DIY Headphone Amplifier

[Rroff]

Yeah in some ways it's more time consuming and difficult I guess due to the extra materials and tools that are needed.

Some simple electrics with a soldering iron is a nice place to start like you have

I kind of regret dropping electronics now, most of my IT qualifications were obsolete 10 years ago whereas a lot of an electronics engineering degree (none the least covers a fair bit of computing also) are still meaningful today. Was struggling to be able to stay on top of studying for both courses though.

However my electronics lecturer at the time though absolutely brilliant at electronics was a total failure as a teacher - I think only 1 person passed (he already had degrees in maths and physics so had a head start) - so it wouldn't have likely produced much either :S

 

[Rroff]

This could be modified to work with the pseudo balanced amp idea to give a high performance pseudo balanced dac/amp combination:

http://users.abo.fi/jskata/JEDAC/

Talking a good £200 in parts for the whole lot and needing multi-layer pcb and a well engineering design to be really worth doing though - think I'll give it a miss for now.

 

[rids57]

Yeah, £200 is a bit steep at this stage

 

[TwsT]

awesome stuff

 

[Rroff]

Yeah, £200 is a bit steep at this stage

Yeah, I'd happily spend the money if I thought I could get most of the performance out of those higher end parts but the reality is you need to be an experienced engineer (especially when it comes to ground design, etc.) to do them anything like justice on prototyping type PCBs and those kind of people wouldn't be building circuits like this on those type of PCBs anyhow :S

i.e. with the PCM1794 I'd probably struggle to top 100db SNR out of its possible ~137db whereas with the PCM2902 I can easily get the top end of its capabilities (around 96db).

 

[Rroff]

Oh **** was hurrying to get the enclosure for the final version of the amp drilled before having to leave for work earlier and managed to cut the panels the wrong way up (not that there is exactly a top and bottom but one looks better as the top than the other) and the LED hole is 1mm off level with the other holes :| only realised after I'd got all the connectors screwed in, etc. :S

 

[rids57]

worth getting another one if the amp is a keeper though

 

[Rroff]

Got a couple of spares of the enclosure - they are quite useful - probably redo it when I've got some time - put it together just now and it works (better than the testing one as well) but seems the RCA inputs are reversed :S (not used this cable before so it might be the cable at fault).

Putting stuff together in a hurry + not enough sleep doesn't produce the best results lol.

EDIT: It is the cable that is made up wrong phew - bit bad as its not a cheapy one. (EDIT2: Easily "fixed" as the L/R markings are just a removable sleeve and the wiring doesn't make any odds which way around it is).

EDIT: As I ended up with the day off work couple of quick pics of how it looks - I'm not really happy though few things ended up out of alignment, etc. due to space constraints i.e. the RCA panel should be inside really so probably replace it with individual sockets as well as the case technically being the wrong way up also some of the spacing is off so as to leave the possibility of adding stuff in like source and gain switches, etc. if I feel like I need them:

Spoiler

The board itself is largely similar to the first pic though JST connectors, jumpers, etc. are gone and layout is better.

 

[Rroff]

While more luck than judgement (though I did spend awhile hand matching resistor values) got a dead even split to 1mv on the virtual ground this time (or atleast as precise as my fairly basic equipment can measure) - sits at exactly +/-6.333v unmoving under any load conditions I tested it with (which is enough to prevent clipping with the 8066 driving 300 ohm - would need a bit more with some other op amps).

Have to say while some question that virtual ground on its engineering merits sonically it seems to deliver and it easily (if you have heatsinks :S) handles upto about 60watts heh - think I'm using like less than 100th of that.

 

[rids57]

Looks a decent job to me and worth extra brownie points for the satisfaction it will give you in knowing you built it. Is that it now for this build or do you have any extras like gain switches planned?

