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Old 26th July 2018, 09:27 PM   #51
wrenchone is offline wrenchone  United States
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Maybe, but it's the headphone I have.
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Old 31st July 2018, 01:09 AM   #52
wrenchone is offline wrenchone  United States
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Here are next-pass PCBs mounted on common heat sink for stereo testing. On the board to the right you can see temporary pots that plug in to two-pin headers on the board for tuning resistor values to optimize bias current and trim offset to minimum.
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Old 6th August 2018, 12:44 PM   #53
Murdoc is offline Murdoc  Germany
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Discrete Headphone Amp.
I have to confess...
...that I'm eagerly waiting for the next post every time!
But I know that all good things need time!
Just to know that people are happy to read here (but you of cause know it for sure)...
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Old 10th August 2018, 07:10 AM   #54
Shrut is offline Shrut
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Time Domain Response of the tweaked board:

Rise Time ~ 400ns
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File Type: jpg Rise_ch2.jpg (99.6 KB, 187 views)
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Old 13th August 2018, 05:29 AM   #55
wrenchone is offline wrenchone  United States
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Shrut and I have verified function for the latest amp modules. I might try tweaking the transient response to higher heights, but failing that I'll be setting up for listening tests when Shrut gets back from a brutal business trip. Some nice tunes would be an excellent "welcome back".
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Old 13th August 2018, 12:27 PM   #56
Mark Johnson is offline Mark Johnson  United States
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Discrete Headphone Amp.
Are the wiggles in the flat portions of post#54 square waves "real" (produced by the amplifier itself)? Or perhaps are they merely imperfections in the test fixture, such that a scope trace of "ground" would show those same wiggles?
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Old 13th August 2018, 04:34 PM   #57
wrenchone is offline wrenchone  United States
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Likely just pickup, as I'm using a "hairball" CMOS 555 oscillator to drive the amp for square wave tests. We're testing in a lab full of switching power supplies under development. I'll be adding proper shielded input cables soon, and a case, once I figure out the logistics.
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Old 17th August 2018, 12:44 AM   #58
wrenchone is offline wrenchone  United States
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Like I said, I tinkered a bit more with the compensation of the preamp and obtained faster rise and fall times. Compensation values are now C2 - 470pF and C3 - 12pF. The same type 61 ferrite beads are used at the gates of C9 and C10 as in previous measurements. The picture tells the rest of the story.

The rise and fall are quite smooth when examined closely, though the digital scope trace shown here appears to show oscillation at the edges. There are no "squiggles" on the top and bottom of the waveform on my bench setup, and an averaged trace looks even cleaner. Using the usual formula, bandwidth clocks in at 0.34/2.64E^-7 = 1.29 MHz.

The amp wiil now go home to get some proper input cables. Next stop - tunes!
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Last edited by wrenchone; 17th August 2018 at 12:59 AM.
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