A version of an O2 Desktop Amp (ODA)

Update 8/11/2014

V2.1 of this ODA project PC board is now available. The materials for V2.1 of this ODA project are at the same Google Drive link immediately below, except at the folder 80x160mm -> 7_9_2014 V2.1. One of the sub-folders has a description of the changes between V2.1 and V2.0. The V2.0 build photos in posts #278 - 290, #301, #302, and #368 still apply since there were so few differences betweeen V2.1 and V2.0. I have a full set of V2.1 build photos posted at the V2.1 Google Drive link, with a text file for each photo section that talks about any differences between the two versions.

Update 4/10/2014

V2.0 of this ODA project PC board is now available. The part of this thread for V2.0 starts at post 272. For all the files go to the Google Drive link here: Go to the 80x160mm folder -> 3_19_2014 V2.0.


Go to the 80x160mm folder -> then the folder "ODA 3_19_2014 V2.0 fabricated" . There you will find:

* Layout and schematic in both png and PDF format. The PDFs can be zoomed up as large as you want to clarity.

* Bill of Materials (BOM) - the stuff you need to buy from Mouser or Digikey. Mouser has all of it but the LT LDO regulators, which you have to get from Digikey. Mouser is out of the pot at the moment but Digikey has those too.

* Gerber files. These are the board layout files you can send to any board fabrication house to have a run of your own boards made. I've been using Seeed Studio in China but OSH park is another good one.

* Build instructions. Plus I've posted a step by step photo log of the build in this thread.

I would advise using this V2.0 rather than any of the past versions at this point since it adds the DC output offset null feature. If you are working on a past version board, please be aware of the one marking error, C51 has polarity reversed ont he board marking. Be sure to flip it. That error is fixed in V2.0.

Please note: the ODA designed evolved over time from the first post in this thread, below, to what is shipping now in V2.0. The current version uses 3 NJM4556AL chips per channel in parallel (6 op amps total per channel), no longer uses an OPA627 at all, uses LME49990 chips for the gain stage, and many other changes. You may want to start with post #272 first to get up to speed on the current stuff, then go back and read some or all of the earlier posts for historical background. :)


NwAvGuy/RocketScientist never did get around to publishing his O2 Desktop Amplifier (ODA) before he disappeared last year. For fun I've taken a few guesses on what might be in an ODA and whipped one up. :D

Below is an LT Spice sim of the ODA circuit on the top half, one channel, and the regulator O2 circuit on the lower half for comparison. From the plots the two outputs are virtually sitting on top of each other, green and blue, which they should be. The next two below are the schematic and about 2/3 of the way through a layout. Once the Chinese PCB shops open up in a week after the holiday I'll get one fabbed and stuffed. Once I finally get a working design I'll post the layout in the wiki section here for anyone who might want to DIY it. Since RocketScientist released his O2 under the gnu license, and this design is a derivation, I'll list it as being covered by the same gnu license for public DIY use.

This goal of this ODA design is to maintain the same basic design philosophy that NwAvGuy/RocketScientist used, but to try to bump up the performance slightly in most areas. Plus add some features that people have posted about and a few of the O2 mods I posted.

I know that one of RocketScientist's goals with the O2 was to make it cheap to be readily affordable. I've kind of thrown that one out here with an eye towards some improvements. :) The OPA627 chips alone are $20 for the low end and $30 each for the high end. I don't have a dScope or AudioPrecision analyzer, so the final result here has not been properly measured, which of course was sort of the whole point of the O2, so fair warning.

