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Old 19th December 2011, 01:52 AM   #31
agdr is offline agdr  United States
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regal -

Definitely post your results of the mod build! I would encourage anybody doing any of these mods to post results and pictures too - whether it works or not. Post a picture of the smoldering remains if that is all that is left afterward, lol!

The whole thing about using +/-15v rails with 32 ohm headphones is pretty interesting. True that the amp would not need to be turned up very much (volume) to get an acceptable sound level out of the phones. But the problem comes in what if the volume does get turned way up - then you get a large voltage swing into the small impedance load and the resultant power could be more than the headphones can handle, if the amp can source a lot of current.

Now a person might think "but that would never happen - I would never be turning the volume up loud enough to cause too much power to the headphones since it would be too loud". But here is a scenario that keeps sticking in my head. Lets say someone has a pair of 600 ohm low-sensitivity headphones that really do need the full 8.5Vrms-ot-so output swing from +/-15v rails to be loud enough. They are listing to their amp that way one evening and eventually switch it off and unplug the phones.

Then two days later they plug in a pair of 32 ohm headphones, which are still lying on the table and can't be heard yet, then switch the amp on, not really remembering where the volume control was last at or which of the two pairs of headphones it was last used with. And just as the amp is switched on the doorbell rings and the person has to run off for 10 minutes. When they come back they find the amp has been pumping out 8.5Vrms into the 32 ohm phones and they are now cooked. Could happen! Just replace "doorbell" with the phone ringing, the cat chucking up a hairball on the furniture, or any other unplanned distraction.

However in the particular case of using the LME49860s they are only capable of about 30mA each, or 60mA in parallel in the O2, which would limit maximum current (over the two NJM4556s which together can do around 140mA). The problem there (with the LME49860s) is that the result, with volume turned up, would be severe clipping going into the headphones. Not good.

So, to summarize, with +/-15Vdc rails even if the output chip dissipation would be within bounds for lower impedance headphones (with their lower voltage swing requirements) I really can't recommend using the modded amp with anything less than 600 ohn headphones. Just too easy to essentially make a mistake with the volume control and feed to much voltage to lower impedance headphones.

One solution here, when powering both a 32ohm and 600 low low-sensitivity set of headphones, is just to make two O2s. One built up the standard way with the 12V rails - or even 9V or 6V rails for safety! - for the 32 ohm headphones. Then a second O2 modded with 15V or 18 volt rails for the 600 ohm cans. There is just too much variation in headphone impedances, it seems, for one amp to cover all bases and still be "safe" from volume control accidents. One of my suggestions to RocketScientist along the way for the desktop O2 was a variable rail voltage control for this very reason, but that wouldn't really solve the "accident" problem either. It would be just as easy to have the rail voltage control in the wrong position when the doorbell rang.

Anyway, just my two cents!

I just realized that I never posted a link to the DIP8 heatsinks I used in the first set of mods in this thread. Here they are:

580100B00000G - Board level heatsinks

https://avnetexpress.avnet.com/store...1&listIndex=-1

I bought mine at Avnet Express and it looks like Newark also has them. Unfortunately Mouser and Digikey don't. The heatsinks worked out very well. I had to just bend one heatsink fin up slightly to avoid a 470uF cap and trim 1/16 off the bottom of the clip to clear a 0.22 cap. I'm going to eventually hot glue that bent fin right to the cap for even more mechanical support.

These heatsinks really work well, too! Fairly warm when I put a finger on them, indicating good thermal transfer from the chip.

I saw your post about the standoffs in the O2 thread. Yes, that would be an interesting way to go. Using a bigger case and mount with standoffs.

The AMB e12 is definitely a great little protection board! But by way of full disclosure I had a small amount of input on the latest version so I'm probably somewhat biased, too.

The ε12 Muting / Protect Circuit and the "history" link on the left...

But seriously that is a very good protection board and it senses actual DC riding on the output signal. AMB has some really good stuff.

