Sunday, March 24, 2019

Velleman K4040, take two

This is the second of three posts on Velleman K4040. Here are the links to the first and third posts.

The sound of Velleman K4040 as built from the kit (see my previous post) was not in line with the price of the kit, so I decided to modify it.
I found two posts on by ecdesigns describing what he has done to K4040. (In case the first link doesn't take to the right place, look for posts #1029 and #1037 on the thread named "Building the ultimate NOS DAC using TDA1541A").
For your convenience, here are the list of the mods:
  1. 100nF/63V capacitor across D23, this will suppress hum on the bias readout.
  2. R5 and R10 lowered to 10K, C29,31 increased to 100uF/100V, 47V zener diode across C29 and C31 (cathode to plus), this will stabilize the bias voltage, regardless of transformer load.
  3. High-quality potentiometers for bias adjustment (Bourns or Spectrol)
  4. V11 changed to ECC83, improved sound quality (ECC83 from phase splitter can be used)
  5. R61=100k, R62=100K, R59=27k, R60=27K, R11=6K8, R16=6K8, this will set the correct gain for V11.
  6. C5=100nF/1KV, C9=100nF/1KV polypropylene (Farnell P/N 106367)
  7. Cathodyne phase splitter replaced by modified Schmidt phasesplitter (see diagram). J508 and J509 are 2mA constant current sources (Farnell P/N 9549951). V9 changed to ECC81 (RS P/N 5011342).
  8. C11...14, C19...22 changed to 100nF/1KV polypropylene (Farnell P/N 106367)
  9. RV1...4, RV5...8 changed to Bourns or Spectrol.
  10. R89...96 changed to 1K Ohm 1W.
  11. R32...34, R51, R29...31 and R38 changed to 1.5 K Ohm.
  12. Overall feedback removed, only using local feedback.
  13. Svetlana EL34's replaced by JJ KT77, clearer sound, better bass and trebles.
  14. 3.3nF/5KV capacitors were placed in parallell with D11...D14 to suppress switch noise.
  15. ZD1=3.9V zener instead of 7V5 (solves mains brownout problem, this is already modified in new kits)
The new schematic is quite different from the original:

In fact, only the output stage with the quad of EL34 is left from the original design.

I implemented the proposed changes rather directly. The only difference is that I used IXCP10M45S current source instead of a pair of J508 current regulator diodes in each channel and replaced the 1N5408 rectifiers with UF5408 instead of adding noise suppressing capacitors in parallel to them. Here are some pictures:

The result was a remarkable improvement in sound!
In 2015, Mark Snape (a.k.a. SNAPEFU made a PCB layout for this mod, although I have never seen it myself.

However, I still had two problems to resolve: (1) I got hum that wasn't present before the mods; and (2) a two-year old running around doesn't get along with eight hot, attractively glowing tubes. The project was shelved until a better time, which happened to came only in December 2020.