Thursday, 3 January 2019

A tale of two products...

... in which I make myself unwelcome at a high-end hi-fi store.

This is a personal experience which happened over a decade ago, but I decided to put it here because it's still relevant.

These are the two products in question:


Audio Alchemy Digital Transmission Interface, US$ 1600


Generic 10/100 8-port auto-sensing ethernet hub, around US$ 40 – 60 (at the time) depending on brand.


On the day in question, I walked into a hi-fi shop and the proprietor greeted me with a wide smile. This was a real high-end place where the demo systems are set up with speaker cables as thick as fire hoses, and just the rack that the system is sitting on has a five-figure price ticket.

We got talking (this was years before I started designing and building gear myself, I was just looking at buying a new pair of speakers which is what drew me into the shop). Pretty soon he had figured out my system and had decided that it would be improved by the addition of the top product, above.

While extolling its many and varied virtues, he inadvertently tripped himself up by completely inaccurately describing the phenomenon of jitter ... seems he hadn't read up enough on the technical manual that accompanied the product.

After a little while, I attempted to summarise my understanding of this product back to him, to show him I'd been listening. He was all ready to sell me one until I started asking some questions which led me to explain the function of the second pictured device...


Conversation went something like this:

"So, this machine will take a PCM data stream at 1.4 megabits per second (Red Book CD standard), store and buffer it, then output an identical data stream according to its own internal clock, which is carefully designed to be high-accuracy and not susceptible to disruption, correct?"

"Yes, and [long spiel about how that makes it sound better, yada yada]"

"And it's $2200." [That was the $NZ price at the time]

"Yes, possibly the best value enhancement you can make to a digital system....."

"OK, so what then would you say about a device that does this at around 70 times the data rate, and not for one but eight separate inputs?"

"Well the DTI represents the cutting edge of digital transmission design, so perhaps in the future something might be designed that could do what you say, but it would be a very high-end piece of equipment, so only the most serious audiophiles would require it, and that's assuming there'd ever be a digital recording standard that would utilise such a bit rate"

"So you're saying it'd be expensive then?"

"Something with over 500 times the processing capacity of this machine? Very!"

At which point I explained the functionality of the 10/100 ethernet hub, and then its price.

He wasn't smiling any more.

In fact, I got the distinct sense I'd outstayed my welcome in that establishment.


– This is why I don't go into high-end hi-fi stores any more. –

Monday, 17 December 2018

PP Fixed Bias circuit design and calculator

The push-pull design for output stages has persisted since times of antiquity. It was one of the very earliest circuit designs, and has persisted until the present day, with modern solid-state linear amplifiers still overwhelmingly using it.

With tubes, a typical topology is given by the circuit below. (Click to magnify). The anode (plate) voltage on either side comes through the primary of the output transformer.

This design uses a pentode tube, which has a screen grid. This is attached to taps on the output transformer to run in Ultra-Linear mode, increasing efficiency and reducing distortion.

In this design, the cathodes are tied to ground through a very low value shunt resistor. The resistor is simply there to provide a small voltage drop from which the current through the tube can be measured. It plays no other role in the circuit, other than being a fuse if the tube red-plates.

Values of one ohm or ten ohms are typical of this arrangement.


Typical implementation of a Fixed-bias, ultralinear Push-Pull output stage with a pair of pentode tubes (EL84 / 6BQ5)


Because the cathode is at (or very close to) ground potential, this requires the control grid to have a negative DC bias voltage applied to it, to regulate the flow of current through the tube.

If there was no negative bias applied to the tube, it would go into full conduction, the plate would glow red hot, fireworks will happen and that would be Bad, mmkay?

So we need to contrive to feed a constant negative voltage into the grid, along with the signal, to achieve the desired regulation.

The voltage required depends on several factors... as a very rough rule-of-thumb, take the screen voltage and divide it by the tube's mu (gain) to get the maximum negative bias voltage likely to be needed

Looking at the circuit above... An EL84 has a mu of 20 and in this implementation the screen voltage is 350, which gives us 17.5 volts. Multiply by -1 since we're dealing with negative volts. So we're likely to need around -17.5 volts.
In this case, our adjustment range is from -12.5 to -21.5 volts.


