Saturday, 24 March 2018

EL84 Amp II: Progress Post 6 – Listening Tests

With electronic construction complete the amplifier has been moved from the workbench to the listening room to get some initial listening impressions. 

The speakers it's running into are floorstanding KEF C95s from around 1990. Efficiency is quoted at 90dB. These are sealed bandpass.

The amp has 3 line inputs and one phono. So the source used for listening tests was the Chromecast Audio and my turntable with its Audio-Technica AT11 MM cartridge.

First up - the line input. An easy test. Passed with flying colours. The measurements on the workbench were showing around 15 watts power into the 8Ohm dummy load before any sign of clipping, and the THD was reading less than 1% rising as the power level approached maximum. So expected behaviour, in other words.

The EL84s are biased to about 8 watts dissipation, which at the B+ of 365V means 22mA. The cathode shunt I am using is 10Ohm so that's a reading of 0.22V at each cathode test point.


EL84s with bias adjusters

The topology of this amp is that the first thing the line-level input signals encounter is the Cathode Follower which forms the first stage of the tone control. This means the input impedance is around half a meg. So a very easy load to drive. Most amplifiers route the input signal to the top of the volume control pot, so the input impedance is effectively the track resistance of that pot, usually 50 or 100K.



Hum levels are acceptable. There is a barely detectable hum if pressing the ear to the front of the speaker cabinet when no music is playing. However since I don't know anyone who would use the amplifier in this manner, I'm not going to allow myself to get too worried about it.

Sound quality assessed subjectively is clean, pleasantly detailed, no trace of any distortion or harshness in the treble, and with plenty of power to the bass. I would be happy having this sound quality as my daily driver.

The tone control behaved exactly as the previous build of this circuit, this is the second time I've built this one.

The phono stage however was not such a stellar performer.

The only problem with this one is an unacceptable level of hum. This is normal with phono stages and simply indicates a signal grounding problem somewhere.

Other than the hum, the RIAA stage sounded well. So I am confident some simple experimentation with the earthing will resolve the hum.

I tested the RIAA compliance on the workbench and found it to be within 1dB from 45Hz to 15KHz. This is acceptable.



One aspect of this build I am less happy with is the power transformer.

This is a Hammond 370FX and it's loaded to its rate current. Around 140mA DC load on the B+ and around 5A load on the 6.3v

However this transformer runs hot. After 30 mins run time it's noticeably warm and after 60 mins you can touch it but if you wrap your hand around it, you'll want to remove it after 3 or 4 seconds.

I found a few discussions on DIYAudio commenting about the heat output of these transformers, and to be honest it makes me reluctant to use them again, either that or I'll be sure to apply a generous de-rating margin next time.

Cosmetically I still have some work to do





The person who I am building the amplifier for has indicated his dismay at the power switch. Something with a chrome toggle is preferred. Finding something that will a) fit without fouling the adjacent EL84 socket and b) fit the existing 20mm hole, is going to be a challenge.

Secondly, on the right hand side, the hole in the front is for the remote control sensor. This needs to be permanently fixed in place internally and a piece of translucent white acrylic press-fitted into the hole.

Electronically I need also to arrange a mute circuit to avoid switching spikes going through to the speakers when toggling the speakers/headphones switch, and to avoid the open-circuit hum going through to the speakers when the switch is set to headphone (thus disconnecting the input to the preamp)

First order of business though: cure the ground-loop hum on the phono stage...

Saturday, 3 March 2018

EL84 Amp II: Progress Post 5 – Confessions!

It's been a little while since I posted an update on this project, and there's been a lot of progress, as well as one or two hiccups.

Thought I'd put up a few photos today since I've been taking plenty.

First up - I've had a few requests from people (offline, as well as on) for some photos of the PCB fabrication process. Since I took photos during the etching process of the power supply PCB, I submit for the admiration of sev'ral viewers [anyone get the obscure reference?] the PCB etching process in stages...


