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Old 17th December 2010, 06:14 AM   #11
opc is offline opc  Canada
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Hi Guys,

Nikon1975:

There's a board here for you if you want it! As you can see from the picture, I've got a good pile of them.

I'll figure out what I want to do for pricing, and I'll post it up here tomorrow.

Frex:

Very valid concern, and one that is easily addressed if need be. There are two options I can see, the first of which is band limiting the LME49600 by lifting pin 1 (not really band limiting since it only swaps between 180 and 110MHz) or soldering small caps in parallel with the two input R's.

Any band limiting does come with phase issues though, which is why I chose not to do anything. The circuit is perfectly stable as is, and unless you're really sloppy with input and output wiring, then there isn't really any issues with RF interference.

I did find that I was able to pick up radio stations with long input cables and a long headphone jack, and with the circuit strewn across the carpet and not in a chassis, but I don't think it would be an issue with any normal setup in a proper chassis.

Just the same, I could calculate the value of two input caps that would help roll off at say 100kHz, at the cost of some phase shift at 20kHz.

abraxalito:

It does depend on what you're driving it with, and if you're running SE then things definitely change. Not sure what the exact impedance is, but it is technically a large flat piece of copper foil, so I'd imagine it's pretty decent when it come to HF.

If you can draw up what you're thinking in terms of input pin current, then I can certainly look into testing it. In my experience though, the serious culprit in crosstalk is almost always the output jack, and even more often the headphone cable itself. I pulled apart a pair AKG 240 MkII headphones the other day and saw that each channel uses a common ground wire that is tied at the headset itself! That means you're got at least 0.5 to 1ohm of ground resistance which is going to results in terrible crosstalk. The jack itself has a decent amount as well.

If I measure crosstalk at the jack with a 32 ohm load, I get between -98 and -102dB which is pretty decent. Higher impedance loads give significantly better results.

Cheers,
Owen
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Old 17th December 2010, 06:56 AM   #12
dggs is offline dggs  United States
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Hi Owen,

I'm interested in getting 1 or 2 sets of the PCB.

Thanks,
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Old 17th December 2010, 01:24 PM   #13
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Quote:
Originally Posted by opc View Post
If you can draw up what you're thinking in terms of input pin current, then I can certainly look into testing it.
How about 100uA RMS with frequencies ranging from 50Hz to 100MHz or is that too tall an order? The audio band output noise floor to be monitored while playing some high level white noise in the band 22-100kHz. That ought to be enough of a torture test to see if noise modulation is occuring

Quote:
In my experience though, the serious culprit in crosstalk...
I'm not so interested in crosstalk as I don't think it has a serious impact on sound quality - LPs have poor crosstalk by design for example. More relevant IME is intermodulation.
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Old 17th December 2010, 05:08 PM   #14
qusp is offline qusp  Australia
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Looks great Owen, i'm looking forward to the PCB, I might even play around with a portable with one, looks small enough. if you havent sent mine, i'll grab another one from you if thats cool.
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Old 18th December 2010, 02:36 AM   #15
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Quote:
Originally Posted by opc View Post
Finally, I've attached the schematic with the top layer layout.

Any comments or questions are welcome, and of anyone is interested in a board, let me know. I made 25 pairs of amp and PSU boards, some of which I'll be keeping for various projects.

Cheers,
Owen
Hello,
If you have a set of PCB's to send south count me in for a pair. I would like to try my hand at surface mount parts.
DT
All just for fun!
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Old 18th December 2010, 03:16 AM   #16
qusp is offline qusp  Australia
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should we GB the buffers and opamps?
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Old 18th December 2010, 04:22 PM   #17
opc is offline opc  Canada
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Hi Guys:

A group buy for the LME parts would indeed be wise. Digikey has the worst pricing for these parts, and they can be had for half as much at other sites.

Either way, I've got 25 boards and that's 150 LME49990 and 50 LME49600.

For 100pcs of LME49990 you've got:

Avnet = 2.57 each
Newark = 3.00 each
Digikey = 4.15 each
Arrow = 2.95 each
Future = 3.12 each

Digikey's single part price is $5.70.

For 50pcs of LME49600 you've got:

Newark = 6.98 each
Digikey = 9.47 each
Arrow = 6.53 each
Future = 6.90 each

Digikey's single part price is $11.56.

I would consider doing a group buy kit for the boards and parts, but I'm not prepared to do it at cost. I just got through that process with 40 DAC boards, and it took easily 50 hours of my time and I ended up more than $200 in the hole.

If there's enough interest though, I'll look into it. The price would end up being less than buying all the parts on your own, but it wouldn't be at cost.

Cheers,
Owen
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Old 18th December 2010, 07:06 PM   #18
qusp is offline qusp  Australia
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yeah see I dont need anything much except the chips, not using the PSU and wont use the input or output connectors, or probably even the same reservoir caps, although those LF series are pretty good value as polymer caps go. i'm sorted for the rest of the parts as far as I can see, except for perhaps specific feedback R's

yeah DKs extortionate prices are why I asked, the buffer is insanely priced

did you get my email about the boards mate?

so yeah I would participate in a GB for the chips, but thats about it I think, unless prehaps you did an SMD only thing.

I dont even want to talk about the time yet to come for me for the GB haha, at least youve got something on the way to offset the 200, hopefully it puts a smile on your face
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Old 18th December 2010, 07:40 PM   #19
johnm is offline johnm  United Kingdom
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Hi John.

I'd like a board if there's any still available please?

Kind regards,

- John
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Old 18th December 2010, 08:38 PM   #20
qusp is offline qusp  Australia
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john?? there is no john here but you
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