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....