Etching is just starting



Copper exposed to the UV light is being slowly etched away


A few more minutes and it's nearly done


Finished! Ready for washing and drilling


The completed Power Supply board was already shown in the previous post, so these photos are a trip back in time *by popular request*

I did make up another little board though, by necessity. The motorised remote control volume control board I bought from AliExpress didn't work, so I had to buy another one, only this was a different type, and of course it needed a different voltage, that I didn't have to hand. 

I needed 9V DC for that board, which I also needed for the signal relay which switches the signal to the headphone stage. So necessity being the mother of invention, this was the result


5vac in - 9V DC out. 

A voltage doubler and regulator board. A couple of diodes, some capacitors, and a 7809 regulator. Plus a switch (on the other side) and on a board that's smaller than a SD card. 

So – on with the amplifier. 

First order of business was to get the top panel of the chassis ready. This involved a lot of measuring and drilling - the mounting holes for the boards and transformers, then the chassis punch for the valve sockets. A lot of swarf ended up on the floor during this process.



After getting the top panel ready, it needed to go to the laser etching workshop before I could do anything with it. This is to get the identifiers for the valves etched on - this design uses four different types, so it's important to know which type goes where!

Once that was back, it was time to begin assembly. Mounting up the transformers and sockets to the top, and circuit boards underneath. A delightful jigsaw puzzle, but everything fit together nicely and it was not necessary to utter any curses.


Transformers and output valve sockets in place



Starting to assemble the business end



All boards in place, ready for wiring up


Next the back panel needed drilling - this design will have four sets of inputs – three line-level and one phono, with the necessary separate earthing point. Also the speaker terminals will expose 4Ohm and 8Ohm outputs, and of course the standard IEC Mains connector.

Everything was mounted onto the back panel, just to make sure it all looked OK and didn't foul anything inside the case when in place (it didn't, so again, no cursing necessary!)

Then it was a case of removing all the terminals and sending the naked panel off to the laser engraver to get the descriptions and other vital pieces of information added to it.


The back panel, before laser engraving


Then, just because it would be remiss not to, it was time for a photo session


Front panel legend - this is a valid design technique and don't let anyone tell you otherwise!






Showing the bias adjusters and test points for the output valves (same design as the last EL84 amp I made)


So at the top of this post I mentioned one or two hiccups. This firmly comes under the category of "learning from mistakes". Those who are more experienced at this may choose to laugh at my misery if they are of a vindictive nature, or sympathise if they are more empathetic... but I screwed up the low-voltage side of this amplifier rather badly and it's going to need a rather ugly (and obvious) rescue.

I will disclose my thinking and why it didn't work here, in the hopes it might help someone.

Warning: There are no more pictures, and it gets a bit technical from here on.

So going by the previous pictures you will see there is a lot of glass here – 13 valves to be exact. This is because of the configuration - a RIAA stage, Tone control, headphone stage, and push-pull output.

Long story short, the amount of 6.3 volt needed exceeded the rating of the transformer. Plus, in a RIAA stage, it is preferable to run the heaters on DC. Handily, the transformer I am using (Hammond 370FX) has a 5v winding. So I thought I could run the RIAA stage off an arrangement like this - the capacitor is 47000µF













Then, to relieve the load on the 6.3v I thought it might be possible to run 3 more filaments from this arrangement, for a total of 5 12A*7 tubes running on DC heaters.

Sadly this arrangement was not suitable – the DC voltage dropped to 5.5V which is too low to run filaments on, plus that diode was running rather warm, to make an understatement.

If I removed all load from the DC except the RIAA stage, the DC voltage was closer to 6.1 which is tolerable. But this leaves me with the following problem:












The transformer is rated at 5A on the 6.3V winding. So, my original plan was to use the DC to take about 900mA of that load. Alas this plan did not work so now I need to find around 1000mA of 6.3v ac from somewhere.

