Wednesday, September 16, 2020

"retro-K465" portable headphone amp


Oatley released a headphone amp kit based on the OPA2134 - K465. A CMoy style simple HP amp based on the OPA2134. Oatley guarantees genuine OPA2134s and I have validated that. I received an early release which was assembled with pot and sockets on board. These were removed and the good input caps replaced with Russian PIO caps. 

A larger ABS enclosure was used than the last two HP amps to accommodate the cct. brd and two 9V lithium batteries. The batteries are converted to differential supply on board - +/- 9V. The enclosure is lined with Al foil top and bottom and is earthed. 

Once more my "safe on" wiring using the changeover switch on the input socket ensures the amp cannot accidentally be left on and drain batteries. The sound is good with excellent clarity, good volume and drive. The kit should be simple to assemble. The kit is complete with 9V battery clips and the ability to also use AA battery holders. You can see I am now spoiled for choice with three HP amps, one with the PT2308 chip and the others with OPA2134 chip.

Sunday, July 26, 2020

Salt Cellar II - 6EM7 triode mono amplifier

Building yet another mono triode Salt Cellar wine box amp I had to make it different. To make even simpler this time just one tube, the 6EM7, which has two triodes in the one bottle. I'm told its like having a 6SL7 and a 45 in the one tube. Once more an external (but different) tranni box was used. This one also works with my two wine box 6L6 SE UL amps.

Again all parts came from da lab. That part of the Salt Cellar initiative was flowed strictly. How simple with just one tube and an Alps stereo attenuator. The output of the attenuator is coupled with two resistors for a mono mix. Strictly using parts on-hand I use a SS double Pi rectifier brd. I had made many years ago. Add a big 10H 125mA choke and yet another stage of filtering you have a dead quiet "is it on" quietness. 

A different finish here from using a staining varnish and again an old world look and feel. The knob goes on top otherwise it would have looked a little baron. All wiring is via solid core Cat 5, motsly as twisted pairs. Early days the amp sounds good but does it compare with the original SC 807 amp. Time will tell.

Thursday, July 16, 2020

Salt Cellar - mono 807 triode strapped integrated amp

You should read the Salt Cellar article from Stereophile online to get a better understanding of why I took on this project. In the article a mono system was hobbled together using low power tube mono amps and high efficiency horns. The system was auditioned in an old salt mine which would add to the acoustic. 

After building a few small tube preamp from parts on hand I decided to build a mono triode integrated amp from parts I had in da lab. Because the amp in the article had tube rectification and the power tube had a top cap plus the amp was held on a wooden frame I went with similar. I've built a few wine box amps and had a few spare boxes on hand. A stain I have used once before was re-used here but instead of multiple coats of hand panted lacquer I went with a spray lacquer. The finish was much more rustic and had the appearance of aged wood.

The power tube is an 807 a pair of which I was given many years ago. On hand was two old RCA 6SN7 tubes and two "80" small rec. tubes. I had to purchase a few high voltage filter caps and a big ass 720V power tranni. The power tranni sits outside on its own matching brd with a multi-pin connection. The tranni was mounted on thick rubber grommets to control vibration and light rubber feet are also fitted. For a bit of tranni bling chromed domed nuts bind down on brass washers. A shared tranni means any other tube amps I build can share the one expensive tranni. Bonus!

The smothing network, in the box, is a doubled Pi. C-L-C-R-C;- 47uf Mundorf MLytic ploy, 10H 125mA choke, 100uf, 1.5K 5W, 200uf. All this fits snuggly into the box. The amp with 98db speakers is absolutely dead quiet, like "not on" dead quiet. 300V is applied to the Edcor 15W OPT and 250V to the 6SN7. The 6SN7 is both preamp and driver tube. Half for the preamp and half for the driver. The whole PS is well snubbed with 0.1uf polies. There is a mix of metal film and carbon film resistors plus 5W wire wounds.

The volume pot sits between the preamp and driver tube not upfront as many do. This means Im not throwing away signal then amplifying what is left. The whole input signal is amplified in the preamp stage then attenuated into the driver stage. There is no input cap and a  0.68 poly connects the preamp to the driver stage and a 0.1uf PIO to the 807 power tube. The preamp section and 807 have no Ck but the driver tube does - a low ESR elctro which is also snubbed.

Beware if you view the original schematic on Diyaudioprojects site pins "2" and "3" are incorrectly labled so check it out yourself.  After solving many small issues including the pin swap and in my case bad pin counting the amp function perfectly with voltages and currents near textbook values.

In my Salt Cellar system you will see a mix of 12" Beyma and Altec 511B/FaitalPro horns. The Beyma provides bass and super tweeter and the horns mid/treble. I'm using the Beyma Xover which Xs at 1100hZ and is expecting 16 ohms on the treble side. Two horns in series gives me that value. Two runs of solid core Cat 5 with mates and pairs split provide the speaker wire.

