Pro Audio Design Forum

Hi Roger,
actually I want to play with DCAO in several spot, as pico output driver, soundcard line driver and headphone amp, and find the best place for it, now you just tell me another spot to try, I'm using Arn's step attenuator too..... looks like 2 board as a start is not enough.....

Hello all,

Had been looking for a really good headphone amp to build for some time and believe this is likely it.

Would just like to ask if anyone has experience in driving Planar headphones with it. I believe that it should have plenty of drive from what I am reading, but as am going to be buying rather expensive headphones, would like to be sure the amp is a good match for such low efficiency headphones. I was planing on getting the amp built and then purchasing one of the three headphones below.

Headphones I am looking at buying are the: HiFiMan HE-500 89db sensitivity & 38 ohms rated
“ HE-560 (new model) 90db & 35 ohms
OPPO PM-2 Headphone (Told will be released for sale any day now – no specs yet, suposedly easier to drive than most planar.)

Any feedback would be welcome,
Greg
Radiology Engineer

Thanks for joining us Greg.

I looked at some of those phones and they are indeed lower sensitivity around 90 dB SPL at 1 mW.
A 120 dB SPL would require 1W...

Comparing them to the Sony MDR-7506 at 1 mW for 106 dB they would require 39 mW to provide the same SPL.

I think you should be able to drive them pretty well though depending on how loud you want to monitor.
The highest level I think I've specified with the dual class-A is 150 mW rms.
I didn't measure higher powers since, until now, I hadn't found phones with that low a sensitivity.
150 mW should get you to about 112 dB SPL with a 90 dB SPL at 1 mW sensitivity with low distortion.

Here is the RMAA results for a 30R load at 26 mW: viewtopic.php?f=6&t=23&start=14
There are some more results here and a few posts down: viewtopic.php?f=6&t=23&start=16

One thing you can do with the dual class-A is directly parallel the outputs to increase the drive.
You can also increase the idle current though I would recommend a larger heatsink than what I show.

I'm tempted to measure higher power levels...

Hi Wayne,

Thank you for the feedback. Seems likely should be able to drive them loud enough as is. As far as biasing further class A, I have collected heat sinks off old boards and such going in the trash for years and have quite a selection already on hand. Haven't yet had one wear out.

Really interested to hear the OPPO -2 headphones as they were stated for July release, but haven't yet seen them available. Supposedly by using a conductor on both sides of the cone, they have greater effiency. If the sound quality comes close to the original model, seems like they would be ideal for what I want.

If I get a chance to hear them, I will post impressions.

Regards,
Greg

Hello,

Was hoping to confirm something before I get to applying power to my new headphone amp. Believe I read somewhere that the BD139 & BD140 transistors used in this project did not need a insulator pad between them and the heat sink. Heat sink is not grounded to the case and I did apply a touch of thermal grease to the transitors. Did a quick on line search on this and looked at the datasheet and didn't see it spelled out. Just trying to insure success rather than smoke. Any help would be appreciated.

Regards,
Greg

Greg - You can double-check by measuring for continuity between the collectors of each transistor and the heatsink. Also, if you have an extra BD139 or 140 look at the backside and you should not see an exposed lead frame.

Hello,

Thanks for the feedback. Did get a chance to check these transistors and they indeed do need insulators. Have some on order and should have the amp working this weekend.

Regards,
Greg

What manufacturer were those BD139/BD140s that were not insulated?

Hello,

Transistors were from Digikey.com, detail on one of them below. Both were from same company and the collector of all the transistor shows a short to the heat sink.

Regards,
Greg

Manufacturer STMicroelectronics
Manufacturer Part Number BD139-16
Description TRANSISTOR NPN 80V 1.5A SOT32

The Fairchild's, also in TO-126, are fully-insulated.

Is there not a black plastic over mold on the back side of the ST Microelectonics part?