The last amplifier I built (the Matariki) has been my Daily Driver for the last 8 months or so, and it hasn’t given me any problems…
…Except for a few “bad manners” that needed attention:
The amp has quite an inrush current thump and hum when the B+ relays close after the heater delay, the hum lasts about 2sec
The amp has a buzz through the speakers when the mute button is activated, this disconnects all the inputs, but since the first stage is a bootstrapped cathode follower, it has a very high effective input impedance.
The Hammond power transformer gets up to about 60ºC which is too hot to comfortably touch, after ~2hrs operation
So I’d filed these ideas into the “if I ever build this one again” bucket.
Fast-forward to Jan 2019. I was contacted by someone from an online entertainment magazine who wanted an amp to write up and review.
So I decided I didn’t want to send this prototype because of the bad manners identified above. Audiophiles tend to be very protective of their speakers, and rightly so. Any amp that puts a thump and brief hum on power-up through the speakers would be disadvantaging itself from the moment the power was turned on.
Not Good. So the plan of building another Matariki was conceived, which would address all of the above “bad manners” as well as add a bit of bling (gold-plated speaker terminals etc)
The reason for building another one, rather than just retro-fitting this one, was that to fix these issues, either an ugly (and obvious) hack would be needed on the PCB, or else a new PCB made. Having already done the design work on the PCB layouts, I saw this as an opportunity to test them.
So. The new design incorporates a better soft-start circuit in the power supply which should avoid inrush issues (and this necessitated putting arc-protection diodes on the phase splitter too). Also I’ve got a bigger power transformer for this one that will be used at only around 60% of its rating. Even though the existing one is nowhere near hot enough to represent any danger, I just didn’t like it running that hot.
And then after it’s reviewed, it’ll be for sale. Which will be a test as to whether it’s possible to sell hand-build valve amplifiers for a cost that exceeds the parts. Watch this space.
The EL84 amp is completed and has been removed from the workbench and is now in the living room where it’s been entertaining us the past few days.
First, a few pretty pictures
This is the best looking amp I’ve made so far. Great care was taken with centering and spacing. The translucent hole to the left of the volume control covers the IR detector for the remote control.
About the name
The amplifier is named “Matariki” which is in the Maori language of New Zealand. Literally translated, it means either “Eyes of God” or alternatively “Little Eyes”.
In more common usage, it is the name given to the Pleiades star cluster, when it becomes visible (which is mid-year, mid winter here) and has traditionally become associated with renewal, the Europeans decided to call it the “Maori New Year”
There was also a rare southern right whale which made an unusual appearance in Wellington Harbour recently, during Matariki, and the whale was thus informally named Matariki.While all this was happening, I was designing this amplifier. Hence the name
The case is aluminium, sourced from AliExpress, of the type I usually use. The front panel is 8mm thick, brushed aluminium. It required pockets being milled on the CNC from the back to accommodate the controls mounted through it.
The lights are 3mm LEDs but I decided I don’t like the bulging appearance they give when pushed through the front panel, so we laser-cut some 2mm clear acrylic into 3mm circles, so the lights on the front could be flat and flush. They press-fitted perfectly and the look was 100% what I was wanting.
The STBY LED is red, and the PWR led is dual-colour, it starts red at power-up and then when the HT switches on after 30 sec, it turns green.
The power switch and input selector are a rotary encoder: push to toggle power, rotate to cycle through the inputs.
Inside the case
Inside the chassis there’s the amplifier mainboard, which contains 6 tubes and is the phono, tone, gain and phase splitter. To the left of that is the base of the output valves, the long thin board contains the bias and cathode shunt resistors, test points, and on the track side, the four trimmers for adjusting the bias voltage.
The green boards are bought-in components: input selector, mains switch/standby, remote volume, and microcontroller.
The power supply contains the usual array of resistors and capacitors needed to provide the various voltages, as well as the usual 30 sec startup delay timer relay circuit I always use.
The DC voltages provided by the power supply are:
+320V +300V +270V +250V +6.0V (DC heaters for Phono stage, rectified from the 5vac secondary with Schottky diodes) -27V for fixed bias
In addition there’s the standby transformer which provides 9vac at around 200mA to power the microcontroller and standby board.
This was the first all-on-one-board amp I’d made and it was successful. Everything worked exactly as expected on the first power-up. All the components fitted on the board, the board itself was a success (first project with the new temperature-controlled PCB etching tank) and the board looks fine (although there’s no soldermask or silkscreen on it, it really is just single-sided naked copper tracks on FR4)
Likewise for the power supply.
The level of tidiness inside the case is better than anything I’ve achieved before, although I don’t think I’ll ever get to the level I am looking for… which is OK, because when you shoot for the moon you’re not gonna hit it, but you will end up in the treetops, which is a whole lot better than being on the ground.