Worse, I also discovered that the tracks on the PCB I'd set up for the 6.3vac were not thick enough for the 5A load. They were dropping around 0.44v which at 5A equates to 2.2 watts of heat.

Having PCB tracks dissipating 2.2 watts of heat is a Very Bad Thing.

Clearly some thought and remedial action required!

The initial idea - swiftly dismissed - was to re-make the PCB with larger tracks. However in the end I opted to rework the connection to the EL84s so that they don't go through the PCB. That will save the board from burning up.

Only problem is that it will look ugly. It will work fine but I am not pleased.

Second problem. Where's that extra 1000mA of 6.3v going to come from?

Only one possible solution. A second, smaller, filament transformer.

As luck will have it, a transformer rated at 6.3v 1000mA was sitting in my box of spare parts. And with a small amount of re-shuffling, will fit inside the chassis.

However. It will add weight, and it will forever bear testimony to the error made in the calculation. So I am doubly not pleased.

However, the happy ending to this awful debacle is that it doesn't set the schedule back too far, and the amplifier will work completely as intended, and I do not need to re-make the board. So not a total disaster.

But definitely some lessons learned for the next project.

Here endeth the confession. 

Ending on a good note

Before the Great Filament Supply Disaster of 2018, I had the output stage and the amp board running and the B+ and other HT voltages were exactly where I planned them to be (so my high voltage design is fine, just the low voltage stuff I messed up!) and the amp was running with a signal source (=old iPod) connected, through a small pair of speakers. It sounded great, and the quick measurements through the oscilloscope with the function generator showed the response and power exactly where they were supposed to be. And there was no hum!

More later when the panels are back and more building is completed...

Sunday, 11 February 2018

EL84 Amp II: Progress Post 4

A bit more activity on the amplifier as time permits, and a few trials and errors later... we have a power supply board.

In the previous post, I showed the design. I made the board, using my new hand-made UV exposure box with 120 UV LEDs in it, and after etching and drilling, began to build the circuit.

I began with the low-voltage parts: the delay switch-on and LED colour reverser.

Long story short, it didn't work. Due to two errors on my part.
  1. I'd omitted the reverse-biased diode across the output of the 555 IC (which I've never used in previous designs, and thus far gotten away with. This time it didn't work)
  2. I'd accidentally used a relay with a 9V coil voltage instead of the required 5V for the LED colour reverser.
So, it didn't work. And worse, in soldering and de-soldering components to test it, I ended up stripping some of the tracks off the board.

So it was back to the design. Make up a new board that rectifies these omissions (I ordered a relay with 5V coil and of course its pin spacing was different)

So, a new board was designed and exposed, developed and etched.

I use the Mega / Farnell UV-sensitive boards, and Ammonium Persulphate as an etchant. These boards are not specified for this solution so while it works, it's very slow, etching a board takes around 30-40 mins. Of course during that time, the etchant bath cools down and the process slows as a result.

//TODO: Buy an etching bath heater!

Anyway, my UV exposure lightbox gives a much more consistent light than my previous approach, which frankly is too embarrassing to describe here. So the boards produced with it look a lot better.

After etching, it was to the drillpress to drill around 145 holes of various sizes, then back to the soldering bench.

First test was to stuff and solder just the components for the low voltage circuit. Make sure the revised delay switch-on and LED colour reverser was working.

Success. It worked as planned! This meant I could then continue to stuff and solder the rest of the components.

This is now done and the results are in the pictures.

Bit of a squeeze to get everything on the board - the size dictated by the chassis. During testing I'll be watching carefully for any heat stress


Closer view of the low-voltage switching section




Dimensions: 140 X 75mm, about the size of your smartphone



47,000µF - to smooth the DC heaters for the phono stage. Because bigger is always better


How clean is this?!


Next steps: Metalwork - case drilling - and a bunch of connectors to make up.

