Some LME49600 implementations - Page 18

This looks to be the right socket if you can't identify one from a regular supplier:

http://www.ebay.com/itm/5pcs-1-8-3-5mm-Stereo-Socket-Headphones-Jack-PCB-Panel-Mount-Connectors-2411-/320960009847?_trksid=p2045573.m2042&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D2258704038311425622%26pid%3D100033%26prg%3D1011%26rk%3D2%26sd%3D221023868653%26

Obviously I can't be certain without having one in my hand, but there are numerous other 5-pin 3.5mm sockets on ebay, all with a chrome ring round the hole, unfortunately they protrude too far from the front of the board to permit it to fit in the enclosure, so be warned. I may change the footprint at the next iteration, but that doesn't help here and now.

Failing any other source, I do have a limited number of the correct sockets in hand, but I think you will find it cheaper to buy 5 from that ebay source than pay the postage from here, I get away with sending the boards letter mail (£2) because they are flat, but package postage is twice that.

w

Ah, I need to run some sims or tests on the servo/relay circuit myself. When does the relay actually drop out in response to a step input? I don't think it does until the DC offset at the servo output gets quite large (quite a few seconds), since the take-off point feeding the comparators is at the servo output, at the midpoint of the filter intended to keep switching spikes from the chopper-stabilized servo opamp out of the audio.

This is not the only function of the relay, it prevents thumps at switch-on and switch-off. Obviously you can speed the whole thing up, but only at the expense of increasing the impact on the FR of the amplifier as a whole. Switching off the output comparatively late is better than never switching it off. This way is better than a (another) simple low-pass filter at the output. Output offset protection is always problematic, there's always a compromise between response time and false triggering due to low frequency program content, I always intended to return to the issue and that of the servo authority at some point, but I've allowed myself to be distracted.

w 

I have this:- http://www.head-fi.org/t/586042/what-i-did-next-with-texas-headamp-chip-a-digital-control-desktop-portable PGA3120/TPA6120 design running from 6 * 16340/CR123A lithiums, but I didn't incorporate the constant current/constant voltage charger in the 49600 amp. I use protected cells in combination with the charger, it requires accurate setting of 2 trimpots, but thus far I've had no problems.

I decided to go with the charging scheme after reading a few datasheets and some articles about lithium battery charging and standby use. The batteries are continuously in circuit even when the wall supply is plugged in.

You'd have to drill an extra hole in the backplate and bring out a wandering lead if you wanted to plug an external battery pack into the 49600 amp, but you'd have to think hard about how you wired it in since there's currently separate regulation for each channel

w

The DC servo integrator offset is critical in determining the overall amplifier output offset, so the offset voltage of the opamp used is critical. Since the integrator uses quite high values of resistance to keep the size of capacitors used manageable, the input offset current has quite a big effect on the output offset and it's necessary to calculate its effect when choosing an opamp. 

It's a close call in performance terms. The two chips are very close in terms of offset voltage and current, and the ADA4638 is cheaper by a good margin (less than half the price). I'd probably use it in future over the Linear part, particularly as it doesn't require additional filtration at the output, although the filter in the amplifier as drawn serves a dual purpose, and should be left as-is if the ADA4638 is used. Other than that, they're so close that in testing either chip might win, depending on the individual examples.

I has a pretty good search round for low offset parts at the time I drew up the design, and ran a spreadsheet to calculate overall offsets, but I didn't see the ADA4638 at the time. Low offset and +/- 16V with a good output swing are the principal requirements, so if you find anything exceeding the performance of the ADA4638 then please let me know.

w

Of course the actual output offset may in practice be perfectly satisfactory with a range of different parts, and in fact it has been argued that the LME49600 amplifiers with no DC servo are entirely adequate. The amount of offset tolerable is open to question, but nobody would really dispute that a low offset is preferable in principle.

I believe the Mouser BOM which Avro_Arrow posted in the thread is accurate, but I prefer 565-1577-ND for the electrolytics as they are 15mm tall. This will ensure that they clear the extrusions in the recommended case. I will move the footprint slightly in a future iteration so that taller caps will also fit.

This is now a well-proven design with some desirable features.

1) It fits an attractive off-the-shelf enclosure and is completely self contained (PSU is on the board).

2) It runs off a readily available wallwart.

3) It has a switch-on and -off relay to protect headphones from any transients (thumps or clicks) and any excessive (beyond correction) input DC offset.

4) It is fully DC coupled throughout with a DC servo and vanishingly small output offset (<0.1mV).

5) It employs active parts enjoying the highest reputation for quality and can be built with the very best passives in the signal path.

No claims are made for the performance of the amplifier beyond the RMAA and oscilloscope results shown, but if they are to be believed (and I think there is little reason to seriously question them) the noise, distortion and crosstalk are all well below the threshold of audibility, the FR is flat, the output impedance is extremely low (<1R) and the output voltage swing is sufficient to drive all but the most demanding of 'phones, and there is even the possibility of building for higher power rails.

There are 3 boards remaining, although I will probably order more when they are gone. Price (non-profit) including worldwide shipping is £6.50 (UKP).

w