How to set up the bias adjustment resistor values

The bias voltage needs to be adjustable. Both tubes need to be drawing the same current, otherwise the net current through the transformer will not be zero, which will lead to magnetisation of the transformer core. This is a most undesirable situation and left unchecked, it will cause quantum fluctuations in the space-time continuum. Well ok maybe not that bad, but the transformer will saturate unevenly and distort the sound.

The usual approach is to use a voltage divider network with a potentiometer, as above.

Couple of points about this design.
  1. The more negative the voltage goes, the lower the current through the tube
  2. In this implementation, if the potentiometer fails, it will fail safe. The most common mode of failure with potentiometers is the wiper lifting off the track. If this happens, effectively the voltage at the grid of the tube will go full negative, reducing the current through the tube to (almost) zero. This is far more desirable than the voltage reaching zero and the tube immediately red-plating.
  3. Expanding on (2) – please don't ever build this circuit with just the pot wiper connected to the grid. When the pot fails (and it will, eventually) it'll likely take the tube with it.
  4. If you're going to build this circuit, it's intuitive to set it up so that clockwise rotation of the pot increases the current through the tube (ie brings the biasing voltage closer to zero)

The next question is - what value resistors will be needed? This is where some trial and error in the calculations is needed. Using Ohms' law, these are the variables:
  • The desired bias voltage adjustment range
  • The input negative voltage from the power supply

From there, you can calculate the values for the resistors and potentiometer to give you the range you need.

This is where a spreadsheet can be mighty useful.


This is a screenshot from a spreadsheet I made that can help with this calculation. You can download this Excel worksheet here.

Put in the numbers in the red. Experiment with the values for R1 and R3 and the potentiometer, until you get the desired voltage range in the "Output V" column.

The "Build-Out R" represents the load seen by the preceding driver or phase-splitter stage, so watch the maximum "Rg-k" from the tube's datasheet isn't exceeded. In the case of an EL84, that value is 300K. 

The columns of this spreadsheet:
  • Step - the setting on the potentiometer
  • Total R - the total resistance from the bias voltage to ground
  • Output V - the negative voltage as fed to the control grid
  • P(R1) - the amount of power dissipated by R1
  • P(R3) - power dissipated by R3
  • P(Pot) - the power dissipated by the potentiometer
  • Pot pwr % - the power dissipated by the pot expressed as a percentage of the pot's total power rating AND the amount of track being used to conduct. 


About Pot pwr %

In the specifications for the potentiometer, there will be a power rating. However that power rating is across the entire length of the track. If the pot is set to half-position (assuming it's a linear taper which in this design it is) then the power handling drops to half.
So, this Pot pwr % column shows how much power the pot is dissipating as a percentage of its maximum considering the wiper position.


Using the circuit

Putting all this together, it's easy to see how it works. Adjust the potentiometer for maximum negative voltage (wiper closest to the left, in this schematic). Power on the circuit and let it stabilize. Measure the voltage across the cathode resistor. Then adjust the potentiometer until the desired current is flowing through the tube.


What is the desired current through the tube?

Glad you asked. This depends on the tube itself, and the B+ voltage, and your preference regarding bias.
As a rule of thumb, around 70% is the sweet spot with most types. If you bias low, that's called "cold" biasing. The current through the tube is low, and the sound may take on a thin, glassy, brittle aesthetic. Also the distortion will increase.
Bias too hot and you'll shorten the life of the tubes.
70% is the goldilocks zone.