Here is a summary:

Best new stuff:

  • · Up tp +/-16Vdc power rails with adjustable regulators for up to a 11Vpeak swing. Useful for high impedance headphones.
  • · Lower noise voltage regulators, LT1963A and LT3015. Probably won’t make any audible difference, though.
  • · Twice the output current capability and power dissipation - 280mA per channel. Useful for low impedance and low sensitivity headphones.
  • · 4 NJM4556A chips to handle the current and +/-16Vdc dissipation, two per channel. Uses the SIP 8 pin inline version, NJM4556AL.
  • · NJM2068 replaced with OPA627, which is now in a feedback loop with the NJM4556 chips to null out DC offset and reduce distortion even further. DC output offset voltage should be around 0.3mV = 300uV per channel.
  • · Has input RCA jacks and output ¼” Neutrik jack in addition to better (Switchcraft) 3.5mm jacks.
  • · Bass boost circuit – switchable on/off.
  • · Rotary gain switch with 4 gain settings.
  • · Relay-based no-thump circuit that waits 2 seconds to switch in the headphones and then drops them out quickly on power switch-off.
  • · Should have even lower background noise than the O2 headphone amp at high gain settings. 4 layer PCB with full middle ground plane.
  • · Volume pot is on the input now rather than the middle of the circuit, so it can attenuate “hot” sources as much as needed. Still no pot turning noise.
  • · Coupling cap is on the input, 4x as large to work with the 10k pot vs. 40.2k resistor in the O2, to block all incoming DC from the source.
This version of the ODA uses the B4-080 case. In the layout below all the power supply stuff is now in the lower right corner, including the power plug. Having the power plug in back is something a lot of folks have posted about. The voltage regulators now line up along the back edge and are heat-sinked to the back panel of the aluminum case, one of the O2 mods I posted. So no finger-burning regulators in this one, although the transformer is now limited to 14Vac or 16vac, nothing higher or lower due to the input limitations of the new regulators.

The 3.5mm jacks are higher quality Switchcraft jacks, with actual springs inside and not just springy metal, another thing a lot of folks have posted about (bad jacks). I've used vertically oriented jacks to make better use of the vertical dimension and leave more front panel space. I've included 0 ohm surface mount jumpers on the back of the board to disable the grounds on the input jack switches if using the RCA jacks - no more cutting board traces as with the O2 to add the external jacks. The design has a companion top board that slides in the top slot of the B3-080 chassis, upside down. That board has a Neutrik 1/4" output jack and RCA jacks that can be wired for input or preamp out. The top board also has space to mount a ODAC.

New stuff on the front panel now includes a rotary 4 position gain switch. Gains are 1x, 2x, 4x, and 6x. There is a bass boost switch for boost on/off. The resistors can be selected for any level of boost, such as 3dB, 6dB, etc. Another one of the O2 mods I posted. The power LED is now on the back panel, one for each supply rail to help diagnose dead power supplies. The unit has a (pico)fuse now too on the AC input and the power switch shuts off the entire power supply now.

I've included the output relay mod I posted for the O2 which delays the headphone output turn-on by about 2 seconds, then switches the relay out quickly when the power is shut off to prevent thumps.

The rest of the new stuff is in the middle. The gain stage now uses an OPA627 with a feedback loop wrapped around 2 NJM4556AL (SIP 8) chips in parallel on each channel, for a total of 4 op amps in parallel now on the output. The pot in the middle had to go to allow the feedback loop (DC) to function. The pot and coupling cap are now on the amp input, with the cap value adjusted up 4x to account for the 10K pot.

The new arrangement should nearly null out (around 300uV vs. 3mV for the O2) any DC offset on the output of the amp, given the exceedingly low 100uV max input offset voltage of the OPA627 and its tiny 5 picoamp input bias current, which results in nearly zero IR drop across the input resistors. This also means the pot won't have any substantial DC through the wiper even though there is no longer the coupling cap on the wiper as with the O2. Just another way around the silent-pot design issue. :)

The cap is still there feeding the pot though to block incoming DC from the source. Highly not recommended to bypass that. I've made sure the frequency response still matches what RocketScientist's design had on the low 10Hz end.

A by-product the the new negative feedback loop design should be even slightly lower distortion. RocketScientist had pointed out that the distortion figures of the output chips swamped that of the NJM2068 input chip. Essentially made the output chips the limiting factor on distortion. They were just a necessary evil to get the high current. The NFB loop should lower that output chip distortion a small amount. The OPA627 has excellent distortion numbers.

