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.
The last EL84 amplifier I built (with the phono stage, tone control and headphone stage) has a Hammond 370FX power transformer. It was noticed this transformer gets uncomfortably hot to touch after about one hour’s use of the amplifier.
Being unfamiliar with how-hot-is-too-hot, I’ve adopted a cautious approach and ordered a higher spec transformer to replace the current unit. However this is a few weeks away (coming from Canada) so in the meantime I decided to measure the temperature rise to gain a deeper insight into the problem.
First thing I tested, before doing any measurements, was to pull the tubes from the headphone stage, thereby relieving the power supply of 44mA of B+ and 1600mA of 6.3volt. As expected, this resulted in a much slower heating up of the transformer.
With all tubes plugged in, the quiescent current on the B+ is 140mAThis is a centre-tapped transformer, so conventional wisdom is that in this mode of usage, the secondary should be rated at 1.2 times the desired DC current, which in this case would be 168mA In the case of the 370FX the secondary is rated at 173mA
On the Low voltage side. the 6.3volt is rated at 5A and the total draw on it is 5.2A so a little over (by 5%)
Assessment: Not ideal (The replacement unit ordered has an extra 1000mA there).
So. Down to the measurements. I ordered a digital pyrometer (infrared surface temperature measurement gun) and when that arrived, I ran the amplifier for 3 hours, measuring the surface temperature on the top of the transformer, every 5 minutes.
Over three hours, this was the result:
The measurements were made each time at the same spot on the top of the transformer, from the same distance.
After around 45-50 mins the transformer became uncomfortably hot to touch if resting the hand on it. This corresponded to a temp of mid-to-high 40s. Once the temperature was in the low to mid 50s it became uncomfortable even to a fingertip.
1) This is an unscientific test with a cheap uncalibrated instrument from AliExpress. I have some confidence in it because the baseline temperature reported (22ºC) at zero minutes was exactly the ambient temperature in the room reported by multiple other thermometers.
2) Electronic components are rated at 105ºC. I do not know what the temperature difference between the windings and and the outside of the transformer would be, so I am going to make a totally wild and uninformed guess of 20°C. Therefore the windings are at around 80-85°
3) From reviewing others’ experiences with Hammond power transformers, it seems a commonly reported phenomenon that they run hot. Therefore this transformer is behaving as expected, although it is causing considerable unease in doing so.
4) Because of my wild assumption in (2) above, I have no confidence that this transformer will be safe or indeed what detrimental impact sustained running at high temperature will have on it.
It’s been a little while since I posted an update on this project, and there’s been a lot of progress, as well as one or two hiccups.
Thought I’d put up a few photos today since I’ve been taking plenty.
Following the previous post, the next order of business was to get the top panel of the chassis ready. This involved a lot of measuring and drilling – the mounting holes for the boards and transformers, then the chassis punch for the valve sockets. A lot of swarf ended up on the floor during this process.
After getting the top panel ready, it needed to go to the laser etching workshop before I could do anything with it. This is to get the identifiers for the valves etched on – this design uses four different types, so it’s important to know which type goes where!
Once that was back, it was time to begin assembly. Mounting up the transformers and sockets to the top, and circuit boards underneath. A delightful jigsaw puzzle, but everything fit together nicely and it was not necessary to utter any curses.
Next it was time to fit the circuit boards inside, for a photo shoot:
Following this photo it was time to take it all apart again for lasering and drilling on the front and back panels. Once that was done it was time to assemble all the boards together and make up all the various connectors.
From here it was time to take it to the listening room for some tests.
Connected to the KEF C-95 floorstanding speakers (early 90s vintage) the sound quality was subjectively is clean, pleasantly detailed, no trace of any distortion or harshness in the treble, and with plenty of power to the bass. I would be happy having this sound quality as my daily driver.
The tone control behaved exactly as the previous build of this circuit, this is the second time I’ve built this one.
There was a small amount of ground loop hum on the photo stage which necessitated a bit of re-design of the earthing point ofr the power supply and the addition of a 100µF capacitor in the power supply very close to the photo stage.
