nelsonvandal said:
No need to use anything that large.
Yes. And that's why I was originally thinking of just using your existing buffer rather than starting from scratch.
What's the input impedance of the buffer you're using now?
It can, yes. But I was looking to keep things simple and just use a 1:2 step-up transformer in conjunction with the buffer you're using now in order to get the 6dB of gain you need.
Question. You say you're going to be using this with your DAP which only has a 1 volt output. Will you be driving this from the headphone output of your DAP? Or does your DAP have a separate line output?
That's doable.
se
The input stage are JFETs (2SK170/2SJ74), so I guess the input impedance is quite high.
Yes the DAP has a separate line out, otherwise this whole amp-thing would be almost or totally meaningless. I've tweaked the output by replacing the tantalum caps to 47 uF BlackGate HiQs. The line out drives my 300 Ohm Sennheiser HD650 very well, so I think a lowish input impedance is OK. The phones sound better straight from line out than via the "common type" of portable headphone amps.
Yes the DAP has a separate line out, otherwise this whole amp-thing would be almost or totally meaningless. I've tweaked the output by replacing the tantalum caps to 47 uF BlackGate HiQs. The line out drives my 300 Ohm Sennheiser HD650 very well, so I think a lowish input impedance is OK. The phones sound better straight from line out than via the "common type" of portable headphone amps.
nelsonvandal said:
Well, unless you haven't any shunt resistor on the gate of the input FET, the input impedance will essentially be the value of that resistor.
True enough. Just checking to see what your actual situation is.
Hmmm. Would it be too much trouble for you to check and see what the DC offset is on your DAP's line outs ahead of the capacitors? If it's not terribly significant (a few millivolts) then you could just remove the caps completely as the transformer would provide protection from DC in the event of a failure.
Thanks. That's good to know.
se
Oh yes I know, they are barely enough for 300 Ohms. There's not enough current either for low impedance phones. I just mentioned it to point out that the line out is powerful enough to drive a fairly low impedance. And also to point out that I'm not happy with a common opamp based headphone ampNordic said:
.Steve Eddy said:
I'm not clever enough to tell if there are such resistors. Here's a schematic. http://headwize.com/ubb/showpage.php?fnum=3&tid=6711
Do you mean the 1 M resistor to ground? I use a 100 k resistor in my amp and a 10 k pot.
I think it's a lot more than a few mV, but I will measure it. It would be an improvement not to have to use DC-blocking caps, but it also feels safe to have them there. I don't know how sensitive the DAC/amp chip in the DAP is. Some of the headphone amps I've done with bipolar input have up to 10 mV from line in. The caps do color the sound. I think they are somewhat grainy and dark, but I'm not sure.Steve Eddy said:
nelsonvandal said:
Yes, I was referring to the 1M resistor.
Not sure what you mean when you say you use a 100k resistor and a 10k pot.
Do you mean you use a 100k resistor instead of the 1M resistor and the 10k pot is located ahead of the 100k resistor?
Thanks.
Yes. Since your buffer can pass DC, if there were a failure in your DAP which put a lot of DC on the input of the buffer, it could potentially fry your headphones.
But by the same token, your buffer doesn't have an output cap (at least none is shown in the schematic) so it's possible for the buffer to fail and put DC on your headphones.
Some of that may be from the base current of the input bipolar devices.
se
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