Velleman K4040 Photo Gallery

 This is one gorgeous looking amplifier!

Velleman K4040, take three

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

Over 10 years ago, I built a Velleman K4040 power amplifier from an (expensive) kit. The amplifier still looks quite impressive:

Not satisfied with the out-of-the-box performance, I modified the amplifier (see my previous post for details). The result was a dramatic improvement in sound. However, without its global feedback, the amplifier had higher measured distortion, higher input sensitivity and more hum.

Ten years later, I (slightly) revised the amplifier. I replaced the resistors in the signal path with mil-spec metal film from Vishay, tidied up the wiring of the phase splitter and added a global feedback loop that encloses the input stage. With it, the amplifier has 0.015% distortion at 1W (that is, one-sixth of its original specification), the hum is much reduced, and the input sensitivity is in line with the output voltage of today's signal sources.

The final schematic:

Measurement results compare well to those of tube power amplifiers made by major brands and priced at up to $10,000, as measured by Stereophile. The modified Vellemn K4040 offers respectable measured performance typical of a classic tube design.

More photos (click for higher resolution):

Aikido Cathode Follower Preamplifier

After all that work removing hum from the Aikido ACF-2 board (see the previous post), it would be a shame not to make a complete preamplifier with it, and this is what I did.

The enclosure is from Modushop with front and rear panels custom made and engraved by Front Panel Express.

The rear panel has three pairs of gold plated RCA input connectors and two pairs of outputs (connected in parallel), plus an AC power inlet.

Inside, besides the ACF-2 board, are a toroidal power transformer, an output muting board from Pete Millett, a volume control, and and input switch. The input switch is mounted in the rear and is connected by a shaft extender to the front panel knob. All connections are made by teflon insulated, sliver plated copper wire.

The front panel has a sub-panel holding the volume control potentiometer (ALPS RK27) and the bearing for the extended switch shaft. The sub-panel allows hiding the bottoms of the knobs inside the front panel for a more professional look.

The tubes are E88CC.

Removing hum from Aikido Cathode Follower

A while ago, I purchased from Glass-Ware.com an Aikido Cathode Follower 2 (ACF-2) All-in-One 9-Pin PCB, designed by John Broskie, the editor of Tube Cad Journal (tubecad.com).

The PCB holds a pair of cathode followers, each loaded by a triode current source, and a respective power supply. In my build, a JJ E88CC with 220ohm cathode resistors shows 0.003% THD @1kHz 1Vrms with pure 2nd harmonic distortion.

As I tested my ACF, I noticed audible buzz at its outputs @0.15% THD+N - that's 1.5mVrms(!), well above the THD, and it was not a grounding problem.

A careful look at the schematic (below), confirmed by some Spice simulation, revealed that the "Aikido" in the ACF-2 relies on the exact match of the voltages across the capacitors C18 and C19, including the hum component. These capacitors are in series for the rectifier (e.g. hum) current and, together with R4/R7, are in parallel for the signal current. In the ideal Aikido world, the hum voltage across C18 would cancel that across C19, providing hum-free signal output.

In reality, ACF-2 provides no rejection of the hum caused by the mismatch of the AC components of B+ vs. B-. A mismatch is easily created by e.g. C18 and C19 having slightly different capacitance. Mismatched electrolytic capacitors are common - they usually have 20% tolerance, and the capacitance changes with time and temperature, so an ideal match never happens.

My SPICE model shows that a 1% mismatch between C18 and C19 would produce about 3.5mV of imbalance in the 120Hz components between the positive and negative rails. In ACF-2, 50% of that imbalance goes to the output - that’s 1.75mV of hum not cancelled by Aikido - and can be easily audible. Larger mismatch would produce more hum. 

One solution (see the schematic below) would be to split C4 into two capacitors, one connected between B+ and the ground, the other between B- and ground - see the attached schematic. That wouldn't eliminate any mismatch but would make it less relevant, as there would be a lower impedance path for the signal current to ground from the anode of U1a (U2a) and the bottom end of R5. The PCB is not designed for this, unfortunately, but one can place two radial capacitors in the space allocated for C4, connecting them to C4 pads and to the ground pad nearby. I ended up installing 2x 470uF 200V caps in each channel and leaving in place C5. This reduced the hum from 0.15% to ~0.001%.

Another solution was suggested by John Broskie himself, and that is to replace R12 to R15 with jumper wires and then place a 100 to 1k ohms resistor in series with the secondary center-tap and the PCB's ground pad. Better still would be to use a choke in place of the series resistor. This would create an RC (LC with the choke) filter with that series resistor/choke and C18/C19. The two solutions are not mutually exclusive and can be used together for even better results.

Note: the attached schematic was designed by John Broskie and published in his Tube CAD Journal. John is the author; I just built his design. I reproduced the original schematic to illustrate the post above.

Grounded Grid Preamp

Built from the kit from Transcendent Sound (Bruce Rozenblit). It sounds very nice, but is not being used - in stock form, it has too much gain for any of my systems.

DCPP aka Engineer's Amplifier

In early 2011, I built a Distortion Cancelling Push Pull (DCPP) amplifier designed by Peter Millett. It sounds very nice, was fairly easy to build, and is rather inexpensive. Thank you Peter for the great design!

Elekit TU-879S

Elekit TU-879S is the best sounding kit I have ever built. With my Fostex BK-20 DIY full range speakers I mentioned a couple of posts earlier, the sound is fantastic.

If you are not into single ended output, low power tube amplifiers, this amplifier may totally turn around your views on music reproduction. It is a classic two stage amplifier with a triode in the first stage and a power beam tetrode in the second with a global negative feedback applied to the cathode of the triode. Many variants of this circuit are available online, as are many suspicious implementations. When built from scratch, it can be sensitive to the layout and wiring and tricky to properly adjust (it will work but may not show its full potential), but thankfully the kit takes this difficulty away.

Sadly, the kit is now out of production, although TubeDepot says there is an alternative.

(better pictures to be posted later)

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 diyAudio.com 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 @diyAudio.com) 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.