Friday, 2 February 2018

EL84 Amp II: Progress Post 3

The design of the amplifier has continued since the last post although slower than hoped. This was die to a few random factors (including but not limited to a computer that died and needed replacing and setting up, and some non-electronics projects that came up)

Anyway, the progress this time is the completion of the headphone board, which is based on a White Cathode Follower using ECC99 tubes.

This completes all of the signal handling boards. Below they are shown in their location on the chassis. 


Headphone board front centre

This represents a significant achievement: The headphone stage was a late addition requested by the customer after the chassis had already been ordered. I was not at all confident that the entire amp would be possible to build in the small chassis. But I persevered, with the approach of taking a long time on the PCB designs to get them as compact as I could, including manually routing them (which gets more complex as each track and component is added)

The last remaining board needing to be designed was the power supply. This is going to sit inside the chassis under the transformers. This dictated its size, which – as with the rest of the amp – needed to be as small as possible.

The final board design measures 140 X 75mm.

The next step is the careful eyeball check before exposure, etching, drilling (141 holes) and stuffing.

This is the layout



On this board we have:

  1. Delay circuit for controlled power-up
  2. driver for the 2-colour power LED that starts red and goes green when the main power kicks in after the warmup delay
  3. Elevated 6.3vac power supply for heaters, with three sets of output terminals
  4. 6V DC power supply for heaters with three sets of output terminals
  5. 370V supply rail for output stage
  6. 265V rail for phase splitter stage
  7. 300V rail for headphone stage
  8. 300V isolated rail for phono stage (to avoid feedback through power rail)
  9. 280V rail with two outputs for tone control and preamp gain stage
  10. double-filtered and isolated negative bias with four outputs (one per output tube)


Regarding point 4: From a technical requirement, it's only strictly necessary to run the phono stage on DC heaters, but due to the complexity of this project it has a high tube count (13) the number of tubes exceeds the rating of the 6.3vac heater winding on the power transformer. This transformer also has a 5vac winding which will be passed through a diode and thereafter smoothed by a 47,000µF 10V capacitor (it's huge... has to lie down on the board!) and this rectified DC heater power will be used for other areas besides the phono stage, just to share the load.

Next steps: Make this board, then metalwork for the chassis...

Wednesday, 3 January 2018

EL84 Amp II: Progress Post 2

The Christmas/New Year holiday has provided plenty of opportunity to progress the design and construction of this project. My approach to this one is to build a library of discrete PCBs which fit together to complete the project, and which can be produced again for a future project.

So this will allow me to create a menu of sorts, with the customer selecting what features they want.

So, we have a separate board for the RIAA/Phono stage, one for the tone control, one for the preamp stage, and one for the headphone stage.

So far, the RIAA board, tone control board, and preamp board, are built. Here, I've laid them out according to where they will sit in the final build



I've done them all the same way as the previous post, with the valve sockets on the copper side of the board.

My method for laying out the boards is to use no automation: These are all completely manually routed. Attempting to use he auto-router was like a bad comedy show. 

Top-Right is the RIAA Phono stage. This one:


In front of that is the tone control

This one:


(Same board as previous post, but I re-made it with lead-free solder, since this amplifier's forever-home is within the European Union)

And the preamp gain stage and phase splitter



This is how the boards look from the under side. These are all made using a laser print onto a transparent sheet then exposed onto the photosensitive board under UV light


Remaining to be done: the headphone board (this will be a challenge as it has some large capacitors on it) and the power supply board.

The challenge with this build was to fit everything into a case with an internal dimension of 300mm (11.8") wide and 225mm (9") deep. The layout shown will accommodate it... just!

Sadly, one of my suppliers has let me down and as a result, I've spent $100 on parts I don't think I'm ever going to see. I will not name them yet as I am extending them the charity of my silence to give them the opportunity to rectify the matter. In the meantime I've had to order some tubes from the regular supplier to replace the missing parts.

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....