INTRODUCTION TO THE PURE DARLING

This is an ultra-low power Single Ended Triode (SET) amplifier. An ideal project for first time tube amplifier builders. This Pure Darling is the "high end" version of the Darlings. It will give you a taste of "Pure Sound", a glimpse into that audio philosophy that focuses on getting the mechanical aspect of audio reproduction as much out of the way as possible. The Darling amplifier has extremely few components. This is not a weakness, but in fact a strength. At this level of "simplicity", or better said, "purity" every component contributes to the sound - you change any component and you will have a noticeable change in the character of the sound. That's why the simplicity is misleading, as each component has so much riding on its shoulders. Think of it as a 3 legged chair. A chair with 3 legs is the most stable of any chair solution. So, when you change / modify a leg, the chair collapses. Complex audio gear are like chairs with dozens of legs. You can change a number of legs, even cut off every second leg, and will have barely an effect.

Some would say, why so low power? Why not make it 5W, 10W so it's more "useful"? Short answer: because a 5-10W DIY amplifier from scratch cannot be produced with such simplicity, nor with such cost-effectiveness, nor with a beginner skillset.

What is this less-than-1W-amplifier good for? At the very least it will teach you what pure, direct sound is. Also, if you are dreaming of a most refined single ended tube amplifier, but cannot afford it, then this will give you that quality of sound reproduction within its power limitations. Which are quite a bit more than what you would expect. With ultra-efficient speakers quite likely you will not want for more power, provided you are a person who values his / her hearing and stays below 100dB listening levels.

Notes to the schematics:

Resistors: -- all KOA Speer (Kiwame) carbon film 2W, unless otherwise noted

• R1, R2: grid stoppers. 200-400R any value will work. Mount resistor body as close to tube pin as possible. Even 1/4W OK, but 1W-2W is better.

• R3: 400R input tube cathode resistor. (1/2W works as well, but try to make this 2W.)

• R4: 1.5K total resistance. Use two 750R resistors in series. Power tube cathode resistor.

• R5: 200K. Down to 1/4W OK, try to use at least 1W. Grid leak resistor. Anything between 100K-300K will work, just make it same for both channels.

• R6: Input tube plate load.

Capacitors:

• C1: Power supply tuning capacitor. Sets the high voltage. Lower value brings voltage down, higher value increases it. Make it so that the high voltage reads between 275-290VDC. Exact value will depend on your actual line AC, transformer choice, choke choice.

• C2, C3: Power supply main filter capacitors. 130uF/500VDC Kemet DCLink. ou may use other capacitors - these are the best balance of sonic performance, size, reliability, cost. Bypass each with Obbligato or Miflex 1uF/600VDC film caps for top performance.

• C4: snubber capacitor. 4nF/2kV-6kV rated, to lower line AC noise. Place it as close to the power transformer body as possible. Can also apply another cap at the power transformer primary. Different value might work better for you, depending on your power line situation. 4nF is a good start.

• C5: Power tube cathode bypass capacitor. Elna Silmic II, 100uF/100VDC . Anything between 50uF-200uF will work if you happen to have one. Just make it 100VDC rated, no compromise on the rating.

• C6: Input tube cathode bypass capacitor. Elna Silmic II, 100uF/100VDC . Anything between 50uF-300uF will work if you happen to have one. Just make it at least 50VDC rated.

• C7: Coupling capacitor. Miflex KPCU-01 0,33uF/600VDC. Try not to compromise, this is critical. You can also use Jupiter, Audio Note, or your favorite coupling cap here. Miflex is a very high quality, natural uncolored sounding copper foil PP/paper oil capacitor at an affordable price. (Same as Duelund CAST, but at a fraction of the price - Miflex is their OEM manufacturer.)

Tubes:

• V1: Input tube is premium version of the 6J4: the US military 8532, 8532W or the British military CV5311. Will need one per channel. (Two total).

• V2: Power tube is 1626. Only available as US NOS tubes, brand does not matter. Will need one per channel, two total.

• V3: Rectifier tube.

• Diodes, in red: optional choice, when you go for vacuum tube bypassed with solid state diodes. This will do two things: 1., you may roll a lot of different rectifier tubes 2., adds quite a bit more midbass bite to the sound, while retaining tube smoothness. C1 tuning cap value is fixed to 1.6uF, with any (or no) rectifier tubes in parallel.