The design uses both surface mount and through hole components and has parts on both sides of the board, so it probably isn't a novice level of soldering type of DIY build, unfortunately.


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So the aim is to get better figures but not better listening experience? Carrying on the 'objective' philosophy but to the next level?

I'd be a bit worried that by sticking 4556s inside the feedback loop of the OPA627 stability is going to be compromised. Also is the 627 being chosen for its figures? If so, which figures?
The OPA627 is chosen primarily for the DC parameters, having both a very low input offset voltage and extremely low input bias current (FET input), along with having good AC parameters for audio. The project originally started out trying to reduce the DC offset of the O2. I posted a servo using the OPA627, then realized I could just put it in a loop around the buffer. TI actually shows this application of the 627, to reduce a current buffer's ouput offset, in figure 14 on the datasheet:

www.ti.com/lit/ds/symlink/opa627.pdf (opens PDF)

I took a look at the OPA211 which comes in a dual package (power pad, which I hate to solder) and also has low input offset. But that has bipolar inputs and still has a hefty input bias current. Having the FET input on the OPA627 lets me directly connect it to the pot wiper without a cap and solves that issue (pot turning noise due to DC current through the wiper). Also means very low IR drop in external input resistors due to input bias current, making the imbalance in impedance looking out the inverting and non-inverting inputs (a pot, which will be varying) insignificant. Interesting to note that the leakage current through the pot wiper coupling cap on the O2 should be about the same, or even more, than the 5pA of input current through the OPA627 input. So the two different designs really should be equivalent.

I've left the 220pF feedback caps from the O2 in to roll the chip's gain off to unity above the audio band. Turns out 220pF was still a good value for this chip.

On the subjective vs. objective thing, I'm in the camp that an amp has to sound good as well as measure well. I do believe an amp can measure well and sound bad. I'll post my subjective thoughts once I get a board stuffed and have a listen.

As for why someone would want 4 paralleled NJM4556s vs. just a single LME49600, BUF634, or even an OPA551 ddpak, beats me other than that seemed to be the logical extension of the O2 to an ODA. :) The ddpak parts make it a bit harder to build a compact amp due to the square inches of heat sink foil needed. I prefer chips that can be heat sinked, or in the case of thse SIPs do a good thermal transfer job themselves. The SIPs are rated at 800mW vs. 700mW for the DIPs in the latest version of the data sheet:


I actually laid out an all-smd version first but decided it would be too hard for folks to solder. Then I laid out an all-through hole version that used two 100 x 80 boards in a B3-160 chasis, with the amp board in front and power supply board in back. After pondering that one a bit I decided it was too physically big. That led to this version with the SIP parts to save space.

BTW, I left the OPA627 as DIPs in sockets rather than the SOIC version due to the senstive FET inputs. That way all the soldering can be done with the socket, then the chip inserted with anti-static mat and wrist strap later. I figured the odds of toasting the $30 chip with hand assembly would go way up with the smd version.

I should also say that I'm not selling anything and have no plans to sell or do a group buy of PC boards. I'll post the layout and PCB gerbers once I get the design verified.
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Lol! :) Hey jcx, I would value any thoughts you might have on the ODA design and layout. You are pretty good at this stuff. :)

I've made one change today. Although the op-amps inside a package are going to be well matched, no so much between packages. So in addition to the 1R resistors on each output op amp output I've added another 1R going from the tie point on those on each chip to the next chip. So (1R || 1R) + 1R = 1.5R on each package, which should yield an output R in parallel somewhere around 0.75R.
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I don't see any consensus for translating "a better listening experience" to amplifier hardware

but there are some tricks for improving linearity - even assuming the proposed chips

I would try Class A push-pull bias of the output op amps ~20-30 mA total to ~ 1/2 the combined packages power rating

the 50-60 mApk Class A range would drive most headphones over 100 dB SPL before leaving Class A - and the the amp would still be able to push 200+ mA peak in Class B if required by the load

