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Old 23rd February 2012, 06:10 PM   #1
Tekmek is offline Tekmek  Canada
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Default how to select portable headphone amp kit/diy?

I recently decided to put colour in my daily commute (train ride) and purchased a pair of philips/O'Neill "the stretch" headphones.

Not exactly audiophile, but excellent in robustness and audio isolation. Bass is somewhat lacking but the sound is still good. Long story short, they do very good for on-the-go music listening, better than anyother headphones I have ever owned


Problem is, my MP3 player doesn't deliver powerful ennough output, as seem to be usual with 32ohm headphones on MP3 players. The "normal" volume setting is almost at maximum at all time.
Anyway, its amp isn't the best so passing by line-out trough a better amp can't be wrong... and should be a fun project.

So I am looking at the next step, a portable headphone amp.

I'd like to build it, but am quite lost about what's available, what's good, what to read, where to go... and mainly "how to choose."

I've considered Chu Moy's design, seems to be a classic. But I am thinking there might be somethign else to look for? I'd be very happy to use LiPo batteries with an onboard charger.
I wouldn't want to spend a huge fortune on that, but would want quality audio. You know, the usual compromise?

Any pointers welcomed. If there's already ongoing discussions on the matter (that I have obviously missed) please let me know.
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Old 23rd February 2012, 08:33 PM   #2
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You can try building an Apheared 47, which is a Cmoy on steroids.
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Old 23rd February 2012, 10:00 PM   #3
bcg27 is offline bcg27  United States
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Maybe amb's mini3. Has rechargeable lipo batteries, readily available pcbs, and great documentation and support.
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Old 23rd February 2012, 10:35 PM   #4
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Probably the best performing portable headphone amp available in kit form is the O2. Details here:- NwAvGuy: O2 Headphone Amp. It's a little bulky, but nothing's perfect and the price is better than reasonable.

You can order a kit from the 'Group Buy' section here.

Last edited by counter culture; 23rd February 2012 at 10:37 PM.
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Old 23rd February 2012, 11:24 PM   #5
Tekmek is offline Tekmek  Canada
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Thanks for the info guys.

Counter, I saw O2's website. This amp seems awesome, but as you said somewhat bulky. I might want to keep a lower footprint and partscount. Unless there's a real gain for doing otherwise..?
My main question remain... how to select? there's many available projects. Still... for some reason, most websites discussing about pocket headphone amps point to Chu Moy's design...


I am not sure I understand the apheared47 amp... its a cmoy tweaked to output stronger signal, with a bigger battery?


I have stumbled upon this site
Various writings, mumblings and dialogs.: June 2011

Which is mainly a schematics to put 16850 lipos to a cmoy amp; anyone has experience with it?

It seems like a good idea.
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Old 24th February 2012, 03:30 AM   #6
qusp is offline qusp  Australia
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i would be wary with lipos as an amateur
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Old 24th February 2012, 06:04 AM   #7
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You don't need lithium-ion. There are still high-capacity Nicads available and they can easily be trickle-charged with a wall wart.

There are lots of choices here, but honestly, why not build a CMOY or Headbanger amp as a starter project? It will cost you peanuts and if you don't like it, you can toss it away. You can make one in any small metal or Radio Shack project box.
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Old 24th February 2012, 03:46 PM   #8
Tekmek is offline Tekmek  Canada
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I might have a few 16850 lithium cells from a laptop that I could use, and have no nicads on-hand... that's mainly why I'd go with Lithium.

Moy's design seems to be generally accepted as a great sounding amp, and seems to be the simplest and cheapest option. I guess I'll look into that.

Would it be a good idea to socket the op amp in order to be able to experiment with various ones? (or does the whole setup rely strongly on the specific op-amp, leading to trouble if one is to change it once the board is made?)
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Old 24th February 2012, 07:04 PM   #9
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Quote:
Originally Posted by Tekmek View Post
I am not sure I understand the apheared47 amp... its a cmoy tweaked to output stronger signal, with a bigger battery?
The Apheared-47 only is about using a second opamp to get a buffer stage. Plus its output impedance is very low, as its feedback loop stretches to the output. Using the two sections of a dual opamp for each channel means it's easy to wire up.

Using a buffer makes an awful lot of sense in a headphone amp. While the maximum output current of a number of opamps can be quite sufficient, they usually aren't too happy driving low-impedance loads.

With an MP3 player as source you can easily use a 10k or even lower volume pot (stereo, log). That reduces distortion due to input impedance nonlinearity and input imbalance as well. The feedback network can be lower in impedance, too - I'd shoot for a sum resistance (Rf + Rg) of maybe 2..3k.

The commonly used OPA2132/4 opamp actually isn't much of a load driving king, in fact it's quite sensitive to output loading (maximum current still is about average). Well-suited candidates in terms of current output would include NJM4556, NJM2114 (assuming you can find one...), NE5532 and NJM4580 in order of preference. With low impedances around them as outlined above, they should work well.

Incidentally, while equal current sharing as in the Apheared-47 maximises available output current, it results in less than optimum nonlinear distortion performance. The opamp that does voltage gain has less spare open-loop gain and hence will do a worse job kepping its output stage distortion at bay. (Essentially the result is equivalent to a single opamp driving 2*Rload then.) The ideal ratio of current sharing resistors thus is about as high as voltage gain.

Here's a little example using a discrete opamp model at a gain of 11 (21 dB) driving 11 Vpp into 64R. Equal current sharing resistors:
Click the image to open in full size.
And 100R vs. 10R.
Click the image to open in full size.
2nd harmonic remains about the same, but all the higher-order stuff decreases dramatically.

With a bit more effort in wiring things up, you can use two dissimilar opamps for gain and buffer, too. Those with beefy output stages tend to fare less well when it comes to gain-bandwidth and noise, so it makes sense to use those as a unity-gain buffer with a low-noise opamp providing gain.
This arrangement is, however, not without its pitfalls in terms of stability, potentially making a little feedback cap across the gain opamp necessary if the buffer is too slow.
(With the two identical and identically-loaded opamps in an Apheared-47, the one used as a buffer will always be faster. At medium gains like 3x you shouldn't run into too much trouble shifting around current-sharing resistor values IMO, but it does potentially become a tradeoff between distortion performance and stability.)
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Old 24th February 2012, 07:26 PM   #10
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Originally Posted by Tekmek View Post
I might have a few 16850 lithium cells from a laptop that I could use, and have no nicads on-hand... that's mainly why I'd go with Lithium.
Do these have integrated protection circuitry? I would never use "naked" cells that don't.

Cmoys are usually powered from 9V blocks, two each for a +/- 9 V supply is most practical. 2x 4 AAA or AA cells would also work quite well.
Quote:
Originally Posted by Tekmek View Post
Would it be a good idea to socket the op amp in order to be able to experiment with various ones? (or does the whole setup rely strongly on the specific op-amp, leading to trouble if one is to change it once the board is made?)
Sockets, while practical, do come with their share of inductance and resistance. Probably not too much of an issue when working with well-behaved opamps on breadboard, but problematic for faster types that are more critical WRT layout. Use the best ones you can find.

Oh, and while it would be no fun, one could even buy a little amp like a FiiO E6 (the bigger E9 is essentially a Minił clone with better performance, btw). Matching something like that in DIY is not at all easy, especially when driving fussy low-impedance loads like BA IEMs.

Last edited by sgrossklass; 24th February 2012 at 07:29 PM.
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