All of 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.
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 🙂
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
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.
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.
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.
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
So the case has been laser-cut and etched, the front and rear panels have been filled, seats added to the top:
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:
The new amp is not going to live with me… it’s being made for a friend.
Topology-wise, it will be a push-pull running fixed bias, ultralinear, just like the previous one. Except it’ll be running EL84s on the output instead of KT88s. And the plate voltage will be 300V instead of 560.
So we’re looking at realistically 12-15 watts, which for the intended usage will be ideal.
Owing to this one having a phono stage which includes a cathode follower, we have some fairly rigourous requirements for the power supply:
B+ of 300V for the output stage with some hefty reservoir capacitance
2 X separate 280V rails for the phase splitter (DC-coupled, hence the slight voltage drop needed)
A separate 300V rail for the phono stage
2 X 250V rails, heavily decoupled to avoid signal feedback through the B+
That’s just the high voltage side. On the low voltage side this amp needs
6.3V for the EL84s
12.6V for the input and phase splitter and cathode follower. This rail needs to be elevated … the 12.6 vac needs to be standing on around 60 VDC to avoid exceeding the Vhk on the follower. (I figured the other valves on that supply wouldn’t mind an elevated heater supply too much either)
25 VDC for the heaters on the two phono input valves (12AX7s with heaters in series to absolutely minimise the mA)
And last but not least a minus supply of about 20V for the fixed bias
As with my previous amplifier, there’s a delay start circuit with a 555 timer and a relay which turns on the HT after about 30 sec from power-up, to allow all the cathodes to be at operating temperature before B+ is applied.
Since I’m challenging myself to build this amp on a small chassis, I decided that the power supply needed to go on the smallest board I could get it on. The design I sketched out in the last entry was the inspiration for the final layout for the board, see below:
Once the design was done, it was time to employ my primitive stone-age PCB manufacturing technique: Print design onto plastic sheet, finely sand copper-coated board, apply plastic sheet face-down to board, then iron it! (Seriously… with a clothes iron).
When all the image is transferred to the board, the board is rinsed then placed into a tray of etchant solution to dissolve away the uncovered copper.
Leaving us with a nice board ready for drilling. The transfer is usually not 100% with this technique, but can be easily touched up with the soldering iron
After the board is drilled it’s a case of assembling the components and soldering them on, with a board of this nature I like to start with the smallest/lowest components first (resistors) and work up in size.
When designing amplifiers, I like to put the power supply on a circuit board.
While I think point-to-point wiring has its place in the amplifier section, the use of a PCB gives a more compact and tidy layout for power supplies.
So in the next amp I’m building the power supply is going to be a little more challenging than the previous, as this one has a phono stage, which adds complexity.
This power supply needs to provide:
B+ 300V for the output stage
280V (X2) for the preamp
300V separate regulated supply for phono stage
250V (X2) off the phono stage rail
-25V regulated DC for the heaters on the phono stage (and doubling as bias supply for output valves)
Elevation voltage of around 60V for the heater supply for the Cathode Follower
555 IC circuit for 30sec B+ delay on power-up
So a number of resistors, capacitors and rails needed. And it all has to fit as compactly as possible onto the PCB since this amp is going into a compact chassis.
So, before starting to design the finished PCB, some thought needs to be given to the layout… in the old days this might have been a pencil-and-paper exercise, but that method isn’t particularly flexible when it comes to making corrections or revisions.
Enter a tablet computer with a pen. This is how I make my rough sketches for PCB layouts… from this file I shall design the final PC board
Next step: designing the board from this sketch, when done I’ll post the layout here