Universal buffer/headamp based on OPA1622

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While the headamp undergoes systematic and thorough testing by a fellow diyer I decided to find a suitable chassis for it. I've got one from ebay which has 100x60x20mm internal dimensions (see here). I decided to make mounting the headamp in such a small chassis easy by adding all necessary peripherals on a 100x60mm PCB. The way I've done it (see attached images) allows separating the main headamp board from the peripherals should the project require different arrangement. The assembly requires soldering a dozen of jumpers but it is easier than wiring all the ins and outs using loose wires.

If someone sees potential improvements to the design please share your thoughts.

Regards,
Oleg
 

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I also made a small headamp PCB without the PSU onboard. It is convenient if one needs to add headphone out to a power amp or a preamp. I also have an earlier design of such headamp where gain setting is implemented around OPA1622.

Regards,
Oleg
 

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Headamp is assembled and plays music! The sound is excellent, no pops or clicks when switching ON or OFF even with signal applied and at full blast! Now only drilling the front and rear panels is left which I'll do on the weekend. I can say that my design goal is now reached and I have a headamp and universal buffer design for all possible situations:)

Regards,
Oleg
 

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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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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
 
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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I was too quick to announce no pops :guilty: 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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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
 
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
 
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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