice, I will leave this at this setting for a few weeks of use and then revisit after that. I want to let the various capacitors, resistors and transistors break-in. The tube bias was quite straight forward. At present, the channel imbalance doesn't seem too bad, and both channels are well balanced.

Once everything was biased and running for around a minute after that, I did the measure of the DC offset at the outputs. It was measured around 0.1mV. Not too bad at all! Everything appeared to be in good order. The heatsinks were getting warm, but only slightly so. With the diamond bias below their recommended levels, I'd assume that it isn't doing as hard work as it was designed to, and running cooler.
Once the final assembly was completed, I hooked them up with my test pear of cheap Sennheiser ear-buds. I first noticed a little hum, but it was not too loud. Connecting my test iPod up to the amp, I waited for the delay relay to kick in. Music! The sigh of relief was immense when sound was coming out of the ear buds! After letting this run in for about ten minutes, I powered off the amp to change to my usual cans, the HD650.
The sound, in short, was amazing. The hum had disappeared. Not sure whether that was from moving the temporary power cables around, or from a different headphone, or whatever, it was gone. Dead quiet, and this was late at night with very little ambient sounds. Fantastic! Powering through music, the first conclusions I made was; I did not like the crossfeed. At this time, I am not sure if the crossfeed was causing the first problem, or whether it was wrong wiring.
Using the crossfeed for the first time, I found that it muted the highs and made the sound very flat and even a little restricted. Turning it off made the sound more fuller and far more enjoyable. I was thinking that I probably should've tried the crossfeed before on another amp and would've saved many hours. Maybe, the caps need some time to open up, but at the moment, I think I will be listening without the crossfeed. I did not notice any widening of the soundstage or it moving from the off position. I think my ears/brain probably doesn't respond to the crossfeed at all.
Regardless of the issue mentioned above, the sound was amazing. The bass was awesome, very authoritative, and I have never really heard such clean and articulate bass before (headphones or speakers). It was almost scary. The mids was warm and clear and the highs crystal clear. All in all, a great sound. I think there will be no need for any bass boost. The balance at this moment, is very good to my ears.
At the end, I used both a Vitamin-Q and a Vishay MK1837 as a bypass on the output capacitors, and though only have less then 1 hour, the sound is already amazing. Looking forward to seeing them break in and get better over time. Part of me still wanted to try the Mundorf Silver-oil caps, but the leads were too thick, and I was in no mood to do any more modifications yet.
Overall, very impressed with the sound and quality. The remaining actions are:

1. Remove Crossfeed
2. Build legs to raise chassis off the ground to allow air flow inwards from bottom.
3. Possibly install a few more LED to create a more glowing effect.

Enjoy some of the progress build Photos
The photos below show the construction of the PCB with the most of the components installed. It also shows all the trimmers and terminal blocks on the bottom to allow for easy access and adjustment without having to remove the entire amp.

Below are the diamond buffer devices mounted on the heatsinks, ready for installation.


Below are some shots of the completed PCB. Note the Vitamin-Q mounted on the bottom of the board.



A few shots of the assembled amp. Note all the hook-up wires from the cross-feed to the switches and the various input and out connections. It was such a shame that I did not like the cross-feed and most likely remove it eventually.





Below the completed amp ready for power-up and first use.

DAC

After much reading in the last year or so, I have finally settled on a DAC Design to begin experimentation. The Analog Devices AD1865 family. To this end, I recently purchased from eBay the AD1865NOS-1.0 kit. As a start, I will hook it up with any modifications and let the unit run for a few months to let it break it and understand the type of sound this DAC is capable of. But the longer term plan is to modify the unit, potentially using a different I/V stage to draw more out of the chip. The long term plan would be to make a poor man's Audio Note DAC 4. This would most likely entail a CS8414 receiver, feeding into the AD1865. This would then be coupled to the tube gain stage via transformer, which the gain stage would be coupled to the world via another transformer. The intent would be similar to Ciuffoli's DAC-End, with the exception of an added transformer between the DAC and the tube. The power supplies at this stage would be Twister Pear Audio's Placid and TubeCAD's Janus.