So. Look at the tube's datasheet. For an EL84 we see the maximum plate dissipation is 12 watts.
70% of 12 watts is 8.4 watts
B+ is 350 volts so using Ohms' law:

we solve for I at 24mA

Across 10 ohms (cathode resistor) our 24mA will give 0.24V

So we want to see 0.24V across the cathode resistor. Adjust the potentiometer until that's the value shown. Then repeat for the other tube in the circuit, then do a final check that they're both the same (or as close as you can get)


Please feel free to use my spreadsheet - I developed it to assist in choosing the resistor network values, and also to ensure the power rating of the potentiometer wasn't being inadvertently exceeded at any setting.


Feedback in the comments please


Wednesday, 14 November 2018

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:



Click to enlarge if necessary.

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.

Saturday, 28 July 2018

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.

This one went fairly well, however there were a few problems.

First, a few pretty pictures


This is the best looking amp I've made so far. Great care was taken with centring and spacing.



The translucent hole to the left of the volume control covers the IR detector

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


Pic taken with phone under workshop fluorescent lights, sorry for quality!

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:

+350V
+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.


Things that went wrong in this build

1. Phase splitter grid current

I have a continuous improvement philosophy in that each build needs to be better than the last. I don't profess perfection in any of these projects, but provided each shows improvement, I am happy.

In this one, I built it essentially to the same circuit as the previous, but owing to finding a supplier of NOS Soviet valves, I changed the circuit replacing the 12AX7 with 6N2 and the 12AU7 with 6N1.

This is where the problems began. The different characteristics of these valves meant that my original choices of operating points and voltages (I like to DC-couple the cathodyne phase-splitter) were wrong. I thought I knew how to work this stuff out, turns out I didn't, and the results on the scope were disappointing.


This was the kind of nonsense I was seeing on the output from the cathodyne


This was the output from the gain stage (yellow) with input inverted, scaled and superimposed (blue)

The initial gain stage was running into clipping quite badly as it went positive. This was disappointing; it meant that I was inducing grid current into the phase splitter and the gain stage couldn't drive it. If I pulled the phase splitter tube the waveform immediately reverted to an undistorted sinewave. Back to the drawing board to figure out what I'd got wrong.

This is how you learn. After some head scratching and calculating, I arrived at a set of voltages and operating points that - while not eliminating this problem - shifted it beyond the range of signal levels that I would need in this amp. So now at 28V p-p it looks like this


Which no doubt purists will jump all over me for, but I can live with it. We're driving EL84s here, so we don't need huge levels.


2. Gain Deficit

So subbing in the 6N2 in place of the 12AX7 resulted in lower overall gain, despite the two having on paper the same µ

The result of this was that when applying 10dB NFB, it was taking something like 1.0 vrms (2.8v p-p) to drive it to full output. On quieter passages of music this meant that even at full volume, it was too quiet.

In the end, I had to reduce the NFB to around 7dB, a figure which according to some is worthless, the advice I had was to try for 20dB but realistically in this amp it would mean adding another gain stage.

This issue is unresolved in that I've left the NFB at 7dB but even at that level, you have to crank the volume control over more than expected. It makes me think twice about building this design again

On the plus side, it sounds absolutely wonderful, and it leaves me asking myself why I really need to add more NFB. Is it to satisfy some purist urge to get to the holy grail 20dB or what? I need to understand more about NFB - in this amp the bass is solid and tight, not flabby at all, and the sound is the purest of any amp I've built so far (to my uneducated 47 year old ears at least)


3. Low voltage timer delay and LED colour reverser circuit

In each of my amps I use the same 555-based startup delay circuit. It's based off a rectified 6.3 volt using a W02 rectifier which then feeds the 555

The output to the power relay is taken from pin 3 of the chip and also this feeds another small DPDT relay which reverses the polarity of the dual-colour LED. Except that this smaller relay when switched on, buzzes like a bumblebee for the first 4 or 5 sec, and I measured its coil voltage, it's only getting 3V which is weird because the 555 is being powered off the rectified 6.3 volt... somewhere there's a ton of voltage drop happening. 
Measured the positive pin of the rectifier, only 5VDC. That was surprising, I'd have expected 7.5 volt, even allowing for the 1.4volt forward voltage drop. Then there's the voltage drop through the 555.
So somehow I need to re-design this circuit for my next amp so that the relay coils are getting 5 volts. This is currently an unresolved, work-in-progress - I'll leave it like this for this one but re-design the circuit for the next one.




