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Old 16th December 2011, 07:57 PM   #1471
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For AKG's with the DMK45 or XXL drivers (K701, K702, Q701, K141, K142, K171, K172, K240, K242, K271, K272) I recommend Hi = 1.6x (up to 1.8x for the 700's), Lo = 1x gain.

The reason is simple. Even with an iPod output, the 1.6x will get very loud (~110dB if I did the math right) up at max volume but when connected to a 2.1V source it will be just at the cusp of maximum power handling, thus not damaging your phones if there's an accident and the pot gets a full rotation.

Unity gain is good for getting high on the volume pot with strong sources.
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Old 16th December 2011, 08:06 PM   #1472
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Quote:
Originally Posted by ethanolson View Post
For AKG's with the DMK45 or XXL drivers (K701, K702, Q701, K141, K142, K171, K172, K240, K242, K271, K272) I recommend Hi = 1.6x (up to 1.8x for the 700's), Lo = 1x gain.

The reason is simple. Even with an iPod output, the 1.6x will get very loud (~110dB if I did the math right) up at max volume but when connected to a 2.1V source it will be just at the cusp of maximum power handling, thus not damaging your phones if there's an accident and the pot gets a full rotation.

Unity gain is good for getting high on the volume pot with strong sources.
I don't agree unless you're talking about a 2 volt home source. Tyll at InnerFidelity measured the K701 at 0.32 volts for 90 dB SPL. That's not too far off AKG's specs.

As a side note, several AMB Mini3 owners now have the O2 because the Mini3's 2.4 volts wasn't enough for their K701s.

So that's 3.2 volts for 110 dB. An iPod LOD typically only puts out 0.5 volts. So you have 3.2/0.5 = 6.4X gain to hit 110 dB SPL.

If you use the iPod headphone out, that clips around 1 volt in 10K, so that's a gain of 3.2/1 = 3.2X to hit 110 dB SPL.

With a 2 volt home source, like a home CD player, you have 3.2/2 = 1.6X
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Old 16th December 2011, 08:13 PM   #1473
Alexium is offline Alexium  Ukraine
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RocketScientist, thanks for the help. That puts everything in place, since all 4 resistorrs are exactly the same according to my DMM.
Is my understanding correct that plated tips won't get corroded? I was in the shop today, looked at the tips, but found them a bit too expensive. Those all seemed to be plated with some white metal. All tips I ever saw or had are bare copper and get corroded in no time (those parts not covered in soldering alloy). If those more expensive tips don't get corroded - I definitely want to get one.
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Old 16th December 2011, 08:18 PM   #1474
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With a plated tip, and a simple iron (no closed loop temp control) the solder on the tip will still turn black but it's just the solder, not the tip itself. A wipe on a wet sponge while it's hot and it's shiny clean. With an unplated tip, the tip itself corrodes.

A closed loop temperature controlled station can go much longer before even the solder turns black because the tip runs much cooler when it's just sitting in the holder.
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Old 16th December 2011, 08:44 PM   #1475
Alexium is offline Alexium  Ukraine
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Great, I'll get one of those tips!
One last question today (already asked, but I think nobody noticed). I got the wall transformer. Seller claimed it to give exactly 20V idle, but actually it's 22.4V. Will O2 survive this voltage? Can I tune it to accomodate it?
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Old 16th December 2011, 08:51 PM   #1476
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@Alexium, the regulators are going to run really hot at that AC voltage--especially if you work the amp hard. I would suggest building it and seeing hot they get with your headphones. If you're worried they're too hot you can either get a different transformer or you could add some 10 - 15 ohm, 3 - 5 watt resistors and put them in series with D3 and D4. Mount D3 and D4 on "end" (standing up) with only one lead in the correct location on the board. Do the same with the resistors in place of the other diode lead. Then solder the free ends together and make sure they don't touch anything--especially the top of the enclosure. Each diode/resistor pair should form "^" shape above the board.
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Old 16th December 2011, 08:54 PM   #1477
Alexium is offline Alexium  Ukraine
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OK, so I can expect O2 to actually work for some time without breaking? Then I'll finish it tomorrow and give it a try. I think I'm going to usually use it at quite low output power, perhaps I'll get away without changing the layout...
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Old 16th December 2011, 08:59 PM   #1478
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Alexium, if your O2 breaks it will be very likely due to your soldering problems not the regulators. If they get too hot, they just shut off.

You need to be very careful with your soldering, make sure all the parts are in the right way, in the correct locations, etc. before you ever apply power. Follow all the testing steps carefully. When it comes time to plug in headphones make sure they are headphones you do not care about in case they are damaged by some problem you're not aware of. Good luck.
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Old 16th December 2011, 11:16 PM   #1479
billyk is offline billyk  United States
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A wall wart I had in my bin that stated 20VAC and read 23.2 unloaded on my meter ran fine but R1 and U6 get quite hot, cannot hold finger on R1, U6 is almost too hot to hold. This is after 20 minutes. The only problem other than the heat was the turn off/on pops were very loud, I feared for the headphones, this was true at any volume setting. I won't use this traffo again, but thought my findings would be of interest.
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Old 16th December 2011, 11:37 PM   #1480
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@Willakan (and @RS) for the K702's you can't really go lower than 2.26kΩ gain resisters on the O2 for redbook-compliant sources (in case you're wondering, I actually do use the number 2.1V) without risking damage to the headphones if AKG's specs are to be believed. I recognize that the K701 may have been less sensitive than the K702 is, but if something's too loud for a person, then the gain needs to come down... and AKG's max specs back up the claim of lower gain values for the K702 and Q701. These are the currently-produced models as the K701 is discontinued. The good news is that on the K702, you should be pumping 115dB at that point. With an iPod, you are, in fact unable, to go higher than 103dB. That's way too loud for me.

So, my 110dB math was wrong and I can't find where I got it wrong (oops!). So... an iPod can't push a K702 past 103dB with a 2.26kΩ gain resister pair; but neither can a redbook-compliant source damage your cans... just your ears.
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