Once this was done, the amp was deemed electronically fit for use, and a very enjoyable few days burn-in testing was had. A few cosmetic finishing touches were completed, the cover for the remote sensor, and a bit of cable dress inside
A bit more activity on the amplifier as time permits, and a few trials and errors later… we have a power supply board.
In the previous post, I showed the design. I made the board, using my new hand-made UV exposure box with 120 UV LEDs in it, and after etching and drilling, began to build the circuit.
I began with the low-voltage parts: the delay switch-on and LED colour reverser. Long story short, it didn’t work. Due to two errors on my part.
I’d omitted the reverse-biased diode across the output of the 555 IC (which I’ve never used in previous designs, and thus far gotten away with. This time it didn’t work)
I’d accidentally used a relay with a 9V coil voltage instead of the required 5V for the LED colour reverser.
So, it didn’t work. And worse, in soldering and de-soldering components to test it, I ended up stripping some of the tracks off the board.
So it was back to the design. Make up a new board that rectifies these omissions (I ordered a relay with 5V coil and of course its pin spacing was different)
So, a new board was designed and exposed, developed and etched.
I use the Mega / Farnell UV-sensitive boards, and Ammonium Persulphate as an etchant. These boards are not specified for this solution so while it works, it’s very slow, etching a board takes around 30-40 mins. Of course during that time, the etchant bath cools down and the process slows as a result. //TODO: Buy an etching bath heater!
Anyway, my UV exposure lightbox gives a much more consistent light than my previous approach, which frankly is too embarrassing to describe here. So the boards produced with it look a lot better.
After etching, it was to the drillpress to drill around 145 holes of various sizes, then back to the soldering bench.
First test was to stuff and solder just the components for the low voltage circuit. Make sure the revised delay switch-on and LED colour reverser was working.
Success. It worked as planned! This meant I could then continue to stuff and solder the rest of the components.
This is now done and the results are in the pictures.
Next steps: Metalwork – case drilling – and a bunch of connectors to make up.
The design of the amplifier has continued since the last post although slower than hoped. This was die to a few random factors (including but not limited to a computer that died and needed replacing and setting up, and some non-electronics projects that came up)
Anyway, the progress this time is the completion of the headphone board, which is based on a White Cathode Follower using ECC99 tubes.
This completes all of the signal handling boards. Below they are shown in their location on the chassis.
This represents a significant achievement: The headphone stage was a late addition requested by the customer after the chassis had already been ordered. I was not at all confident that the entire amp would be possible to build in the small chassis. But I persevered, with the approach of taking a long time on the PCB designs to get them as compact as I could, including manually routing them (which gets more complex as each track and component is added)
The last remaining board needing to be designed was the power supply. This is going to sit inside the chassis under the transformers. This dictated its size, which – as with the rest of the amp – needed to be as small as possible.
The final board design measures 140 X 75mm.
The next step is the careful eyeball check before exposure, etching, drilling (141 holes) and stuffing.
This is the layout
On this board we have:
Delay circuit for controlled power-up
driver for the 2-colour power LED that starts red and goes green when the main power kicks in after the warmup delay
Elevated 6.3vac power supply for heaters, with three sets of output terminals
6V DC power supply for heaters with three sets of output terminals
370V supply rail for output stage
265V rail for phase splitter stage
300V rail for headphone stage
300V isolated rail for phono stage (to avoid feedback through power rail)
280V rail with two outputs for tone control and preamp gain stage
double-filtered and isolated negative bias with four outputs (one per output tube)
Regarding point 4: From a technical requirement, it’s only strictly necessary to run the phono stage on DC heaters, but due to the complexity of this project it has a high tube count (13) the number of tubes exceeds the rating of the 6.3vac heater winding on the power transformer. This transformer also has a 5vac winding which will be passed through a diode and thereafter smoothed by a 47,000µF 10V capacitor (it’s huge… has to lie down on the board!) and this rectified DC heater power will be used for other areas besides the phono stage, just to share the load.
