Saturday, 16 December 2017

EL84 Amp II: progress post 1

Progress is happening with the new amplifier... this design is more modular as I have decided to design standard boards for tone controls, headphone output, and phono RIAA.
Having standard boards for these means I can easily accommodate future builds, and provide a "menu" of sorts.

The process has not been entirely smooth sailing, owing to the somewhat hit-and-miss nature of home PCB fabrication. Until now, my method has been to print the PCB design onto an iron-on transfer which then gets pressed onto the board (and then touched up with the etch resist pen) before going into the etchant.

This process has been unreliable and time consuming, and expensive, owing to the high reject rate. So a new technique was called for.

I've decided to move to a photosensitive board workflow. The design is printed onto transparency, which is then plaed over a light-sensitive board and exposed under a UV light, thereafter a two-step chemical process: Developing then Etching.

The first board I designed for this project is the tone control. This is using the same circuit as the previous project, except I had two changes:
  1. I needed to reduce the size of the board 
  2. I needed to put the tubes on the copper side of the board
So, I re-designed it to be 120mm X 65mm (down from 150 X 75) and attempted to fabricate the board... with less than spectacular results


Yeah. Not enough light. 


This board failed because I did not expose the photosensitive layer sufficiently.

Lesson learned, I did a second attempt, which looks much better. So I went ahead and drilled and stuffed it.

Result:

There are four topside wire links on this board. I always try to design with as few of those as possible. It's a challenge!



The copper looks a bit more messy than I'd like because the balance of exposure and development and etching was still not quite right, but this board is usable at least.


The tubes are on the copper side because of the customer's preferred aesthetic of having the tubes visible. This design will be applied to the other boards in this amp as well.

In the process, I have become a lot more familiar with the operation of my PCB software: namely DesignSpark from RS. Also its quirks and foibles, such as less-than-ideal behaviour when moving things around, and its ability to have "invisible" track that isn't visible in design but is when you print. As a result of this, the board above needs to have one track cut with the dremel and re-routed with a short jumper on the track side. Yeah I hate doing that!

Lesson: Inspect the board VERY carefully in print-preview before fabrication.

Or, to use an appropriate engineering axiom: Measure Twice, Cut Once!

I also built the bias boards for the EL84s. Owing to the amount of heat these produce, I am not mounting them on boards, but the voltage divider and potentiometers for the negative bias voltage, and the cathode shunt, can be put on a board. So drawing on my earlier design, these are the bias boards, made using the same technique:



Next up: A two-triode RIAA stage, I'm planning this on a board on 100 X 65mm.

There's a reason I want these boards as small as I can get them: The size of the chassis


Internal Dimensions 300 X 225mm

This chassis is going to represent a challenge to fit everything into it... this design will have 13 tubes: The RIAA stage, tone controls, headphone stage, as well as the amplifier itself. And size is a consideration since it will be packed up and sent overseas when it's finished.

Next update when I have more boards to show...

Monday, 4 December 2017

New project: Another EL84 Amp

Following an approach from a new customer, a new design has emerged...
This customer had a well-defined set of requirements:
  • Usage situation dictated an EL84 PP design would be suitable
  • MM Phono peamp required
  • Line-level inputs required
  • Tone controls required
  • Headphone output required
  • Remote control volume adjustment required
Fairly rapidly I decided this amp could be based on the previous EL84 amp I made at the start of the year, with some additions.

Power Transformer

Firstly, I intend using an off-the-shelf power transformer. The custom-wound transformers are handy, but they're an industrial product, and as such the aesthetics in their design limit their use in a piece of equipment where they are going to be on display. Sadly the manufacturer was unwilling to work with me on this aspect, so my transformers will have to come from Canada now, instead of being locally made.

The power transformer I selected for this job is the Hammond 370FX. 172mA at 275v, 3A at 5v and 5A at 6.3v, with a 50v bias tap. Everything I need.


Tone Control

The previous tone control worked well enough for it to be included in this project without modification. Except I'll redesign the circuit board.


Headphone Stage

This customer was adamant this amplifier have a headphone socket. This was a non-negotiable requirement since their musical taste is not shared with other members of their household. This is provided by an ECC99 SRPP-based OTL design borrowed from the internet. It simulates well in LTSpice down to 32 Ohm headhones, and will drive into 16Ohms as well, although with greater distortion and a lower level.