The CRC filter is an interesting thing. My main purpose for it was filtering out power line noise that might happen to be in the upper audio band. Unfortunately the filter caps are around 20 times too small to filter out the 120hz and 180hz power line harmonics. The filter would have some value for power line noise from around 2khz on up that might happen to make it through the transformer via core or winding parasitics.

But the CRC filter has that voltage drop across the resistor and a reduction in ripple going into the regulator, both of which would reduce the regulator input voltage and make them run a bit cooler. I've seen a post or two on Head-Fi where folks seem to be using the filter mainly for that reason. Like you say, not a bad idea for 20VAC input to knock the voltage down a bit before it hits the regulators. I just noticed this morning that RocketScientist has added a mod in the O2 thread that puts a 10-15R resistor in series with the rectifier diodes to do a similar thing, although putting it betwen the caps here gives some filtering too - but is much more of a DIY project, having to cut traces and all!

I'll be quite curious to hear how your +/-15V mod does with your 600R cans, if you decide to build it!

Last edited by agdr; 19th December 2011 at 02:00 AM.
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Old 19th December 2011, 02:51 AM   #32
regal is offline regal  United States
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Quote:
Originally Posted by agdr View Post
regal -

. When they come back they find the amp has been pumping out 8.5Vrms into the 32 ohm phones and they are now cooked. Could happen! Just replace "doorbell" with the phone ringing, the cat chucking up a hairball on the furniture, or any other unplanned distraction.
!


All the new high power commercial amps have this issue, Heck even the B22 which folks have been using for years.
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Old 19th December 2011, 08:39 AM   #33
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So much for "harmless" excess I suppose!
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Old 19th December 2011, 09:13 AM   #34
regal is offline regal  United States
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Originally Posted by Willakan View Post
So much for "harmless" excess I suppose!
Its not an issue if you are on old timer, we always turn the amp down all the way when changing phones. Not doing that would be like walking under a ladder
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Old 19th December 2011, 09:08 PM   #35
agdr is offline agdr  United States
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Quote:
Originally Posted by regal View Post
Its not an issue if you are on old timer, we always turn the amp down all the way when changing phones. Not doing that would be like walking under a ladder
Exactly! That would solve the problem. I know that AMB keeps mentioning turning the volume all the way down between headphone changes over on his forum, for any of the amps. Also solves the issue of TRS plugs shorting on their way in and out.
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Old 23rd December 2011, 07:29 PM   #36
agdr is offline agdr  United States
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regal - I've discovered what could be another hiccup in using the higher voltage +/-15V or +/-18V rails with lower headphone impedances. In your example of the 32R Grados needing a maximum output swing of 0.4Vrms, even if the chip dissipation were OK at that level (they probably are) and even if the volume control were turned fully down between headphone changes, the volume control rotation would be only about 1/20th of full rotation. Probably too small to be practically set. The problem is that the first stage would still have to have a gain of 7x-or-so to produce the output voltage swing needed for the 600R phones. So the first stage output would be the 8.84Vrms (15V rails) or 10.6 (18Vrms rails) and only 0.4V is needed.

One possible solution is set the gain switch up so that high gain is 7x for the 600R headphones and low gain is the 1x minimum for the 32R phones. But even then the maximum rotation would only be 1/4, or less with the non-linear tapers in the BOM. But at least that would probably be usable. There is probably also some clever way around the pot rotation issue too out there somewhere.

I think that I'm still a fan of two separate O2s. The +/-15V or +/-18V O2 for the 600R phones, and a stock +/-12V, or even down to +/-9V or +/-6V O2 for the 32R phones.
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Old 23rd December 2011, 07:38 PM   #37
agdr is offline agdr  United States
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Default +/-15V and +/-18V revised chip dissipation calculations

I just realized that I did, in fact, muck up the output chip power dissipation calculations for the 600R headphones with +/-15 V rails (NJM4556 chips) and +/-18V rails (LME49860 chips), in a post above. My wording was right, input power from the rails minus the power delivered to the headphones (and 1R resistor dissipation), but then my power dissipation calculations went in an odd direction using maximum chip voltage swing.

Here are the corrected calculations. The output chip dissipation roughly doubles from what I posted above, but is still well within bounds for either case.

