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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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  • 2 weeks later...
1 hour ago, starcat said:

Hi there, 

I know the difference between the A..D versions of the LSK 389 but how does the LSK389 without a letter differ to the versions with a letter? Thanks for any insights!

lsk389 generic name when you don't need/want to talk about the hfe gain.

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46 minutes ago, jamesmking said:

lsk389 generic name when you don't need/want to talk about the hfe gain.

This answer makes no sense, as hfe is the forward current gain of a BJT transistor. LSK389 is a pair of JFETs that have no appreciable gate current, so hfe is not applicable here.

Instead, the letter specifies the drain saturation current group:

Copy-201122-LSK389-Datasheet-Rev-A24-202

The drain saturation current is the current that flows through JFET when you short gate to source and apply the specified voltage (here, 10V) between source and drain. The parameter has wide dispersion in the normal manufacturing process, so each manufactured JFET is measured and marked according to its saturation current.

1 hour ago, starcat said:

I know the difference between the A..D versions of the LSK 389 but how does the LSK389 without a letter differ to the versions with a letter? Thanks for any insights!

I have not seen LSK389 without a letter - are they genuine? According to the datasheet, every transistor should be marked with a letter:

marking.png

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11 minutes ago, alexcp said:

This answer makes no sense, as hfe is the forward current gain of a BJT transistor. LSK389 is pair of JFETs that have no appreciable gate current, so hfe is not applicable here.

Instead, the letter specifies the drain saturation current group:

Copy-201122-LSK389-Datasheet-Rev-A24-202

The drain saturation current is the current that flows through JFET when you short gate to source and apply the specified voltage (here, 10V) between source and drain. The parameter has wide dispersion in the normal manufacturing process, so each manufactured JFET is measured and marked according to its saturation current.

I have not seen LSK389 without a letter - are they genuine? According to the datasheet, every transistor should be marked with a letter:

marking.png

1. I assume the poster was referring to a circuit diagram NOT the marking on the case of a transistor.

2. I could not be bothered to see if the transistor in question was a bjt, mosfet or whatever. I probably should have put a / between the hfe and the word gain to cover more options.

3. Almost all of kevins circuit diagrams do not specify the transistor gain variant.... 

4. I just had a conversation today with someone about transistors and almost every transistor we mentioned we did not put the gain letters on the end because it did not matter for the purposes of the conversation.

5. I completely agree if we are talking case markings then there should be a letter code on lsk389.

 

 

2 hours ago, starcat said:

Hi there, 

I know the difference between the A..D versions of the LSK 389 but how does the LSK389 without a letter differ to the versions with a letter? Thanks for any insights!

starcat where you referring to transistor cases without the gain letter or just lsk389 in a circuit diagram or in conversation?

 

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2 hours ago, jamesmking said:

 I probably should have put a / between the hfe and the word gain to cover more options.

That letter and the saturation current it signifies are not related to gain, either.

One thing you're spot on is that in that particular place on KSA-5 clone schematic, the letter does not matter. In fact, JFETs can be replaced there with a pair of BJTs with a benefit.

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  • 2 weeks later...
On 1/24/2021 at 11:19 PM, jamesmking said:

1. I assume the poster was referring to a circuit diagram NOT the marking on the case of a transistor.

2. I could not be bothered to see if the transistor in question was a bjt, mosfet or whatever. I probably should have put a / between the hfe and the word gain to cover more options.

3. Almost all of kevins circuit diagrams do not specify the transistor gain variant.... 

4. I just had a conversation today with someone about transistors and almost every transistor we mentioned we did not put the gain letters on the end because it did not matter for the purposes of the conversation.

5. I completely agree if we are talking case markings then there should be a letter code on lsk389.

 

 

starcat where you referring to transistor cases without the gain letter or just lsk389 in a circuit diagram or in conversation?

 

Yes, I was referring to LSK389 TO-71 markings on the case, someone was selling on ebay, some 5 pcs or so. They were pictured well and there was no letter at all after the LSK389. Might have been fakes... price was though pretty original-like. 

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51 minutes ago, starcat said:

Yes, I was referring to LSK389 TO-71 markings on the case, someone was selling on ebay, some 5 pcs or so. They were pictured well and there was no letter at all after the LSK389. Might have been fakes... price was though pretty original-like. 

if the case has no letter/other designator for the grading then it's almost certainly a fake, (or possibly a reject?). The only time cases for transistors don't have an indication of Beta/hfe/gain or whatever the manufacturer uses to bin and sort them is if the manufacturer does not bin and sort them... e.g. 10m90s, C2M1000170D etc etc.

 

Edited by jamesmking
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I had the opportunity to buy the LSK389s in person from the guy who's selling them on ebay. They all tested good on my curve tracer with Idss in the reasonable range for most applications. I've not purchased any from the official channel such as a distributor. Whoever did that can chime in and let us know where the binning code is printed.

I suspect Linear Systems had some sort of lab floor sweeping event about 2 years ago. The above is not the only source which suddenly appeared in the local market. Can you imagine someone approaching you with a gallon-sized ziploc bag full of LSK389s for sale? I was able to get a fistful of them, second-hand. Most of them checked out okay, although very few were in the preferable Idss range. They could very well be factory rejects, but they've served me well.

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I have certified four LSK389A from Micross, and are definitely Linear Systems products. Because I'm in the UK I had to sign all sorts of export waiver forms to get them out of the USA.

They all measure bang on and are very well matched and very close to Idss of 5mA.

I've just looked at the package markings, and there is nothing to say that they are A grade. They are just marked with LSK389 1509C, and "Philippines". So LS either send their bare die to the Philippines for packaging, or use a foundry out there.

I also have similarly certified LSK489 and LSK689 and both of those are marked Philippines too. These were purchased after quite a long discussion with Linear Systems by email.

Anyway, if you have bought a few of different grades, don't mix them up - you have no way of telling from the device markings what grade each one is. A bit like Z-foil resistors; no markings at all. So you have no idea what the value is, or the tolerance.

Deliberate error - LSJ689

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