DIY Headphone Amplifier

Rroff · 28 Apr 2015 at 01:56

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.

Rroff · 28 Apr 2015 at 03:09

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.

Rroff · 28 Apr 2015 at 12:33

Looks like the 4 gang version of the one I'm using, they seem to be about as good as it gets without spending a lot of money.

Might be some potential with that - gives 4 options to try and match the channels closest.

Rroff · 28 Apr 2015 at 20:56

£8-10 on mouser, farnell only has the £25 ones - shame I didn't think about it last night before I finalised my order on mouser.

Rroff · 30 Apr 2015 at 15:33

Quandary... Nichicon KWs stand upright in the case halving the footprint of the power circuitry but they are inferior IMO to the Panasonic FMs (which have to lie down to fit) I've been using. Don't really want to go to a bigger case as the size is convenient for me :S (Using Nichicon FGs elsewhere in the circuit).

Rroff · 30 Apr 2015 at 21:21

Farnell suddenly has stock of the same case but 1.5cm taller version

Rroff · 9 May 2015 at 13:39

All the bits turned up for another go at this... kind of tempted to try a buffered design with either an OPA2228 gain stage and AD8066 buffer or AD8066 for both but its new ground for me :S having the extra enclosure height means I can reduce the power supply footprint and use more space for the actual amp side of it.

Rroff · 10 May 2015 at 01:27

Haven't really done much today - been trying to organise the layout so as to eliminate or have as few wires crossing/top of the board as possible but can't really see a way around it in a couple of places.

EDIT: And as I posted that I figured out the one that has been kicking my arse all day :S

Rroff · 12 May 2015 at 20:46

Hmmm kind of made it up as I was going along :S but it works - OPA2228P gain stage, AD8066 buffer:

Didn't make the best use of the board and I'm hopeless at matching up board mounted sockets/volume control with the frontpanel :S gonna go back to having them floating I seem cursed however much I measure and double check (bought enough for another go anyhow as I wasn't sure on the double op amp setup).

Its an interesting combination - many positives - but not yet convinced its the one I want - bass is nice but maybe loses a tiny bit of the 8066's edge I think though on the flipside the veiled sound of the 8066 is minimised, mid-range is ridiculously detailed and much better use of the volume control but think I'm detecting a slight hint of sibilance and don't think the sound stage is as defined as with just the 8066.

End of the day it was just a bit of experimenting with pushing the boundaries before potentially making a final choice with the 8066.

EDIT: Also pushed the input caps upto 0.47uf with higher quality ones, I think what I might be thinking is sibilance I'm sometimes catching is maybe compression artefacts on mp3s, etc.

Rroff · 12 May 2015 at 22:07

Uh... since I started this I've spent £255 at mouser, £85 at farnell and another ~£300 at Amazon/ebay/similar - that does include buying new soldering and testing equipment, cables and stuff for the USB DAC side as well. (I had a pretty decent payout to work the recent bank holidays/additional overtime so not been too careful with costs).

Quite liking this latest circuit from listening to various sources including FLAC from originally recorded sources I think most of the issues I'm hearing are compression artefacts in MP3s, etc. though need a bit more testing yet - it certainly has a lot of positives - clarity, lifelike note to instruments, very decent bass, but some stuff I'm not sure on.

Quick run down on it:

As before the virtual ground is based on goldpoint's regulated one here: http://www.head-fi.org/t/654485/virtual-ground-regulated-and-rail-splitter-circuits (1085 + 317/336/337 version) this is pretty much essential to get these particular op amps properly stable.

All caps in the power supply stage are Panasonic FM series (if your using battery power then Nichicon KW series will give a warmer sound if thats your thing but don't clean up anything like as good as the FMs if your using a DC adapter).

The big caps are all 470uf (bigger better if you have space for it but don't go below 470), the rest 22uf as per the original design.

I'm using 15v into the 1085 with 220/2.2K resistors on the adjust circuit and 2.8K on the divider givingly slightly less than 7v per rail.

