CHANGSTAR: Audiophile Headphone Reviews and Early 90s Style BBS

Lobby => DIY => Topic started by: Hroðulf on January 30, 2013, 09:38:09 AM

Title: BUF634 in opamp feedback as a DAC current buffer.
Post by: Hroðulf on January 30, 2013, 09:38:09 AM: Just had a revelation when I bypassed the MOSFET based current buffers on my Parasound D/AC-1600. The sound was much cleaner and devoid of the usual romantic midbass. Initially I had a plan to swap out the MOSFET buffer with a OPA134/BUF634 composite circuit ass seen here- http://www.ti.com/lit/an/sboa065/sboa065.pdf

On the paper it looks like a perfect solution as it is a very wide bandwidth high current buffer. Also the distortion figures look really nice. Another matter is how it will work in real life.

A more perfect solution, however would be using this- http://www.diyaudio.com/forums/digital-source/121394-discrete-i-v-converter-board.html

I'll see if I'll be able to make a PCB.
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: AstralStorm on January 30, 2013, 03:11:50 PM: Redoing the classic PIMETA design, eh? Both parts are quite decent choices, but obviously the devil is in the details and implementation.

The main drawback of this specific buffer is the quite large voltage drop if large amounts of current are sourced - so do remember that this won't really work to source 250 mA if you give it 3.7V or 5V supply. This is typically not an issue in headphone loads, as high current low impedance headphones are usually sensitive enough. Do note that the result will be current mode drive - balanced armatures respond differently to voltage and current drive and voltage is the more common mode in headphone amps.

Note that V/I converters need a matching I/V converter and amplifier. It makes no sense to use these for short cable runs - they're made for PA systems.
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: gurubhai on January 30, 2013, 03:41:34 PM: Nice to see someone else enjoy a PCM63 based DAC.:)

@RW : I noticed that the Parasound 1600 also uses the CS8412 as receiver.Do take a look at Wildmonkeysect's loop filter mod for this chip.It was probably the most significant mod that I did on my DAC apart from switching to a modern opamp.The improvement in bass impact & imaging wasn't subtle at all to my ears.
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: Hroðulf on April 21, 2013, 09:50:01 PM: Finally soldered the suckers in! Ended up using the same heatsinks the MOSFETs used.

All in all I like it better than the no-buffer solution. There most certainly is more bass and stereopanoramics have improved as well.

I still can't get my head around why would Parasound ruin a clean opamp output with such a crude buffering solution.
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: Hroðulf on October 29, 2014, 04:14:05 PM: I enjoyed good PCM63, before it was cool... Right Sneis?
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: Marvey on October 29, 2014, 05:14:06 PM: you most certainly did.
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: firev1 on October 29, 2014, 05:18:09 PM: LOL! damned hippy!
Title: Re: BUF634 in opamp feedback as a DAC current buffer.
Post by: Hroðulf on October 29, 2014, 06:08:25 PM: Btw, it was your pals' Amos's old DAC. Now it's dead-ish, but your latest findings make me want try bringing it back into the realm of living. ...all the hours I've spent peering into specsheets and schematics, makes me shudder.