"Class A" should at least be worth some expectation effect listening experience enhancement - toss in Nelson Pass "First Watt" musings and point to the GedLee Metric to add the Guru factor too

and Class A operation helps reduce distortion from poor power, gnd layout - which definitely needs some work

preventing RF/EMI power line problems in an objective manner requires a definition of the “problem” - threat environment and sensitivity of both circuits and audibility consequence

verification requires expensive equipment and some knowledge of how to use it
Looks to me (from the 20kHz THD plot) that the linearity problems are down to the NJM4556's undegenerated input stage. Also there's no PSRR nor open loop gain/phase plot provided, I think some data could be inferred from the 40dB gain reponse plot though with reasonable assumptions being made.
Looks to me (from the 20kHz THD plot) that the linearity problems are down to the NJM4556's undegenerated input stage. Also there's no PSRR nor open loop gain/phase plot provided, I think some data could be inferred from the 40dB gain reponse plot though with reasonable assumptions being made.

NwAvGuy: Cmoy eBay Headphone Amp
shows ~ unity gain performance of the 4556 - the 20 kHz distortion is much lower than the datasheet Av 30 dB example, ~ 30x lower diff input V yeilds ~ 60 dB lower distortion generation from gm nonlinearity

psrr can be hugely improved in a multiloop - sub regulate the input/global feedback op amp supply

this can require gain in the output buffer if you use much headroom for the regulation - a gain of +2x may be reasonable but does slow the output stage - can be OK as long as the overall closed loop gain is higher, but the local input op amp feedback C may be needed
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I would try Class A push-pull bias of the output op amps ~20-30 mA total to ~ 1/2 the combined packages power rating

Good thoughts! I kind of like that idea. I keep looking for things to make this ODA-version a bit different/upgraded from an O2.

The class A bias would probably put this thing in the Lehmann amp category in terms of heat generation but I wouldn't necessarily be opposed to it. I would think about going back to the previous all through-hole layout though, below, that uses the amp board in front and a power supply/ODAC board in back in a longer B3-160 case. On that one I used the DIP8 packages for the NJM4556s and spaced them apart enough to be able to use the Thermalloy DIP8 heat sinks on each.

580100B00000G | 8 Pin Dip Package 30.00 C/W Thermal Resistance Slide On Heat Sink | AAVID THERMALLOY - Future Electronics

I used those on one of my O2 builds and they worked very well. A bigger case would also give the heat generation more surface area to dissipate into the aluminum case. On this through hole version I had enough board space to include the trim pots on the offset-null pins on the OPA627. Squeezing even more DC accuracy out of the pricey chips. :)

I'm kind of partial to using DN2540 depletion mosfets for CCS although that might be on the noisy side (for the class A biasing). Something to ponder. :)


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shows my push-pull scheme - you need a small stable Vref riding the 1st op op amp output to bias the output buffer pairs against each other
smaller dV, but greater regulation can be had with a Bandgap scheme - differing current ratio in identical diodes

I think multiloop performance with with high frequency noise in the input signal is mostly up to the input device performance - it has plenty of "authority" to servo out IMD products in the audio range to below the O2 -80 dB criteria for IMD coming from sub % output buffer nonlinearity in absence of deadband - and added Class A output bias should eliminate deadband

the input diff pair linearity may be where problems could arise with sufficeintly high out of band signal - the 627 is pretty good
a ~200 kHz low pass input shouldn't harm audio, gives some atteuation of RF

but if you want to see alarming out of band hash - look at SACD DSD with a single bit DAC - they really need 6th order filtering
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jcx - that heatsink in your link is an absolute work of art! That is fantastic. :) I didn't know about that thread from way back when. Very good stuff. Thanks for the pointer. I'm on the lunch break now but I'll read more tonight. I see in general, though. Your bias scheme is not just the typical ccs but a push-pull class A biasing.

Yeah the more I think about it your class A biasing idea really rocks. That is something that RocketScientist couldn't have even considered with the O2 due to battery drain. For an AC power only desktop-only version here though it fits right in with the theme, I think.

