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Old 11th June 2006, 02:36 AM   #1
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Default use D1 I/V with voltage output dac

Go easy with me on this as I am relatively new to all of this.

The question is how to use the D1 output stage with a pcm1751, voltage out dac.

I know that a transistor is a current amp.
I know that adding a resistor before the first transistor can convert v to I (and vice versa).
The dac puts out 2.5v
To calculate the resistor:
V=IR, so R = V/I
If we set the desired current to 2ma, then
R= 2.5/.002 = 1250 ohms

Does this seem salient? Is there anything else I should do, like couple with a cap as well?

Thanks
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Old 11th June 2006, 03:46 AM   #2
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Why do you want to do this? The D1 is a I -> V stage, you already have a voltage out, why convert the voltage to a current and then back to a voltage again?

What you need is a buffer and/or some filtering. Look into one of the Pass preamps if you are really set on a Pass design.
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Old 11th June 2006, 04:51 AM   #3
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true, but I already have the I/V board made and working. I had planned on using it with an art dIO DAC (current out) but have decided to leave that one stock. Since I need to connect it with a lead to the dac anyway, adding a resistor is the easiest option. I would like to adapt this output board to various DACs that I have, including a couple of DVD players, an MSB link II, etc. Adding a little resistor noise should not be harmful and probably not an issue.

Additionally, the output stage is extremely similar to his balanced line preamp with the addition of another transistor. One transistor pulls the gate of the other, and with it the higher voltage. I would only need one of these to make a gain stage as the internal opamp can pull the amplifying transistor. I suppose that I could remove the top transistor and replace with some jumpers to create a new circuit. But, adding a resistor would be easier (if possible).

So was my math correct in the first post. You did not answer my question
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Old 11th June 2006, 05:16 AM   #4
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Yes, the math is OK, have fun.
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Old 11th June 2006, 09:23 AM   #5
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Haven't seen it clearly from your post, but use the resistor in series between the Dac Output and the Mosfet Source. No need of a coupling cap.
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Old 11th June 2006, 02:12 PM   #6
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Thanks.
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Old 11th June 2006, 07:47 PM   #7
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I had 1.5k resistors on hand and it is working fine and sounds great. Question, though, the data sheet recommends using a 10uf polarized cap at the voltage output of the dac. I tried inserting the D1 circuit before the cap but it did not work (bad noises) unless after the cap. The data sheet does not describe the reason, only that it is needed and that the dac is ac coupled. Is this blocking DC? Any thoughts? Is there a reason that it would sound as good as it does with some budget cap in the signal path?
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