Mini Max Millet Part 2

Finally, everything is ready to go, every part, socket, tube, panel, etc is on-hand. See below the timber frame ready to receive the amp. Note that I am using a XLR input (as is my new standard for all my own equipment) rather than 2 RCA jacks. Also note the use of a lock TRS jack for the headphones. You can see below a few photos of the arrived panel on the timber chassis. The Front Panel Express panel is fantastic. The quality of finish and precision is very good. I will continue to use them for all future projects.

It was a tough choice, but as you can see, I decided to use the orange text on the panel. Combined with an orange LED under the chassis, I am hoping the whole unit will have a very warm colour glow during use, particularly at night, when I will be using it the most.

You can see below the partially completed crossfeed board. For the moment, I have only use stock caps, but I might change that some time in the near future. But the Panasonic ECQ caps are pretty decent. The trouble will be finding a cap of the required size to fit.

The capacitors used are Nichicon Muse for the large signal output capacitor with a Mundorf Silve/Oil as the high quality bypass cap. The power supply capacitors are Nichicon Fine Gold. The other parts are the stock parts as recommended on the MiniMax website.

Construction of the amp will begin after a final check to make sure everything fits. I anticipate that the construction will take around a full day, and I am eagerly looking forward to its completion. Over the last few years, I have built a small collection of 12AE6/12FK6/12FM6 tubes from a range of different manufacturers. By far the best sounding has been the Brimar. I am desperately trying to source more of the Brimar 12AE6A, but to date, no avail. Since I will be using Sennheiser HD650, I'd think that the higher gain tubes would be preferred and ultimately be the permanent tube in the amp. Look for the next part of progress build photos!

Mini Max Millet Part 1

In the last few months, due to family, I have had a resurgent interest in Headphones. Having not really listened to my Sennheiser HD650 for a few months, I thought it time to dust them off and relax. During those months away from the headphones, I also took the task of upgrading my computer, during which I decided on a sound card upgrade. This was in the form of a Asus Xonar Essence STX. Whilst mainly functioning as a gaming PC, my music library is stored on this computer and shared over iTunes.

Naturally, I started to enjoy some headphone music through this setup. Initially, the sound was a little dark, but as the hours stacked up, became increasingly more open and less laid back. This combination is both easy on the ears and non-fatiguing. The sound stage for me was more forward, instead of in the middle of my head, it was at the front, near the forehead. The details that you get from headphones is so much more then through speakers. Although I find at this stage of the burn-in period, the Asus lacks some of the dynamics and 'sparkle' of my PPA headphone amp, I am sure that with time, this will improve.

Working on the premise that most of my audio enjoyment will probably come from headphones for the near future, I started looking into building a high quality headphone amp. Naturally my criteria was a tube based design. My first DIY Audio was Pete Millet's hybrid headphone amp. That was a rewarding build and a nice little amp. It combines a tube voltage gain and a IC buffer stage. Some would say combining the strengths of both technologies. However, having built that many years ago, the I decided it was time to see how that design has progressed.

Surely enough, there is a thriving community of Millet amp builders and has spawned several evolutionary designs. Looking through the forums and the net, it seems that most of the improvements lie in the output stage (employing a diamond buffer) and the biasing of the tube (i.e., using a CCS). Having the fore-sight to ensure that whatever I build be as reasonably compact as possible, I settled on the MiniMax Millet. Some of the progressions.

1. Original Millet
2. Max Millet
3. Mini Max Millet
4. Head-fi Iteration

Having ordered the various parts over the last month or so, the build is ready to begin. This time around, the chassis design will be a standard DIY Tube affair. 4 timber side walls with a metal top plate. The timber will be home made with a beeswax finish. The top metal plate has been laid out and ordered through Front Panel Express. The design allows for ventilation slots above the heatsinks with the tubes projecting past the panel. Further, most of the control are also on the top panel. In this design, I have also integrated the Tangentsoft's Modified Linkwitz Crossfeed.

