All posts by ATRAD Audio

A better time delay startup circuit

The amplifiers I’ve built so far have all incorporated a delayed turn-on circuit for the high voltage supply. The intention is to allow the 6 volt supply to turn on first and allow the valves to reach operating temperature before turning on the high voltage supply.

This is accomplished with a simple circuit based around a 555 timer IC in monostable mode, set up to a delay of around 25 seconds.

The circuit I’ve been using, while functioning, had a few problems. Driving a relay directly from the output of a 555 IC resulted in a lot of voltage drop through the IC and the relay coil voltage being low, for a 5V relay it was getting around 3.5 volts, fortunately this is still enough to trigger it, but less than ideal.

My re-design of the circuit was prompted by my addition of a 2-colour LED to the design, to glow red at initial turn-on but change to green when the timer activates and the HT voltage is turned on.

These LEDs are 2-pin, they work by reversing the polarity into them. So they’re 2 LEDs in one envelope, and depending on the polarity of the applied voltage, one will be forward biased and glowing, the other reverse biased and dark.

After breadboarding it and measuring carefully, this is the circuit I designed:

Note in this diagram my symbol library for the MOSFET is wrong… if you’re gonna use this same MOSFET be very aware its pinout (viewed from top) is S-G-D instead of G-D-S. So my pin numbers are wrong. Sorry about that.

The 7805 voltage regulator is not strictly necessary but it does result in a nice 4.9V across the relay coil.

The 330K and 68µF cap provide the time constant for the timer IC. The formula in this mode is:

T = 1.1 x R x C

The MOSFET Q1 buffers the output of the IC switching the negative on or off to the relay based on the voltage at the gate, which comes from the output of the IC at pin 3. This starts low until 25sec elapses then goes high and stays high until power down.

The two 330R resistors form a voltage divider, at the mid-point the voltage is 2.5V. When the relay is off, the + voltage will flow through the coil (which is around 62 Ohms) and then into the LED, then to ground through the lower 330R resistor. This results in a voltage drop of around 0.2 volts across the relay coil, not enough to turn it on.

When the IC turns on, the voltage appears at the gate of the MOSFET, switching the transistor on. This effectively shorts the Drain and Source, causing the negative to connect to the relay and the LED. At which point the return path for the LED is through the top 330R resistor, so this reverses the polarity across the LED causing it to change colour.

The reverse-biased diode across the relay is for flyback suppression.

After breadboarding, I’ve designed a single-layer PCB layout for this circuit which is 35mm x 35mm utilising a W02 rectifier.

On my board design I’ve also added a header for a regulated 5V power supply, in case it’s needed elsewhere (such as a tone control bypass relay for example).

The current and dissipation is such that no heatsinks are necessary on either the voltage regulator or MOSFET.

Be sure to put the relay on the AC side of the rectifier diodes, relays have a much easier time switching AC than DC and this is reflected in the voltage rating on the datasheet.

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.

Advice comes at a cost

This post is a bit of a rant, and also a warning to those embarking on this craft and seeking the advice of experienced or expert designers and builders.


No pictures in this one sorry.


I’ve debated whether to post this for a while, but recent events have compelled me to.
When I started this blog, I was completely new to designing and building amplifiers and valve gear in general. I was delighted to see all of the resources available on the internet, and I joined one or two of the more popular forums. After sitting and watching for a while, and reading as much as I could, I started posting up a few questions, and a couple of schematics I’d designed, to get some input and opinion from the wise and experienced folks.


The input and opinion I got was not quite what I was expecting or hoping for. In my mind I’d imagined that the experienced folks would be tolerant of – or even welcoming – to the newbie, and take time to give explanations or point to resources to further my understanding.


Instead I was the recipient of sarcasm, scorn and ridicule. Both on the boards, and in private messages. It became quickly apparent to me that the prevailing attitude seemed to be that unless you know all of the common topologies by heart, you have no business even picking up a soldering iron. 


