Dual O2-Inspired, SMD Headphone Amp - diyAudio
Go Back   Home > Forums > Amplifiers > Headphone Systems

Headphone Systems Everything to do with Headphones

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th May 2013, 06:06 PM   #1
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
Default Dual O2-Inspired, SMD Headphone Amp

I just realized that this Headphone Amp section of the forum existed! My previous copy of this post was incorrectly put in the Solid State section.

I'm trying to improve my analog layout and circuit design - please provide feedback! I designed and laid-out a dual headphone amplifier based on nwavguy's O2 amp:
NwAvGuy: O2 Details
There is a ton of open space on the board, so clearly, this design can be miniaturized further. I wanted to make something nice for listening to music or watching a movie together in an airport / plane / quiet place.

One of the most notable differences is that there is only one 4556 output stage opamp per headphone output, meaning this amp can deliver about half the current of the O2 per channel. The thing I'm most concerned about are the thin-film surface mount resistors. I'm using Yageo 1% carbon thin-films, eg RT0603FRE07100RL and others from that series. I know others have used Susumu thin films and I'd be interested to know what the difference is between those and the ones I picked.
I was really excited about getting an initial prototype built, so I omitted some of the more practical features present in the O2 such as:
-switchable gain
-wall power
-low voltage battery protection: this device could become dangerous near the end of uneven battery discharge
-power LED indicator
-the ability to remove power from one of the headphone output stages (n/a to the O2)

The 3 PCBs I ordered from OSHPark are arriving tomorrow! For the next revision, I am considering using a single Li-Po cell and boost / inverting circuitry to generate the dual voltage rails. If proper filtering precautions are taken for the switch-mode converters, I think it will introduce minimal noise into the system as well as dramatically decrease the board space currently required by the batteries.

Bill of Materials
https://docs.google.com/spreadsheet/...3c&usp=sharing

Schematic
https://docs.google.com/file/d/0B1Ul...it?usp=sharing

Layout
Bottom layer is a ground pour
Click the image to open in full size.

3D render
Click the image to open in full size.
__________________
Bipole Transmission Line TangBand 871s

Last edited by gastro54; 27th May 2013 at 06:36 PM.
  Reply With Quote
Old 27th May 2013, 07:30 PM   #2
agdr is offline agdr  United States
diyAudio Member
 
agdr's Avatar
 
Join Date: Sep 2010
Good work! There have been a couple of posts in the main O2 thread over the last year or two from people wanting a dual "his and hers" O2. One quick solution in the past was to stick two O2 boards back to back in a longer B2-160 case and use a Y 3.5mm splitter cable on the input. Yours is a more elegant solution though, especially if someone has phones that don't need the current from the paralleled NJM4556A.

Not having the battery low voltage protection and/or charging circuit would have been a problem, but not since you are going to switch to LiPo. Apparently it is possible to keep the DC-DC noise out of the audio. Take a look at JDS labs C5 if you haven't already:

JDS Labs - C5 Headphone Amplifier

He has a converter in there and has performed the same dScope tests that NwAvGuy did, plus an FFT at the switching frequency. Everything looks good.

One suggestion, since you have some board space. I see why you changed the NJM4556A input ground return resistors to 100K from 40.2K. You only have one half chip's worth of input bias current draw through each instead of two, hence half the IR voltage. And that 100K lets you shrink the coupling cap to 1uF keeping the corner frequency the same.

But... the 100K resistor may give you more Johnson / thermal noise. I would suggest backing those resistor back down to 40.2K and put two 1.0uF's in parallel on each circuit to go back to the 2.0 (2.2) uF. Or just a single 2.2uF SMD cap if one will fit.

I would also suggest putting your gain resistors R6 and R19 in header sockets, the ones that NwAvGuy has listed at the bottom of his BOM. That would make it easy to change the resistors, or pull them out entirely for 1x gain, especially since you don't have a gain switch.

It is kind of slick that your design does away with the need for the output (1R) balancing resistors! Even lower effective output impedance from the amp. Since the NJM4556A are internally current limited they don't need the resistors to protect against the TRS jack/plug shorting, of course.

Last edited by agdr; 27th May 2013 at 07:45 PM.
  Reply With Quote
Old 27th May 2013, 08:28 PM   #3
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
Hi agdr - thanks for the suggestions! I have a lot to learn about analog design and any feedback is very helpful.

