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Simple op amp + BUF634 HPA
Simple op amp + BUF634 HPA
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Old 2nd December 2018, 09:35 PM   #11
xrk971 is offline xrk971  United States
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Simple op amp + BUF634 HPA
Lots of good recommendations Abrax. But why so down on the OP’s titled opamp buffer? I suppose the LME49600 could be used - about same price and have 500mA and less distortion. But many of those designs floating around.

I recently used an OPA1688 for voltage gain and used a pair of OPA1688 in parallel for the buffer. It’s working out really well. With 18v rails and supply decoupling via 10uF and 100nF x7r’s right on pins basically it is very quiet and sounds great.

Click the image to open in full size.

Top view (using low ESR OSCON rail caps):
Click the image to open in full size.

One can take it an extreme of 8 in parallel:
Click the image to open in full size.

It worked so well I had to box it up and it’s in constant use now. Great as a Preamp too.
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Last edited by xrk971; 2nd December 2018 at 09:39 PM.
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Old 4th December 2018, 05:30 AM   #12
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by mercedese63 View Post
result in high DC offset

Ok, being scared by dozens ov uV typical offset we could use AD8067 with typical less than a pA input bias current.


Quote:
Originally Posted by mercedese63 View Post
Input bias current is quite high for a lot of the better bipolar op-amps.

You know first hand we talk about very different parameters. Of course i will exchange that DC offset for high feedback depth through audioband. One could prefer to keep null grounded.
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Old 5th December 2018, 05:09 AM   #13
Dibya is offline Dibya
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OPA1688 is my most favourite op amp for driving headphones.
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Old 5th December 2018, 04:37 PM   #14
DontHertzMe is offline DontHertzMe  South Africa
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Simple op amp + BUF634 HPA
I know there are plenty of op amp based variants amps out there. I was attracted to the simplicity of this combination, and wanted to give it a try.

I'm currently putting together a through-hole version of The Wire, will post more when it's ready.
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Old 11th January 2019, 03:07 PM   #15
ANCdiy is offline ANCdiy
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Yas BesPav, thank you for a really good post!
But there is some stuff here, that I don't understand.

In theory an inverting, buffered OpAmp circuit should be very simple (?) as in the picture below:
Click the image to open in full size.

In your opinion, what kinds of downfalls would this way of wiring it up have?
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Old 12th January 2019, 12:01 AM   #16
sgrossklass is offline sgrossklass  Germany
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Originally Posted by ANCdiy View Post
In theory an inverting, buffered OpAmp circuit should be very simple (?) as in the picture below:
Click the image to open in full size.

In your opinion, what kinds of downfalls would this way of wiring it up have?
Your input impedance is only 1 kOhm. Probably fine for any device that can drive headphones by itself (DAP, soundcard etc.), but still lower than what's considered normal in the hi-fi world (which would be 10k and up).

Noise gain of an inverting circuit also is 1 higher than signal gain, which becomes rather noticeable by the time you want lowish gains and highish input impedance.

Last edited by sgrossklass; 12th January 2019 at 12:04 AM.
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Old 12th January 2019, 03:23 PM   #17
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by ANCdiy View Post
Yas BesPav, thank you for a really good post!

Nothing for!

Quote:
Originally Posted by ANCdiy View Post
In your opinion, what kinds of downfalls would this way of wiring it up have?

Letís analyze.
1. Comparatively low input impedance will result in lower noise due to straight impedance lowering and higher distortion due to higher loadind of previous stage. Iíll prefer 2k, too low for noise and too high for previous module.
2. Gain of -10 is too high for headphone use, in my opinion, especially with typical opamp-level supply not more than 15-18 V. High-impedance and low sensitivity headphones could demand higher signal levels with that or even higher gain, but they also demands higher signal voltage which couldnít be achieved with opamp-level supply. Maybe fully-differential opamp like THS4131 could be used in this case, doing this will double output voltage.
3. Noninverting input straight on ground will lower CMRR, but in case of high input bias current could lower output offset.
4. Feedback circuit shown could be used with opamp stable at gain of >~10. But if you doesnít need so high gain and prefer to lower distortion up to 20-30 dB there could be used low gain of -1 with uncompensated high-GBW opamp. So all of excessive gain of uncompensated opamp will be straightly exchanged to higher feedback depth and lower distortion figures, but demand some stabilization circuitry like shown.
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Old 15th January 2019, 05:50 AM   #18
ANCdiy is offline ANCdiy
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Thanks for the feedback gentlemen!
This has gone into my current build thread on this forum
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