For the NJM4556 chips running +/-15V rails into 600R headphones the maximum swing is still 8.84Vrms and the corresponding maximum rms current 14.7mA, as I posted earlier. But it is important to be a bit more precise that 8.84Vrms is the output of the O2 into the headphones, ie after the 1R resistors. The total power dissipation in the 1R resistors would then be (14.7mA * 0.5R) = 7.35mW.

The NJM4556 power dissipation at the maximum output voltage swing would then be P(rail) P(1R resistors) P(headphone) = (15V * 14.7mA) 7.35mW 120mW = 93.1mW. Still well within even the de-rated 400mW power handling capability of the NJM4556 chip. A double-check on peak current delivery is also in order. I(peak) would be (12.5v [peak output voltage swing]) / 600R = 20.8mA, well within the 140mA capability of the two paralleled chip halves. I haven't done the math, but at just 93mW chip dissipation at full output voltage swing I would guess that 300R headphones may also fall within chip dissipation limits with +/-15V rails.

For the LME49860 chips running +/-18V rails into 600R headphones the maximum swing is still 10.6Vrms and the corresponding maximum rms current 17.7mA, as I posted earlier. The 10.6Vrms is again after the 1R resistors into the headphone load. The total power dissipation in the 1R resistors would then be (17.7mA * 0.5R) = 8.85mW.

The LME49860 power dissipation at the maximum output voltage swing would then be P(rail) P(1R resistors) P(headphone) = (18V * 17.7mA) 8.85mW 187mW = 123mW. Also still well within even the de-rated 400mW power handling capability of the LME49860 chip. A double-check on peak current delivery again: I(peak) would be (15.0v [peak output voltage swing]) / 600R = 25mA. That is 25mA total over the two paralleled op amp sections, or 12.5mA per section, well within the data sheet maximum of 26mA. Again, given these numbers, I would say there is a good chance that 300R phones may also be within chip dissipation and peak current limits at +/-18V rails.
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Old 24th December 2011, 04:14 AM   #38
regal is offline regal  United States
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Quote:
Originally Posted by agdr View Post
regal - I've discovered what could be another hiccup in using the higher voltage +/-15V or +/-18V rails with lower headphone impedances. In your example of the 32R Grados needing a maximum output swing of 0.4Vrms, even if the chip dissipation were OK at that level (they probably are) and even if the volume control were turned fully down between headphone changes, the volume control rotation would be only about 1/20th of full rotation. Probably too small to be practically set. The problem is that the first stage would still have to have a gain of 7x-or-so to produce the output voltage swing needed for the 600R phones. So the first stage output would be the 8.84Vrms (15V rails) or 10.6 (18Vrms rails) and only 0.4V is needed.

One possible solution is set the gain switch up so that high gain is 7x for the 600R headphones and low gain is the 1x minimum for the 32R phones. But even then the maximum rotation would only be 1/4, or less with the non-linear tapers in the BOM. But at least that would probably be usable. There is probably also some clever way around the pot rotation issue too out there somewhere.

I think that I'm still a fan of two separate O2s. The +/-15V or +/-18V O2 for the 600R phones, and a stock +/-12V, or even down to +/-9V or +/-6V O2 for the 32R phones.

Thanks again for the info. I know Grados are commonly used with amplifiers that have a gain as high as 8x or more with the same series of alps pots (I think), the 1x should be fine, but you are right that ideally one would make an O2 for the low impedance cans separate/dedicated.

The smart thing to do with the 1x gain would be to eliminate the first opamp gain stage all together, all it is doing is adding noise and distortion to headphones like Grados that max out at .5Vrms (assuming a normal 2vrms source.) Really only need the buffer section of the O2 for these phones.
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Old 24th December 2011, 05:01 PM   #39
agdr is offline agdr  United States
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Originally Posted by regal View Post
The smart thing to do with the 1x gain would be to eliminate the first opamp gain stage all together, all it is doing is adding noise and distortion to headphones like Grados that max out at .5Vrms (assuming a normal 2vrms source.) Really only need the buffer section of the O2 for these phones.
I agree! That is a good idea. All the first stage parts between the 220pF RF filter capacitors and the pot could be left out, including the 10K input resistor. Just jumper the 220pF cap right over to that channel's pot. The 10k pot then becomes the "input resistor" and the RF filter still works as before. The O2 signal path then reduces to the typical pot-in-front design feeding the second stage current buffer. The second stage still has the input current blocking capacitors, so still no DC through the pot. The only thing lost is gain, which like you say just isn't needed if someone knows upfront that they only need 1x.