Printed Circuit Boards

This amplifier will be designed on several PCBs:
  • Power Supply
  • Phono Stage
  • Tone Control
  • Headphone stage
  • Bias adjusters for EL84s
The circuit boards will differ from the previous tone control board in that the tube sockets will be on the opposite side to the discrete components, to facilitate the boards being mounted upside-down in the chassis, allowing the tubes to rise from the top of the chassis as in a point-to-point design.

The EL84s will be chassis-mounted, as in the previous EL84 design on this site.


First stage of development, we have a circuit.

Full circuit. Click to enlarge, right-click to download.


By way of explanation:

V1 + V2 are the MM cartridge phono amplifier stage. RIAA equalisation is given by the RC network giving NFB to the stage

V3 is a Cathode Follower, necessary because the preceding phono stage has a high output impedance, and also to provide additional current capability to any line-level signals at the input, to drive the tone stage.

V4 provides around 20dB gain to compensate for the losses in the tone stage, restoring the entire stage to unity gain. This is the same circuit as the previous "Tone Control" project on this site

V5 is the gain stage for the amplifier proper
V6 is the concertina phase splitter. This needs an elevated heater.

This stage encompassing V5 and V6 is borrowed from the Fisher X-100.

V7 and V8 are the PP output stage with the EL84s, running in Ultralinear configuration into Hammond 1650E output transformers. Fixed bias is employed with the cathode resistors providing the reference voltage for adjustment.

V9 is the gain stage for the headhone amplifier, V10 and V11 the SRPP current driver stage to power the headhone output.

The power supply will incorporate the same 30-sec startup delay on the B+ as the previous amplifier projects on this site.


Owing to the current capability of the low-voltage secondaries, we have a split, with some of the tubes receiving DC heater voltage and others receiving an elevated AC.


Parts are ordered, next stage is PCB design. to be continued....

Wednesday, 29 November 2017

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


All assembled and in service


Full frontal. Yeah a bit of a Star Trek vibe in the labelling. The LED is just a power indicator


The transparent acrylic top gives a nice view of the insides.

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 the same very important SAF* Modifier as the integrated amp: 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


* SAF := Spousal Acceptance Factor

Thursday, 9 November 2017

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.

Tuesday, 7 November 2017

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...

Saturday, 28 October 2017

KT88 Amp Freq Response measurements

With the big KT88 amplifier nearing one year old, it occurred to me that I had never performed a frequency response measurement on the whole amplifier.... I'd done plenty of them when I was prototyping the preamp and driver stages, but I hadn't done anything other than cursory checks with the oscilloscope over the entire amplifier.

Today I set about rectifying that omission. My method was simple. Move the amplifier to my (newly tidied) workbench, attach the function generator to the input, dummy load to the output, set up a sine sweep from 50Hz to 50hKz at 200mV RMS, open the taps until the amp is producing about 5v RMS on the output, then start the FFT process.

This resulted in the following:




So the yellow trace reveals that the input signal is not exactly flat... about 1dB down by 30kHz. This is why I showed the input signal, for a reference.

The blue plot is the speaker terminals. the load is an 8 Ohm carbon resistor on a heatsink.

So what does this tell us? I read this as the amplifier being reasonably flat (within 1dB) to 40kHz whereafter it drops rather quickly. 

My 'scope does have a "Math" facility where I can supposedly reference one signal against the other. This functionality seems buggy however, which is why I avoided it.

I am happy with this frequency response.

Monday, 16 October 2017

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.)

Saturday, 30 September 2017

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

Thursday, 28 September 2017

Philosophy

Until the late '60s, vacuum tube/valve amplifiers were not an expensive or exclusive option, because there was no alternative. They were mass-produced, margins were slim, and they were manufactured to a price. Quality was variable and short-cuts were taken in the interests of cost-containment.

In the late '60s the first solid-state (=transistor-based) amplifiers became available. The increased reliability, smaller size, and greater power capabilities of the new gear quickly won favour and by the late 70s it looked like valve gear was relegated to history.

However, In the 1980s there began a resurgence in interest in valve gear, driven partly by nostalgia and partly because of the lingering dismay felt by audiophiles in particular at the sound of the early transistor amplifiers, which to be charitable, sounded bloody awful.

So the concept of valve amplification as a high-end audiophile alternative began to take root. And then it found a market and grew.... exponentially.

Fast-forward to 2017 and we now have hysterically over-engineered behemoths from Kronzilla and the like, using ridiculously overpriced tubes like the absolutely monstrous T1610. Try buying these, a snip at €3500 a pair.

