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"Light Touch" Class A Amp
"Light Touch" Class A Amp
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Old 11th May 2017, 06:57 AM   #21
wrenchone is offline wrenchone  United States
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The results of all this messing around will also be applied to my "Amp Cramp" project, which would benefit. I wonder if Papa still has the Kleinhorns - I'd like to turn the little cramper loose on them.
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Old 11th May 2017, 05:03 PM   #22
Nelson Pass is offline Nelson Pass  United States
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"Light Touch" Class A Amp
The Kleinhorns were donated to an artists collective in the Bay Area some
years ago. These days I would hook it up to the Tannoys at 95 dB. They
get reasonably loud at 1 watt.
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Old 11th May 2017, 05:36 PM   #23
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Attached Images
File Type: jpg tannfun.jpg (43.8 KB, 314 views)
File Type: gif smesna.gif (16.0 KB, 387 views)
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Old 11th May 2017, 05:37 PM   #24
wrenchone is offline wrenchone  United States
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Too bad, but the Tannoys probably occupy a heck of a lot less cubic.

As promised, here's a simulation with the values of feedback resistors R1 and R3 whacked in half - R3 goes from 1.8k to 910R, and R1 from 100R to 47R. The gain remains the same at 18dB, or 8X. Current in the p-channel rises to 16 mA. Dissipation in the p-channel is a whopping 60mW, totally ok even for a plain TO-92, cruising for the Zetex/Diodes, Inc. E-line package, which is good for about 0.5W for a sane temperature rise.

As can be seen, distortion goes down compared to the previous example and is still dominated by 2nd and 3rd harmonics. However, it doesn't go down that much, leading me to believe that I'm approaching the point of diminishing returns - time to declare victory and go home..
Attached Images
File Type: gif LT_LOIMP.GIF (13.9 KB, 312 views)
File Type: gif LT_LOIMP_DIS.GIF (9.2 KB, 289 views)

Last edited by wrenchone; 11th May 2017 at 05:53 PM.
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Old 11th May 2017, 05:42 PM   #25
Vix is offline Vix  Yugoslavia
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Somebody could think that Tannoy stands for a tiny speaker Everything is relative
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Old 11th May 2017, 05:43 PM   #26
wrenchone is offline wrenchone  United States
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Lots of things would look small compared to the Kleinhorns...

https://www.passdiy.com/project/spea...einhorn-part-1

Last edited by wrenchone; 11th May 2017 at 05:47 PM.
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Old 12th May 2017, 12:40 AM   #27
wrenchone is offline wrenchone  United States
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Post #24 has the wrong schematic and the right harmonic profile - let's try again, shall we?
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File Type: gif LT_LOIMP_SCH.GIF (13.9 KB, 276 views)
File Type: gif LT_LOIMP_DIS.GIF (9.2 KB, 275 views)
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Old 13th May 2017, 12:29 AM   #28
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I haven't used IRFP360 before, I quite like IRFP250.
Have you tried 250 vs 360?
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Old 13th May 2017, 12:45 AM   #29
wrenchone is offline wrenchone  United States
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The IRFP360 in the simulation is a placeholder for the IRFP260, my go-to monster fet. I have some IRFP150s, but would rather use the 260 to get the better junction-to-case thermal resistance and higher transconductance (one size larger die). The 150 would work, but I imagine the bias point would be a little different. Adding the variable bias setting in the bottom of the input resistor would likely make switching between output devices relatively painless.
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Old 13th May 2017, 12:59 AM   #30
wrenchone is offline wrenchone  United States
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I just finished running the latest circuit at 16 ohms and 4 ohms load - the default so far has been 8 ohms. At 16 ohms, the gain goes from X8 to X10, and the THD goes from ~0.2% to 0.48%. At 4 ohms load, the gain goes down to X6 from X8, and the distortion goes up to 1.14%, still dominated by 2nd and 3rd harmonic.
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