I've remembered why RS wound up with the NJM4556As now over the OPA551. I think he posted somewhere that the OPA551 was somewhat unstable into capacitive loads, as was the AD8397, but he didn't get that with the NJM4556A.

I've spent a boatload of time over the weekend with noise analysis on the front end, given the pot up front. Nearest I can tell the increased Johnson noise isn't really going to be audible, especially from the various posts that people seem to be using their O2 at 1x and 2.5x gain. The 6.5x wasn't as popular as RS initially thought.

From what I can tell it seems that maximum Johnson noise comes at the midpoint of the 10K pot, with 5K on either side of the wiper. Less rotation and the resistance & noise is less than 4K. More rotation and now the voltage divider starts doing its thing and reducing the noise along with the signal. If anyone has worked that out (noise figures from a pot at the front of the circuit) and has some thoughts to share, please post. At least the current-induced resistor noise will be negligible with just 5pA of input bias current.
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It works!

The crazy thing works! The PCB's arrived from China on Monday and package of parts from Mouser today.

The project has actually exceeded my expectations and I'm not entirely sure why at the moment. It is completely noiseless, at all pot settings, with a gain up to 12x. The O2 headphone amp has a extremely small background hiss at 6.5x gain and the volume all the way up (input grounded), so this thing actually has less even less hiss than the O2. That is a surprise, given what should be the increased Johnson noise with the pot at the input and the FET input op amp. I just wasn't sure from the noise calculations. Hard to translate that into what is actually going to be audible. And that is with my senstive AKG K550 cans that are full volume at about 80mV.

The 12x gain was a mistake too, :) I had forgotten the feedback resistor is now 3K for the bass boost circuit, rather than 1.5K in the O2 amp, therefore the resistors to ground need to be doubled. So instead of the 1x, 2x, 4x, and 6x I had intended with the rotary switch I have 1x, 4x, 8x, and 12x. But worked out well for hiss testing. Just a matter of pulling out those resistors and stuffing in double the values.

Nuking the O2's 3mV or so of DC offset was the primary goal of my project and it certainly has achieved that, as the photos below show. I was expecting a low of about 300uV (0.3mV). Instead the 1x position in the photo is just 30uV! The DMM is good for 1uV resolution so that should be fairly accurate. The 4x position is 71uV, the 8x position 98uV, and the 12x gain setting 112uV. Woohoo!

No oscillation at all up to the 100Mhz limit of my scope. I even tried a parallel 680pF capacitor across the headphones to try inducing instability. Nothing. The bass boost didn't de-stabilize it either, even though that is putting a 0.1uF in the feedback loop which causes phase shift. I was really expecting some stability trouble with the bass boost, especially at the high gain setting with the increased load capacitance, but not so.

The schematic and layout that went out to fab is below. Since then I've traded the 1/8" output jack for a 1/4" output jack which seems to be more standard on desktop headamps. I managed to get a front panel RCA jack wedged into this one along side the input 1/8" jack. I used solid polymer electrolytics for the power rail bypasses (390uF) in this one rather than the wet electrolytics.

I also manged to stuff in a third 4.7uF film capacitor in parallel for the input blocking capacitors on each channel, bringing that to 15uF. That was important since I decided to lower the pot from the 10K used in the O2 to 5K (audio taper). The lower pot value may be one of the things that has led to the exceptionally low hiss. But to keep the LPF corner frequency the same as the O2 between the blocking cap and the pot I had to up the blocking caps to 15uF.

The output balancing resistor scheme seems to have worked out very well, too. The O2 uses one 1R resistor from each op amp output to the headphone output to balance the two op amps in the single package. In addition to that I've added another 1R resistor between packages, as the schematic shows. The theory was that the op amps in a package would be well matched but not so much between packages. Measurements this evening tend to bear that out. I measured around 300uV across the two 1R resistors in one NJM4556A package, while both in the other package were around -100uV.