Complementing this is the hi-fi cable industry. It's possible to spend €9000 on a 1.5 metre power cord. Interconnect cables and speaker wire runs to similar prices.

All of these excesses are targeted at one thing only: Relieving audiophiles of their money as quickly as possible. Nothing more.

The claimed benefits from the use of audiophile-grade cables and components are entirely quoted in pseudoscience terminology. There is no human alive who could discern the difference between a standard IEC power cord and something that costs more than most people earn in a month.

They are merely a very expensive placebo.

This point is worth exploring further because it's quite a frequent occurrence that someone will buy some high-quality gear, hook it up with ridiculously expensive cable, then credit the cable with the beautiful sound they experience, instead of the audio components. This is highly insulting to the engineers and designers who designed the equipment. It is somewhat like the religious family with a child sick with a life-threatening condition which they take to hospital, and the medical staff work tirelessly to help, which is ultimately successful and the child recovers. Then the family publicly thank their god for the child's recovery instead of the medical staff.

I make a parallel with religion very deliberately; because belief in the purported benefits of hyper-expensive cables has much in common with it. 

Compounding this delusion is my personal view that an audiophile derives 80% of the pleasure from his equipment in bragging to others how much it cost him, and only about 20% actually enjoying the sound.

My use of the gender-specific pronoun is deliberate; I don't know any women who would be this misguided or frivoloous.

In fact, I suggest that Hi-Fi boutiques should not display prices on their equipment. Instead, as you walk in the door, the assistant asks "How much would Sir like to brag his system cost to his friends?" - you say $100,000 please - so the salesman puts together a system and charges you $100k.

The equation probably goes something like this:

Cost of manufacture of equipment: $3000
Cost of Sale: $2000
Rights to brag that your system cost 100K: $95000


Unsurprisingly, I utterly reject this form of thinking. I enjoy the sound quality from a well designed and constructed amplifier, and I enjoy the design and build process. But I dismiss all the pseudoscience.

With the amplifiers I build and provide to others, in the accompanying instruction leaflet, I include the following statement:


Monday, 17 July 2017

Fine tuning the big amplifier

So I've lived with the big KT88 amplifier for the last few months and have been mostly happy with it, though there were 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

Wednesday, 17 May 2017

Last piece of the EL84 Amp design

So the circuit sketches and various measurements on assorted bits of paper have finally been consolidated into a coherent schematic... thanks in no small part to an incredibly patient and supportive wife who endured being an electronics widow for one final evening on this project!

The schematic is "as built" and there is one part I am less than happy with and if this amp is ever back on my workbench it'll get fixed: I am not happy with the resistance of R3 and the resulting voltage on the low side of it.

This leads to the voltage on the anodes of the RIAA stage for the second valve and the cathode follower. A little low, but notwithstanding, it passes the listening test with flying colours. 

But if it's ever back on my bench, R3 is getting swapped out for a 27K quick-smart.

Apart from that, you can see the implementation of the 555-based delayed HT switch-on circuit, the DC heaters for the first two valves of the RIAA stage, and the sneaky re-use of that voltage for the bias for the EL84s, and the 270K / 68K Voltage Divider for elevating the 12.6V heaters - the cathode follower needs it.

The Gain and phase splitter stages (V4 / V5) are based on the Fisher X100, but with higher quiescent current on the 12AU7.

This is the circuit. You'll need to click it to see full size and download / print / zoom / scroll, or whatever.




You'll probably need to download this to see it clearly

Monday, 15 May 2017

EL84 amp completed

So the EL84 amp is complete and has been delivered to its new owner, who has compared it favourably to his 60wpc Harmon Kardon Solid State amplifier.

The aesthetics of this one are much more favourable than the previous build. In testament to this, the new owner reports a high Spousal Acceptance Factor :) 

Looking good next to the turntable

In the end the chassis was about 5mm too narrow to fit the power transformer and the output transformers across the back in the usual configuration. So it had to be non-symmetrical


Making a virtue out of necessity: The non-symmetrical theme carried through from the transformers to the placement of the valves, and the controls on the front panel.



The front three small-signal valves form the RIAA Phono stage, the rear two are the line-level voltage gain and phase splitter.

