Category Archives: Construction posts

“Model7” – amp-on-a-board

This is a project that was born from a visit to a fellow amp-building enthusiast. There was a small output transformer sitting in a pile of wires and upon closer inspection it was connected to a breadboarded pushpull stage, with the driver stage sitting across the room connected by a long cable.

The size of the output transformer and the diminutive size of the tubes struck me, particularly because they were at odds with the full, rich sound I was hearing through the Klipsch speaker.

The driver tubes were EL95 and the output transformers were custom-wound, 9.1K Ra-a with 20% ultralinear taps.

I quickly acquired a set of EL95s and a pair of the output transformers. Then I returned home and sat on them for a while before deciding on a circuit topology. I opted for a preamp stage based around the 6N1 tube since I had plenty in stock. So I designed a simple topology based around a gain stage followed by a long-tail pair phase splitter. I calculated this would give me around 15dB more open-loop gain than I needed to get full output at 1v P-P input. Thus allowing 15dB global NFB, which is a good amount for a push-pull design.

(click to enlarge)

The blue numbers in the schematic represent actual measured values in the built circuit.

The next step after designing the circuit was to bread-board the initial stages, and fine-tune the resistor values with experimentation. Note especially the Long-Tail Pair, the first half has 14K load resistor while the second half has 15K. This was calculated using Blencowe’s method to provide the best balance, which was validated by measurement. To make 14K I parallelled a 39K and a 22K resistor.

After breadboard validation, it was time to lay out the PCB. I had one very specific consideration for this amplifier: I wanted to fit it into a small form-factor size case, the same size as my previous 3-ch preamp project, in fact. This necessitated a limit on the PCB size, so I opted for 160 X 100mm. Therein lay the challenge: To fit the whole amp, power supply included, onto this size board.

In the end, I was successful, and after review I sent the board out to be manufactured. When it came back, I built it:

After building, it was time for some testing. This was accomplished using a 8R resistive dummyload and Room EQ Wizard, using a 100R : 20R voltage divider on the speaker output to allow accurate measurements at 1W and at full rated power (7W)

This shows freq response at 1W and full power, noise floor at 1W and full power, and THD again at 1W and Full Power. (Noise Floor levels as references to output level). (This was with the amp totally unshielded sitting on the bench with long flying wires everywhere, so I expect the noise floor to improve when it’s built into a chassis)

Now, it’s just a case of putting it into a chassis. The chassis design has been completed – the CNC pattern files in QCad and the PDF of the laser engraving in LibreOffice Draw. Once the chassis is back from the sheetmetal workshop, I’ll complete the assembly. Stay Tuned…

Build completed

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:

Nasty ringing through the NFB

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:

NFB after phase compensation added

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.

PCB Party

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.

More photos as the build progresses.

Hammond Power Transformer temperature problem

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

Assessment: OK

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.

(Posed photo. Te measurement target shown was not the actual measurement location due to radiant heat from the output tubes).

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.

Conclusions

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.

Tone Control Finished

After much waiting on parts, the tone control is now finished and in service.

The 250K Potentiometers took three weeks from order to arrival, in the meantime I’d been using components of the wrong value, so the characteristics were not correct.

Also the front panel has been an epic test of the patience to get the printing onto it. Several techniques were tried:

  • Using thermal transfer film – the same method I use for making PCBs – with the iron. Result: Design and lettering failed to transfer cleanly.
  • Using a cold-transfer method with a laser-printed design and chemistry (mix of alcohol and acetone). Result: A highly flammable and volatile mix of chemicals, complete failure to transfer lettering
  • Print onto paper, transfer paper, transparency (smooth and rough side), experiment with printer settings regarding toner etc – all to no avail.

In the end, the method that was the least dreadful involved covering the front panel with adhesive masking tape, and using a laser-cutter to cut the outline of the letters, then peeling them off with the tweezers to create a stencil, through which several coats of black spray paint were applied, before peeling off the adhesive then applying several coats of clear lacquer to protect the paint.

The results are not fantastic, but they are tolerable in the face of the spectacular failure of the previous methods attempted.

High on my To Do list is to devise a better method of printing onto aluminium.

Anyway, this is the device as completed

The transparent acrylic top gives a nice view of the insides. I went for a bit of a Star Trek vibe in the labelling.