I'm not entirely sure why those DC offset drops are so much lower than the O2's 3mV. I know half of that 3mV was NJM4556A input bias current going through the 40.2K resistor to ground, which is all removed here. The OPA627 will effectively has zero ohms out, of course. But that leaves 1.5mV of inherent DC offset at the chip's input. Apparently the feedback loop with the OPA627 is working extremely well to null that remaining NJM4556 input offset, even though there are 4 op amps now on each channel over two packages.

No significant heating of any of the chips either. Lol - only stuffed one channel to see if it was going to oscillate itself to death or what. :)

In the photo I'm tapping off the +/-12Vdc from an O2 since I don't have the power supply board for this thing out to fab yet. So all of these numbers are with just the O2's power supply.

I also haven't stuffed the offset trimmer pot holes yet, and at this point I probably won't. I know that using the trimmer adjustment on op amps is one of the quickest ways to kill the PSRR and unbalance the op amp's input stage. With just 30uV of overall DC offset there really is no point adding the trimmer circuit.

jcx - if you happen to read this - I decided that clever class A scheme was a bit more than I wanted to roll into this one, but I have added a position on the PCB for the simple resistor on each package output (after the 1R resistors) to a rail to bias the NJM4556A's into class A that way. I haven't tried it yet but probably will. Looks like 750R or so would do the job. :)


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...and the matching power supply board that goes in the other half of a B3-160 aluminum case. I've already posted about this in a thread in the power supply section here. In the latest incarnation LM317/LM337's are acting as pre-regulators for low(er) noise LT1963A's and LT3015's. +/-16Vdc.

I've labelled the various sections for any hobbyists out there learning some electronics. :)


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Pleased to hear that's doing fine - RS swore blind that paralleling opamps in more than one package could never work due to offset mismatch :D

Lol! :D Yeah I pitched more NJM4556A's to him also once via PM or email a year or two ago and got pushback too, but no specifics other than he didn't think it was workable. I'm guessing one problem may have been his 40.2K return resistors. With 4 op amps rather than two that input bias current would double and the voltage drop across the 40.2k would double. The DC offset of the O2 may have gone up to 6mV. :)

This thing also proves that a FET input on the pot wiper is as good as a capacitor-coupled bipolar op amp input, in terms of keeping DC out of the pot wiper. I'm getting absolutely zero pot turning noise, just as with the O2. I really expected that would work since the pico-amp FET bias current is probably about the same as the leakage current of the O2's coupling cap.

I was a bit concerned about what the pot/FET design would do from a DC standpoint, since turning the pot changes the impedance looking out the non-inverting OAP627 pin from 0R to 5k, vs. less than 1k looking out the inverting input at the feedback network. But in real life measurements last night the DC effect of turning the pot is a couple of micro-volts variation in the amp output DC offset, which then stabilizes after a second or two. A non-event due to that miniscule pico-amp bias current on both inputs.

I just managed to wedge in 4 film coupling caps on each channel on the current layout. That will drop the corner frequency on the input LPF down below that of the O2 by 3Hz or so. :) One of my goals with this thing is to try to slightly exceed the O2 parameters in a few places, at least those that I can measure/verify with a DMM and scope. I'm thinking about setting the latest up with a single DIP8 socket for the gain amp(s) and use a dual-soic-to-DIP adapter to use 2 OPA827s instead of the 627. I didn't know those things (dual soic adapters) existed until a posting in the O2 thread a week or two ago. That should result in a further small upgrade. The OPA827's only come in SOIC last I looked. But still the dual assembly will be in socket in case one or both fry due to static electricity during assembly, for easy replacement. Either that or do the same with 2 soic OPA627's. The adapter and single DIP8 socket would also save one socket of layout space.

I'm still planning on having the final layout done as 4 layer (this one is just 2 layer and still is completely hiss-free!) with one layer as nothing but a full ground plane, single-point grounded at the output jack ground connection which is essentially the star point. The hope/expectation is the ground plane will help with all those parameters I can't measure, like distortion.

Also thinking of adding a clipping indicator LED. I've always thought that is something the O2 needed.
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