The Bias test points sit between the EL84 output valves, with recessed trimmers to adjust, and test points to measure the voltage across the 10Ohm cathode shunt.
The Slovak-made JJ EL84 output valves on this one have quite a pleasing amount of light-leakage from the filaments and cathodes. Unlike the Russian Electro-Harmonix small-signal valves which are hard to see any filament glow from at all


Inside, the amp is crowded. Point to point wiring inside a tight working space.

Polyethylene Film capacitors are used for inter-stage coupling, and also in the RIAA stage, which as at top left in this photo. The power supply board sits under the output transformers, The power supply board has my usual 555 timer-based circuit to delay the B+ turn-on be 30sec giving the valves plenty of time to warm up first. By use of this circuit, combined with heater elevation for the small-signal valves, I can get away with avoiding the diode on the cathode follower in the RIAA stage, which is DC-coupled to the previous gain stage.

6 PCBs inside this case, including 5 home-made ones

Some specs and tech details

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 370V Plate Voltage.
Valve complement
Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
50 KOhms
Input Sensitivity
300mV rms for rated power
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal


RIAA Phono Preamp

Topology
Phono-level amplifier, cascaded grounded cathode gain stage, DC-coupled cathode follower, RIAA equalisation
Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Valve complement
Gain stage: 2 X 12AX7 (ECC83)
Cathode Follower: 1 X 12AU7 (ECC82)


Listening tests

The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the 12AX7 gain and 12AU7 phase splitter, though this design runs that 12AU7 closer to its 5mA sweet spot for linearity from a 300V B+ than the Fisher does.

The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)

The RIAA stage works well. The Cathode follower is needed to drive the volume control which represents a 50K load across the input. The noise floor is low, hum is non-existent, and distortion does not occur even on the loudest passages. 


Lessons learned

During the construction of this amp several lessons were learned...

1) Hum was a constant problem
The amp was built backwards, with the output stages and transformers being wired up first, powered on to test, then the preceding stage, right back to the RIAA stage.

The 12AU7 Phase Splitter was putting a nasty hum into the output. After chasing that down and much testing with the oscilloscope etc, it was determined that the hum was on the anode but not the cathode. Many solutions to this common problem are available on the internet, in the end I opted for an additional level of decoupling in the power supply with a 10K resistor and 220µF capacitor, this fixed the problem. The lesson is that Phase Splitters have NO PSRR on the anode side. That power needs to have no trace of ripple on it


2) Grounding needs close attention in a RIAA stage
You can read as many books as you like but it's only when you build an RIAA stage that you truly get to appreciate how to ground the incoming signal... and how not to. This one had a nasty hum which was coming in through the Earth side, it would only manifest when there was a source plugged into the phono input. If the jacks were empty, the stage did not hum. But plug any source in, the hum appeared ... after bridging the input with a 1K2 resistor to simulate the cartridge, it was observed the hum was injecting into the live from the earth through the source. 
After moving all the signal Earth to a common point - which was the star earth off the first valve in the phono stage - suddenly it went dead quiet.


3) Take care with design and placement
There were a few too many near-misses with things fitting much tighter than planned, or almost overlapping other parts, 

There are also some minor changes I'll be making to the design of my bias boards and power supply boards for the next build, which is not currently planned.

Next post... I'll put up the as-implemented circuit schematic.

Monday, 24 April 2017

Some progress on the EL84 amp

The EL84 amp is well underway now. First of all, the name:

Iwa Orotuanaki Wairua

This is is the Maori language (explanation for people outside New Zealand: The Maori are the indigenous people of New Zealand).

It means (approximately) "Nine Echoes" or more accurately "Nine Sound Spirits"

Why this name? Nine because it has nine valves (tubes) and it's going to a friend who is involved in paranormal investigations

Progress

So the case has been laser-cut and etched, the front and rear panels have been filled, seats added to the top:





A slight error with the laser etching means we seem to have two sets of "Speakers A" ... oh well, it adds character

Circuit boards made up – the power supply shown in the previous post, plus the relay board for the speaker switcher, the feedback filter board for the Phono stage, and the bias adjusters for the output stage:


Feedback/RC Filter board for the Phono stage
Relay board for the speaker switch (Speakers A/B, 4Ohm and 8Ohm)



The Bias adjust boards, already mounted up. Yes it does say "Star Wars" inside the amp; this was a test of the laser etching before potentially messing up the top!


Construction


Bias boards mounted, tag strips, power supply and feedback board... starting to see how much of a squeeze this build is gonna be!


Wiring up the low voltage parts....



So that we can do this!

More later.