The level of aesthetic appeal on this one is better than any of my previous projects as well. I am completely happy with that aspect.
From a technical standpoint, on this build I’d designed the board to allow phase compensation into the NFB loop. This is because NFB produces high-frequency ringing which you can see on the oscilloscope if you put a 10KHz squarewave into the input. At the output you get something like this:
The prescribed method to resolve this is to phase-compensate the NFB with resistors and capacitors, the values of which are determined by experimentation. After doing this, the 10KHz squarewave output now looks like this:
Finally, one aspect I am well pleased with is the listening test. Subjectively this is the cleanest sounding amp I have built to date.
Schematic as built
Click to enlarge. Might need to download / Save-As, to be able to read it
Editorial Mar 2019: This is a schematic for the second Matariki I built, with a few improvements over the first. I felt it prudent to replace the old schematic with this one.
The demo amp is taking shape… the chassis is back from laser engraving and milling, the back panel is assembled, and the PCBs have been made.
This time we tried a different approach to the lettering on the aluminium. Instead of using paint, we used adhesive vinyl sheet which was applied over the front panel, then the outline of the letters was cut by the laser engraver, and the vinyl carefully peeled off. Then the enclosed letters (eg. “e”, “R”, “O”, “P” etc – there’s lots of them!!) needed to have the inside fill carefully removed with tweezers, a steady hand, and a magnifying glass.
After the front panel was doie I sprayed two coats of clear protective lacquer over it, to prevent the letters from falling off later on.
Some photos for now.
The PCBs were made using photo resist method, with a UV lightbox I made up for the task (with hundreds of UV LEDs on stripboard, yes it took a long time!) but it was far more accurate and consistent than the previous PCB project. Also I got a cheap drill press for doing the PCBs instead of using the hand drill in a stand.
It’s been too long without a project on the workbench, and I’ve got a few leftover parts from previous projects. Plus, I happened across some NOS Soviet military-spec 6N1 and 6N2 tubes. It would have been a grave sin of omission not to do something with them.
So, the idea of building a new amplifier took shape. This one doesn’t have a new owner waiting for it, but rather I’m making it as a demo unit. Idea being to use it to hopefully drum up a few orders and to test the market to see if I can sell it at a price that recovers the parts cost and makes a profit.
Topology-wise this will be a tried-and-true amp, I’m not breaking any new ground electronically with this one, but I am refining the construction as far as my skills will allow, and hopefully the results at the end will be worth the effort.
So, we’re looking at (yet another) EL84 push-pull amp in ultralinear with fixed bias, a split-load phase splitter, preceded by a gain stage, the same active tone control as I’ve built twice before, and a Phono (RIAA) stage, again the same one as I made before.
This time, however, I’ve spent a bit of time on the board design. My photosensitive board blanks are 160mm X 100mm, so I decided to see if I could fit the RIAA stage, tone controls, gain and phase splitter stages, all on that board.
Several hours of editing on the PC later, and I had a design which has passed 3 stringent eyeball checks. I am happy to build it and see what happens.
Circuit-wise it’s the same as the previous one I made but those were all on separate boards. Also in the Gain stage I’ve incorporated phase compensation in the NFB both on the cathode and the load resistor.
I’m even using the exact same chassis as the last one. So, the first job was to work out the component placement.
So, I printed out my PCBs onto paper at 100% size and placed them in the chassis. Then I added the PCBs for the remote control volume, standby, and input selector (thanks Aliexpress!) Finally, the connectors and other things that go inside the case to complete the job. It’s all a big jigsaw puzzle, and I find this the easiest way to visualise what the inside of the case will look like, and whether there’s anything that’ll need re-arranging.
Luckily there’s enough room and I don’t need to stand anything on its edge. This case only has 50mm height so this is good news.
So the printed board at top left is the RIAA / Amp / Tone Control board. That has 6 tubes on it in two rows of three, with 50mm spacing. The sockets for the EL84s are next, proceeding clockwise, and the long thin printed board is the bias board. Same design as I’ve used previously each time.then we have the volume control which will be mounted to the front panel.
Continuing clockwise, this is a cardboard cut-out of the 9V transformer which will supply standby power for the remote control board, giving us the ability to turn the amp on remotely. Then there’s the mains relay.
The 100 X 100mm printed board is the power supply incorporating all the resistors and capacitors and usual power supply things. It also incorporates my usual 555-based startup delay with the driver for the 2-colour LED, like in the previous project. (It turns on red to begin with but then changes to green when the high voltage switches on)
The remaining two boards are the input selector and the driver board for the remote control receiver.
My next job is to score up the case and cut the holes needed, then make up the three boards.
I got tired of using a dish for etching boards, it takes too long and is a bit hit-and-miss. So I bought an etching tank with a heater:
The heater keeps the etchant at the correct temperature and should improve the process. When I get to making these boards, I’ll do a video of it to publish here.