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About alexcp

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    FL, USA
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  • Location
    FL, USA
  • Interests
    DIY audio
  • Headphones
    Grado GS1000i; Beyerdynamic T1; Sennheiser HD800
  • Headphone Amps
    Musical Fidelity X-CANv8 and assortment of DIY headamps
  • Sources
    Sony XA-20ES, NAD M51 DAC
  1. 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.
  2. 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: 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 hav
  3. Thank you! The final mod, that of the front end, is only slightly more complicated. Here is the updated schematic with both mods: The list of changes vs. the original schematic: Replace R1, R2, R6, R7 with 100 ohm resistors Replace R5 and R8 with 332 ohm resistors Replace R9, R10, R11, R12 with 274 ohm resistors Replace R16 and R17 with 22 ohm resistors Replace R19 with a 562 ohm resistor (reuse one of R5/R8) Replace R23 with a 1 Megohm resistor Replace R24 with a 68pF 50V NP0/C0G ceramic capacitor Replace R33, R34, R35 and R36
  4. 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: Here is the performance of the fully modifi
  5. 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: 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 t
  6. To see what and how to improve, let's have a look at the original schematic. 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-
  7. 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. As usual, at a higher output level the distortion percentage improves somewhat:
  8. With a 32 ohm load, Class A extends to 6.4 volts peak, so at a lower level of 4Vpeak the performance is quite similar:
  9. Let me start with a few measurements of the unmodified KSA-5. They seem to confirm the listening experience described above. With a 100 ohm load, KSA-5 never leaves Class A, and the performance is decent, with 0.02% THD and dominant 2nd and 3rd harmonics:
  10. I built my KSA-5 clone back in 2013 (see my post dated August 9, 2013 in this thread on page 8) but was never quite satisfied with it. I made every effort to make it look good and perform well, and it was well within the original KSA-5's specs. Yet, although it worked well with my 32ohm Grado headphones, it could not compete, in my subjective opinion, with Musical Fidelity's X-CANv8. Connected to a pair of 8ohm speakers, the clone would become rather confused with anything but simplest music. Because of this, the amplifier fell into disuse and was gathering dust on my rack. Until this wee
  11. I matched the quad of Q23-Q26 by hFE, and separately matched the octet Q15-Q22. However, my main reason for matching was because I could. You cannot get perfect symmetry between NPN and PNP transistors no matter what. In my case, I could not match MJE15030 with MJE15031 from the batch I had, so I replaced them with D44H and D45H, respectively - these matched much better.
  12. 50C is normal for power semiconductors, you don’t need to make it cooler.
  13. A CRC filter is something along these lines: In that diyAudio post, both capacitors were 4700uF and the resistor was 4ohm. Have a look e.g. here: http://www.learnabout-electronics.org/PSU/psu12.php for the theory behind it.
  14. FIf you are working with the "new" power supply, the one with opamps, then each opamp compares the reference 12V voltage (supplied by D1 or D2) with the portion of the output voltage from the R8/R7 or R9/R10 voltage divider. If the voltages differ, the opamp drives the pass transistor so that the voltages become equal. With the 511/750 ohm divider, V(out) = 12V * (511+750) / 750 = approx. 21.2V. If you want a lower output voltage, you can reduce the value of the top resistor in each divider (R8 and R9). For example, 499ohm instead of 511ohm would give you 12V * (499+750) / 750 = appr
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