Sunday, January 10, 2021

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.

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