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BURSON BANG mod/transformation

trl

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#1
I got a spare PCB for Burson BANG amplifier and started to change it from driving speakers to drive headphones instead. This should probably make it the cheapest good quality amp able to drive Hifiman HE-5 (too bad I don't have one to test it).

What I did:
- Added two Pentium 3 blue heatsinks, one for each LM3886TF chip.
WP_20180825_16_34_42_Pro.jpg

- Added 4 x WIMA 220pF for decoupling (not necessary, but I felt like I need to do this).
WP_20180825_16_35_00_Pro.jpg

- Removed and bypassed the input opamp to lower the gain and reduce the noise.
WP_20180801_21_11_36_Pro (1).jpg

- Replaced the 2 gain resistors (20K with 10K) to lower the gain from 20X to 10X (about 5W/50-Ohms, enough for all planars, even for Hifiman HE-5); it outputs about 15-16V RMS.

- Connected XLR plug instead of speakers plug (for use with powerful balanced headphones) via shielded wires.
WP_20180825_15_47_09_Pro.jpg

- Did a small external resistive adapter to divide the output power to match sensitive cans like IEM’s (similar with lowering output gain to about 2.4X) and connected to a standard 6.3 mm jack; it outputs a bit less than 5V RMS. Thanks to Frans (aka solderdude) for that: https://diyaudioheaven.wordpress.com/tutorials/power-amp-adapter/.
WP_20180825_15_20_32_Pro.jpg


Now it looks like that:
WP_20180825_16_32_07_Pro.jpg



And the outputs:
WP_20180825_16_30_51_Pro.jpg



It measures very well, there is no background noise on the jack plug (about 5V RMS) and very little noise on the XLR plug (about 15V RMS).
 

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trl

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#2
Some scope measurements. Red is audio generator, blue is modded BANG's output:

Screen Shot 2018-08-23 at 18.44.36 copy.png

20 Hz sine


Screen Shot 2018-08-23 at 18.43.46 copy.png

1 KHz sine

Screen Shot 2018-08-23 at 18.43.00 copy.png

20 KHz sine


Screen Shot 2018-08-23 at 18.44.31 copy.png

20 Hz square (you can see it's AC-coupled)


Screen Shot 2018-08-23 at 18.43.42 copy.png

1 KHz square

Screen Shot 2018-08-23 at 18.42.50 copy.png

20 KHz square
 

trl

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#3
BANG_headamp_dynamic.png

Dynamic measured after the resistive divider from output=104.7dB, very low noise headamp.

BANG_dynamic.png

Dynamic measured before resistive divider (speakers output): 90.3dB
 
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DonH56

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#4
Do you know why the LF noise is so much worse in the left channel?

Also guessing your mains are 50 Hz based on the plots...

Curious - Don
 

trl

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#6
Do you know why the LF noise is so much worse in the left channel?
Also guessing your mains are 50 Hz based on the plots...
Curious - Don
No, but will try looking into this. I'm also using really cheap RCA cables to measure this...maybe is from the cables.
Yes, 50Hz...:)

Lots of high frequency spuria going on there. Is the amp stable?
It's from the cheapo RCA cables and the ADC's input (ASUS U7, sorry I don't have a better one).
The original design was tested here with 0.33uF on outputs: https://audiosciencereview.com/foru...ide-pics-technical-stuff-etc.4302/#post-98644 and it was rock stable. After lowering the gain from 20X to 10X the amp is stable for capacitances lower than 0.15uF. AFAIK headphones cables are only having few tens of pF, so I don't see an issue here, for the moment.

Thank you for comments!

P.S.: When this amp gets unstable and starts to oscillate, the 250VA isolation transfomers I'm using to power this amplifier does a nasty louder noise. Also, I'm always using the scope after doing a modification.
 
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