Also there’s a few extra photos here if you need to see more.

The circuit as built in the end. Note I changed the R and C values in the treble arm to even up the response on both sides
What LTSpice (Circuit Simulator) says this circuit should do at various control positions

Observations

  • This circuit works extremely well; listening tests reveal a completely neutral sonic signature, and that is using the cheap Shuguang 12AX7 tubes (all I had to hand)
  • The circuit is “quiet as the grave” – hum and hiss are inaudible even with the amplifier on maximum volume and ears pressed right up to speakers
  • The boost and cut levels measured on the oscilloscope (see earlier post) match closely with the predictions in LTSpice

I am very pleased with this circuit since it was my first attempt at designing an audio circuit on a PCB. Previously, my PCBs were limited to power supplies.

Waiting to be done: Distortion and noise floor measurements. Rainy Day activity.

This unit is now in service in my listening room, sitting between my RIAA stage and integrated amplifier.

This unit is fitted with a power-pass port, to allow the main amp to turn on the tone control and RIAA stage, so that multiple power switches don’t need to be toggled to play some vinyl

FFT Tests on the Tone Control

With the tone control board and power supply built, but the case still in the machine shop, it seemed a good opportunity to run some performance tests. The site I got the circuit from only had Spice simulations rather than measured results. So I don’t know if this is the first time actual test results from this circuit are available.

Anyway, my testing method was to run a sine sweep into the input from 50Hz to 40kHz, then connect input and output to the oscilloscope, running the output into a 100K dummy load (to simulate the volume control of the amplifier it would be running into)

I ran FFT transforms at tone flat, full treble lift, full treble cut, full bass lift and full bass cut. The results are below. 

Please ignore anything below around 70Hz; my FFT process loses all resolution at that frequency.

There are two traces on each plot; the yellow trace is the input signal, for reference. The blue trace is the output from the tone control.

First up – the testing setup

Ain’t it beautiful? Careful where you put your fingers!
The 9V is for the signal relay; if 9V is present the contacts close and the circuit is engaged. If no 9V, then the relay bypasses the circuit and bridges the input and output. When the case is made I’ll wire up the 9V supply properly but for now it’s Energizer-power
Tone controls flat. As mentioned in the text, ignore the region below 70Hz; my FFT process doesn’t have resolution there
Bass full cut
Bass full lift
Treble full cut
Treble full lift

In each graph, scale is dB on the millivolt, 5dB per vertical division, yellow is input trace (for reference) and blue is output. (My FFT math capability to reference one signal off the other doesn’t work properly, hence the presentation of both signals).

For completeness, I ran the same tests on the other channel. Results were identical.

Conclusion

This tone control is working exactly as anticipated, and is not far from the predictions on the source site.

Something seems to have gone right.

Gestation photos of the Tone Control project

Just a few photos of the project’s gestation. Click each to make it bigger 🙂

The circuit I decided on. Simple tone control (bass and treble) with feedback. Uses an initial cathode follower and a final gain stage to compensate for the losses in the tone control section. Because it’s an Active tone control, it doesn’t need audio taper pots. Linear ones work fine. I managed to find some with a centre detent as well!
I decided to make this on a PCB just because I hadn’t done this before. So being a complete novice at PCB design, everything is done by hand. No auto routing or anything else. This was my initial sketch
Then I designed the board. Measuring the components I intended to use by hand with the micrometer. Yeah, when I said “First Principles” I meant it.
The board, ready for etching. The design was made according to the size of the board blanks I had to hand, to avoid having to do any cutting. The transfer process involves printing the design with a laser printer onto a plastic sheet, then using a clothes iron to transfer it on to the board blank. Usually this requires a bit of touch-up with a Sharpie-pen but in this case it was almost perfect.
Work in progress. Starting to build the board. I discovered to my dismay that I did not have all the resistors I need, so there are some unfilled holes in this board. 

Also there’s the power supply board which I made the same way, it’s very simple and boring.

This whole project started because I bought the power transformer from the local auction site for $9. It has 213 – 0 – 213v secondaries, plus 6.3v. This gives a nice B+ of around 300VDC.