So when do you start building a tube amp? You know you want to

 

[Rroff]

I've left room to accommodate stuff like gain switching but still on the fence on that - I'm not sure on the technical side if there is any potential for damage, etc. but even IEMs run fine off it aslong as I turn the volume down without any frequency response issues I can hear (maybe a tiny amount to the bass). I'm using a gain of 6 which seems to work best over a range of headphones and I can't personally hear any quality loss between that and 11 using the 600s.

Tube amps have never really been my thing - I like a harsher "analytical" rather than warmer sound and prefer the bass from solid state.

Gonna put together a USB DAC (still based on the PCM2902) in the same enclosure to go with it though won't be my main audio source.

Quite tempted though to pickup an STX II seeing the chips they use in there and try putting some 8066s in :S

 

[rids57]

couldn't you do that in the older and cheaper STX?

 

[Rroff]

^^ Not really given it much thought.

Had a bit of a play with the 2nd PCM2902 I'd bought over the weekend but looks like converting the design to be properly externally powered is beyond me right now :S its not really designed for it anyhow.

(Doing a little playing around with the original PCM2902 + the amp puts SNR at ~89dB which is why I'm a bit reluctant to go crazy on the pseudo balanced setup - some basic tests on the amp put THD @ 1KHz at 0.003% which is pretty good but the other results are a bit mixed, frequency response I'm having a hard time getting an accurate figure for with one channel seeming a bit off and crosstalk also having some issues getting an accurate figure for but generally seems to be around -70db).

 

[Rroff]

Ooops - course the amp + usb dac combo was ~89db as I was looping it for that test and the ADC is rated for 89db *facepalm*.

Forgot I'd done some bank holidays, etc. last month so got a bit of spare cash so decided as I'm not entirely happy with the results (aside from OCD about the case being "upside down") to redo it with a bunch of Nichicon FG series caps for the audio parts though sticking with the Panasonic FMs for the power supply - think I can improve the amp quite a bit as there are a few bits of wire between connectors I've since thought of ways to get rid of.

 

[rids57]

is it still on protoboard or have you etched a PCB?

 

[Rroff]

Still on prototype board - doesn't look like DIY single layer etched PCB would have much advantages performance wise and not really put much thought into proper multi-layer which would probably mean using a service that makes them based on a provided layout.

EDIT: The footprint required by that virtual ground (in combination with the fairly small enclosure) is holding me back a bit - but I like this enclosure and so far testing the other options TLE2426, Sijosae discrete splitter, using another op amp i.e. LM386, etc. for rail splitter/virtual grounds definitely aren't as pleasing sonically as the regulator version in combination with the 8066 though can't tell the difference when using them with the OPA2132, 2227, etc.

 

[rids57]

as you're building it yourself you can put it in whatever sized enclosure you want - personally I'd prefer a bigger, heavier enclosure that's not going to slide about too much. You could also use a larger, high quality Alps potentiometer with a nice heavy volume control knob hanging off the end for that extra bit of refinement.

 

[Rroff]

Using the Alps RK0971221Z05 in there - its not amazingly good - notice a bit of difference between the 2 channels when testing it but anything else I've looked at is a LOT worse other than huge/expensive (>£70) ones :| any ideas of higher quality ones would be welcome.

I kind of like the enclosure - with some decent feet on it not had a problem with slipping about - if it wasn't for the ridiculously over the top power supply (the regulators are specced for upwards of 1500ma and I'm not even using 50ma) the size of it wouldn't be too much of an issue.

 

[rids57]

Don't know the model number but the one on the Asus Essence One is a beauty. Considering the cost of the Essence One and the buying power of Asus, it's probably one of the >£70 ones.

 

[Ninja Pirate]

Don't know the model number but the one on the Asus Essence One is a beauty. Considering the cost of the Essence One and the buying power of Asus, it's probably one of the >£70 ones.

Looks like it's one of the cheaper Alps models (B10k ~$10):

http://jimmyauw.com/2013/04/21/demystifying-asus-xonar-essence-one-potentiometer-volume-control/