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With an 8 ohm load, Class A ends at 1.6V peak, so 1W performance is not so good anymore. Curiously, the Krell brochure mentions that the amplifier is good for 5W into 8ohm, but the owner's manual warns agains connecting it to any loudspeakers. I understand the 8ohm was meant for driving STAX via a transformer.

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As usual, at a higher output level the distortion percentage improves somewhat:

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My Krell:        

Got it cased.  Pretty standard.  I apologize for the power switch--I couldn't resist.  

I completed the full modification (output stage and front end) on both channels and listened to the amp briefly. The headphones were Grado GS1000 and Sennheiser HD595, the 8ohm speaker were B&W 60

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To see what and how to improve, let's have a look at the original schematic.

ksa5-marked.png

KSA-5 is designed along the lines of "moderate feedback", that is, it uses very little to no global feedback but lots of local feedback a.k.a degeneration.

The pair of input JFET buffers (Q1, red box on the schematic above) run independently of each other and outside of the global feedback loop. With low loop gain, they see very different signal levels, so the differential stage downstream doesn't cancel their distortion. (BTW, because of this JFETs need not be matched. Also, the expensive and hard-to-find JFETs can be easily replaced here with BJTs.) Still, a JFET follower loaded by a current source has 100% degeneration and relatively low distortion, at least at low signal levels, so the buffers are not the biggest problem.

The pair of differential stages (Q2+Q3, Q7+Q8, orange box) is heavily degenerated by 680ohm emitter resistors and produce R10/(R1+R2) = 2 = 6dB of gain.

The pair of common emitter stages (Q12, Q13, purple box) is also heavily generated by 402ohm emitter resistors and, with the low load of R23 and R24, provides R23/R16 = 9 = 19dB of gain.

Since the output stage (blue box) is a double emitter follower with approximately unity gain, the total open loop gain of KSA-5 is 2x9 = 18 = 25dB. The feedback divider (R45-R47) attenuates the output signal by a factor of 9 (19dB), which leaves 18/9 =2 (6dB) of global feedback. That is, the global feedback loop attenuates the distortion of the output stage by a small factor of 1+2 = 3.

The output stage, meanwhile, is a large source of distortion. Although Krell claimed that KSA-5 runs in "pure Class A", in reality it can easily slide into Class AB. The output pairs run at only 50mA of quiescent current each and leave Class A (that is, one half of the output stage stops conducting current) when the output current reaches 200mA. The driver quads (Q15-Q22) also run in Class AB (R37 and R38 are connected to the output), which means they stop conducting at that point, too. With a 100ohm load, it would happen at 20V peak output voltage, so the amp never leaves Class A with such a load. However, with 32ohm, KSA-5 leaves Class A at 6.4V peak; with 8ohm, at 1.6V.

Even within Class A region, the output stage is not very linear, especially with low impedance loads. It uses paralleled transistors with relatively large emitter resistors to ensure current sharing. The dark side of large emitter resistors is that they make the output impedance of the emitter follower large and nonlinear in the crossover region (see e.g. Douglas Self and his "wingspread" diagrams). Since the output impedance forms a voltage divider with the load, its nonlinearity makes the gain of the emitter follower nonlinear, adding crossover distortion and negating the benefit of the large bias current.

Overall, KSA-5 has a nice and linear front end followed by a not-so-linear output stage, with little feedback to let the former help the latter stay linear.

The game plan, then, is to improve the output stage and add more feedback. 

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Let me first modify the output stage. The mod affects only the performance with lower impedance loads, and even there it can take us only so far, but it is a start. The front end modification that increases the feedback loop gain will be posted separately.

The changes are simple. Here is the schematic:

KSA-5-EF-mod.png

R33-R36 are replaced with 0.22ohm  3W resistors, R19 is reduced to 470..560ohm to allow proper biasing, and R37-R38 are replaced by a single 47..51ohm resistor. The bias will need to be re-adjusted. Note that 50mA per transistor that resulted in 100mV between the test points in the original will now give 11mV - still easy to measure with a DVM.

The performance improvements are as follows. With 1W and 5W into an 8ohm load:

KSA-5-EFmod-8ohm-4-Vpeak-IMD.gif
KSA-5-EFmod-8ohm-4-Vpeak-THD.gif
KSA-5-EFmod-8ohm-9-Vpeak-IMD.gif
KSA-5-EFmod-8ohm-9-Vpeak-THD.gif
 

With 4Vpeak (2.8Vrms) into a 33ohm load the effect is much smaller:

KSA-5-EFmod-33ohm-4-Vpeak-IMD.gif
KSA-5-EFmod-33ohm-4-Vpeak-THD.gif

Distortion into 100ohm does not change appreciably, so I don't show it here.

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I completed the full modification (output stage and front end) on both channels and listened to the amp briefly. The headphones were Grado GS1000 and Sennheiser HD595, the 8ohm speaker were B&W 602.5 floorstanders. The amplifier performed very well in each case and sounded immaculate. It is a HUGE upgrade over the original and over the output stage mod alone. I compared it against Musical Fidelity X-CANv8, and they performed equaly well.

I took some measurements. Here it is delivering 1W and 5W with an 8ohm load:

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Here is the performance of the fully modified KSA-5 into a 32 ohm load:

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and into 100ohm:

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Thank you!

The final mod, that of the front end, is only slightly more complicated. Here is the updated schematic with both mods:

ksa5-changes.png

The list of changes vs. 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 photo 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!

The list of parts required to modify two channels:

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

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. 

KSA-5-PCB-replace-small.png

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 like this:

air-core-inductor.png

then connect a 10ohm 2-3W resistor in parallel to it.

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