I tried the speaker setup with two horns but later felt a single horn on the Beyma on the floor the more balanced configuration. The horn is tilted up slightly and padded down a few db. Still lots of mid but the bass is felt more. I watched a 4K movie with this configuration and really enjoyed it. Missed nothing and gained a lot more. Who needs stereo?

The sound is light on bass but heavy on detail, immediacy and transients are ridiculous. So much more revealed in tracks I know very well. Part of the SC system is to hobble one together from parts on hand. I achieved that about 90%. I have enough parts for a second amp.

Stereo is mixed down to mono in the amp with a "Y" network of three 47K resistors. The amps produces about 5W and plays plenty loud through the high efficiency speakers (98db). I haven't tried how loud yet as at about half vol. it is already deafening with no distortion.

Friday, June 19, 2020

“Rosso” - E88CC SRPP Preamp

After building an excellent sounding preamp ("Continuum") based on the same Gold Lion E88CC gold pin tubes I thought I would build another. Main reason is I have another pair of perfectly matched tubes. Too make this a visually standout preamp I painted this one red and thus the name "Rosso". 

As with Continuum it has two selectable inputs. Rosso has an Alps pot and 4oz solid brass gold plated knob. Similar componentry under the bonnet. Russian PIO output caps but this time snubbed with a 0.1uf poly. Metal film resistors and silver plated wire-warp wire as hookup. My universal external PS just plugs straight into this one also via an XLR socket.

Where this one does vary from Continuum is it has been redesigned. Bruce Heran of American oddwatt fame, who provided the original design, re-tweaked the whole preamp. Much lower HT and lower value resistors. I'm sure this one sounds even better than Continuum which sold. It must be the redesign as so much of Rosso is similar to Continuum.

Treble has lifted here and transparency has taken a big step forward. Gian is high in both preamps because the original design was for a lower gain tube. In operation I have found this not to be a problem, just bloody great sounding. Combined with my DIY 300W SS power amp these two are truly a matched made in sonic heaven. 

Tuesday, May 5, 2020

Pass F5 turbo extreme build

I was commissioned to build this F5 Turbo for another. The kit is from Diyaudio shop and comprises a 5U extra heavy enclosure with steel subchassis, two 300VA trannies and double PSs. This allows the construction of a dual mono amp. There is a total 192,000uf of filtering audio grade caps and 16 dual high performance diode bridges all on their own heat sinks. Each diode bridge is capable of 15As.

The F5 boards looked first class. The components were all high-end and selected by the owner. Riedon resistors featured strongly and a few other resistors were beefed up to bring the amp to turbo specs. Also large power thermistors are placed on both legs of the mains to allow for a soft start. Matched power FETs and matched high performance driver FETs were also chosen.

To push the amp to turbo the idle current is raised to 1.6A across each power FET. Though the heat sinks are massive they still get to 50c and can only be touch briefly. This high current mode was requested by the owner. Bias is fiddley and a little vague but with persistence you can get the current up and the DC offset down.

The amp is extremely quiet and has no sound at all when on. It is beyond any of the quietest amps I have ever built. It has a dead back background. From this back hole of nothingness leaps extreme dynamics with a sweet and detailed mid and treble. Bass is deep and swings a little to the bloomy side but not unpleasantly so. This is amp is to drive 100db sensitive Zu Druids. I wish I could be there to hear it. In my system is was an expensive but extreme giant killer. May build one myself. 

“Zeus” - Holton inspired power amp

"Zeus" is named by the owner who commissioned me to make the amp. He wanted the more expensive 4U enclosure but with a silver face plate. So that's what he got. Inside is 1KW tranni and 80,000uf of 100V filter caps. The Holton amp modules are special in as much as they are 200W modules with upgraded dual die power FETs to produce 300/500W.

As usual a RFI/EFI power filter is included and a 240V speaker protection module. Amp weighs a ton and looks real pro. And what's more sounds excellent. Heavy as this amp delivers better than it looks. The owner and I are both happy with the result. 

Thursday, April 23, 2020

"Continuum" - E88CC Gold Lion SRPP preamp

I had bought four Gold Lion E88CC gold pin tubes for another who had one of my tube preamps and wanted to upgrade it. They decided not to upgrade and I got stack with the tubes. They remained unopened  for the last 25 years until a few days back. I had most parts on hand to build a whole preamp from scratch and use these very nice tubes. And here it is. Colour is Deep Indian Red epoxy. Sound is full bodied and appears very balanced top to bottom.


  • Hammond wood cheek chassis
  • Ceramic sockets
  • Silver plated fine copper hookup wire
  • Russian PIO output caps.
  • External DC PS
  • WBT silver solder
  • Alps Blue Velvet pot
  • Vintage style Vernia gold knob
  • Two selectable inputs
  • 4uf PIO final HT cap internal to the preamp
  • Low ESR BP Ck
  • Metal film resistors
  • DC on heaters.