Aikido Line Stage Build Begins!

Finally got around the designing and building the chassis for the Aikido line stage. Have a look at some of the photos during the planning phase. The chassis will be the usual timber sides and metal plate on top with the tubes and regulator heat sinks sticking through the top plate. The knobs and connectors are all on the front face.

I have decided to use XLR connectors in lieu of the 'Audiophile' type uber-expensive RCA sockets. It was actually the Eichmann plugs that gave me the inspiration. They were talking about the eddy currents, etc that happens when there is a sleeve type connection. And well, that does kind of make sense. But why not go one step further and remove ANY sleeve or ring type connection in the signal chain? The XLR are pin connection only, and being a professional audio product, designed for high numbers of inserts and removals. It is also silver plated connectors, with cable locks. All this had to around $10 each. Further, with the 3 pin standard connection, I can run stereo RCA, with the grounds tied together (which is what happens anyway inside the amp), and thus half the number of cables in the system.

You can see the generic layout of the chassis, with plugs all front access. Transformers and power stage at the back. 3 inputs and 4 outputs. The outputs will be arranged so that 1 will have a High Pass filter (around 150 Hz) and the other 3 will be full range with the RC f-3db point set around 1 Hz.

Close up of the DACT Stepped Attenuator. There will be enough room around for a potential upgrade to the Acoustic Dimension 41 step ladder attenuator.

Close up of the DACT Selector. It will switch both channels and ground. Since it is a 5 position switch and you can see I only have three inputs, it will be arranged that the two unused positions will act as mute function between each active position.

Some additional parts ordered to replace the standard kit components. Tantalum resistors in the important areas and Takman resistors in others. Note also the large Elna Tonrex caps for the heaters. Moulded Teflon sockets for all tubes and Obbligato film capacitors. The choice of output stage capacitors will be Mundorf Tin foil, with a potential for one of the output to have some kind of oil cap (Teflon, Copper, Silver/oil or Beeswax).

The power transformers, one for heaters (Hammond 185D12 6.3/12,6V 43VA) and one for B+ (Hammond 185E230 115Vx2/115Vx2 80VA). The IEC inlet has both dual fuse and EMI line filter. The heater transformer is slightly over-rated. With 5687 and 12AU7 as the tubes, the current draw should be around (5687 @ 12.6V = 0.45A and 12AU7 @12.6V = 0.15A) 1.2A total draw. With an available 3.4A supply from the 185D12, there should be enough head room.

Kids!

This is what happens when you leave speaker without protection from young kids! My no longer very nice Fostex FE127e dipole! Well, it may be just the excuse to try to phase plug replacement. And yes, it was done to both speakers, but luckily only to the front speakers, and not to the rear facing pair.

Perfect Speaker

Requirements for a perfect speaker for my 2 speaker (FR + Sub) setup.

1. Paper cone.
2. Between 5" to 6", or 120mm to 150mm diameter.
3. High Efficiency of above 94 dB/W.
4. Frequency response of at least 150 Hz through to 20 kHz.
5. (Optional) AlNiCo Magnet.

Possible Contenders

1. Lowther DX45/55/65
2. Lowther A45/A55
3. Supravox 165GMF (but a little shy of 20 kHz response)
4. Supravox 165-2000 (With Alnico)
5. Fostex 167E, Sigma 167E, 166E
6. Fostex F120A (too low efficiency, but AlNiCo)

New Open Baffle Speaker Concept

Thinking about OB and Full Range speakers. The most apparent critical aspect is the omission of the cross over in the critical frequency range that humans are most sensitive to. This kind of conclusion is widely postulated on the web by a range of different people. Whether this is true or not can only be determined by iterative testing by the listener and the speaker setup. If we take the assumption that around the 200 Hz to 10 kHz range is the most imperative for removing cross overs, then a three way design can be explored.

This new concept is centered around 3 drivers driving the 20-200 Hz, 200-10,000 Hz and 10-40 kHz range, fully covering music from 20 Hz to 40 kHz.

