Add headphone output to preamp

whitelabrat · 24th January 2008, 01:53 PM

I've currently got a pair of headphones rated for 250ohms. Nothing fancy (Koss Pro4aa). I'm wondering what changes are needed to add a headphone output to my preamp. Output caps such as those using in Morgan Jones' design?

http://headwize.com/projects/showfil...=cmoy5_prj.htm

I've attached a schematic of one channel of the preamp.

ErikdeBest · 24th January 2008, 06:01 PM

Hi

In theory, yes, add a large valued output cap parallel to C4 and you could use a headphone. The problem is distortion: although your headphones have a relatively high impedance, asking a 12AU7, even in CF, to drive it is asking for high distortion figures.

One possibility is to make a WCF of that output stage, just as in the project by Morgan Jones you linked to. But that's not that simple, as you would need another tube, which means another hole in the chassis, higher filament current...

What you could do is get rid of the second 12AX7 stage's that is, IMHO , superfluos anyway, and use the hole and filament current for another 12AU7 to make a WCF. And as you are at it, change the 12AU7's for E88CC's... But that is lot of reworking...

Good luck! Erik

AndreasS · 24th January 2008, 07:36 PM

Hello whitelabrat,

you can use this schematic for a headphone with little modifications:

Take the feedback path from the output of the CF, in the best way connect the cathode of V1B to a tap on R11 and omit C2, R7, R8 and R2 (a "ring of three").

Regards Andreas

24th January 2008, 06:01 PM	#2
ErikdeBest mostly thinkerer diyAudio Member Join Date: Mar 2003 Location: Zürich	Hi In theory, yes, add a large valued output cap parallel to C4 and you could use a headphone. The problem is distortion: although your headphones have a relatively high impedance, asking a 12AU7, even in CF, to drive it is asking for high distortion figures. One possibility is to make a WCF of that output stage, just as in the project by Morgan Jones you linked to. But that's not that simple, as you would need another tube, which means another hole in the chassis, higher filament current... What you could do is get rid of the second 12AX7 stage's that is, IMHO , superfluos anyway, and use the hole and filament current for another 12AU7 to make a WCF. And as you are at it, change the 12AU7's for E88CC's... But that is lot of reworking... Good luck! Erik __________________ my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!

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24th January 2008, 07:36 PM	#3
AndreasS diyAudio Member Join Date: May 2005 Location: near Duesseldorf	Hello whitelabrat, you can use this schematic for a headphone with little modifications: Take the feedback path from the output of the CF, in the best way connect the cathode of V1B to a tap on R11 and omit C2, R7, R8 and R2 (a "ring of three"). Regards Andreas