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Old 23rd November 2010, 05:02 PM   #1
PMA is offline PMA  Europe
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Default Audio Buffer 8 years after

I made some modifications of my 8 years old project, that can be seen in

Audio Buffer in english

Please forgive the old text, I would write it quite different today
Anyway - what are the current mods. I needed gain, so now I have the gain of +10x, i.e. 20dB. I decided to use Analog Devices ADA4898-1 opamp. This is a very nice part, which has very very low noise 0.9nV/rtHz, enough slew rate of 55V/us and very low distortion up to 100kHz.

Some distortion results measured just below clipping, and 100kHz square response in 400ns/div:
Attached Images
File Type: jpg buffer with gain 20dB.JPG (80.0 KB, 1098 views)
File Type: png ada + buffer.PNG (35.6 KB, 1055 views)
File Type: png buffer gain10x sq100kHz.PNG (30.1 KB, 1041 views)
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Old 24th November 2010, 11:34 AM   #2
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Hi Pavel, Thanks for the update.
I have a question about the choice of the buffer and any advantage it has over single or parallel opamps, in the same position. Given that distortion levels in current opamp designs are also considerably lower, is there a particular reason to use it?
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Old 24th November 2010, 05:36 PM   #3
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Ian,

please take into account that the plots are for gain 20dB, and 13Vp. Also, take into account CCIF 19+20kHz plot, and the time plot in 400ns/div.
For 5Vrms, I get 0.00068%, which is just the bottom I am able to measure, so in fact I do not know distortion at 5Vrms.

IMO buffering is very important.
1) you have high output stage idle current (15mA in this particular case)
2) you are able to drive any capacitive load, and resistor load down to 100ohm. This may sound useless, but rugged output IS important.
3) the output is immune to HF EMI interference

This goals are very important for my design philosophy, and sonic result is much better with buffer, to me. It just works

The opamp used is http://www.analog.com/static/importe.../ADA4898-1.pdf
to me this one of the best opamps for audio ever designed.

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Last edited by PMA; 24th November 2010 at 05:40 PM.
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Old 24th November 2010, 06:43 PM   #4
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I agree with you Pavel 100%. Buffering is the key to good sound with Opamps, be the cable long or short, be the termination high impedance or low and this toutches only the surface. Another chip to watch is the LME49990 and the older but good LT1468.
I prefer an open loop buffer though, outside the feedback loop.
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Old 24th November 2010, 06:44 PM   #5
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Pavel, regarding the results above, into what load were the measurements made? Unless the same distortion applies into any load down to your 100 ohm example. I'd be most interested in distortion results into a typical load like headphones, for instance, 32-64 ohms, of an output voltage of about 3V p-p.
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Old 24th November 2010, 07:28 PM   #6
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The measurements were made at 13Vp/3kohm. I can repeat them at 3Vp-p/32ohm.

I have one measurement at 0.5Vrms, but it just attacks my measurement limit.
Attached Images
File Type: png buff thd1.PNG (35.3 KB, 939 views)
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Last edited by PMA; 24th November 2010 at 07:32 PM.
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Old 24th November 2010, 07:51 PM   #7
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These 2 measurements are for 50 ohm load at 3Vp-p
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Edit: in fact the result is my soundcard again, and not the buffer. When I add the divider before the card input, I get 0.00062% at 3Vp-p/50 ohm, and I have reached my limits again. The 3rd image.
Attached Images
File Type: png 3Vp-p 50R CCIF.PNG (37.0 KB, 923 views)
File Type: png 3Vp-p 50R THD.PNG (37.6 KB, 97 views)
File Type: png 3Vp-p 50R THD div.PNG (35.1 KB, 87 views)
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Last edited by PMA; 24th November 2010 at 08:02 PM.
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Old 24th November 2010, 08:10 PM   #8
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I have tried to tune the divider ratio in front of the card input (stepped attenuator with 1.5dB steps), this is the best I can get:
Attached Images
File Type: png 3Vp-p 50R THD div2.PNG (35.3 KB, 96 views)
File Type: png 3Vp-p 50R CCIF divider.PNG (34.9 KB, 64 views)
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Last edited by PMA; 24th November 2010 at 08:15 PM.
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Old 24th November 2010, 08:16 PM   #9
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Thank you Pavel. I think these are excellent distortion results.

And speaking of that, why would anyone try to do better? It really begs the question, why one would go to the trouble of building a tube headphone amp, hugely inefficient, with every kind of distortion higher than this one?

If we were to put into volts the amount of 3rd order distortion from the second graph, which looks to be around 105dB down from 3V p-p, it would amount to about 17uV p-p.

May I ask what sound card you're using for these measurements?
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Old 24th November 2010, 08:30 PM   #10
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The soundcard is a cheap M-Audio FastTrack USB. I have to run it 6-12dB under full scale input to get lowest distortion. Also, I often have to use input signal transformer, to convert card input signal to balanced - this is the only way how to keep low distortion of the card at higher frequencies, and to avoid ground loop.
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