Hello diyAudio forum,
I was looking at the SE/SE Wire headphone amp recently but I found out that the LME49610/49600 buffer isn't in stock anymore so I looked at alternatives and found the BUF634A.
In it's datasheet there is an example headphone amp circuit using the OPA1656. I drew up a full schematic in Kicad of the circuit What are your thoughts on it? Should I change anyhting about the schematic before I go ahead and design a PCB for it so I can test it out? Also, would running it off of two buck boost converter modules to create a dual rail PSU be a good idea to power this? Kind of like how this person does it for his eurorack PSU.
I was looking at the SE/SE Wire headphone amp recently but I found out that the LME49610/49600 buffer isn't in stock anymore so I looked at alternatives and found the BUF634A.
In it's datasheet there is an example headphone amp circuit using the OPA1656. I drew up a full schematic in Kicad of the circuit What are your thoughts on it? Should I change anyhting about the schematic before I go ahead and design a PCB for it so I can test it out? Also, would running it off of two buck boost converter modules to create a dual rail PSU be a good idea to power this? Kind of like how this person does it for his eurorack PSU.
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@diyralf That's absolute nonsense. The BUF634A as used here is capable of very respectable audio performance. It doesn't need added resistance on the output to keep it stable in most cases. Don't run a headphone cable long enough to reach your neighbors house and it will be fine.
I agree - have used the BUF634 like this, and for headphone amplifiers, and confirm there is no issue if layout & decoupling are considered - without output isolation R.
Do ensure you set it for 'high bandwidth ' operation - so that it runs full c. 180MHz bandwidth than the opamp. It is 'slow' buffers inside the f/back loop of very fast opamps, that will give issues. Achieving this is a simple tie of the BUF634 'BW' pin, to -ve supply.
It is also simple to stabilise such a simple thing against any conveable capcititative load, with the added output resistance inside the feedback loop plus one very small capacitor per channel. Standard composite amplifier techniques apply.
Do ensure you set it for 'high bandwidth ' operation - so that it runs full c. 180MHz bandwidth than the opamp. It is 'slow' buffers inside the f/back loop of very fast opamps, that will give issues. Achieving this is a simple tie of the BUF634 'BW' pin, to -ve supply.
It is also simple to stabilise such a simple thing against any conveable capcititative load, with the added output resistance inside the feedback loop plus one very small capacitor per channel. Standard composite amplifier techniques apply.
I used BUF634A as one of my interchangeable back-end modules for the Azul headphone amp*. Azul is a composite amplifier, where an opamp and a high current buffer (in this case, BUF634A) are both enclosed within a single global feedback loop. Any "errors" or distortions introduced by the BUF634A, are reduced by NFB in the global loop; in effect the opamp "error corrects" the BUF634A. Works great and sounds great too, in my opinion.
*demonstrated at Burning Amp Festival 2019 in San Francisco.
Full disclosure: At the time I felt BUF634A was the second-best sounding back-end module for Azul. The very best, in my opinion anyway, was the LH0033. Although it has been discontinued, there still are NewOldStock parts available through brokers, even today. Check octopart.
*demonstrated at Burning Amp Festival 2019 in San Francisco.
Full disclosure: At the time I felt BUF634A was the second-best sounding back-end module for Azul. The very best, in my opinion anyway, was the LH0033. Although it has been discontinued, there still are NewOldStock parts available through brokers, even today. Check octopart.
I have to clearly disagree with the previous speakers
The LME49600 is far superior to the BUF634.
https://nihtila.com/2018/02/21/hp-buf-improvements-500-mw-into-32-ω-headphones-with-0-00015-thdn/
Zero ohm outputs have the disadvantage that under certain circumstances it starts to oscillate.
The LME49600 is far superior to the BUF634.
https://nihtila.com/2018/02/21/hp-buf-improvements-500-mw-into-32-ω-headphones-with-0-00015-thdn/
Zero ohm outputs have the disadvantage that under certain circumstances it starts to oscillate.
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Non-zero output impedance has the disadvantage that'll be load dependent.
And the frequency response will be modified by the amp.
Also the current distortion will be significant higher with real transducer loading.
https://www.ti.com/lit/pdf/slyt630
Both LME49600 and BUF634/BUF634A are very good buffers I believe.
In OPA1656's datasheet TI also shows the the combination of OPA1656/BUF634A w/o output resistor and states it's stable.
And the frequency response will be modified by the amp.
Also the current distortion will be significant higher with real transducer loading.
https://www.ti.com/lit/pdf/slyt630
Both LME49600 and BUF634/BUF634A are very good buffers I believe.
In OPA1656's datasheet TI also shows the the combination of OPA1656/BUF634A w/o output resistor and states it's stable.
.. the hidden 'gotcha' is why.
Look at the BUF634's output impedance, open-loop: it is none-zero, but c 5-7ohms depending on how much output idle current it is running (..that 'BW' pin' - hethe datasheet even shows you how that works)
And the datasheet's 'serving-suggestions' circuits, include one using a tiny cap inside the F/B loop - to stabilise against capacitative loadings, exploiting this - as I suggested above.
Honestly - there's no magic in such an old, but very, very good part: and examining the datasheet, tells the how and why - very clearly.
Look at the BUF634's output impedance, open-loop: it is none-zero, but c 5-7ohms depending on how much output idle current it is running (..that 'BW' pin' - hethe datasheet even shows you how that works)
And the datasheet's 'serving-suggestions' circuits, include one using a tiny cap inside the F/B loop - to stabilise against capacitative loadings, exploiting this - as I suggested above.
Honestly - there's no magic in such an old, but very, very good part: and examining the datasheet, tells the how and why - very clearly.
Certainly not on its own despite internet lore and Aunt Corey's fans . Corrected by an opamp it performs a lot better, even Wadia used it at the time. Which was a really long time ago.
Hi Mark,
Kindest regards,
M
I asked about the version with LH0033 a while ago, since I have quite a few of them, but you indicated that the design was not finalized yet. So, I was wondering if it is now, and whether you will be publishing it?
Kindest regards,
M