Next steps: Make this board, then metalwork for the chassis…
The Christmas/New Year holiday has provided plenty of opportunity to progress the design and construction of this project. My approach to this one is to build a library of discrete PCBs which fit together to complete the project, and which can be produced again for a future project.
So this will allow me to create a menu of sorts, with the customer selecting what features they want.
So, we have a separate board for the RIAA/Phono stage, one for the tone control, one for the preamp stage, and one for the headphone stage.
So far, the RIAA board, tone control board, and preamp board, are built. Here, I’ve laid them out according to where they will sit in the final build
I’ve done them all the same way as the previous post, with the valve sockets on the copper side of the board.
My method for laying out the boards is to use no automation: These are all completely manually routed. Attempting to use the auto-router was like a bad comedy show.
Top-Right is the RIAA Phono stage. This one:
In front of that is the tone control
And the preamp gain stage and phase splitter
This is how the boards look from the under side. These are all made using a laser print onto a transparent sheet then exposed onto the photosensitive board under UV light
Remaining to be done: the headphone board (this will be a challenge as it has some large capacitors on it) and the power supply board.
The challenge with this build was to fit everything into a case with an internal dimension of 300mm (11.8″) wide and 225mm (9″) deep. The layout shown will accommodate it… just!
Sadly, one of my suppliers has let me down and as a result, I’ve spent $100 on parts I don’t think I’m ever going to see. I will not name them yet as I am extending them the charity of my silence to give them the opportunity to rectify the matter. In the meantime I’ve had to order some tubes from the regular supplier to replace the missing parts.
Progress is happening with the new amplifier… this design is more modular as I have decided to design standard boards for tone controls, headphone output, and phono RIAA.Having standard boards for these means I can more easily accommodate future builds, shoudl they be requested.
The process has not been entirely smooth sailing, owing to the somewhat hit-and-miss nature of home PCB fabrication. Until now, my method has been to print the PCB design onto an iron-on transfer which then gets pressed onto the board (and then touched up with the etch resist pen) before going into the etchant.
This process has been unreliable and time consuming, and expensive, owing to the high reject rate. So a new technique was called for.
I’ve decided to move to a photosensitive board workflow. The design is printed onto transparency, which is then plaed over a light-sensitive board and exposed under a UV light, thereafter a two-step chemical process: Developing then Etching.
The first board I designed for this project is the tone control. This is using the same circuit as the previous project, except I had two changes:
I needed to reduce the size of the board
I needed to put the tubes on the copper side of the board
So, I re-designed it to be 120mm X 65mm (down from 150 X 75) and attempted to fabricate the board… with less than spectacular results
This board failed because I did not expose the photosensitive layer sufficiently.
Lesson learned, I did a second attempt, which looks much better. So I went ahead and drilled and stuffed it.
The tubes are on the copper side because of the customer’s preferred aesthetic of having the tubes visible. This design will be applied to the other boards in this amp as well.
In the process, I have become a lot more familiar with the operation of my PCB software: namely DesignSpark from RS. Also its quirks and foibles, such as less-than-ideal behaviour when moving things around, and its ability to have “invisible” track that isn’t visible in design but is when you print. As a result of this, the board above needs to have one track cut with the dremel and re-routed with a short jumper on the track side. Yeah I hate doing that!
Lesson: Inspect the board VERY carefully in print-preview before fabrication.
Or, to use an appropriate engineering axiom: Measure Twice, Cut Once!
I also built the bias boards for the EL84s. Owing to the amount of heat these produce, I am not mounting them on boards, but the voltage divider and potentiometers for the negative bias voltage, and the cathode shunt, can be put on a board. So drawing on my earlier design, these are the bias boards, made using the same technique:
Next up: A two-triode RIAA stage, I’m planning this on a board on 100 X 65mm.
There’s a reason I want these boards as small as I can get them: The size of the chassis
This chassis is going to represent a challenge to fit everything into it… this design will have 13 tubes: The RIAA stage, tone controls, headphone stage, as well as the amplifier itself. And size is a consideration since it will be packed up and sent overseas when it’s finished.