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.
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.
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.
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
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
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.
This tone control is working exactly as anticipated, and is not far from the predictions on the source site.
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:
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.)
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Sign into our website and create an account for yourself (or use your Google or Facebook ID)
With your amplifier cool, unplug the valves/tubes one-by-one, and make a note of the tube type, manufacturer, and the date stamp or code on the tube
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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.
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:
This was the result:
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
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
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.