Lower Frequency 20 Hz to 200 Hz (approx.)

Eminence Alpha 15A in a U baffle with approximate internal dimensions of 40cm x 40cm with a 13.5cm depth side wall. This most likely will be actively crossover with a 2nd order Linkwitz-Riley at somewhere between 100 to 200 Hz.

Mid Range Frequency 200 Hz to 10 kHz (approx)

Visaton B200 in an open baffle size of around 40cm by 120cm high. Using either 1 or 2 drivers. By having the baffle this size, and the speakers offset, it appears from EDGE simulations that the baffle boost around 200 Hz to 1000 Hz would compensate for the lowered response of the B200. This could lead to a relatively flat response from 200 Hz to 10 Khz, of which the rising mids can be balanced on the lower frequency by the baffle step response. The natural falloff of the baffle would be boosted with either 1st order or 2nd order filter to match closely with the Alpha 15A.

High Frequency 10 kHz to 40 kHz (approx.)

Fostex FT17H or other super tweeter that is relatively flat up to 40 kHz. This should be filtered with an inline capacitor.

More to follow!

Updates

In the past few months, work has been quite hectic as well as the family side of things, and haven't had much of a chance to do anything audio. Primarily, still enjoying my music through the Zen amp with the Fostex 127e. On one note, I have begun to be sensitive to the beaming of these speakers. Going to have to investigate ways to reduce this beaming... The plot thickens.

Recently, I purchased a Dayton Audio Class T amp from Decibel Hifi. This was very cheap and came with an power supply as well. This will be an interesting test to see how it sounds as it compares to my Zen amp and the Chipamp. Over the course of time, I am sure I will dismantle the unit and upgrade components where I can. I see already that the speaker connections need work along with the input and volume control. I would also like to build a new case and potentially use SLA batteries to power this puppy.

Still haven't had the opportunity to build the Linestage yet, nor have I even had the time to order the components for the Aikido stage. Shame on me. However, I will get around to it eventually and that should finally give me the missing link in my audio chain.

Been looking around the net for information on Open Baffle speakers. From my cursory reading it appears that OB are quite popular and have a good reputation for being a revealing speaker design. The alternate design I am also looking into are compression drivers mated to horns. In either scenario, a LF driver and amp would probably need to be used below 100-300Hz. At this point, it is interesting to read all the various opinions and technical background on the two designs, and should form the basis of my next speaker project. To that end, an order was placed for a pair of Eminence Alpha 15A speakers from Essential Audio. For the low end, I am currently thinking of a 2nd order Linkwitz-Riley active crossover at 150 Hz, with the Alpha's in a U frame open baffle. The construction and use of this should also assist in my current Fostex MLTL. The U frame should be quick to build, and I will mate them with the chipamp initially, but maybe target a 200W LF amp in the future.

Hopefully have some more updates later and actually get some more stuff done next time.

Aikido Revised Output Capactitors

The following table is updated to use the 100 K resistor to ground for the output RC part.

Frequency	C	R	V-Cap Price
1 Hz	1.59 uF	100 K	US$251.99 (300V) 1.5uF
3.3 Hz	0.48 uF	100 K	US$139.99 (600V) 0.47uF
5 Hz	0.32 uF	100 K	US$114.99 (600V) 0.33ufF
10 Hz	0.16 uF	100 K	US$72.99 (600V) 0.15uF
150 Hz	0.011 uF	100 K	US$34.99 (600V) 0.01uF
180 Hz	0.008 uF	100 K	n/a
200 Hz	0.008 uF	100 K	n/a

With the revised output impedance of 100K, it makes the the cost equation a little more sensible. The table below shows if the R is changed to 1M.

Frequency	C	R	V-Cap Price
1 Hz	0.16 uF	1 M	US$72.99 (600V) 0.15uF
3.3 Hz	0.048 uF	1 M	US$44.99 (600V) 0.047uF
5 Hz	0.032 uF	1 M	US$41.99 (600V) 0.033uF
10 Hz	0.016 uF	1 M	US$36.99 (600V) 0.015uF