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Everything posted by simmconn

  1. The Boonton should be measuring THD+n instead of purely THD given its vintage and price range. So if your one half has a noise problem it will also show up in the “Distortion” reading. Might be easier to figure out if your unit had the 400Hz high-pass filter.
  2. The Boonton 1121 should allow you to do fully differential measurement up to 300V. Did you try that? I measured my unit with SYS2722 and the second harmonic was the only one visible that sits comfortably below -100dB. Too bad I didn't save the test result and the unit is too heavy to move around for retest. The only complaint I had was that 1mV wide-band noise. Compared to the Stax amps this is very good already. I don't remember any single-ended test results. Perhaps I did single-ended input but never single-ended output since that would not make sense. By the way my PSU is set at +
  3. Thanks @Blueman2 for the tips. Here is the internal picture (the file attachment allowance of this forum is ridiculously small!). You can see that I had to cut notches on the chassis and modify a few other things to make them fit. Also low-profile HV film caps are used due to the transformer posts protruding from the top cover. The hum levels of the assembled amp (2.5mV to 5mV) are way below the audible threshold. For the hum to be audible, it needs to be somewhere near 50mV. The wide-band noise of this amps is quite low (<1mV). So higher S/N ratio is possible if the hum is under
  4. I finished my first SRX+ build yesterday. It is overall quite satisfying. It doesn't have the breathtaking bass or unforgiving resolution of KGSSHV Carbon, but is rather non-fatiguing for long-term listening. I took the SRX+ Gerber file Kevin created (Thanks, Kevin!), added larger footprints in order to use the cheaper tube sockets, TO-92 footprints for the DN2540s and moved the heater-lift resistor dividers to the amp board so the wiring between this and the PSU board can be more streamlined. I also changed the small tubes to use 6.3V heater in order to match the surplus power tran
  5. 550-380=170 not 230. Either you got your zener diode string order wrong or the top 110V zener is way off.
  6. @dingding123 Why don't you measure the voltage drop on each zener diode in the chain, and find out which one is out of tolerance and giving you a higher voltage than it should? You didn't tell us the part number of the zener diodes you used. Let's just use the On-semi 1N5952B (130V, 3W) as an example. The datasheet has the nominal voltage 130V, +/- 5% specified at 2.9mA: https://www.mouser.com/datasheet/2/308/1N5913B-D-1801544.pdf and the curve below shows the zener voltage goes lower as the current drops (1N5952B would be somewhere between the first and the second curve f
  7. Have you tried swapping the channels and see if the problem follows the left channel amp or the left phone? If some leakage exists in the phone, raising the offset reduces the effective bias and hence you get less arc-overs.
  8. Finally finished the installation of my AliExpress KGSSHV Carbon into a proper enclosure. I wanted to use a somewhat slim chassis so I skipped those taller ones (100mm+) and ended up with the Breeze Audio SD4309B. It's inexpensive (about $57) and has a pretty solid build. The internal space is just enough for the AliExpress/eBay Carbon PCBs, but a bit shallower than the transformers I have. I had to mill 1mm-deep 'pockets' on the bottom cover for the transformers to fit right below the top cover. I'm not a big fan of the heat spreaders so those sections of the PCB were cut off. Alumi
  9. @cloudhead Thank you for putting together the netlist. I appreciate your time. If I may nit-picking a little bit. If you would like to do harmonic analysis on the differential signal, you could use '.four V(out+,out-)'. Using .four V(out+)-V(out-) will get you the analysis for the single-ended V(out+). You can see the session error log for details. The HV power supplies starts at full voltage and drops to zero within a few ns at the start of the simulation. This causes a large current spike through the body diode of the SiC devices. This is probably an artifact due to the HV supplies
  10. I'm not sure if I understand your reply. In the Megatron the first stage uses 12AU7, and the second stage uses 12AX7. What do you mean by "provide 24db gain to drive pair 12au7s"?
  11. Are you talking about the partially overlapped double holes for the C2M1000170D? I guess non-plated, oblong holes would work better, but somehow the layout person got sloppy and slapped on two plated holes (pads).
  12. I was wondering... why the first and second stages are designed at such low operating points. 12AU7 is at 1.2mA and 12AX7 at 0.5mA. Wouldn't higher operating points give better distortion and lower output impedance and hence better frequency response at the higher end?
  13. Oh I'll definitely put these in a proper enclosure with volume control and input selection, etc. This is just temporary. By the way, do you have any suggestions to reduce the gain? Increasing the NFB would certainly do it. I'm more looking for a way to reduce the open loop gain I guess.
  14. I built the amp on a heat sink taken from an old Denon home theater receiver, and decided to try it out before investing more time and $$ on a 'real' enclosure. It just so happens that the heat sink has the exact length for two boards mounted side-by-side, and width to mount the transistors on both the PSU and amp boards. The big electrolytic caps are mounted on the opposite side of the PSU board to double as temporary mechanical support. You can see the green Soviet military style HV transformer. I got it a number of years ago on ebay when the international shipping wasn't that crazy. It's be
  15. I just finished building the KGSSHV Carbon and Golden reference PSUs, well, using the PCBs from AliExpress. The performance is superb. It registers 0.0006x% THD+N up to 583Vrms and the distortion profile is very well behaved (2nd order slightly higher than 3rd order). The noise floor is at 1.1mVrms wide-band without power-line frequency harmonics. I found that my comfortable listening level is around 10Vrms and would probably never need that kind of dynamic headroom. Is there a lower gain option or mod that can bring the noise down?
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