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BURSON FUN mods

trl

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#1
This headamp has lot of power, 2 x 2 Watts @ 32 Ohms, pure Class-A, powered from a +/-17V dual supply. With 100db sensitive IEMs noise is almost non existent (default NE5534 opamps in voltage gain).

However, I felt like sticking my scope inside, and here's what I found.

FUN_+17V.png

The +17V has less than 1mV AC RMS, a decent figure, especially when thinking it's coming from a boost-converter 12V-to-17V with an SMPS of 12V/6A in front.


FUN_-17V.png

The -17V instead has over 7mV of AC RMS ripple and noise, kinda big for my taste. The spikes are really there, scope measurements were done @10Mhz with spire around the probe, to minimize interferences. So, almost 10 times more noisy than the +17V rail...interesting.


FUN_-17V_mod_ch2.png

After adding few more caps (six actually: 2x1000uF + 4x100uF) AC RMS decreased about 4-5 times and now it's about 2 times more noisy than the +17V, quite acceptable I'd say.


FUN_headphones-min-volume_isolation-transformer.png

Noise spectrum @ headphones output plug seems OK to me, no wonder there's almost no noise, even with sensitive cans. So, probably the PSRR/CMRR is able to reject most of this noise, so good news.


WP_20190110_21_32_24_Pro.jpg

2 x 1000uF/35V Nichicon FC + 4 x 100uF/35V Nichicon Audio KA are able to settle down the ripple & noise.

WP_20190110_21_34_28_Pro.jpg WP_20190110_21_34_47_Pro.jpg WP_20190110_21_35_09_Pro.jpg
 
Last edited:

maxxevv

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#2
Perhaps you can try sticking some LT1028 and/or AD797 in there for a comparison ?
 

trl

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#3
AD797 is just a bit less quiet, but hard to tell anyway...even at night and with IEMs in my ears. Besides that...sound is not changing, or at lest I can't figure it out; after all, that's not much gain in this voltage amplification stage, but I'll give a try again. Too bad I don't have another FUN to A/B test the 2 different sets of opamps.
 

maxxevv

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#4
The LT1028 is the most interest to me as it supports up to 18V rails.
 

restorer-john

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#5
Did you test the noise prior to your 'mod' and if so, present that FFT so we can see if anything useful came of it.
 

trl

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#6
Yes John, but it looks identical. I probably need a better ADC to see any difference or the little improvement to negative rails are not showing any improvements on the headphones jack.
 
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