I like SMD a lot. Once you are used to it, its much faster to populate SMD boards by hand compared to through-hole, which is why I tried to choose SMD components wherever possible. Also, if you ever decide to get the board manufactured by a third-party, it is much cheaper to assemble SMD boards because its easier for the assembly house to setup automated pick and place -> reflow. Unless you are getting a LOT of boards manufactured, you're basically paying for someone to individually hand-solder your through-hole components.

That said... SMD film caps are ridiculously expensive and generally hard to find. For example, this is the only 2.2uF film cap available at Mouser and it costs $3.33 in single quantities, and its a larger footprint, so its not really an option from a cost-perspective. If I chose to go with 2uF for AC-coupling, I'd do 2 of the 1uF caps in parallel as you suggested, which would add about $2 to the BOM parts cost (would need 4 additional caps). With the current PCB, I could probably stack the caps on top of each other.

Part of my design choice for the current 1uF configuration was influenced by this paragraph in nwavguy's O2 Details post:
Quote:
R12 & R13 TRADE-OFF: R12 and R13 provide DC bias for the output op amps. Their value wants to be large to push the low frequency roll off from the C13 and C14 down close to DC. But their value wants to be small to minimize DC offset at the headphones. So it’s a classic trade off. Larger value film caps also take up more space and are more expensive. So 1 uF caps with 100K would be cheaper but the amp would have over twice the DC offset. I opted to go the more expensive route at around 40K and 2.2 uF. This means – 3 dB at only 1.8 hz and results in a typical DC offset of under 4 mV—both excellent specs.
I figured a 4-5mV difference in DC offset wasn't going to be a big deal. 5mV / 16ohm load = 300uA. If it ends up being an issue - I'll gladly change the design.

Its kind of funny how the parts cost of a design like this is dominated by the electro-mechanical components (switches, pot, connectors, batteryholder). These parts cost $13.92 of the $20.83 parts total, thats 67%!

I hadn't seen the C5! Thanks for the link. It looks like they're boost / inverting the Li-Po to greater than +/- 7V, then using the LDOs for their final output, which is a well-documented method for obtaining a high efficiency and low-noise supply. I'd love to see a picture of the disassembled C5
__________________
Bipole Transmission Line TangBand 871s

Last edited by gastro54; 27th May 2013 at 08:56 PM.
  Reply With Quote
Old 27th May 2013, 09:25 PM   #4
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
Quote:
I would also suggest putting your gain resistors R6 and R19 in header sockets, the ones that NwAvGuy has listed at the bottom of his BOM. That would make it easy to change the resistors, or pull them out entirely for 1x gain, especially since you don't have a gain switch.
In the next revision, there will definitely be a gain switch (or two actually, one for each headphone output). More electro-mechanicals noooo. Unless I go digital.
Quote:
But... the 100K resistor may give you more Johnson / thermal noise. I would suggest backing those resistor back down to 40.2K and put two 1.0uF's in parallel on each circuit to go back to the 2.0 (2.2) uF. Or just a single 2.2uF SMD cap if one will fit.
The more I think about it, I'm gravitating towards using 2uF, 42kOhm in the AC-coupling section for peace of mind.
__________________
Bipole Transmission Line TangBand 871s
  Reply With Quote
Old 27th May 2013, 11:52 PM   #5
agdr is offline agdr  United States
diyAudio Member
 
agdr's Avatar
 
Join Date: Sep 2010
Here is a picture of the innards of the C5: Search down for "reference level performance" and the picture is just below.

JDS Labs Blog | Premium Headphone Amplifiers

In your case you wouldn't get twice the output DC offset with 100K since you only have one half of the NJM4556A connected to each 100K resistor. In the O2 both halves go through each resistor, so that is twice the current draw and twice the I * R voltage which then gets reflected to the op amp output by the 1x buffer stage. So if the O2 amp has about 3mV of output DC offset with two NJM4556A inputs pulling through one 40.2K resistor, you should also have about 3mV with one NJM4556A input pulling through one 100K resistor. Half the current times about twice the resistance.

But NwAvGuy didn't really get into the Johnson noise issue with that resistor, probably since it is feeding a 1x voltage gain stage. At 1x at least the noise isn't amplified, just passed right through. He did talk about the Johnson noise at the gain stage, the 274R resistor, since that noise would get amplified by any voltage gain in that first stage.

So... 40.2K + 2uF would give you less noise that would in turn get sent though, 1x, to the output. The Johnson noise is just related to the resistance of the resistor (more is noisier) and the ambient temperature.

To keep the corner frequency at 1.8Hz with the 2uF instead of 2.2uF you would actually need 44.2K instead of 40.2k.

