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Old 8th August 2011, 04:18 AM   #1
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Unhappy Cmoy with low Z phones.. Problem

I bought parts to make a few BBCmoy amps. Well I've knocked one together on the fabulous JDSLabs board and... It sounds great!!! Until you get past 1/4 turn on the pot..

Then it begins to sound like my headbanger when the battery is low. I know these things can have problems with low Z phones. There are several mods that can be made to help with this problem..

I have read this passage on the JDS labs site and I have some questions about it.

Virtual Ground:
The cMoyBB is normally assembled with a single Texas Instruments TLE2426 "rail splitter" placed in spot U2. These highly accurate rail splitters have been used in CMoys since 1998 for their superior performance over resistive voltage dividers. The majority of users will find listening satisfaction from a single TLE2426, but in some cases (very low impedance headphones played at very high volumes), more current is demanded than any CMoy can continuously supply. If you require even higher volumes, a second TLE2426 can be added to double current handling.


Does this work? I would think I would need a buffer stage to fix this problem. The provisions are on the board already and I have the extra parts. It doesn't seem like this would help that much. I am just used to having the headroom for a bit louder volume levels. This amp is clearly having problems powering these headphones. 32-Ohm Senn eh150.

The headbanger is nearly 1w. It's a shame really. I hope that this mod will work as advertised. The Cmoy sounds so much better. It just starts to break up about 1/3 into the dial.

I guess this could be a component issue. I am using a OPA2227P. Gain of 6. My input caps are huge 1uf poly film. I have also OPA2132 available if that might help.

Any advice would be helpful.

EDIT:
Just to clarify, I don't consider the level to be very loud when this happens.

Last edited by Rembrant; 8th August 2011 at 04:28 AM.
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Old 8th August 2011, 06:09 AM   #2
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You're using a single 9V battery for power? +/-4.5V rails don't appear to leave a lot of operating room for the OPA2227s. A rail2rail op amp should improve things a bit if this is the source of the problem. The OPA2132 isn't R2R but does seem a little better in the Vout spec.
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Old 8th August 2011, 07:16 PM   #3
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I am using a 12v RC pack right now. The intention is to use two 9v NiMh in series. The RC pack is at about 11.8v on full charge and two 9v NiMh should be about 14-16v. Good to know that the 2132 might help a bit. I'll give it a shot.

I was really just wondering if the extra tle2426mod was worth it. They are cheap enough but from my limited understanding, I don't see that helping the situation much.
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Old 9th August 2011, 08:33 AM   #4
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LOL sofa you were 100% correct. This thing really needs more voltage. Also JDS was right. Adding the second TLE chip helped somewhat. So the chip did need a bit more current also. I believe that could have been overcome by a little more capacitance easy.

Apparently, my battery was bad and not charging. It kept reading 10v and falling after a fresh charge. It should be at least 11.5. The charger by it self puts out about 13v. So I unplugged the battery and hooked it straight to the charger with some gator clips. I got about another 1/4 turn on even the nastiest compressed junk. Like game sounds. I would say that this is nearly sufficient. It's way too loud on music now but voice aps and the worst of game sounds still need a little headroom.

I have no idea how people can get away with one 9v with this design. It needs at least 14v to function within the realm of useful for more than just the best source material.
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Old 9th August 2011, 09:19 AM   #5
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Glad to see you've sorted it out.
For my headphone amp, I simply chose the resistor divider method. Nothing's perfect, but it has advantages. Not the least of which is the supply voltage range virtually equal to the op amp parameters. Certain apps might nudge me toward a splitter chip, but the Cmoy isn't one of them. Adding a second battery which get you further with the final project IMO. As for just one 9V for power, and sidestepping the "does it sound different?" debate, different op amps do perform differently in any given circuit. The Cmoy shines as a simple "use what ya got" project that gives good results. Output current from most op amps is still a limitation with popular 'phones though.
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Old 9th August 2011, 09:20 AM   #6
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Quote:
Originally Posted by sofaspud View Post
Glad to see you've sorted it out.
For my headphone amp, I simply chose the resistor divider method. Nothing's perfect, but it has advantages. Not the least of which is the supply voltage range virtually equal to the op amp parameters. Certain apps might nudge me toward a splitter chip, but the Cmoy isn't one of them. Adding a second battery which get you further with the final project IMO. As for just one 9V for power, and sidestepping the "does it sound different?" debate, different op amps do perform differently in any given circuit. The Cmoy shines as a simple "use what ya got" project that gives good results. Output current from most op amps is still a limitation with popular 'phones though.

After I busted out the trusty meter again. I made some more measurements. The Batt charger output is actually around 20V even. The amp with 2xtle-2426 drags it down to 17.8v. Voltage offset RT channel .9mv. Offset left cahnnel .4mv. I would say it almost isn't worth it. If it didn't sound like angels licking my eardums.
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