Also I had two 12AX7 tubes left over from a previous project – these are horrible cheap Chinese ones, but they work OK. Sufficient to test it, if I like it I might put some JJ or EH ones in.

More photos later when I get the case back from the CNC and laser etching…

New project – Tone Control

After building the last two amplifiers, I’ve had several months of not building anything, while I wait for the next firm customer. Lots of people expressing interest, but no-one ready to put any money down… yet.

So in the meanwhile I’ve been keeping an eye on the local auction site and pouncing whenever anything that looked suitable for a future project came up.

Latest score was a small power transformer with 213-0-213 secondaries (so good for 300V B+) and a handy 6.3v also. Size suitable only for a preamp, this transformer cost the grand total of $8.00

With a couple of 12AX7s left over from my last project, thoughts turned to the possibility of making a headphone amplifier, nice idea except I didn’t really need one.

Then I saw a site with some tone controls. My phono cartridge is a little down on treble so it seemed like a good idea to build a tone control based around a pair of 12AX7s. Idea being to put this between my RIAA stage and the integrated amp.

This is the schematic:

(Heaters don’t need elevating; it’s an error in the notes)

Just to challenge myself, I plan to build this on a PCB rather than on a chassis with point-to-point. The reason for this decision is so that I can at some future point take this PCB and transplant it into something else, if needed. Or alternatively, make up another one quickly if needed.

The next challenge is to arrive at a PCB layout. Being a complete novice at PCB layout design, my preferred design method is the same as the constipated accountant: Work it out with a pen and paper.

Or in my case, because I am not a complete luddite – my Surface Pro computer with the drawing pen.

So. This is the concept drawing of the PCB – obviously I’ll duplicate it for stereo – and there will also be a pair of relays to disconnect the inputs and outputs from the circuit and tie them together for a tone bypass.

Next step will be to measure the components and design the PCB.

Work in progress.

Just for reference, my Phono preamp I am using is the 3-triode “Little Bear”

This machine uses 6N2 tubes, two for gain and one on the output as a cathode follower.
I’ve tested its RIAA response using the FFT function on my USB oscilloscope:

Blue is input 5mV RMS Sine sweep,  yellow is output. Scale is dB on the millivolt. Ignore the input below 70Hz, hum due to unshielded leads.

From this it’s clear that the RIAA stage is behaving itself, so the blame for the slight treble loss from the turntable must be with the cartridge. An Audio-Technica ATS-11 with a band new Shibata stylus that cost considerably more than this tone control will.

(The treble loss is ascertained by listening tests by doing an A/B comparison with the same music played from vinyl and a digital source, synchronized at playback. Other than that, the vinyl sounds fantastic.)

Fine-tuning the NFB

Having lived with the big KT88 amplifier for the last few months and been mostly happy with it, there were still some niggles that I resolved to get around to. 

Specifically, the amp ran hot, and needed more ventilation holes drilled. 

Also, there wasn’t enough Negative Feedback (NFB). I wasn’t too worried about this until I built the EL84 amp featured elsewhere on this blog, which had more NFB. On hearing the difference, I resolved to correct the situation in the big amplifier, but this would need some equipment I didn’t have.

So, first up was a shopping trip online to get some power metal film resistors for a dummy load, it’s very important to have a non-inductive load for tuning NFB. These were duly mounted to a large heatsink.

Next I needed a pair of old-style variable capacitors, the kind you’ll find in an old valve radio. eBay to the rescue, and these eventually came all the way from Bulgaria.
The method I intended to use for fine tuning the NFB was from Morgan Jones “Building Valve Amplifiers” p.290-291. 

Today, I managed to get the amp back onto the workbench. Pulled out all the valves and attacked it with the drill, to make some new ventilation holes. Problem 1 fixed, and it remains to leave the amp on for several hours to determine its effectiveness.

Problem 2 was also resolved, though this was a good deal more time-consuming. Utilising Morgan Jones’ method, and armed with a healthy stock of film capacitors of various values, I started making the necessary modifications to the circuit, first with potentiometers and variable capacitors, before subbing in fixed components.

