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Universal buffer/headamp based on OPA1622
Universal buffer/headamp based on OPA1622
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Old 2nd November 2017, 06:45 PM   #271
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
Thanks, JP!
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Old 3rd November 2017, 06:41 AM   #272
hallodeletue is offline hallodeletue  Germany
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Join Date: Oct 2011
Location: Goettingen
Universal buffer/headamp based on OPA1622
Nice work indeed. Are you planning to sell a few boards? I would take one....

BR hallodeletue
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Old 3rd November 2017, 08:24 AM   #273
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
Hi hallodeletue,

Yes, I have some boards to offer. Please PM me for details.

Regards,
Oleg
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Old 4th November 2017, 09:08 AM   #274
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
One note for potential builders. I forgot to make two small cutouts for the phone connectors in the PCB design (see attached image). I overlooked the fact that the phone connectors need them to flush mount with the PCB surface. In the end I made them using a small file.

Regards,
Oleg
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File Type: jpg MissingCutOut.jpg (423.9 KB, 197 views)
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Old 4th November 2017, 09:33 AM   #275
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
I've put together a project BOM at mouser to help estimating the build costs. The selected gain setting resistors are for the gain of 2. Input RF filter components (RC network) are 1k+100p which if driven by a 10k volume pot result in cut off frequency between 450kHz and 1.6MHz depending on the volume setting.

Regards,
Oleg
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Old 4th November 2017, 08:31 PM   #276
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
Almost ready! Just need to install 3mm power indicator LED and done.

I made a small mistake while drilling the hole for the input jack and had to enlarge it by 1 mm to fit the connector. Other than that all is fine
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File Type: jpg Headamp.Rear.jpg (307.5 KB, 170 views)
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Old 5th November 2017, 07:35 PM   #277
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
I was too quick to announce no pops While playing with the boxed headamp I just realized that I hear a faint pop when turning it ON without the signal applied. Nothing ear punching but it is there. I guess before I only tried it with the signal applied, and indeed when signal is applied the pop is totally masked by the music. Interestingly, the pop is there even when the volume pot is at its lowest position. After a bit of thinking I guess I found an explanation. The headamp has around 1mVDC output offset, so I guess that is what I hear when I turn it ON: 0mV to 1mV step. On my first board I used OPA2140 which resulted in literally zero output offset voltage and I had no pop there as far as I remember. I asked a fellow diyer who has that board for testing to check it again to be sure. In this headamp I used OPA1642 which can have up to 3.5mVDC input offset voltage. I am probably lucky since output offset voltage of this headamp even with the gain of two is only around 1mVDC (my DMM has low accuracy in this range, so I would not take 1mVDC literally). I'll report here if the output offset origin of the pop is confirmed or not. Stay tuned!

If someone has better explanation for the origin of this pop please let me know.

Regards,
Oleg
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Old 5th November 2017, 09:24 PM   #278
AIM65 is offline AIM65  France
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Join Date: Feb 2015
Location: Paris
Universal buffer/headamp based on OPA1622
Default too much 'Plop' noise !

Hi Oleg,

Have a look to post #135 of Alex about my Ampcasq, he describe same behaviour as yours.
I tend to believe that that mute function of the 1622 is not fully effective. If I remember you use the circuitry of the datasheet or appnote (a divider), and I use a different way : mute pin is grounded by power switch through a Shottky diode. Both should work fine but both exhibits same issue. I think that mute stop being effective when supply get below a specific value, and as we put large cap on the supply (as zealed DIYA members...) the chip may stay in this 'bad' supply voltage area a while, thus producing this sound.
On my amp, but not yours as you have the same DC/DC converter on the 2 rails, the -5V supply disappear before the +5V, this may be another cause of parasitic noise, but at the very end the question of mute pin effectiveness remain.
If you have a digital storage scope it could be interesting to catch V+, V- and Mute pin and one output at power down, that may give some clues.

You did a great build with your new amp!

Chris
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Old 5th November 2017, 09:42 PM   #279
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
Thanks for the hints and the compliments, Chris! I am using the blue LED (around 3.2Vforward) from the positive rail plus 1k current limiting resistor in series with it to GND as the voltage divider. The idea was to delay the unmute until the positive rail is at least +4V. On power down the pop is faintly heard only at max volume and no signal is applied. In any case the pop is not really disturbing but I hoped it would not be there at all. But maybe you're right about the DC/DC converters not starting simultaneously. I will investigate further. Probably it is time to learn to properly use my scope

Regards,
Oleg
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Old 6th November 2017, 04:38 PM   #280
OlegSh is offline OlegSh
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Join Date: Dec 2010
Location: Germany
Universal buffer/headamp based on OPA1622
I haven't pulled out my oscilloscope yet, but while playing with the headamp I noticed that the pop becomes nearly inaudible if I wait a bit longer before turning the headamp on again after power off. I only notice the pop because I know it should be there. The pop becomes clearly audible (but faint and non-disturbing anyways) only if I turn the power ON fast after turning it OFF which is not a typical use condition. So it looks like some caps charging/discharging has an effect here and I strongly suspect part to part turn-on delay variance between the DC-DC converters to be the cause of such behaviour. There is no pop at power down though. I think I will not investigate it any further for now because it does not disturb me anymore. If I ever make revision 2 board I'll think of adding turn on delay network monitoring both power rails, but for now it will stay as it is.

Regards,
Oleg
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