The gain stage is based off the tried and tested cmoy: http://tangentsoft.net/audio/cmoy/

The audio input capacitors are Nichicon 0.47uf polypropylene film caps.

I'm using 0.1uf ceramic and 10uf Nichicon FG series bypass on both the + and - op amp supply pins (the Nichicons seem to offset the slight hardening of the sound from the ceramics) for both the gain op amp and the buffer op amp.

The gain stage op amp is an OPA2228P (not PA though it doesn't make much odds) with 10K and 2K resistors in the feedback loop for a gain of 6 (it won't be properly stable much under that).

The AD8066 is hanging off the 2228's outputs as a unity gain buffer stage using a simple feedback loop with just a 22 ohm resistor in the loop and 47 ohm resistors on its outputs - the feedback loop branches before the 47 ohm resistors.

EDIT: Think I'm sold on this setup - do lose a tiny bit of the 8066's bass and sound stage but trade off is overall more airy sound without the veiled signature of the 8066 and even more lifelike instruments than the 8066 alone even, so far the "issues" I'm hearing seem to be due to how unforgiving it is with lossy compression.

Rroff · 13 May 2015 at 01:06

Facepalm moment - wondered why the regulators were sometimes warming up (nothing problematic) while most of the time normal temperatures... while the op amps, etc. nominal draw a few of 10s of ma max I'd overlooked that the big caps can pull a fair bit of current while charging :S

Rroff · 13 May 2015 at 17:25

Boxed up:

Overall pretty happy with where its at now seems to be the best approach for the sound and features I want - going to redo it (from scratch) with the LED moved back to the right and the connectors floating rather than soldered to the PCB as I don't have the tools really to cut it precise enough to reliably line up (some cosmetic damage from where I got frustrated with it :S) and the 3.5mm port is flaky from when I had to bodge the 6.35mm to be able to screw the front panel on - also OCD about the wires inside especially the ones to the on/off switch part of the volume control trailing around the board but now I know how it all goes and can plan better.

Got another project for the next week or so, so won't get to it for a little while but its fully functional for driving my HD600s in the meantime.

Rroff · 13 May 2015 at 18:20

Its not badly out enough to bother me and gives me far more space to play with inside (when I plan it properly) a larger volume control would be a good idea though aesthetically.

I seem to have messed the 6.35mm socket up a bit though - doesn't feed the 3.5mm properly (by design the 3.5mm is chained off it so that when something is plugged into the 6.35mm it cuts the 3.5mm off) and getting static on the 6.35mm sometimes if the plug is at the wrong angle/gets knocked :S

Rroff · 15 May 2015 at 17:16

Sigh really like the sound from the 2 stage circuit but getting some random interference - might just be from bodging the headphone socket as I've not got time to investigate atm and/or from the wiring/layout mess but I'm suspecting the prototype board just can't provide the performance needed for this circuit especially without a proper ground plane and I lack the knowledge to implement the appropriate filters :S

Highly recommend it to anyone with the ability to build/design it proper though - the sound is smooth, detailed, well balanced with a slight warmth that helps to round it out without too much colouration.

Rroff · 17 May 2015 at 21:25

Not had time to test it proper but poked around with a screwdriver tonight and so far not had a recurrence of interference since yet... so hopefully it is the socket.

One thing I'm a bit undecided on - this circuit gives even the 600s fast punchy bass (how?) which on the one hand is great on the other I like their signature unhurried low range. The almost tube like element to the mid-range and very much solid state bass is an interesting combination.

Rroff · 19 Jun 2015 at 15:07

For those who might be interested this is what I'm upto schematic wise (showing 1 of 2 audio channels)

For a higher end design a la the O2 it would be useful to move the dc filter and volume control between the output of the gain stage and input of the buffer but this is about as good as it gets performance wise on prototype board (and I don't really have the experience to know all the implications if the pot becomes a load on the gain stage).