First order of business was to reduce the NFB resistor from the (fairly useless) 100K to something lower. After experimenting with the input sensitivity, I dropped this to 33K.
This got the amplifier’s gain to where it needed to be, and eliminated the problem of the very touchy volume control.It did however introduce another very serious problem: high-frequency ringing. The Williamson design is prone to this, and adding NFB in any quantity will exacerbate it.

This was the result (red trace) at the speaker terminals of dropping a 10kHz square wave into the amp, after reducing the NFB resistor from 100K to 33K

Yeah 🙁

Don’t know about you but I don’t want to listen to that amplifier like that. Apart from anything else, it’ll set all the dogs in the neighbourhood howling. And things will be getting mighty hot with all that high frequency energy to dissipate.

So clearly some compensation needed.

So watching the trace on the scope, using Morgan Jones’ method, I arrived at these changes to the circuit:

Added compensating capacitor and resistor parallel to the anode resistor in the first gain stage
Dropped feedback resistor from 100K to 33K
Added compensating capacitor and resistor parallel to feedback resistor

This was the result:

Speaker trace in red

Yeah, I forgot to clip the CH1 probe back on to the input. No matter; it’s the same signal at the same amplitude.

So far so good, this is all textbook from Morgan Jones. However in the course of my experiments I discovered something else that Morgan Jones apparently neglected to mention which I pass on here in the hopes that it may help someone.
Specifically – Jones’ method calls for the feedback resistor to be bypassed by a variable capacitor and resistor, which I did, and I noted that the resultant waveform at the speakers looked pretty much like the above already. 

Thinking I wouldn’t end up needing anything bypassing the anode resistor, I acted on Jones’ recommendation and put a 220nF capacitor across the speaker terminals as a test, and watched the output go absolutely crazy. It looked much worse than the amp with no correction at all, and in fact was on the very edge of falling over into uncontrolled oscillation.

I then decided to bypass the anode resistor in the manner suggested, this resulted in some fine tuning of values as these are all inter-dependent. Eventually I got it to approximately the same level of cleanliness on the output as I’d seen with just the feedback resistor bypassed,

Then I tried the capacitor-across-the-output trick again.

Result: The amplifier barely even noticed the capacitor. A complete fix of the problem 🙂
Conclusion: the anode load resistor bypass doesn’t do much to alter the oscillation into a resistive load, but it makes the amplifier much more stable into a capacitive load.
Morgan Jones did not mention this anywhere I could find.

So for reference this is the circuit diagram of the amplifier now (click to see full size)

The power supply implements the timer circuit which is not shown here for clarity, refer the circuit diagram of the EL84 amplifier on this blog for details on that.

This is the last modification or fine tuning I expect to make to this amplifier.

References
Morgan Jones “Building Valve Amplifiers”, Newnes press, 4th. Edition. pp. 290-291

Shopping begins

So this being the first amplifier I’ve designed and built, there are a few tools I’m going to need.

Firstly, a decent quality voltmeter – better than the cheapie I’m currently using, and most importantly something rated to handle the voltages I’ll be measuring – up to 500V DC and 400V AC.

Secondly – a signal generator. Very necessary for putting test signals into the amp to determine if it’s doing what it should. Normally these things can cost tens to several hundred dollars, depending on how full-featured and robustly built you want them. In my case since I am only interested in audio frequencies, an app on the Android tablet will serve the purpose

Keuwlsoft Function Generator – Free and without advertising.

Next: An oscilloscope. After looking at the options available I’ve decided on a dual-channel “soft” oscilloscope – one that uses a laptop for the control and display.

Instrustar Oscilloscope hardware – the rest of it’s in software.

Finally, I’ve also got a power transformer on the way. After evaluating all of the commercial off-the-shelf products and becoming increasingly dismayed by the cost, I decided to see if a local manufacturer could make me one based on my spec.

Luckily they could, and for about half the price of buying one from an overseas manufacturer. The power transformer is the single largest and heaviest item in the whole project, I expect it to weigh in at around 10-12 kg.

This was the spec I passed to the manufacturer:

Handily drawn on my Surface Pro. I love that computer!

Luckily this was enough of a spec for them and the transformer will be wound and sent to me, expected arrival 2 to 3 weeks.

Next steps: Finish designing the preamp stage. I’ve decided on the topology and just need to draw up the circuit, which involves cursing at the rather primitive diagramming software. I’ll post that up next.