Last edited by agdr; 27th May 2013 at 11:57 PM.
  Reply With Quote
Old 28th May 2013, 04:59 AM   #6
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
Ah - good call. This design has half the input bias current per channel. I wasn't thinking about that.

f_c = 1/(2PiRC) ! You're right. 44.2K it is.

Thanks agdr.
__________________
Bipole Transmission Line TangBand 871s
  Reply With Quote
Old 29th May 2013, 02:55 AM   #7
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
woohoo! The rest of the parts will be in by Friday. The rightmost mounting hole for the power switch is way too close to the board edge for comfort. This, along with other significant design issues will be addressed in the next board revision.
Click the image to open in full size.
__________________
Bipole Transmission Line TangBand 871s
  Reply With Quote
Old 30th May 2013, 04:31 AM   #8
agdr is offline agdr  United States
diyAudio Member
 
agdr's Avatar
 
Join Date: Sep 2010
Looks good!

If you go with the LiPo power supply your amp will have one big leg-up over the C5: more output current capability. He is just using a single op amp stage for everything, gain and output. I forget which chip at the moment but I think it had the typical 24mA or so maximum out into the typical 2K or so. I believe the lowest he has measured with the dScope was 150R or so (details are in that blog post of his). Yours probably will be good down to 32R or so load. I think NwAvGuy went down to 15R with the two NJM455A sections paralleled.

You will need 2.5x the current capability on your LiPo circuit though, vs. the C5.

Another suggestion just hit me. I just realized you are using the surface mount version of the NJM4556A, which only has 300mW of power dissipation from the data sheet. You would get 700mW by switching to the DIP, or 800mW with the inline SIP - I'm using the SIP in a project in another thread. Looks like you may have enough vertical height and board space for either one.

Last edited by agdr; 30th May 2013 at 04:38 AM.
  Reply With Quote
Old 30th May 2013, 07:17 PM   #9
diyAudio Member
 
gastro54's Avatar
 
Join Date: Nov 2003
Location: Atlanta, GA
Thanks!

The C5 uses the SO-8 version of the OPA2227 according to the product picture.

I'm not too concerned about the limited power dissipation of the SO-8 package. Worst case power dissipation in the 4556 opamps will occur at 5.7V p-to-p, or 4.0Vrms because the amp has a bipolar 9V rail.
Power dissipation in the opamp driving ... at 4Vrms
  • 600ohm: 27mW
  • 250ohm: 65mW
  • 54ohm: 300mW
  • 32ohm: 506mW
  • 16ohm: 1010mW
So there is a problem driving loads under 54 ohms using this worst-case, 4Vrms output, but can you name any headphones with impedance under 54ohms that require anywhere near 4Vrms to function within their normal operating range?

For example, limiting the power dissipation in the amplifier to 300mW, you could drive a 32ohm load with 1.6Vrms.
(1.6Vrms)^2 / 32ohm = 80mW. Are there any 32ohm headphones that require >=80mW to reach full loudness (100-130dB)? I haven't found any, but if they exist, I'd like to know!

I used this appnote for the calculations http://www.ti.com/lit/an/sboa022/sboa022.pdf
__________________
Bipole Transmission Line TangBand 871s

Last edited by gastro54; 30th May 2013 at 07:29 PM.
  Reply With Quote
Old 30th May 2013, 10:23 PM   #10
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Orthos can be quite demanding. Hifiman HE-6s are pretty notorious, for example. They measured 53 ohms @ 1 kHz and needed 1.0 Vrms or 20 mW for 90 dB SPL.

Those would still make it to a bit over 100 dB on 4 Vrms, though honestly I'd be more worried about distortion at this point. Poor ol' 4556. NwAvGuy didn't even spec the O2 to drive these (though people have apparently reported pretty good results), and that's with twice the output current available.

BTW, 4.0 Vrms != 5.7 Vpp (but rather 5.7 Vp or 11.4 Vpp).
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
An "O3" - 35% smaller SMD RS O2 headphone amp version agdr Headphone Systems 16 2nd September 2013 10:03 PM
Dual O2-Inspired, SMT Headphone Amp gastro54 Solid State 0 16th May 2013 11:35 PM
Nelson Pass inspired headphone amp Fenris Headphone Systems 39 21st April 2012 10:36 AM
FS: O2 Headphone Amp its_bacon12 Swap Meet 1 10th March 2012 04:21 PM
O2 Headphone amp for sale! agdr Vendor's Bazaar 10 9th December 2011 03:05 AM


New To Site? Need Help?

All times are GMT. The time now is 08:04 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2