Good stuff!
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Old 11th January 2012, 04:35 PM   #40
agdr is offline agdr  United States
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Default O2 amp - DC power jack mod for battery charging only

Quote:
Originally Posted by Nikon1975 View Post
Would it be possible to feed the amp with DC, mainly to charge the batteries. I don't mean necessary on the onboard AC connector, I could tap someware and add an extra DC input.
I'm posting this reply from the O2 thread here so as not to muck up RocketScientist's thread with modifications.

I should also note right up front that this project may not buy a whole lot over simply using the WAU12-200 AC adapter for the O2 that RocketScientist has specified in the BOM. That adapter is about the same size as most 24Vdc 100mA adapters. If all you are doing is charging the batteries it has plenty of power.

Here is one way to do it, although RocketScientist may have a slicker way. A DC input jack (J4) and Schottky diode (D8) could be connected as shown below to the unused pins 1 and 4 on the power switch, then fed with a DC adapter that is 24Vdc and at least 100mA capable. The adapter should be 24Vdc only - rated nothing higher or lower. This arrangement works for charging the batteries - your request - but would not run the amp to produce sound. You can get to the unused pins 1 and 4 on the power switch underneath the O2 PCB, then run the wires back up through one of the PCB holes if you need to wherever you intend to mount the DC jack.

Here are the details. This one is essentially a modification. This mod works because the DPDT power switch that RocketScientist specified is non-shorting, meaning that it breaks the connection to the rest of the O2 (power management circuit and signal path) before making the connection to the single-rail DC adapter. Since there is no ground wire coming in from the power source in this case (not a dual rail power adaptor) there would be no way to run the rest of the O2 properly, so the rest of the circuitry has to be switched out like this.

Then Maha Powerex tech support tells me that it is OK to charge two 8.4V NiMH cell in series like this if the charging currents are limited to essentially trickle rates, which is what the two 220 ohm resistors R1 and R2 that RocketScientist have in the circuit do. In this arrangement the power flows from the DC jack around the reverse-biased D2 and D6, through the 220R resistors to the batteries, just exactly as happens from the O2's voltage regulators when those are used on AC.

In this case through instead of 12Vdc across each battery individually with each having a 220R current limiter, we are applying 24Vdc across both batteries in series and using two 220Rs in series to limit the current. Equivalent circuits either way. The ground no longer works, or is used, but it doesn't matter just for charging in this arrangement.

D1 and D5 are reversed biased in this setup and block current flow back into the voltage regulator circuits. A new Schottky diode D8 is added to prevent current flow from the voltage regulators back to the DC adapter under a special circumstance. The issue here is that if both voltage regulators were at the high end of their specifications, +12.2V and -12.2V, that would give 24.4Vdc from negative to positive rail on the input side of D1 and D5. Minus the 0.2V Schottky drop that leaves 24Vdc on the other side of D1 and D5. If the DC adapter were then less than this, such as 23.6Vdc, current would flow from the O2 voltage regulators into the DC adapter.

So with D8 added it doesn't matter if the AC adapter is plugged in or not when the DC adaptor is plugged in. The Schottky diodes will figure it out. If the O2 voltage regulators produce a higher voltage they will be used to charge the batteries. Otherwise the DC adaptor will will win (which should be the case with the vast majority of "24Vdc" adapters from my experience, they usually run at 24.5Vdc - 28Vdc) and charge the batteries.

Please post a picture if you build it!
Attached Images
File Type: png O2 add DC port 1.png (14.1 KB, 387 views)

Last edited by agdr; 11th January 2012 at 04:50 PM.
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