Krell KSA-5 Clone - Revised Schematic and Build Guide

In the previous two posts on the topic, I discussed my approach to upgrading the KSA-5 and demonstrated the massive distortion improvement I was able to achieve. 

Without further ado, here is the revised schematic:

The list of parts required to modify two channels:

1. 8x 0.22 ohm 2W or 3W resistors
2. 4x 22 ohm resistors
3. 6x 47 ohm resistors
4. 8x 100 ohm resistors
5. 8x 274 ohm resistors
6. 4x 332 ohm resistors
7. 2x 562 ohm resistors (not needed if you can reuse R5/R8)
8. 2x 3.92k resistors
9. 2x 1 Megohm resistors
10. 2x 68pF 50V capacitors, NP0/C0G ceramic or mica
11. 4x 4.7nF film capacitors
12. Some 18 AWG single core insulated wire and 2x 10 ohm 2-3W resistors for the output RL network

The list of changes to the original schematic:

1. Replace R1, R2, R6, R7 with 100 ohm resistors
2. Replace R5 and R8 with 332 ohm resistors
3. Replace R9, R10, R11, R12 with 274 ohm resistors
4. Replace R16 and R17 with 22 ohm resistors
5. Replace R19 with a 562 ohm resistor (reuse one of R5/R8)
6. Replace R23 with a 1 Megohm resistor
7. Replace R24 with a 68pF 50V NP0/C0G ceramic capacitor
8. Replace R33, R34, R35 and R36 with 0.22 ohm 2W or 3W resistors
9. Replace R37 and R38 with one 47 ohm resistor connected between bases of Q23/Q24 and Q25/26. Make sure to connect the new resistor correctly - see the build guide below - or your output transistors are at risk.
10. Replace R47 with a 3.92k resistor
11. Remove C2 and C3
12. Add a 4.7nF film capacitor and a 47 ohm resistor, connected in series, between the collectors of Q2 and Q3. Add another 4.7nF film capacitor and a 47 ohm resistor, also connected in series, between the collectors of Q7 and Q8. Place the new parts on the underside of the board if you like.

Repeat for the other channel. That's it!

As resistors's names are not marked on the board, here is a photo showing what to replace with what. Note that the additional parts from step 12 above are not shown - they are under the PCB. Click the picture to open it in full resolution.

After assembly, take the usual precautions before powering the amplifier up, as if it were a newly assembled board. Turn the bias adjustment trimpots all the way counterclockwise, connect a current limited +/-21V power supply with the current limit set at 0.5A per rail.

With power on, check the output for a possible oscillation, then adjust the bias and re-check for oscillations. The bias level needs to be at about 20mA per transistor - with 0.22 ohm emitter resistors, it corresponds to 4mV between the test points, which should be easy to measure with a DVM. Higher bias levels are possible, but the distortion will be slightly higher. 

Let the amplifier warm up for 10-15 minutes and readjust the bias - it should go down as the output transistors warm up.

As with any feedback amplifier, capacitive loads may affect stability. In my testing, the amplifier remained stable with capacitive loads of up to 100nF. Consider adding the usual RL network between the output of each channel and the load to ensure stability. Make 20-30 turns of 18 AWG single core insulated wire on a 1/2 inch (12mm) former - a Sharpie will work - to make an air core inductor, then connect a 10ohm 2-3W resistor in parallel to it.

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):

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.