Transformers:

• Output Transformer (OPT): 10K primary, 20-30W core size, secondary for your needs (4,8, or 16R). Preferably Hashimoto. Can go for Edcor CXSE25W 10K or Hammond 1638SE for budget versions.

• Chokes (L1,L2): each choke should be: 2H-10H, preferably 100ohms DCR or lower.

• Power transformer: needed: 6.3VAC/1A (input tube filament), 12.6VAC/1A (power tube filament), 5VAC/2-3A (rectifier filament) 500VCT/200+mA (high voltage). This massively higher current rating than absolute minimal necessary ensures dynamics and speed. Option 1: Hammond KX370 (322mA! high voltage) + 12Vfl. Option 2: Hashimoto PT150.

• note: Hashimoto PT150 option: use two 6.3V filament windings in series for 12.6V filament supply for the power tubes.

• note 1 on filament voltage: depending on your parts choices and actual line AC, the filament voltages may be higher than the desired 6.3VDC for input tubes, and 12.6VDC for power tubes. In these cases add a small resistor in series to drop the voltage to the desired voltage.

• note 2 on filament voltage: 6.3VDC and 12.6VDC will yield optimal sound. However, going for slightly lower voltages will yield longer tube life. I usually like to set them to 6.15-6.2VDC and 12.2-12.4VDC.

Special note on tubes:

You can change the input and power tube complement to other tubes, and still can chance on a good sounding amp. However, this combination (6J4/1626) has a special synergy, and you might experiment for many years with dozens of tubes and not come close to the synergy these two have. The successful marriage of tubes is a vital aspect of any amplifier, the very foundation of a successful amplifier. Change the tubes, and it's not a Darling amplifier anymore. The rest of the parts have pretty much free range to play around with.

Internal wiring for the Darling amplifiers AWG30 or AWG32 dead soft annealed fine silver. Multi threads. I recommend Rio Grande Supplies as source, they have AWG32 dead soft fine silver. Use teflon tubing for insulation (AWG14 for heavier gauge, AWG20 for thin gauges). You may also use heavier gauge silver for ground & heaters: AWG22 dead soft fine silver. Not as good, but much less hassle. Use 6 x AWG22 per heater, and 13xAWG22 bundle for the ground.

• All twists: light twist (use a drill for twisting), clockwise.

• Insulation: titanium tape to prevent oxidation

• Teflon tubing for voltage insulation (for high voltage and filament wires)

AWG30:

• Signal: Twisted pair

• General: 8 wires twisted

• Heaters: 2 times 8 twist per conductor

• Ground: 4 times 8 twist

AWG32:

• Signal: Twisted pair

• General: 13 wires twisted

• Heaters: 2 times 13 twist per conductor

• Ground: 4 times 13 twist

Turning on Darling amps the first time, sequence:

1. No tubes first, variac set to 20VAC output. See if there is a short (little to no current drawn). Increase slowly to 100VAC (little under line voltage), and see if you get approximately the voltages you will need (around 6.3VAC filament, 250VDC B+, 5VAC rectifier filament)

2. Insert signal tubes. Start variac from 0 VAC, and ramp it up slowly to 60VAC. Should be very little current drawn. If you are drawing too much current (such as 1A or more) then you have a problem, turn it off and troubleshoot. If current consumption stays very low, at 60VAC input check filament voltage, high voltage. Should be about half needed. (around 3VAC and 120VDC).

3. Increase variac slowly to 120VAC. Check filament voltage, high voltage, plate voltage. Filament should read 6.1-6.3VAC. B+ will be over specified because there is no power tube. Plate of input tube will be between 110-135V.

4. Also add power tubes, and add 8R/20+W resistors as loads to the speaker binding posts.

5. Slowly ramp up variac to120VAC. Measure voltages again, and keep these values in your notebook for future reference.

6. Hook up to system, turn on, do not play any music yet. Listen for hum, and excessive hum. If no hum, crackles, static, then you are good to proceed.

7. Test with music.

8. Allow 100 hrs to break in before you form a final opinion. It will go through radical changes, esp in power delivery and bass handling.