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Old 5th October 2011, 09:55 AM   #1
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Default Increasing gain in ebay PCB

Can anyone tell me the easiest/best way of increasing the gain in this ebay headphone PCB? currently using with LME49710 and diamond buffer, but I don't have enough gain to drive my Grado's:

High current high speed modular headphone PCB stereo !! | eBay

I guess it's best to increase the gain of the op-amp stage. A gain of 10x would be about right I think. Schematics etc are attached.
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Old 5th October 2011, 11:05 AM   #2
Mooly is online now Mooly  United Kingdom
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Are your Grados 600 ohm ? You may have trouble driving these if so.

Although you can alter the gain of the headphone amp you can never get more output than is determined by the supply rails.

Reducing R5 or increasing R6 will alter the gain.

As it is the gain is approximately R5+R6/R6 or 10k plus 1k divided by 1k which is 11

It is approx due to the 1meg effectively in parallel with R6 but the error is small having only a minute effect making R6 9901ohms.

So actual gain is 9901+1000/1000 which is 10.9 rather than 11
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Old 7th October 2011, 01:21 AM   #3
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Sounds like the source is a "Euro-capped" MP3 player, right? It is not uncommon to find maximum output levels of less than 50 mV in these. Most Grados are 32 ohm jobs and not terribly hard to drive.

Artificial volume limitations can be circumvented in some players. You'll be out of luck with anything Sony, but for any of the fruity players (or phones) you could get a line-out dock, and persuading a Sandisk player to enable higher volume levels is very easy, too (even when not running Rockbox on one).
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Old 7th October 2011, 09:30 PM   #4
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Thanks for the help guys, looks like I need to go back and check my resistor values. Sources are CD, tuner etc - I have build a combined Pre-amp/DAC/Phono/Headphone amp all in one case, so it is getting full line level and a good +15/-15V supply to work from.
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Old 7th October 2011, 10:07 PM   #5
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A 3k3 output resistor won't help, when it's supposed to be 47R :/ Never mind, it's sorted now. Just using the diamond buffer circuit now, I am waiting on my LME49600's to arrive. Sounds good already though, much better than the headphone stage in my Nakamichi tape deck after a quick comparison. I am powering mine from a +15/-15V Coffin "super" reg.
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Old 8th October 2011, 06:24 AM   #6
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Pleased to hear it's working OK
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Old 8th October 2011, 12:39 PM   #7
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Quote:
Originally Posted by df_genius View Post
A 3k3 output resistor won't help, when it's supposed to be 47R :/
LOL no, definitely not - nice 40 dB output attenuator there. Even 47R seems higher than necessary, a circuit like this should easily get by with 10 ohms or less. Most of the usual Grados aren't overly fussy when it comes to output impedance and would still be fine with 47 ohms, only the various "1000" models appreciate a lower one. Try 10 ohms, that should give the amp reasonably universal appeal as far as conventional headphones go.

Speaking of resistor values, those in the input and feedback network seem just a touch high for best noise performance, depending on what kind of opamp you use. You can safely scale down all of R1..R6 by a factor of 3. A gain of 21 dB also seems fairly generous, but that's up to you to decide. Otherwise it's a nice concept, Jung multiloop topology and all.

Last edited by sgrossklass; 8th October 2011 at 12:53 PM.
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