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Old 19th February 2012, 11:22 PM   #1
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Join Date: Apr 2003
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Default headphone pcb

Hi,
I'm on with optimizing my philips cd940 as a transport but I would like to improve the headphone output.
I use AKG240 600ohm phones are there any improvements to be made to the headphone pcb to make the best of these phones

op-amp change resistor values etc.

circuit and pcb below
Click the image to open in full size.

Click the image to open in full size.
this is the analogue section of the main board, the headphone feed is taken from the connector at the bottom.

Click the image to open in full size.

Last edited by j baldam; 20th February 2012 at 12:03 AM. Reason: links
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Old 20th February 2012, 07:53 PM   #2
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Join Date: Nov 2004
Location: Germany
Honestly, the output already is best suited for 600 ohm cans like yours as-is. You might want to reduce the value of 3386 and 3387 (120R, once considered sort of a standard) to maybe 10..47 ohms, but I doubt you'd hear much of a difference with your AKGs.

Voltage gain seems near-ideal for the given supplies, too. I would have chosen somewhat lower-value gain setting resistors than 15k and 10k though, since impedance seen by the inverting input (namely 6k) is always higher than that seen by the non-inverting input (varying between 0 and 5k depending on volume setting), and hence the output DC offset caused by input bias current never goes to zero. In any case the offset remains very small though, as in a few mV, and I don't think there'll be a lot of input impedance modulation going on either.
A perfectionist might use 10k/6k8, 7k5/4k7 or 5k1/3k3, maybe even 3k3/2k2 if they are mainly listening at low volumes with very sensitive headphones. (That would cut noise by a maximum of 2..3 dB. But even as-is, it would hardly be an issue, at an estimated 5 µVrms across the audible spectrum.)

So overall, really not a lot to improve there. The guys at Philips did a good job. (Even those LM833s don't seem worth swapping out after a look at the schematic - they're not used in non-inverting mode.)

Last edited by sgrossklass; 20th February 2012 at 07:55 PM.
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Old 20th February 2012, 09:06 PM   #3
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Hi, thanks for the reply.
I was pretty shocked to hear how good the headphones sounded when I bought the cd940 compared to the marantz 63 output.
I suppose I thought that philips have skimped with the design of the output, but maybe in this case they haven't.

cheers
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Old 21st February 2012, 08:22 PM   #4
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Actually I don't see why the CD63 would sound much worse - apart from not having a volume control, which makes the output a joke to begin with. (I looked at the service docs for the newer "plain" CD63 with SM5872, not Mk II or SE.)

Last edited by sgrossklass; 21st February 2012 at 08:29 PM.
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Old 21st February 2012, 08:35 PM   #5
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Hi, there is a large difference on two fronts.
first right from the outset, clarity, no mud here.
secondly dynamics, snare drums have a crack to them that makes them sound like cardboard on the 63(standard).
Probably due to a match for the 600 ohm akgs as the difference is there but less marked through the speakers via analogue out.
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