Things that went right in this build

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

Which would again probably prompt some scorn and ridicule from purists, but it represents around a 99.7% improvement, which I am happy with.

Finally, one aspect I am well pleased with is the listening test. Subjectively this is the cleanest sounding amp I have built to date.


Remaining to do

The last steps before I can call this one finished is just to complete the performance measurements. Input sensitivity, Freq response, noise, and THD all need to be measured.

Thereafter, the remaining to-dos are:
  • Fix the Low-voltage 555-based circuit so the relay coils get 5v not 3v (or else just use 3v relays if they exist, that'd be a quick and ugly hack!)
  • Track down and understand the source of the remaining hum in the RIAA stage and modify the next design accordingly
  • Increase the gain so the NFB can be increased
When performance measurements are completed I'll post up the final project page.


Schematic as built

Click to enlarge. Might need to download / Save-As, to be able to read it






Tuesday, 17 July 2018

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.





Saturday, 14 July 2018

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.

Some photos for now.


The amp main board.

 About 10 hours design work went into this at the PC in several sessions. Then another hour on the exposure and etching (hooray for the new Etching Tank!) then about two hours on the drill press (I do this all manually and this one has about 346 holes) then about four hours stuffing and soldering.
  • The right-hand third of this board is the phono/RIAA stage. 
  • Bottom half of the left two-thirds is the Active tone controls
  • Top half of the left two-thirds is the gain and phase splitter stages



Flip side

The tubes are at 50mm spacing. As you can see it's a single-sided board. The unfilled holes at the bottom left (in this view) are for the phase correction in the NFB loop. These component values will be determined by experimentation



The total size of this board is 160 X 100mm (or 4" X 6.25" if you insist)


I'm using Soviet military NOS surplus tubes which I found a supply of. So this is configured for 6N1 / 6N2 tubes.


We also have a power supply. This one is a bit of a squeeze because I had a 100 X 100mm cutoff bit of PCB which I thought of using. Ideally it should be bigger but I didn't feel like cutting anything.

The supply contains my usual start-up delay (that's the IC and relay you can see) where the AC power to the rectifier diodes is switched on after about 25 sec, to give the tubes warm-up time. I've also got my usual 2-colour LED driver (it reverses the polarity to the LED when the B+ power comes on, turning the LED from Red to Green. I designed this in the last amp and I liked it, so I bought a few 2-colour LEDs and this can now be a permanent feature in my designs)



Touch the capacitor with blue writing and you'll jump across the room. These are 2 caps of 470µF in series (since they're only rated 250V and my B+ is 330). They have Balancing Resistors.



The cap lying down is too high to fit into the case upright. It's 47000µF at 10V, it's for the DC heater supply for the phono stage. The top right semiconductors are the schottky rectifiers for it (since I'm using the 5V secondary for this, low voltage drop diodes will keep my filament supply within voltage spec)


Loving how clean the tracks are using the new etching tank




Finally there's the chassis. The drilling and punching was done by hand on the top panel, since this is a one-off the setup time on the CNC would have been not worth it. If I am gonna make 5 of these amps then I'll CNC it.

The front panel is adhesive vinyl, laser-cut then peeled, and then there's three coats of clear lacquer to protect the vinyl letters.


Not all of the controls are installed yet.


This one will have a motor-driven remote controlled volume and the input selector is a Rotary Encoder which will cycle through the inputs, with indicator lights to show the selection. The knob will also push to turn the main power on. This is also accessible through the remote control, the perspex window for which is to the left of the volume control.


Around the back

The back panel, nothing especially amazing here. The lettering is laser-etched into the aluminium.

The transformers will be Hammond. 370FX for power and 1650E for OPTs.

More photos as the build progresses.



Thursday, 5 July 2018

Measure twice, cut once

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.

More entries as this build progresses...