My particular approach has been that I don’t want to just find a schematic and build it, I want to understand how it works. I’ll only build something I can describe the working of to another person. So I’m gonna ask questions… that’s how you learn.
Besides the condescending remarks, another thing I had to contend with was opinion stated as fact. Some examples:

  • “Hammond Sucks. Edcor all the way”
  • “No audio circuit has any business using the 12AU7, it’s so non-linear.”

So one of the first skills I had to pick up was the ability to discern fact from strongly-held and expressed beliefs.


The next problem I encountered was a peculiar way of offering recommendations. The most recent example was concerning the use of a Constant-Current Source for preamp tubes. This particular recommendation was given to me in an email by another old-timer in a way that implied that any amplifier without a CCS is some kind of useless toy. When I questioned this, my question was taken as a challenge, and I received an insulting and profanity-laden email in return.


Here’s the thing, though. If someone tells me I need a CCS – or any other such recommendation – they should expect me to ask why. This is not to challenge or disagree – but rather because I want to know the reasoning. I need to know if this is another opinion-stated-as-fact, or whether there is some basis for the recommendation. I want to know:

  • Why would I need a CCS?
  • What problem does it solve?
  • How bad is that problem?

This helps me build understanding and further my knowledge. I did not profess to be an expert in this area – it remains a hobby which I fit around a career and a family. I do strive to learn something from each project, and make each one better than the last.


To that effect, I have made a decision which I should have made back in 2016 and this is the reason for this longwinded post. From now on, I am receiving my knowledge from books, or the small number of personal sources I trust, and I recommend anyone else starting out do likewise. 


Either that or develop a thick skin against the attitude you’re likely to encounter.
For my part, if anyone asks me for my knowledge, I’ll happily share it without condescension, such as it is.

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.)

Amplifier Horoscope eCommerce Service

Today I am pleased to announce my latest offering to the audiophile community.

  • Are you looking for a new way to give your system that “edge”?
  • Do you already have the best cables connecting your system?
  • Are your CD players already on isolating feet? 
  • Have you already traced the edges of your CDs with the special pen? 
  • Does your turntable weigh more than a small car already?

Have you experienced the crushing disappointment when you describe all the audiophile tweaks you’ve made, only to find someone else who has already done exactly the same?

Here at ATR Audio Designs, we feel you. The frustration is real. If only there was something more you could do, to give your system an extra advantage.

Well now, there is. At least for those of you with valve/vacuum-tube based amplifiers, that is.

For the first time ever, we have brought the benefits of the ancient art of astrology to the audiophile community.

Using a patented process combining astrology with hysterio-magnetic resonance analysis and flux emission tomographic spectral emissivity coupling, we can now offer horoscopes for individual amplifier components with unprecedented accuracy.
In simple terms, here’s how it works:

  1. Sign into our website and create an account for yourself (or use your Google or Facebook ID)
  2. 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
  3. Using a compass, determine the magnetic heading your amplifier is pointing to
  4. Input this information into our site 

That’s all it takes! Our patented algorithms will get right to work, calculating the optimum time to listen to your amplifier, based on the manufacture date of the tubes and the influence of the Earth’s magnetic field at your given location, in the electron emissions inside the tube.

We will then produce a report which amounts to a horoscope for your amplifier. Complete with predictions for the next 3 months of which days you can expect the best results from your amplifier.

You will also have the option of subscribing to our site, this will add you to our mailing list and every three months we will automatically send you another horoscope as long as your subscription is active. 

Price list

Amplifier Type: Single Ended

Output tube type: 6550, KT66, KT88, 6L6GC, EL34 etc:
Initial Report: $US 1995.00
Monthly Subscription: $US295

Output Tube Type: 300B
Initial Report: $US4995.00
Monthly Subscription: $US495

Output Tube Type: T1610
Initial Report: $US19,995.00
Monthly Subscription: $US1995.00

Amplifier Type: Push-pull
Output tube type: EL84, 6550, KT66, KT88, 6L6GC, EL34 etc
Initial report: $US99.00
Monthly Subscription: Please make a donation to the Onion or Clickhole