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Old 30th March 2009, 02:40 AM   #1
kuroguy is offline kuroguy  Germany
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Default Help me design my DAC/Line Preamp.

I've started building my line DAC/preamp. I've attached a couple of photos. The DAC sits on the left side. The R-core transformer serves the DAC and a custom relay/Microcontroller board that controls the volume and manages the relay board to select inputs, reset the DAC, and control a 64 step attenuator. There will be a set of 4 buttons located in front of the LCD display which is mounted in the box at the left front of the paner.

The rear panel will be drilled for necessary input and output plugs.

I would like to build in some sort of tube headphone amp/buffer on the right side of the chassis. Can anybody recommend a circuit for the headphone amp. I'm not above a hybrid. I've got a starving student and the thought had crossed my mind to build one of them. My only concern would be the skyrocketing price of the 19j6. Whatever the amp is, I'd prefer to not have to buy output transformers.

Do I need a tube buffer in this stage?

Please ignore the current fasteners. I plan to get the hardware store for proper fasteners and to also polish the aluminum panel after I finish machining it.

Any ideas/comments are welcome as I'm kinda winging it here.
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Old 30th March 2009, 02:42 AM   #2
kuroguy is offline kuroguy  Germany
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The rear....
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Old 30th March 2009, 02:43 AM   #3
kuroguy is offline kuroguy  Germany
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The underside....
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Old 31st March 2009, 12:03 PM   #4
kuroguy is offline kuroguy  Germany
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87 views and no opinions? Help a brother out. Do I need a buffer? I have no need for a phono amp.
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Old 31st March 2009, 12:18 PM   #5
SY is offline SY  United States
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Perhaps some info on the headphones might help...
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Old 31st March 2009, 12:36 PM   #6
kuroguy is offline kuroguy  Germany
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Thanks SY. the phones are Beyerdynamic DT770/80 pro. These have an 80 Ohm impedance.
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Old 2nd April 2009, 06:42 PM   #7
kuroguy is offline kuroguy  Germany
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Please! I'm just looking for some opinions. Do I need a buffer in this line preamp? I don't need any more amplification. I'll be plugging in about 3 amps downstream of this line preamp. If not, I'll just go ahead with the headphone amp part and finish machining the panel.

Thanks
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Old 2nd April 2009, 08:31 PM   #8
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Do some measurements -- see what sort of voltage your headphones need to be as loud as you like, then measure what sort of voltage the DAC outputs (you can do this with a 60Hz tone and a cheap multimeter set to AC). If these line up such that you don't need any gain, then I'd suggest a mosfet follower (basically the output stage of the starving student -- or see the Szekeres project at Headwize.)

If you have a balanced DAC, then the following modification to the mosfet follower might work for you. As shown, I've been using this with TDA1543's and it is lovely both for powering phones and as a preamp. Any dac chip that can give you sufficient voltage and that is balanced will work, too. The output transformers I used cost $18 each from Cinemag (they say Jensen 123 on the side).

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