Ideally I'd like to move atleast the dc filter between the 2 stages - but don't fully understand how that affects the input impedance of the gain stage as the 100K to ground in the dc filter also has an impact on the load aspect of the volume control. I'm guessing would mean I should stick a 10-100K or higher resistor to ground after the volume control and reduce the value of the resistor in the now moved dc filter to allow for a lower cornering frequency (though the combination of 0.47uf and 100K contains most of the phase distortion below 20Hz) but I don't really understand it (the impedance side of it) heh.

Been a bit too busy IRL to commit much time to it lately but built a new board with the power circuitry ready for the rest when I get time/happy with the circuit.

EDIT: The bypass caps might seem excessive to those used to building op amps but the 0.1 on both supply pins is recommended by the manufacturer for the 8066 anyway and while the 10ufs aren't essential for audio use subjectively seem to produce better results when they are there than without and don't seem to harm the performance of other op amps.

Rroff · 1 Jul 2015 at 03:31

Hmm managed to get what will be the finally version in this format at any rate (to really do an optimal job needs a 13-15x7cm board not the 10x7cm I'm working from :S but doubt I will revisit it any time soon unless this ones dies) built over the weekend:

Zero background noise from the full sweep of the volume control which is nice (EDIT: Tell a lie - really listening for it there is a very slight hiss in the last 5% of the volume control but I had to really listen for it) - been playing with a few op amp combinations:

8066 gain, 8066 buffer: Very detailed with articulated bass that nothing else I've heard comes close to - the veiled signature however gets a bit much for longer session

2228 gain, 8066 buffer: Un-natural bass - while it doesn't have the articulation of the above setup its deeper and punchier - the 2228 has really nice mid-range a tube-a-like sound very smooth and sounds detailed despite being fairly forgiving on the source, treble can be a bit aggressive (and less forgiving on poor quality sources).

2228 gain, 4556 buffer: The best combination for sustained listening - while it doesn't have the deep bass of the above it still has the punchy bass, great mid-range and none of the veiled signature that comes from the 8066 giving an overall very crisp and clean sound - maybe a touch bright but not as bright as the O2.

The 4556 scares me a little - none the least being bipolar rather than FET it would be easy to run into dc offset issues even with its good stats in that regard but the power output alone just feels like it could destroy headphones heh.

I really like the 2228 which generally isn't a popular choice as people struggle to get it stable - compared to the ubiquitous OPA2132/2134 its like the difference between someone playing the music very accurately but for the sake of it and someone playing with real passion and the extra little nuances that brings to it.

Rroff · 1 Jul 2015 at 11:23

Mostly the hd-600s, played around with some others but it was mainly made for the 600s as everything else I own are below 300 ohm.

The bass on the 8066 isn't so much enhanced - its pretty neutral but seems to preserve "directionality" on low frequencies better than most other popular op amps which does wonders for the soundstage in general and perception of the bass.

2228 + 8066 for some reason has "thicker" bass and might not be entirely neutral to the source but very satisfying whereas when using the 4556 buffer it doesn't seem to "pass through" the deeper bass as much for some reason.

EDIT: Could also be the 4556 needs a little while to "burn in" I noticed 1-2 op amps seem to improve with a few dozen hours of use.

Rroff · 1 Jul 2015 at 15:30

Its kind of hard to describe (not sure if you've listened to op amps with a veiled signature or not) but its like a slight "damping"/less dynamic sound on upper frequencies - how noticeable it is depends a lot on the type of music and headphones used. Overall I really like the detail and bass of the 8066 but for longer sessions it can get a bit much.

Would love an op amp that had the 8066's bass, 2228's mid-range and something a little smoother and more airey for treble - neither are bad for treble but both can be a touch metallic and aggressive for treble at times.

Rroff · 1 Jul 2015 at 21:23

Looks like I might have been right about "burn in" - left it playing music while at work and already there is a definite improvement (both the 2228 and 4556 in this build are new ones not used in the other builds).